Astronomy PDF

02 Observing the local sky ========================== **1. Patterns in the sky** **Keywords:** Periodic events, Day - Night cycle ↔ The sky, Patterns in the sky » Constellations, Celestial Sphere - Day ↔ Night » **the sky**: stars, planets, moon, - 2000 - 3000 stars are visible to the naked...

02 Observing the local sky ========================== **1. Patterns in the sky** **Keywords:** Periodic events, Day - Night cycle ↔ The sky, Patterns in the sky » Constellations, Celestial Sphere - Day ↔ Night » **the sky**: stars, planets, moon, - 2000 - 3000 stars are visible to the naked eye during the night. - People of nearly every culture gave names to patterns in the sky: - the patterns of stars seen in the sky → **constellations** » **[Early Astronomers]** used constellations for navigational purposes -or- for calendars to predict planting/harvesting » **[Today\'s Astronomers]** however use the constellations to refer to a large area of the sky (like continents) - During the night the constellations seem to move smoothly across the sky from east to west. » but relative locations of stars remain unchanged as the sky move » the stars must be firmly attached to a **Celestial Sphere** surrounding the Earth (see the picture on the right). **Celestial Sphere:** Earth is at the center. The Sun is having a path on the sphere (yellow). Constellation patterns are drawn on the sphere. Even Milky Way is distributed on the surface of the sphere. - **[Modern Standpoint:]** apparent motion of stars is the result of the spin (rotation - not of the celestial sphere but of the earth) » Even though Celestial Sphere is an incorrect description we still use the idea as a convenient fiction to visualize the position of the stars in the sky. - **[Illusion:]** The stars in a particular constellation may appear to lie close to one another » but they may be quite far apart in reality because they may lie at very different distances from Earth. » due to lack of perception when we look into space » we should be concluding that **the stars are so far away** **Celestial Sphere:** A clear view of the sphere with only the Sun\'s motion (ecliptic: red line). Poles and equator of the sphere are emphasized as well as rotation axis of the Earth. **2. Circling the sky** **Keywords:** Perception: motion of objects on the sky, Coordinate System-1: Local sky, Measurements: angles, Angular units, Coordinate System-2: Location on Earth, Coordinate System-3: Path of Stars on sky, Annual changes. **Angle Measurements** - Due to lack of perception in the sky we cannot tell the true **sizes of objects** or the **true distances** between objects just by looking at them. » Instead we measure **angles** in the sky: - Angles are measured in sexadecimal units: **Angles:** Measuring angular size of an object. **Angles:** Measuring angular distances between a pair of objects. **The Local Sky** - Stars rise and set - like the sun, moon and planets » In reality, we are the ones who are moving. - Objects appear to rise in the **east** and set in the **west** » because Earth rotates in the opposite direction: **west → east** - If you could view the celestial sphere from outside, » the daily motion of the stars would appear as **circles** - Since we live [on] Earth, we see only **[half]** of the celestial sphere. - It is called the local sky. - Two angle measurements are required to locate any object in the local sky. - Each angle has a **zero point.** 1. Azimuth (direction) - Starts from North, measured along East, South, West and back to North 2. Altitude (elevation) - Starts from the horizon rises up to Zenith (direction where your head is pointing). **The Local Sky:** The sky as seen from wherever you happen to be standing. The zenith is where your head is pointing upwards when you stand. The meridian is an imaginary half circle: North → Zenith → South. **Geographical Coordinate System** - The local sky can be stretched to the whole Earth surface. » creating a new coordinate system: **geographical**  » Similarly, a location on Earth has to be represented with two angle measurements: 1. Longitude (along the rotation direction of Earth) - Starts from Greenwich, plus values towards East, minus values towards West. 2. Latitude (subtended from Equator) - Starts from Equator, plus values towards North, minus values towards South. **Geographical Coordinate System.** **Celestial Coordinate System** - The local sky can be extended further until it reaches to the **celestial sphere** which has a radius of infinity. - When the local sky is ***overlayed*** to the celestial sphere » **motion of objects** in the sky can be observed. » creating **paths** on the celestial sphere. - Circumpolar stars never rise or set but instead make daily counterclockwise circles around NCP - Non-circumpolar stars never rise and remain constantly below the horizon. - The rest of the stars daily rise in the east and set in the west. - The paths of the Sun, Moon and planets follow these rules as well. - From the geometry and by observing the local sky, **[Geographic Location on Earth]** can be calculated: » **Altitude of the Celestial Pole = Your Geographic Latitude**  - Similarly each object on the celestial sphere can be identified from two angles. 1. Right Ascension 2. Declination See [Celestial Coordinate System](https://www.wikiwand.com/en/Celestial_coordinate_system) [**⤤**](https://www.wikiwand.com/en/Celestial_coordinate_system) for more details.  **Celestial Sphere:** Local sky is overlayed (and streched) to the celestial sphere. **Annual Changes** - As we orbit the Sun over the course of a year it appears to move against the background stars in the constellations. - If we could see both the Sun and the stars we would notice the Sun gradually moving [eastward along the ecliptic.] - The constellations along [the ecliptic] are called constellations of the **Zodiac.** **3. The reason for seasons** **Keywords:** Earth centered view vs Sun centered view, Definition: Ecliptic, Definition: Solstices and Equinoxes, Definition: Seasons, Calculating periodic changes =\> Tropical Year. **Celestial Sphere:** Earth centered view **Solar System:** Sun centered view **Definitions** - The apparent motion of the sun on the sky over the course of a year relative to the stars, defines a path on the celestial sphere known as **[ecliptic]** - Earth\'s axis remains pointed in the same direction in space (to the star called **[Polaris]**) - The axis doesn\'t change the direction in which it is pointing. The change in which the hemisphere is tipped toward the sun occurs only because Earth moves between opposite sides of the sun in its orbit. That\'s why the **[two hemispheres experience opposite seasons]**. - The Northern Hemisphere tilt of the Earth\'s axis causes warmer summers and cooler winters. Thus, due to the height of the sun above horizon and length of the day » **[we feel the seasons]** **Solstices and Equinoxes** - **[Summer Solstice]**: the point on the ecliptic where the Sun is at its Northernmost point above the celestial equator. - **[Equinoxes]**: the two points where the ecliptic intersects the celestial equator (short for **equal day and night**) - **[Vernal (spring) Equinox:]** - associated with the end of winter - the start of a new growing season - time keeping: **Annual Changes:** Maximum and minimum elevations of the Sun corresponds to solstices, and in between equinoxes occur. **4. The precession of Earth\'s axis** **Keywords:** Definition: Precession, Earth\'s Precession, Axis tilt. - We can notice the daily and annual changes. - However a much longer cycle exist: a gradual change in the direction that Earth\'s axis points in space: **precession** - Precession can occur with any **[rotating object.]** - Each cycle of Earth\'s precession takes about **[26 000 years]** - In about 13 000 years, the axis will point to the star Vega - Axis **[tilt remains close to 23.5 degree]** throughout the cycle. - Why precession:  - It is caused by gravity\'s effect on a tilted rotating object - Object should not be a perfect sphere - Law of conservation of angular momentum - for Earth:  - We observe pulls from the Sun and Moon; they **[try to reduce the tilt]**. - However since Earth continuously rotates around the same axis this tug of war between gravity and rotation precesses the axis. **Precession:** One cannot easily feel an annual change in Earth axis. However, it also wobbles on a much longer scale (26 000 years). The region around the celestial pole is given on the right panel. The path of precession of Earth\'s axis (yellow) is given starting from 1 AD. **5. The Moon: Our constant companion** **Keywords:** The Moon, Change in its appearance: phases, Periodic changes: Synodic/Sidereal Month, Seeing the same face of the Moon, Eclipses, Eclipse: Lunar, Eclipse: Solar. - Nearest object to Earth and second bright in the sky - We observe two types of **change** in the Moon\'s motion in the sky. - Appearance - Rise and set times » these changes lead to → **Lunar Phases** - Distinctive Phases: - New Moon: not visible (The Moon cannot be distinguished from Sun\'s light) - First and Third Quarters: The angle between Sun - Earth - Moon is 90 degrees - Full Moon: The disk of the Moon is fully illuminated by the Sun. **Lunar Phases.** Experiment: Circle around yourself a spherical object while it is lit by an homogeneous light source. Observe how dark regions (not lit) appear, change and disappear. **Periods and motion of the Moon** **Sidereal Month:** The time required to complete one revolution around Earth with respect to background stars → 27.3 days (360 degree rotation around the Earth). **Synodic Month:** The period of completing phase cycle (eg. New Moon to New Moon) → 29.5 days (takes longer than Sidereal Month). **The Moon\'s phase.** We always see (nearly) the same face of the Moon. This is due to the synchronous rotation of the Moon with Earth. Therefore, the Moon\'s 1 rotation around itself (rotational period) is equal to 1 rotation around Earth (orbital period). **Eclipses** **Definition of an eclipse:** Any time one astronomical object casts a shadow on another. Due to the geometry of lunar phases one would expect the followings:  - A new moon always blocks our view of the Sun. - Earth would always prevent sunlight from reaching a full Moon. - So, why there is **not** always a solar/lunar eclipse every new/full moon? » **Moon\'s orbit is inclined** to the ecliptic plane by \~ 5 degrees. **Eclipse.** Casting a shadow on an object. **Definition of Nodes:** The two points in each orbit (Moon\'s around the Earth and Earth\'s around the Sun) at which the Moon crosses the ecliptic plane. - The nodes of the Moon\'s orbit must be nearly aligned with the Sun and Earth. - The phase of the Moon at this alignment must be either new or full. **Predictions:** The times when the lines of nodes are directed towards the sun are favorable. » eclipse seasons \~ 18 years, 11 1/5 days → **Saros Cycle** **Nodes.** Eclipse occur only when three objects are aligned at nodes. **Lunar Eclipse.** Because of the shadow size Lunar eclipses takes hours. **Solar Eclipses.** Since the shadow of the Moon is narrow, its cast on Earth moves fast (Shadow speed: \>1700 km/hr \~ 470 m/s) and lasts shorter. Thus, creating three different types of solar eclipses: Total (Moon covers the sun totally), Partial (Moon passes over Sun with an offset), Annual (Moon is closer to Earth, creating a ring on the sun). **6. Celestial timekeeping** **Keywords:** Sidereal day vs Solar day, Synodic month vs Sidereal month, Tropical year vs Sidereal year, Planetary Periods. A **sidereal day** is the time it takes any star to make a circuit of the local sky. A **solar day** is measured similarly but by timing the sun rather than a star. **Solar vs Sidereal Day** - Solar Day = 24h 00m 00s (noon/midnight to noon/midnight) - Sidereal Day = 23h 56m 4.098s **Synodic vs Sidereal Month** - Synodic Month = 29.5 days (due to phases) - Sidereal Month = 27.3 days **Tropical vs Sidereal Year** - Tropical Year = 365 days - 20 minutes (due to seasons) - **Reason**: Precession of Earth\'s axis. Each year the location of the equinoxes and solstices among the stars shifts about 1/26 000 of the way around the orbit. This amounts to 1/26 000 \~ 20 minutes - Sidereal Year = 365 days **Planetary Periods** - **Sidereal Period:** It is the time it takes to orbit the Sun. - **Synodic Period:** It is the time between being lined up with the Sun in our sky one time, and the next similar alignment. - **Conjunction:** elongation of 0 degree (inferior: **Sun** - Planet - Earth; superior: Planet - **Sun** - Earth) - **Opposition:** elongation of 180 degrees - **Quadrature:** elongation of 90 degrees 03 Law of Radiation =================== **1. Light & Radiation** How do astronomers **know anything** about objects far from Earth\ ***without traveling to them***?  How do we **obtain detailed information** about any planet, star, galaxy\ ***too distant to travel*** or to perform controlled experiment? **Andromeda Galaxy.** What do you see in the picture? **Interpret the electromagnetic radiation emitted by these objects** - **Radiation:** transmitting energy between two points **with no physical contact** between the points - **Electromagnetic:** Energy is carried in the form of **electric** and **magnetic** fields. For example: - **Visible Light:** part of EM spectrum that human eye is sensitive. - **Invisible Light:** part of EM spectrum undetected by human eye. (Radio, Infrared, Ultraviolet, X-rays, Gamma-rays). **EM spectrum.** It contains both visible (to human eye) region and invisible regions. Therefore, the following terms *refer almost to the same thing:* **LIGHT - RAYS - RADIATION - WAVES** **2. Wave motion** All EM radiation travel through space in the form of waves. - **Wave** is a way in which energy is transferred from one place to another without the physical movement of material from one location to another. - **Wave motion:** The energy is carried by a disturbance of some sort. - A wave is **not** a physical object: ➤ **Pattern** of up and down motion. If a wave moves at **HIGH** (*SLOW*) speed the number of crests/through passing any given point per unit time is **LARGE** (*SMALL*): **Waves of Radiation   ≇   Waves of Water/Sound** (**doesn\'t** need medium)                                               (**does** need medium) **3. Light** - Color  =  frequency/wavelength of light\ (e.g. Red: 700 nm, Violet: 400 nm) - Measuring Wavelength: **4. Charged Particles** - a charged particle :   MASS  +  **CHARGE** (note that mass and charge are properties of the matter) - electron (e) or proton (p): - building blocks of atoms - they carry the basic unit of charge - this is true for **every charged** particle and for **every *other* charged** particle in the universe **Electric Force** (attractive or repulsive)   **≈    Gravitational Force** ≉                     (always attractive) **5. Transmission of electric force** **Electric Field** **Electric Field and Charged Particles.** Electric field lines extends from the charged particles. These lines are a measure of its force exerted to other charged particles **Transferring Information Through Waves.** Particles vibrates (**heat, collision etc.**): →  position changes →  electric field changes →  the force exerted on other particles changes By measuring variations (i.e. on waves) of Electric Field on the **distant** charges one can gather information from actual vibrating particles. **Magnetic Field** - Magnetic Field must accompany every changing Electric Field. - Magnetic Field exerts forces on **moving electric charges** (i.e. electric currents). - Conversely, moving charges **create** magnetic fields. Thus, Electric and Magnetic Fields are linked to one another. A change in either one necessarily creates the other. Note also that: - moving charge create **disturbance** (consists of both E and B). - E and B are perpendicular to one another. - They don\'t exist as independent entities. - They are different aspects of the same phenomenon: **ELECTROMAGNETISM**. **Speed of Transmission** - How quickly does one charge **feel** the ***change*** in the EM field when another begins to move? - EM wave moves at **c** (speed of light):  c = 299 792.458 km/s   \~   300 000 km/s - It is *large* but **finite** ! - We can never observe the universe **as it is**, only ***as it was***. **6. EM spectrum** **Electromagnetic Spectrum.** Comparing all at once: colors, bands, frequency, wavelength, size, atmosphere and opacity. **Opacity** - The more opaque an object is, the less radiation gets through it (opposite of transparency). - Opacity varies; why? - Certain atmospheric gases absorb radiation at some wavelengths. - H~2~ and O~2~      →  absorbs Radio waves ( ƛ \< 1 cm) - H~2~O and CO~2~   →  absorbs Infrared waves - Ozone layer    →  absorbs UV, X-ray, Gamma rays - Ionosphere     →  reflects ƛ \> 10 m. (the layer is at 100 km above sea level) **7. Thermal radiation** - All macroscopic objects emit radiation at all times regardless of their size, shape or chemical composition. - **Temperature:** Temperature of an object is a direct measure of the microscopic motion within it. - **Intensity:** The amount/strength of radiation at any point in space. - Energy is generally spread out over a range of frequencies. - not just one frequency  →  ***distribution***  →  ***properties*** of the object can be reached. **BLACK BODY CURVE** An object that ***absorbs*** **all radiation** falling on it and it must **re-emit** the same amount of energy it absorbs. - No real object absorbs and radiates as a perfect black body. However, the blackbody curve is a good approximation to the reality. - As the object\'s temperature increases → radiation\'s peak frequency shifts → however shape of the curve remains the same - Well known experience: as the temperature of the object **increases ** the color of the object **changes**: → normal color → beginning to glow in red → red hot → white hot **Wien\'s Law** So, the relation between wavelength and absolute temperature is: (where **ƛ** is wavelength of the peak emission) **Reality and Applications in Astronomy** - **No** natural object reach temperatures high enough to emit radiation at **very-high-frequencies** - e.g. thermonuclear explosions  ➨ peak in **X-ray** / **Gamma-ray** - e.g. high-frequency radiation from the most human invented devices ➨ objects cannot attain high temperatures ➨ i.e **non-thermal** radiation - Many extraterrestrial objects ➨ radiation in **UV**, **X-ray** even Gamma-ray Thus, black body curves are used as thermometers to determine the **temperature of distant objects** **8. Doppler Effect** When either the observer or the object is in motion the EM waves received by the observer shifts according to the direction of the motion. **Examples:** - **observing stars** which rotate with very high velocities - listening **sound waves** near the traffic. **9. Spectroscopy** **Spectrum:** a splitting of the incoming radiation into its component wavelengths. - All spectra **deviate** from its idealized form (i.e. Black Body Curve). - However; this deviation **contains** the detailed information about physical conditions in the source of the radiation. **Spectroscope:** the instrument to analyze the radiation. **Type of Spectrums** **Continuous Spectrum** Radiation in all wavelengths with an intensity distribution that is well described by the blackbody curve **Check the screen:** - observe rainbow of colors without interruptions **Absorption Spectrum** Wavelengths of light that have been removed (absorbed) by the gas between the source and the detector. **Check the screen:** - Absorption lines associated with a gas occur at precisely the same wavelengths as the emission lines produced when the gas is heated. **Emission Spectrum** The particular pattern of light **emitted** by a gas of a give chemical composition. **Check the screen:** - a few narrow well defined lines - with a black background (i.e not emitted by Hydrogen) **Note:** - Intensity can be altered but not the wavelength. **Kirchoff Laws (1859)** 1. A luminous solid or liquid, or a sufficiently dense gas, emits light of all wavelengths and so produces a continuous spectrum 2. A low density, hot gas emits light whose spectrum consists of a series of bright emission lines that are characteristic of the chemical composition of the gas. 3. A cool, thin gas absorbs certain wavelength from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum. **10. Atomic Structure** **Classical Atom** **Modern Atom** - **How:** ground state  ➨  excited state by - atom absorbs EM radiation - matter collides with another matter - **How long:**  \~ 10^-8^ s then drops to ground state - **How much:** Energy   **∝**  frequency   ➨  E = h f (where h=6.63 × 10^-24^ J) **Spectral Information** - Observe **Peak Frequency** (for continuous spectrum only)   ➨  Derive ***Temperature*** (using Wien\'s Law) - Observe **Lines** or **Line Intensities**  ➨  Derive ***Composition Temperature*** - Observe **Line Width**   ➨  Derive ***Temperature*** and/or ***Rotation Speed*** and/or ***Density*** - Observe **Doppler Shift**  ➨  Derive ***Line of Sight Velocity*** 04 Observational Techniques =========================== **1. Recording Light** A light collector whose main function is to capture as many photons as possible from a given region of the sky and concentrate them into a focused beam for analysis. **point   ➤  collect   ➤  focus** **Optical Telescope:** It is the one designed to collect wavelengths of radiation that are visible to human eye. **Mirror size \"increases\"   ➤   amount of collected light \"increases\"** **Telescope Types** - **Refracting:** a ***lens*** focuses the light - **Reflecting:** a ***mirror*** focuses the light **Instrument Types** - Photometer. - Measures \"**light**\". It measures the total amount of light received in all or part of the image. - Spectrometer. - Measures \"**spectrum**\". It distributes the total energy into its wavelengths creating a spectrum. **2. Telescopes** **Refraction vs Reflection** - Large lenses cannot be constructed - the lens in a refracting telescope focuses Red and Blue light differently. This deficiency is known as chromatic aberration. - You can correct but cannot eliminate - As light passes through the lens some of it is absorbed by the glass. This is important for IR and UV observations. - No such effect occurs for mirrors - Lenses are heavy - Both sides of lenses have to be processed but for mirrors only one side has to be managed. **Effect of Refraction.** Red/Blue light focuses at different points after it is refracted. This is called chormatic aberration. **Types of Telescope Designs** - Prime Focus - Single reflection from the primary mirror. Difficult to orient and observe - Newtonian. - Double reflection. The light beam is directed outside of the tube, just before it is focused using a secondary mirror. - Cassegrain. - Double reflection. The same as Newtonian however the light beam is reflected back to the primary which passes through the hole at the center of the primary. - Nasmyth / Coude. - Triple reflection. The same as Cassegrain however the light beam is deflected outside the tube before it reaches to the primary. **Special Telescopes** - Schmidt. - A coma (bright central object having a tailed structure) appears as we move away from the center of the field of view. Its size increases further away from the center. - To correct the coma a correcting lens introduced right before the beam enters to the tube: Schmidt Telescopes. - Their constructions are expensive (mirror + lens). However, produced image quality is high. They are mostly used in surveys and astro-photography. **Telescope Size** Mirror size **increases**\ ➤ Amount of collected light **increases** **Therefore:** Observed brightness\ **∝** Area of the mirror (D x D) **Faster Collection:** Mirror is said to be collecting light fast if its mirror size is large **Size Comparison.** (b) Taken with a mirror size twice as (a) **3. Power of Telescopes** **Resolving Power** - The ability of any optical device to form **distinct**, **separate images** of objects lying close together in field of view. Finer Resolution ➤ Better distinguishing the objects - **Angular Resolution:** It is the factor that determines our ability to see the fine structure. Objects \"close together\" ➤ Objects are separated by \"a small angle\" - Resolution - is proportional with **wavelength** and - is inversely proportional with to **mirror size** **Effect of Improving Resolution.** Resolutions of 10 arcmin, 1 arcmin, 5 arcsec, 1 arcsec from (a) to (d), respectively. **Resolving Power.** From (a) to (c) resolving power of the telescope increases. **Limit of Resolution** - Diffraction (bending of light around corners) causes parallel beam of light to spread out slightly ➤ beam is not focused to a sharp point ➤ a fuzziness created **Degree of Fuzziness   ➤   Determines the angular resolution of telescope** Therefore; - As **wavelength increases** (i.e. observing the objects in red part of the spectrum) ➤ diffraction **increases** ➤ angular resolution (as a value) **increases** **➤** therefore resolving the objects **gets worse** **4. Very Large Telescopes** - Mirrors are made from quartz blocks - One needs years of engineering to construct just the mirror of the telescope. - The largest ***single mirror*** telescope is **BTA-6** (6 meters) in Zelenchukskaya, Caucasus build in 1976. Next largest is Hale Telescope (5 meters) in Palomar Observatory, California constructed in 1948! **How to increase the diameter of a telescope today?** - Use smaller sized mirrors as segments - Combine them in an hexagonal (like a honeycomb) construction and align them to focus like a single mirror. - This type of telescopes are called **segmented mirror telescopes**. - Using this type of construction mirror size can be increased tens of meters. Examples: - twin Keck telescopes (Mauna Kea, Hawaii - 36 x 1.8 m = 10 m) - constructed in 1992/1996 - GTC (Canary Island, Spain - 36 segments = 10 m) - constructed in 2009 © 2023 ATASAM - DAG (Doğu Anadolu Gözlemevi - Eastern Anatolia Observatory). 4 m Telescope with 2 Nasmyth Platforms. **5. High Resolution Observations** In theory you can reach 0.02\" with a 5 meter telescope. But in reality you cannot do better than 1\". - Reason: Earth\'s turbulent atmosphere which blurs the image even before the light reaches our instrument. - Blurring: The light from the star is refracted slightly in the atmosphere. ➤ the stellar image dances around on the detector (or on our retina) ➤ creating **twinkling** of stars. So; - on a good night - at the best observing site - the best angular resolution is slightly **\< 1\"** This creates what is called ***seeing***: - If you take the photo of this twinkling (see the Figure): - The disk where the star\'s light is spread over is called **seeing disk**. - So, to achieve the best possible seeing, telescopes are sited - on mountain tops, - in regions of the world where atmosphere is known to be fairly stable and - relatively free of dust, - free of moisture and - away from the light pollution from cities. **Current best resolutions** - Hubble Space Telescope - 2.4 meter - 0.05\" - NTT (Chile) - 3.5 meter - 0.5\" (active optics) - Keck Telescopes - 10.0 meter - 0.25\" **Active Optics** Control mirrors based on temperature and orientation  **Adaptive Optics** Track atmospheric changes with laser; adjust mirrors in real time. **Adaptive Optics in Action** (a) The improvement in image quality produced by such systems can be seen in these images acquired by the 8-m Gemini telescope atop Mauna Kea in Hawaii. The uncorrected visible-light image (left) of the star cluster NGC 6934 is resolved to a little less than 1". With adaptive optics applied (right), the resolution in the infrared is improved by nearly a factor of 10, allowing more stars to be seen more clearly. (b) These visible-light images were acquired at a military observatory atop Mount Haleakala in Maui, Hawaii. The uncorrected image (left) of the double star Castor is a blur spread over several arc seconds, giving only a hint of its binary nature. With adaptive compensation applied (right), the resolution is improved to a mere 0.1", and the two stars are clearly separated. **6. Radio Astronomy** Radio Telescopes collects photons at radio frequencies: - [Jansky discovered (in 1931)](https://www.wikiwand.com/en/Karl_Guthe_Jansky#Radio_astronomy) a faint static \"hiss\" that had no apparent terrestrial source. - It is then identified as a space source which is now known that it was the Galactic Center. - Their working principles are the same as reflecting telescopes - Radio telescopes use prime focus. - However to change the frequency you have to re-tune the instrument. - However, since radio wavelengths are longer than visible they are less sensitive to imperfections and therefore they can be made very large: Resolution \~ Wavelength / Diameter - The best angular resolution is around 10\". - Note that the radio radiation arriving at Earth is \< E^-12^ W. [**100 m Robert C. Byrd Green Bank Telescope.**](https://greenbankobservatory.org/) [**Arecibo Observatory**](https://www.wikiwand.com/en/Arecibo_Observatory). An aerial photograph of the 300-m-diameter dish at the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico. The receivers that detect the focused radiation are suspended nearly 150 m above the center of the dish.  Longer wavelength means poor angular resolution. However, radio astronomy has different advantages too: - Observations can be carried out for **24 hours**. - Cloud, rain and snow **don\'t interfere** the observation - It creates another world at an **invisible** wavelength to human vision. **7. Interferometry** - **Definition:** Combine information from several widely spread radio telescopes as if they came from a single dish (like a segmented optical telescope) - **Resolution** will be that of dish whose diameter = largest separation between dishes.g - Interferometry involves combining signals from **two receivers**; the amount of interference depends on the direction of the signal. - Interferometry can get radio images whose resolution is close to optical (similar to using adaptive optics) - Interferometry can also be done with visible light but is much more difficult due to shorter wavelengths. **8. Multi-wavelength Astronomy** **9. Space-based Astronomy** 05 Solar System =============== **1. The inventory** **Early Astronomers** - Moon - Planets: *Mercury, Venus, Mars, Jupiter, Saturn* - Comets - Meteors - *shooting stars* They didn\'t have any idea about the big picture ***Do you have the big picture in this century?*** **Galileo Galilei (17 cc)** - He used a telescope (a very simple one) - Noticed phases of Venus - Discovered moons of Jupiter - Found something unusual around Saturn **End of 19 cc** - Saturn\'s ring system (1659) - Uranus (1781) - Neptune (1846) - Many planetary moons - The first asteroid: Ceres (1801) **20 cc** - Pluto - Ring systems around Jupiter, Uranus and Neptune - Dozens of moons - Thousands of asteroids **Also** - Non-optical astronomy (radio, infrared) - Spacecraft exploration - Exploration of the Moon - Unmanned probes to other planets **The Final Inventory** - **1** Star - **8** Planets (4 Terrestrial, 4 Jovian) - **5** Dwarf Planet - 1801 - Ceres (asteroid belt) - 1930 - Pluto - 2004 - Haumea - 2005 - Makemake & Eris - **194** Moons (and counting) - **\~700 000** Asteroids in total (and counting) - **10** Large Asteroids - Ceres, Pallas, Juna, Vesta, Astraea,\ Hebe, Iris, Flora, Metis, Hygenia - **239** Asteroids (\> 100 km in diameter) - **1 trillion** Comets (estimated) - **6339** known Comets (as of 2018) - **Countless** Meteoroids **2. Measuring the Planets** **Distance** - Known by **Kepler\'s Law** **Orbital Period** - Obtained by **observation**: repeated observation of its location on the sky **Radius** - Known by its **angular size** **Mass** - Known by **Newton\'s Law**: gravity of moon\'s orbits around the planets **Rotation Period** - Obtained by **observation**: repeated observation of the disk of planets **Density** - Obtained by **calculation**: if radius and mass known - **Mercury** and **Venus** are difficult to determine. They produce small but measurable effects on each other\'s orbits as well as that of Earth i.e wobbles. - **Ceres** is the most difficult one because of its very weak gravity. - Now **space probes** are used to measure these parameters - **Sun** occupies **99.9%** of the solar system! **3. The layout** - Sun - Neptune: 30 AU (1 AU = 150 million km) - 1 500 000 x Earth\'s Radius -        15 000 x Earth - Moon distance - Even with these huge distances all ***planets lie close to the Sun*** - All the planets lie on the same plane (almost) - **exception**: Mercury: 7 degree (Pluto: 17 degree) - All paths of planets are **ellipses** (Sun is at one foci) - Most of them have low eccentricity (how much different than a circle) - **exception**: Mercury (and Pluto) - All the planets orbit the Sun in Counter Clockwise (CCW) **Titius - Bode Law (1766)** - Orbits are **not evenly spaced** - They get farther and farther apart as distance increases - However, there exist some kind of **regularity** - This is experimental; No simple explanation exists! **4. Grouping** **Terrestrial Planets** - Naming: *earth land; Earth like* - Inner: Mercury, Venus, Earth, Mars - Small - Dense - Rocky **Jovian Planets** - Naming: *Jove; god Jupiter; Jupiter like* - Outer: Jupiter Saturn Uranus, Neptune - Large - Low Density - Gaseous **Notes in Terrestrial Planets** - All have atmospheres - All atmospheres are completely different - Mercury: near vacuum - Venus: dense inferno - Earth: Oxygen in the atmosphere, liquid water on the surface - Earth and Mars have similar spin rates (i.e 24 hours) - Spin rates of Mercury and Venus are *in months* - Venus rotates in Clockwise - Earth and Mars have moons - Earth and Mercury have measurable **magnetic fields** - Venus and Mars have *none* **Kuiper Belt** - View of the solar system \"beyond the Neptune\" (including exploration history) - The belt is extended from Neptune\'s orbit (30 AU) to approximately 50 AU. **Oort Cloud** - Cross-sectional view of the solar system (distances are in AU) **5. Interplanetary Matter** **Relatively Large Bodies** - Asteroids (in between Mars and Jupiter) - Kuiper Belt objects (beyond Neptune) **Small Bodies** - Comets (at the end the solar system) - Meteoroids (everywhere in the solar system) - Dust: Created by continuous collision of large bodies **6. Main Structure of the Solar System** **Terrestrial Planets** - The four terrestrial planets (Mercury, Venus, Earth, Mars). - They are relatively small, have solid, rocky surfaces, and have an abundance of metals deep in their interiors. - They have few moons, if any, and none have rings. - We often count our Moon as a fifth terrestrial world, because it shares these general characteristics, although it\'s not technically a planet.  **Jovian Planets** - The Jovian planets have little in common with the terrestrial planets. - They are much larger in size and lower in average density than the terrestrial planets, and each has rings and numerous moons. - Their composition is very different from that of the terrestrial worlds. - The Jovian planets are made mostly of hydrogen, helium, and hydrogen compounds--compounds containing hydrogen, such as water (H~2~O), ammonia (NH~3~), and methane (CH~4~). **Asteroids and Comets** - Asteroids are small, rocky bodies that orbit the Sun much like planets, but they are much smaller than planets. - Even the largest of the asteroids have radii of only a few hundred kilometers. - Most asteroids are found within the relatively wide gap between the orbits of Mars and Jupiter that marks the asteroid belt. - They orbit the Sun in the same direction and nearly in the same plane as the planets. - More than 10,000 asteroids have been identified and cataloged, but these are probably only the largest among a much greater number of small asteroids.  **7. Asteroids** - The main asteroid belt, along with the orbits of Earth, Mars, and Jupiter. - Note the Trojan asteroids at two locations in Jupiter's orbit. - Apollo orbits: Earth-crossing. - Amor orbits: Mars-crossing. - The largest: - Ceres - 940 km - Pallas - 580 km - Vesta - 540 km (showing volcanism) **Top:** Orbits of planets (blue lines) around the asteroid belt (white dots) and Jupiter trojans (green and red dots).\ **Right, upper panel:** Apollo orbits (green shade).\ **Right, lower panel:** Amor orbits (green shade) with Mars trojans (brown shades).\ **M:** Mars, **V:** Venus, **E:** Earth, **H:** Mercury. **Asteroid types:** - C-type: Carbonaceous, dark - S-type: Silicate (rocky) - M-type: Metallic; iron and nickel \(a) The S-type asteroid Gaspra, as seen from a distance of 1600 km by the space probe *Galileo* on its way to Jupiter. \(b) The S-type asteroid Ida, photographed by *Galileo* from a distance of 3400 km. (Ida's moon, Dactyl, is visible at right.) **Right Panel:** A mosaic of detailed images of the asteroid Eros, as seen by the *NEAR* spacecraft (which actually landed on this asteroid). Craters of all sizes, ranging from 50 m (the resolution of the image) to 5 km, pit the surface. The inset shows a close-up image of a "young" section of the surface, where loose material from recent impacts has apparently filled in and erased all trace of older craters.  **Kirkwood Gap** Some asteroids, called Trojan asteroids, orbit at the L~4~ and L~5~ Lagrangian points of Jupiter's orbit \(a) The distribution of asteroid semi-major axes shows some prominent gaps caused by resonances with Jupiter's orbital motion. Note, for example, the prominent gap at 3.3 AU, which corresponds to the 2:1 resonance---the orbital period is 5.9 years, exactly half that of Jupiter. \(b) An asteroid in a 2:1 resonance with Jupiter receives a strong gravitational tug from the planet each time they are closest together (as in panels 1 and 3). Because the asteroid's period is precisely half that of Jupiter, the tugs come at exactly the same point in every other orbit, and their effects reinforce each other. **8. Comets** Comets move in highly eccentric paths that carry them far beyond the known planets. Halley's comet has a smaller orbital path and a shorter period than most comets, but its orbital orientation is not typical of a short-period comet. Sometime in the past, this comet must have encountered a Jovian planet (probably Jupiter itself), which threw it into a tighter orbit that extends not to the Oort cloud, but merely a little beyond Neptune. Edmund Halley applied Newton's law of gravity to predict this comet's return  - Typical cometary mass: 10^12^ to 10^16^ kg - Each trip close to the Sun removes some material; Halley's comet, for example, is expected to last about another 40,000 years - Sometimes a comet's nucleus can disintegrate violently, as comet LINEAR did. \(a) Diagram of a typical comet, showing the nucleus, coma, hydrogen envelope, and tail. The tail is not a sudden streak in time across the sky, as in the case of meteors or fireworks. Instead, it travels along with the rest of the comet (as long as the comet is sufficiently close to the Sun for the tail to exist). Note how the invisible hydrogen envelope is usually larger than the visible extent of the comet; it is often even much larger than drawn here.  \(b) Halley's Comet in 1986, about one month before it rounded the Sun at perihelion.  \(a) A comet with a primarily ion tail. Called comet Giacobini--Zinner and seen here in 1959, its coma measured 70,000 km across; its tail was well over 500,000 km long. \(b) Photograph of a comet having both an ion tail (dark blue) and a dust tail (white blue), both marked in the inset, showing the gentle curvature and inherent fuzziness of the dust. This is comet Hale--Bopp in 1997  \(a) Halley's comet as it appeared in 1910. Top, on May 10, with a 30° tail, bottom, on May 12, with a 40° tail. (b) Halley on its return and photographed with higher resolution on March 14, 1986.  As it approaches the Sun, a comet develops an ion tail, which is always directed away from the Sun. Closer in, a curved dust tail, also directed generally away from the Sun, may appear. Notice that the ion tail always points directly away from the Sun on both the inbound and the outgoing portions of the orbit. The dust tail has a marked curvature and tends to lag behind the ion tail.  **Origins of Comets:** **They come from two distinct regions of space.** 1. The first region, called the **Kuiper belt** extends roughly from the orbit of Neptune to about three times Neptune\'s distance from the Sun (that is, from about 30 to 100 AU). - The Kuiper belt comets orbit the Sun in the same direction as the planets, and their orbits generally lie close to the plane of planetary orbits. - In fact, some of the known Kuiper belt comets are nearly as large as Pluto, leading scientists to suspect that Pluto may simply be an unusually large member of this group. 2. The second and much larger region, called the **Oort cloud**, may extend more than one-fourth of the way to the nearest stars. - Comets in this region have orbits around the Sun that are inclined at all angles to the plane of planetary orbits and go in every possible direction around the Sun. - Thus, the Oort cloud would look roughly spherical in shape if we could see it. - It is so vast, however, that even with a trillion comets each comet is typically separated from the next by more than a billion kilometers.  **Summary of the group** **9. Meteors** A bright streak called a meteor is produced when a fragment of interplanetary debris plunges into the atmosphere, heating the air to incandescence. \(a) A small meteor photographed against stars and the Northern Lights provide a stunning background for a bright meteor trail. \(b) These meteors (one with a red smoke trail) streaked across the sky during the height of the Leonid meteor storm of November 2001  A meteoroid swarm associated with a given comet intersects Earth's orbit at specific locations, giving rise to meteor showers at certain fixed times of the year. - A portion of the comet breaks up as it rounds the Sun, at the point marked 1. - Fragments continue along the comet's orbit, gradually spreading out (points 2 and 3). - The rate at which the debris disperses around the orbit is much slower than depicted here. It actually takes many orbits for the material to disperse as shown, but eventually the fragments extend all around the orbit, more or less uniformly. - If the orbit happens to intersect Earth's, the result is a meteor shower each time Earth passes through the intersection (point 4). Larger meteoroids are usually loners from the asteroid belt and have produced most of the visible craters in the Solar System.  The Earth has about 100 craters more than 0.1 km in diameter; erosion has made most of them hard to discern. This one which is one of the largest, is in Canada. **10. Summary of the Solar System** **11. Formation of Solar System** **Summary of the system** 1. Each planet is relatively isolated in space 2. The orbits of the planets are nearly circular 3. They all lie in nearly the same plane 4. They circle the sun in CCW direction which is the same as the sun itself. - This is true for a planet and moon of a planet. 5. It is highly differentiated (inner, outer) 6. Asteroids are very old. Their properties differs very much from either inner or outer planets or moons. 7. Kuiper belt is a collection of asteroid-sized icy bodies orbiting beyond Neptune. 8. Oort cloud comets are primitive icy fragments. They don\'t orbit on the ecliptic plane and their reside at large distances from the sun. **Nebular Contraction** - The idea dates back to **Descartes** (17 cc) - A large cloud of interstellar gas began to collapse under the influence of its own gravity - Getting **smaller**   ➤  Getting **denser**  ➤  Getting **hotter**  ➤  STAR - Outer cooler parts: Planets, Moons i.e. debris - This swirling mass is called the **solar nebula** **Nebular Theory** - **Laplace** improved this idea - Solar nebula must spin faster as it contracts - decrease in size must be balanced with increase in rotation speed - increase in rotation speed  ➤ changes nebula\'s shape - cloud  ➤  bulge   ➤  disk - As the contraction continues - it leaves behind a series of concentric rings each of which orbiting a central proto-sun - each ring then clumped into a proto-planet - Therefore the idea that planets formed from a disk is called **nebular theory** **Condensation Theory** - Today, a disk of warm gas would **not** form clumps of matter that would subsequently evolve into planets. In fact, **just the opposite is predicted**. - Condensation Theory = old Nebular Theory + [interstellar chemistry] - The key is **interstellar dust** in the solar nebula. - Interstellar space is littered with microscopic dust grains. - They are chunks of icy and rocky matter having typical sizes of about 0.1 micron - **Dust helps to cool warm matter** by efficiently radiating its heat away in [infrared] - Cooling of matter  ➤  reduces the pressure - Reduced pressure  ➤  allows the gas to collapse more easily by gravity - Furthermore  ➤  they speed up the process of collecting enough atoms to form a planet - **Condensation Nuclei:** microscopic platforms to which other atoms can attach, forming larger and larger balls of matter   ➤  Planets 06 Inner Planets ================ **1. Summary of Terrestrial Planets** The surfaces of all five terrestrial worlds (Mercury, Venus, Earth, the Moon, and Mars) must have looked quite similar when they were young. All five were made of rocky material that had condensed in the solar nebula, and all five were subjected early on to the impacts of the heavy bombardment. ***The great differences in their present-day appearance must therefore be the result of changes that have occurred through time.*** Ultimately, these changes must be traceable to fundamental properties of the planets.  **2. Mercury (\#1)** Phases of Mercury can be seen best when Mercury is at its maximum elongation. Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false. Rather, Mercury's day and year are in a 3:2 resonance; Mercury rotates **three times** while going around the Sun **twice**. **Scarp (cliff)**, several hundred kilometers long and up to 3 km high. **Caloris Basin**, very large impact feature; weird terrain on opposite side of planet "Weird terrain" is thought to result from focusing of seismic waves. **Formation** - Formed about 4.6 billion years ago - Melted due to bombardment, cooled slowly - It\'s crust is shrank and crumpling. **3. Venus (\#2)** - Venus is much brighter than Mercury, and can be seen farther from the Sun - Called **morning or evening star**, as it is still "tied" to Sun - **Brightest object** in the sky, after Sun and Moon Apparent brightness of Venus varies, due to changes in phase and distance from Earth. Slow, retrograde rotation of Venus results in large difference between solar day (117 Earth days) and sidereal day (243 Earth days); both are large compared to the Venus year (225 Earth days) . Dense atmosphere and thick clouds make surface impossible to see. Surface temperature is about 730 K---hotter than Mercury! Even probes flying near Venus, using ultraviolet or infrared, can see only a little deeper into the clouds. - Surface is relatively smooth - Two continent-like features: Ishtar Terra and Aphrodite Terra - No plate tectonics - Mountains, a few craters, many volcanoes and large lava flows \(a) Radar map of the surface of Venus, based on *Pioneer Venus* data. Color represents elevation, with white the highest areas and blue the lowest. (b) A similar map of Earth, at the same spatial resolution. (c) Another version of (a), with major surface features labeled. \(a) A *Venera* orbiter image of a plateau known as Lakshmi Planum in Ishtar Terra. - The Maxwell Montes mountain range (red) lies on the western margin of the plain, near the right-hand edge of the image. - A meteor crater named Cleopatra is visible on the western slope of the Maxwell range. - Note the two larger craters in the center of the plain itself. \(b) A *Magellan* image of Cleopatra showing a double-ringed structure that identifies the feature to geologists as an impact crater. \(a) A *Magellan* image of Ovda Regio, part of Aphrodite Terra. - The intersecting ridges indicate repeated compression and buckling of the surface. - The dark areas represent regions that have been flooded by lava up-welling from cracks like those shown in the right panel. \(b) This lava channel in Venus's south polar region, known as Lada Terra, extends for nearly 200 km.  \(a) Two larger volcanoes, known as Sif Mons (left) and Gula Mons, appear in this *Magellan* image. - Color indicates height above a nominal planetary radius of 6052 km and ranges from purple (1 km, the level of the surrounding plain) to orange (corresponding to an altitude of about 4 km). - The two volcanic calderas at the summits are about 100 km across. \(b) A computer-generated view of Sif Mons, as seen from ground level. \(c) Gula Mons, as seen from ground level. - In (b) and (c), the colors are based on data returned from Soviet landers, and the vertical scales have been greatly exaggerated (by about a factor of 40), so these mountains look much taller relative to their widths than they actually are; Venus is actually a remarkably flat place.  **Lava Dome** \(a) These dome-shaped structures resulted when viscous molten rock bulged out of the ground and then retreated, leaving behind a thin, solid crust that subsequently cracked and subsided. *Magellan* found features like these in several locations on Venus. \(b) A three-dimensional representation of four of the domes. This computer-generated view is looking toward the right from near the center of the image in part (a). Colors in (b) are based on data returned by Soviet *Venera* landers. **Venus Corona** - This corona, called Aine, lies in the plains south of Aphrodite Terra and is about 300 km across. - Coronae probably result from up-welling mantle material, causing the surface to bulge outward. - Note the pancake-shaped lava domes at top, the many fractures in the crust around the corona, and the large impact craters with their surrounding white (rough) ejecta blankets that stud the region  **Atmosphere** - Venus's atmosphere is very dense - Solid cloud bank 50--70 km above surface - Atmosphere is mostly carbon dioxide; clouds are sulfuric acid - Upper atmosphere of Venus has high winds, but atmosphere near surface is almost calm - There are also permanent vortices at the poles; the origin of the double-lobed structure is a mystery - No magnetic field, probably because rotation is so slow - No evidence for plate tectonics - Venus resembles a young Earth (1 billion years)---no asthenosphere (soft sphere), thin crust **Runaway Greenhouse Effect** - Because Venus's atmosphere is much deeper and denser than Earth's, a much smaller fraction of the infrared radiation leaving the planet's surface escapes into space. - The result is a much stronger greenhouse effect than on Earth and a correspondingly hotter planet. - The outgoing infrared radiation is not absorbed at a single point in the atmosphere; instead, absorption occurs at all atmospheric levels. - The arrows indicate only that absorption occurs, not that it occurs at one specific level; the arrow thickness is proportional to the amount of radiation moving in and out. **Landing on Venus** \(a) The first direct view of the surface of Venus, radioed back to Earth from the Soviet *Venera 9* spacecraft, which made a soft landing on the planet in 1975. The amount of sunlight penetrating Venus's cloud cover is about the same as that reaching Earth's surface on a heavily overcast day. \(b) Another view of Venus, in true color, from *Venera 14.* Flat rocks like those visible in part (a) are seen among many smaller rocks and even fine soil on the surface. This landing site is not far from the *Venera 9* site shown in (a). The peculiar filtering effects of whatever light does penetrate the clouds make Venus's air and ground appear peach colored---in reality, they are most likely gray, like rocks on Earth.  **4. Earth (\#3)** **Properties** - Surface Temperature = 290 K - Inclination = 23.45 degree - Escape Velocity = 11.2 km/s - Earth = **99.9%**: Differentiated Layers + **0.1%**: Atmosphere+Magnetosphere **Atmosphere** - Nitrogen = 78 % - Oxygen = 21 % - Argon = 0.9 % - Carbon dioxide = 0.03 % - Water vapor = 0.1 - 0.3 % The content of the atmosphere, excluding Nitrogen, makes the Earth\'s atmosphere very distinct from the others Troposphere - The layer where the ***convection*** is the energy transport mechanism - A constant motion of **warm air** and the concurrent downward flow of **cooler air**  to take its place. - Therefore convection is the process that physically transfers heat from a lower (hotter) to a higher (cooler) level. - Convection creates a circulation pattern - This pattern is named as convection cells ➤ rise/fall of air ➤ surface winds ➤ **weather** **Layers of the Atmosphere** **Ozone** - Absorbs UV radiation - Contains Oxygen, Ozone (O+O+O), Nitrogen **Ionosphere** - Top most part of the atmosphere is ionized by the Sun\'s radiation (molecules ➤ atoms ➤ ions) - As the elevation increases, ionization increases - ions  ➤ conductor ➤ reflective to certain wavelengths **Surface Heating** - Most of the **Sun\'s radiation** manages to penetrate Earth\'s atmosphere: - Stage-1: Both reflected and absorbed by the clouds - Stage-2: Absorbed by Earth\'s surface - Therefore the surface is heated during the day - Stage-3: The surface is re-radiated the absorbed energy (a blackbody curve) - Therefore increase in surface Temperature causes much more increase in the released energy - The balance is at -23 C degree; and due to Wien\'s Law, re-radiated energy will in IR (heat) - Stage-4: IR is partially blocked (due to water vapor and carbon dioxide) - Only small amount escapes - trapped radiation causes temperature in the layer to increase ➤ **Greenhouse Effect** **Origin of Earth\'s Atmosphere** **Primary Atmosphere** - Common to whole solar system - H, He, methane, ammonia, water vapor - H, He escaped 1/2 billion years ago **Secondary Atmosphere** - Outgased from planet\'s interior (e.g volcanic activity) - **Volcanic gases** are rich in water vapor, methane, CO~2~, SO~2~, compounds - **UV radiation** from the Sun decomposes the lighter and H-rich gases allowing H to escape - Temperature drops  ➤ Water vapor condenses  ➤ **oceans are created** - Much of **CO~2~ and SO~2~** become dissolved in the oceans or combined with the rocks - **Oxygen** was removed as quickly as it is formed (it is a reactive gas) - **Nitrogen** dominated atmosphere is formed **Life** - It appeared in oceans 3.5 billion years ago - Organism eventually began to produce atmospheric oxygen - Ozone layer formed - Life spread to the land So, oxygen containing atmosphere is due to the evolution of life on Earth **Earth\'s Interiors** - One cannot drill beyond 10 km depth because of the pressure. - So, to study the interiors of the Earth **Seismic Waves** are used Earthquake cause the entire planet to vibrate a little. These vibrations are not random. They are systematic waves called seismic waves. They move outward from the site of the quake. - They carry information (EM radiation is carried with waves) - This information can be detected and recorded - The devices used to record these signals are called seismographs **Types of Seismic Waves** **P-waves (pressure)** - Alternately expand and compress the material medium through which they move - Their speed is around 5-6 km/s - They can travel through both liquids and solids **S-waves (Shear)** - They come shortly after P-waves - Unlike P-waves they cause side-to-side motion - Example: waves in a guitar string - The speed of the wave depend on the density of the matter - They don\'t travel in straight lines - The velocity varies with depth - Waves bend as they move through the interior - Earthquakes - P + S waves - They create shadow zones - Observing seismic wave propagation and surface rocks one can model the interiors **From the Analysis of Seismic Waves** - Mantle \~ 3000 km thick (density: 5000 km/m3) - Crust \~ 15 km (8 km under ocean; 20-50 km under continents with a density: 3000 km/m3) - Core: Ni, Fe and some other lighter elements) - It is liquid - However pressure near the center forces the material into solid state (even T is high) **Differentiation** - Earth is **not** homogeneous; it has a layered structure. - Density and compositions are different for different layers: - CRUST - Low Density - MANTLE - Medium Density - CORE - High Density **Why?** - Earth was molten at some time. - Higher-density parts sank to the core - Lower-density parts displaced toward the surface - Therefore Earth\'s central temperature become hot like the surface of the Sun. **What heated the Earth\'s center?** - Earth grew by capturing material from its surroundings - This increased its gravitational field and therefore speed of material striking to surface increased - Therefore generated heat made the Earth molten - Earth began to differentiate. - As heavy material sank to the center; - More gravitational energy released, ➤ which caused interior temperature to increase further. - Bombardment continued: ➤ a thin layer of surface continue to be molten **Radioactivity** - Heavy elements like Uranium, Thorium and Plutonium release energy as they decay (break up) into lighter elements. - This energy also heated the Earth - Released energy from the decay is very small. However: - Large amount of these elements stored in Earth. - Earth had enough time to accumulate this energy - Created energy at interior parts couldn\'t leak into space because rocky surface was opaque to the heat (rock is a poor conductor) - Earth\'s crust solidified around 700 million years after Earth\'s formation - Heating continued however since radioactive heating is one directional, it gets lower and lower through time. - So, the Earth has been cooling for the past 4 billion years. - Cooling is from the outside in like a hot potato - Regions closest to the surface will release more energy into space. - Therefore this cooling also created current differentiated layer structure. **Earth\'s Magnetosphere** - It is discovered by artificial satellites in late 1950s - It is created by the planet\'s magnetic field. - Our magnetosphere extends far beyond the atmosphere. - It will look like a bar magnet when observed close to the Earth. - The magnetic field lines runs from South to North and they intersect the Earth\'s surface vertically. - The poles are roughly aligned with Earth\'s spin axis. - The poles are not fixed and they drift with 10 km/year - North Pole is at 80 degree North (northern Canada) - South Pole is at 60 degree South (off the coast of Antarctica) **Van Allen Belts** - The magnetosphere contains **two doughnut-shape zones** of high energetic charged particles above the surface 1. Located about 3 000 km - catching **heavier protons** 2. Located about 20 000 km - catching mostly **electrons** - They are named as belts because - their efficiency are the highest at the equator - they completely surround the planet - The particles in the belts originated from the Sun and they are carried by the solar wind. - Neutral particles and EM radiation are unaffected by the Earth\'s magnetism - But charged particles (electrons and protons) are strongly influenced - They spiral around the magnetic field lines and they are trapped in the Earth\'s magnetic field creating the zones (aka. belts). - Therefore, the belts creates an invisible shield to protect the planet surface from energetic charged particles. **Aurora** The particles that can escape from Van Allen belts penetrates into lower layers of the atmosphere. - Particles collide with air molecules - They fall back to their ground states - Their energy re-emit in visible spectrum; named as **Aurora** - This event are observed mostly in higher latitude regions. - The event can be observed on other planets too. **Shape of Magnetosphere** - It is not symmetric - It looks like a tear-drop - Sunlit side is compressed by the solar wind; - It extends to 10 Earth radii from the Earth; named as *magnetopause* **The tides** - Ocean level fluctuates in daily base. - This is due to **gravitational influence** of the Moon and the Sun on Earth - This deforms the Earth\'s surface depending on the distance separating any two objects - The Moon\'s gravitational attraction is greater on the side of Earth that faces the Moon than on the opposite side. - **Earth becomes slightly elongated** with the long axis of the distortion pointing towards the Moon. - Earth\'s oceans undergo the greatest deformation because liquid can most easily move around on Earth. - The ocean becomes: - a little deeper in some places (along the equator) - shallower in others (closer to the poles) - Adding the Sun\'s effect: - Sun is farther away but its mass is so much greater. - Its tidal influence is about **half of Moon\'s** tidal influence **Tidal Force** - Average gravitational interaction determines the object\'s orbit - Adding tidal force to the gravitational influence **deforms** both bodies - This deformation can be estimated as 1 / r^3^  **Earth\'s Rotation** - Earth\'s spin is slowing due to tidal bulge offset - The rate of slow down is 1.5 msec / year - Therefore the Moon is spiraling slowly away from the Earth by 4 cm / year - This slow down will stop when the offset becomes zero - then Earth\'s rotation period will become 47 days - and the distance to the Moon will become 550 000 km. **Why the sky is blue** It is due to scattering of the sunlight by the air molecules and dust particles - radiation is absorbed - and then re-radiated by the material in air This is called **Rayleigh scattering**. The scattering is wavelength dependent. - Blue light is much more easily scattered than the red light. - Blue  light (\~400 nm) frequency \~ Size of air molecules - **Not** red light - Scattering ∝ 1 / ƛ^4^ **When the Sun is high:** - Blue component of light will be scattered so blue light will be removed from our line of sight. - Red or yellow components will be little scattered and arrives along the line of sight ➤ **direction** of the Sun is *reddened* slightly ➤ **away from** the Sun appears *blue* **When the Sun is low** - Blue component is gone completely - Red diminished in intensity ➤ The Sun becomes **orange** in color **5. Mars (\#4)** - Radius = 3400 km - Two moons(?) : Phobos, Deimos - very small compared to the Moon - Mars day = 24.6 hours (similar to Earth\'s) - Least eccentric orbit (circular than ellipse) **Mars from Earth** Only the polar caps can be seen. They grow and shrink with the seasons. Polar caps contain frozen carbon dioxide. Water ice is permanently frozen under the surface. **Main Surface Features** - Dust cover on the surface shifts regularly. Creating frequent **dust storms** with high winds. - The major feature: **Tharsis bulge** (size \~ North America; 10 km above surroundings) - There is no evidence of plate tectonics **Hemispheres** - Northern: rolling volcanic terrain - Southern: heavily cratered highlands (on average \~5 km above Northern hemisphere) - Probably Northern is younger: - it mush have been lowered in elevation - and then flooded with lava **Valles Marineris** - Huge canyon which is created by crustal forces - 4000 km long - 120 km wide (max) - 7 km deep - (On the left) complexity of valley walls and dry branches - (On the top right) Comparison with Grand Canyon in USA **Olympus Mons** - **The largest volcano** in the solar system - 700 km base diameter - 80 km caldera diameter (fallen in part of volcano) - 25 km high - Volcano is extinct for several 100s millions of year - Comparison with Earth: - The largest is in Hawaii: Mauna Loa - 120 km across - 9 km above ocean base **Water on Mars** - Runoff channels resemble those on Earth **Left**: Mars, **Right**: Mississippi River \(a) An **outflow channel** near the Martian equator bears witness to a catastrophic flood that occurred about 3 billion years ago. (b) The **onrushing water** that carved out the outflow channels was responsible for forming these oddly shaped "islands" as the flow encountered obstacles---impact craters---in its path. Each "island" is about 40 km long.  This makes us to think that \"*Open water once existed on Mars*\". This may be an ancient Martian **river delta**. (Upper panel) Much of northern hemisphere may have been **ocean**. Blue color indicates lower elevations. (Lower panel) Evidence of **erosion** by standing water in the crater\'s floor (140 km across) Impact craters less than 5 km across have mostly been eroded away. Analysis of craters allows estimation of age of surface. \(a) The large lunar impact crater **Copernicus** is typical of those found on Earth's Moon. Its ejecta blanket appears to be composed of dry, powdery material. (b) The ejecta from Mars's crater Yuty (18 km in diameter) evidently was once liquid. This type of crater is sometimes called a \"**splosh crater**\". \(a) This high-resolution *Mars Global Surveyor* view (left) of a crater wall (right) near the Mariner Valley shows evidence of "gullies" apparently formed by running water in the relatively recent past. **Recent Martian Outflow** This comparison between two *Mars Global Surveyor* images, taken in 1999 and 2005, of a Martian impact crater shows that something---the white streak (lower right), possibly water---flowed across the surface during that 6-year period: **the activity is ongoing!** **Martian Polar Caps** The southern (a) and northern (b) polar caps of Mars are shown to scale in these mosaics of *Mariner 9* images. These are the residual caps, seen here during their respective summers half a Martian year apart. The southern cap is some 350 km across and is made up mostly of frozen carbon dioxide. The northern cap is about 1000 km across and is composed mostly of water ice. The inset shows greater detail in the southern cap. **Exploration of Mars** **Viking 1** This is the view from the *Viking 1* spacecraft now parked on the surface of Mars. The fine-grained soil and the reddish rock-strewn terrain stretching toward the horizon contains substantial amounts of iron ore; the surface of Mars is literally rusting away. The sky is a pale pink color, the result of airborne dust.  **Viking 2** Another view of the Martian surface, this one rock strewn and flat, as seen through the camera aboard the *Viking 2* robot that soft-landed on the northern Utopian plains. The discarded canister is about 20 cm long. The 0.5-m scars in the dirt were made by the robot's shovel.  **Opportunity Rover** (a) A panoramic view of the terrain near where NASA's *Opportunity* rover landed on Mars in 2004. This is Endurance crater, roughly 130 m across  ©2024 NASA. NASA's Perseverance Mars rover captured this mosaic showing the Ingenuity Mars Helicopter at its final airfield on Feb. 4, 2024.  **Mars Atmosphere** The troposphere, which rises to an altitude of about 30 km in the daytime, occasionally contains clouds of **water ice** or, more frequently, **dust** during the planet wide dust storms that occur each year. But it mostly contains **carbon dioxide** and it is **very thin**. Above the troposphere lies the stratosphere. Note the absence of a higher temperature zone in the stratosphere, indicating the absence of an ozone layer. Fog can form in low-lying areas as sunlight strikes **Atmospheric Change.** Mars may be victim of runaway greenhouse effect in the opposite sense of Venus's: As water ice froze, - Mars became more and more reflective - and its atmosphere thinner and thinner, - freezing more and more water - and eventually carbon dioxide as well. As a result, Mars may have had a thicker atmosphere and liquid water in the past, but they are now gone. **Martian Moons** \(a) A *Mars Express* photograph of the **potato-shaped** Phobos, not much larger than Manhattan Island. The prominent crater (called Stickney) at left is about 10 km across. \(b) Like Phobos, the smaller moon, Deimos, has a composition unlike that of Mars. Both moons are probably **captured asteroids**. This close-up photograph of Deimos was taken by a *Viking* orbiter. The field of view is only 2 km across, and most of the boulders shown are about the size of a house.  07 Outer Planets ================ **1. Summary of Jovian Planets** **Basics of Jovians** - Massive - Low density - Large - Gaseous (Hydrogen, Helium and their compounds) - Far from the Sun - Fast rotation - High Magnetic Fields - Having Ring Systems **Interiors** - Divided into two main configuration: - Jupiter and Saturn - Uranus and Neptune - Mainly: - Similar layers as Terrastials - However the content is purely Hydrogen - Hydrogen changes state and becomes liquid and metallic in lower layers. - Upper layers contain clouds of ammonia and their compounds **Atmosphere** - Meaning of surface is very weak (see the figure) - Jupiter and Saturn atmospheres are studied by probes where they penetrated deep into the atmosphere - Therefore surface is defined where a temperature change (reflected from a pressure change) from cold to hot. - All four Jovians shows similar atmospheric conditions **Fast Rotation** - When a ball of gas rotates it will deform its structure. - The deformation makes the object **elongated**. - Faster along equator - Slower on poles - Fast rotation of gas ball is also the reason of having a high magnetic field: **Dynamo Effect** **Magnetic Fields** - They all have high magnetic fields - The largest is Jupiter and it extends until the Saturn\'s orbit - Magnetic Field axis not always alligned with the rotation axis. - Since Jovians are gaseous, centers of the magnetic fields (with respect to magnetic poles) don\'t always coincide with the planet\'s core; mostly for Uranus and Neptune **Moons of Jovians** - Since Jovians are massive they will surely attract many minor objects beyond the planets. - Current Moon population - Jupiter: **4** major - **79** in total - Saturn: **7** major - **62** in total - Uranus: **5** major - **27** in total - Neptune: **1** major - **14** in total - They are mostly frozen rock. - Exceptions: - Io (Jupiter) - Lava flow - Titan (Saturn) - Atmosphere, Methane in Liquid form - Triton (Neptune) - Atmosphere **Ring System** - All distances are expressed in planetary radii. - The red line represents the Roche limit, and all the rings lie within (or very close to) this limit of their parent planets. However, it is just an approximation **2. Jupiter (\#5)** **Jupiter's Convection** The colored bands in Jupiter's atmosphere are associated with vertical convective motion. - Upwelling warm gas results in zones of lighter color; - The darker bands overlie regions of lower pressure where cooler gas sinks back down into the atmosphere. As on Earth, surface winds tend to blow from high- to low-pressure regions. Jupiter's rapid rotation channels those winds into an east--west flow pattern, as indicated by the three yellow-red arrows drawn atop the belts and zones. **Zonal Flow** The wind speed in Jupiter's atmosphere, measured relative to the planet's internal rotation rate. Alternations in wind direction are associated with the atmospheric band structure. Jupiter\'s Rotation Period: \~ 10 hours **Atmosphere** - Mostly molecular hydrogen and helium - Small amounts of methane, ammonia, and water vapor - Color is probably due to complex chemical interactions. - No solid surface; take top of troposphere to be at 0 km - Lowest cloud layer cannot be seen by optical telescopes - High wind speeds even at great depth (probably due to heating from planet, not from Sun) - Cloud color and cloud content reflect the chemistry of the layer **Great Red Spot** - It has existed for at least 300 years, possibly much longer. - Color and energy source still not understood. - Complex turbulence to the left of both the Red Spot and the smaller white oval below it. **Bottom Left:** The cyclic motion of the Great Red Spot, imaged by the [Cassini](https://www.wikiwand.com/en/Cassini-Huygens) spacecraft. **Right Panel:** Closeup detail of the Great Red Spot taken by [NASA](https://www.wikiwand.com/en/NASA)\'s [*Juno*](https://www.wikiwand.com/en/Juno_%28spacecraft%29) on 11 July 2017. **Red Spot Junior** (a) Between 1997 and 2000, astronomers watched as three white ovals in Jupiter's southern hemisphere merged to form a single large storm. Each oval is about half the size of Earth. \(b) In early 2006 the white oval turned red, producing a second red spot! The color change may indicate that the storm is intensifying.  **Brown Oval** A break in upper cloud layer (northern hemisphere). Deeper atmosphere looks brown. The oval's length is approximately equal to Earth's diameter. **Internal Structure** - Jupiter radiates more energy than it receives from the Sun - Core is still cooling off from heating during gravitational compression. - Could Jupiter have been a star? - No; it is far too cool and too small for that. - It would need to be about 80 times more massive to be even a very faint star. - The density and temperature increase with depth, and the atmosphere gradually liquefies at a depth of a few thousand kilometers. - Below a depth of 20,000 km, the hydrogen behaves like a liquid metal. - At the center of the planet lies a large rocky core, somewhat terrestrial in composition. **Jupiter\'s Magnetosphere** - It is 30 million km across. - The plasma torus, a ring of charged particles associated with the moon Io. - Intrinsic field strength is 20,000 times that of Earth. - It can extend beyond the orbit of Saturn **Ring System** - It has a faint ring. It is photographed (nearly edge-on) by *Voyager 2*. - Made of dark fragments of rock and dust possibly chipped off the innermost moons by meteorites. - It lies in Jupiter's equatorial plane, only 50,000 km above the cloud tops.  **Galileon Moons** - Four major moons: - Io - Europa - Ganymede - Callisto - They have similarities to terrestrial planets: - Orbits have low eccentricity - Density decreases as distance from Jupiter increases **Io**. Its surface is kept smooth and brightly colored by the moon's constant volcanism. Orange color is probably from sulfur compounds in the ejecta. Io is very close to Jupiter and also experiences gravitational forces from Europa. The tidal forces are huge and provide the energy for the volcanoes. The plume measures about 150 km high and 300 km across. **Europa.** (a) It has no craters; surface is water ice, possibly with liquid water below. Tidal forces stress and crack ice; water flows, keeping surface relatively flat. \(b) Europa's icy surface is only lightly cratered, indicating that some ongoing process must be obliterating impact craters soon after they form. The origin of the cracks crisscrossing the surface is uncertain. \(c) This picture shows a smooth yet tangled surface resembling the huge ice floes that cover Earth's polar regions. This region is called Conamara Chaos. \(d) This picture shows \"pulled apart\" terrain that suggests liquid water upwelling from the interior and freezing, filling in the gaps between separating surface ice sheets. **Ganymede ** It is the largest moon in the solar system; larger than Pluto and Mercury Its history similar to Earth's Moon, but it contains water ice instead of lunar rock. \(a) and (b) The dark regions are the oldest parts of the moon's surface and probably represent its original icy crust. The lighter, younger regions are the result of flooding and freezing that occurred within a billion years or so of Ganymede's formation. The light-colored spots are recent impact craters. \(c) Grooved terrain on Ganymede may have been caused by a process similar to plate tectonics on Earth. The image suggests erosion of some sort, possibly even caused by water  **Callisto** \(a) It is similar to Ganymede in overall composition, but is more heavily cratered. The large series of concentric ridges visible at left is known as Valhalla. Extending nearly 1500 km from the basin center, the ridges formed when \"ripples\" from a large meteoritic impact froze before they could disperse completely. \(b) This picture displays more clearly its heavy cratering.  **3. Saturn (\#6)** - **Density**: 700 kg/m^3^  (less than water!) - **Rotation**: Rapid and differential, enough to flatten Saturn considerably - **Rings**: Very prominent; wide but extremely thin **Atmosphere** - It also shows zone and band structure, but coloration is much more subdued than Jupiter's - Mostly molecular hydrogen, helium, methane, and ammonia - Helium fraction is much less than on Jupiter - Similar to Jupiter's, except pressure is lower - Three cloud layers - Saturn's weaker gravity results in thicker clouds and a more uniform appearance. We see only top layer. - The colors shown are intended to represent Saturn's visible-light appearance. **Saturn's Zonal Flow** Winds on Saturn reach speeds even greater than those on Jupiter. As on Jupiter, the visible bands appear to be associated with variations in wind speed. **Saturn Storms** (a) Circulating and evolving cloud systems on Saturn, imaged at approximately 2-hour intervals. (b) This infrared image, displayed in false color: - Blue coloration indicates regions where the atmosphere is relatively free of haze; - Green and yellow indicate increasing levels of haze; - Red and orange indicate high-level clouds. - Two small storm systems near the equator appear whitish. **Dragon Storm I**t generated bursts of radio waves resembling the static created by lightning on Earth. **Polar Vortex** It is at south pole. It has a well-developed eye wall and calm winds at its center. The size of this vortex is slightly larger than the entire Earth, and its winds are about twice that of a category 5 hurricane on Earth  **Interiors** - Similar to Jupiter - It also radiates more energy than it gets from the Sun, but not because of cooling: - Helium and hydrogen are not well mixed - Helium tends to condense into droplets and then fall - Gravitational field compresses helium and heats it up - It has a strong magnetic field, but only 5% as strong as Jupiter's - It creates aurorae **Ring System** - Ring particles range in size from fractions of a millimeter to tens of meters. - Composition: Water ice---similar to snowballs - Why rings: Too close to planet for moon to form---tidal forces would tear it apart. - Details of formation are unknown: - Too active to have lasted since birth of solar system - Either must be continually replenished, or are the result of a catastrophic event - The ringlets in the B ring, spread over several thousand kilometers. - Note the large number of tiny ringlets visible in the main rings. - The inset is an overhead view of a portion of the B ring. **Spokes in the Rings** Saturn's B ring showed a series of dark temporary "spokes". The spokes are caused by small particles suspended just above the ring plane. **Shepherd Moon** (a) Saturn's narrow F ring appears to contain kinks and braids. Its thinness can be explained by the effects of two shepherd satellites that orbit near the ring---one a few hundred kilometers inside, the other a similar distance outside. \(b) One of the potato-shaped shepherd satellites (Prometheus here roughly 100 km across) can be seen at the right of this enlarged view.  **F Ring Structure** Kinks, waves, and other substructure in the F ring can be seen. A back-and-forth dance of the shepherd moons gravitationally sculpts clumps in the core of the rings. **Moons** - Many of them are water ice - Siz medium sized: **Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus** - One large: **Titan** **Titan** - Larger than Mercury and roughly half the size of Earth. - In the infrared some large-scale surface features can be seen. - The bright regions are thought to be highlands, possibly covered with frozen methane; nearly 4000 km across. The structure of Titan's atmosphere. The solid blue line represents temperature at different altitudes. The inset shows the haze layers in Titan's upper atmosphere, depicted in false-color green  **Titan Revealed** - *Cassini'*s infrared telescopes revealed this infrared, false-color view of Titan's surface in late 2004. - The circular area near the center may be an old impact basin. - The dark linear feature to its northwest perhaps mountain ranges caused by ancient tectonic activity. - Image on the left shows a surface feature thought to be an icy volcano, further suggesting some geological activity on this icy moon's surface. - *Huygens'*s view (right panel) of its landing site, in approximately true color. The foreground rocklike objects are only a few centimeters across. **Titan's Interior** It appears to be largely a rocky-silicate mix. The subsurface layer of liquid water, similar to that hypothesized on Jupiter's Europa and Ganymede. **4. Uranus (\#7)** **Uranus** - Discovered in 1781 by Herschel - The first planet to be discovered in more than 2000 years. - Little detail can be seen from Earth - Left arrow: Triton, Right arrow: Nereid. - (Right panel) almost planet's true color. - But shows virtually no detail in the nearly featureless upper atmosphere. - Exceptions: a few wispy clouds in the northern hemisphere. **Seasons on Uranus** - Uranus's axial tilt is 98° - Therefore the planet experiences the most extreme seasons known in the solar system. - Each year the equatorial regions have - Two \"summers\": warm seasons, around the times of the two equinoxes. - Two "winters": cool seasons, at the solstices. - The poles are alternately plunged into darkness for 42 Earth years at a time. **5. Neptune (\#8)** In the closer view, resolved to about 10 km, shows cloud streaks ranging in width from 50 km to 200 km. **Neptune's Dark Spot.** Similar in structure to Jupiter's Great Red Spot. The entire dark spot is roughly the size of planet Earth.  The cloud features (mostly methane ice crystals) are tinted pink because they were imaged in the infrared, but they are really white in visible light. Note that the Great Dark Spot has disappeared in recent years.  **Moons of Uranus and Neptune** **Miranda (Uranus)** It has a strange, fractured surface suggestive of a violent past, but the cause of the grooves and cracks is currently unknown  **Triton (Neptune)** - It is in retrograde motion. - The south polar region: deep ridges and gashes - Lakes of frozen water, all indicative of past surface activity. - The pinkish region at lower right is nitrogen frost, forming the moon's polar cap.  - The long black streaks at bottom left were probably formed by geysers of liquid nitrogen on the surface. - (Right panel) This lake like structure may have been caused by the eruption of an ice volcano. The water "lava" has since solidified, leaving a smooth surface. The absence of craters implies that this eruption was a relatively recent event.