Earth as a Planet Overview

Earth as a Planet

• The Earth is a rocky planet, one astronomical unit from the Sun.
• Its diameter is 13,000 km.
• It has five main layers.
• The outer layer is a thin atmosphere/ocean.
• The breathable part of the atmosphere is only 3-5 km.
• The ocean's maximum depth is about 8km.
• Rocky layers are either liquid or solid, depending on pressure, temperature, and elements present.
• Pressure increases with depth, increasing melting points.
• Temperature increases with depth.
• Temperature must be higher than pressure-related melting point for a layer to melt.
• Silicate rock melts at a lower temperature than iron rich material.
• The outer layer is the crust.
• Land crust is approximately 80 km thick, thinner under oceans at about 8 km.
• The crust is mainly composed of silicate rock rich in lighter elements (calcium, magnesium, aluminium).
• The mantle lies beneath the crust and is approximately 2800 km thick.
• It is solid near the bottom, and behaves like a liquid in lower pressure areas.
• The bottom of the crust and top of the mantle are linked by the Mohorovicic discontinuity (Moho).
• It is marked by a change in chemical composition.
• The mantle is partially melted in nature.
• Heat rises in the mantle via convection plumes, pushing tectonic plates.
• Heat plumes cause volcanoes.
• The outer core is mainly iron and liquid.
• The inner core is pure iron under immense pressure, not melting despite the high temperature.
• Approximately 5,000°C.

Sidereal and Synodic Days and Moon Orbits

• Stars are extremely distant from Earth.
• Apparent angle to stars changes very little.
• Measuring the Moon's orbit against fixed stars gives a 27.3-day period (sidereal period).
• Measuring the Moon's orbit against Earth-Sun positions gives a 29.5-day period (synodic period).
• This difference is due to the Earth's movement.
• The Earth orbits the Sun in 365.25 days (sidereal).
• Orbit movement is approximately 1 degree per day.
• The Sun appears to move westwards from daily perspective due to Earth's rotation.
• Culmination is the Sun at its highest point(local noon).
• The Earth must rotate an extra degree to account for the Sun's movement, taking ~4 minutes.
• A sidereal day is 23 hours and 56 minutes.
• A synodic day is exactly 24 hours.

Moon's Surface

• The Moon formed from ejected material from a collision between Earth and a Mars-sized body (Theia).
• Evidence includes dehydrated lunar soil (regolith), imbalances in isotopes, and kreep-rich rocks.
• The Moon's visible surface shows a contrast between dark, flat seas (maria) and lighter, rough highlands.
• Highlands are primarily overlapping craters, formed from high-speed impacts.
• The craters are mostly circular due to shockwave travel from impact.
• Craters are categorized by size (simple, complex, basins).
• Larger craters have terraced walls, flat bottoms (basins) and central peaks.
• Impact basins are huge and fill with molten lava, creating flat maria.
• Some additional features include rilles (linear features), wrinkle ridges, and mountain ranges.
• These features are associated with past cratering and cooling.
• Apollo missions have mapped the lunar surface.
• Some specific sites (e.g., Sea of Tranquility, Ocean of Storms, Tycho) are notable.