Terrestrial Planets

Earth has a layered interior probed by seismic waves. P (pressure) waves travel through liquids and solids; S (shear) waves can't propagate through liquid. Reflection and refraction patterns at layer boundaries reveal the structure: solid inner core and liquid outer core (both iron/nickel), a thick rocky mantle, and a thin crust. The convecting liquid outer core generates Earth's magnetic field, which traps charged particles from the solar wind in the Van Allen belts and produces auroras near the poles.

The atmosphere has five layers (troposphere → stratosphere → mesosphere → thermosphere → exosphere), with weather confined to the troposphere (~78 % N₂, 21 % O₂). The greenhouse effect — sunlight in, infrared trapped by CO₂, H₂O, CH₄ — keeps Earth's surface ~33 °C warmer than it would otherwise be. Enhanced greenhouse gases drive global warming. Venus shows the runaway version: a 730 K surface.

The Moon's differential gravitational pull raises two tidal bulges on Earth — the near side is tugged harder than the far side. Spring tides (highest) occur at new/full moon when Sun and Moon align; neap tides (lowest) at quarter moons when their pulls partially cancel. Tidal drag has locked the Moon's rotation to its orbit, so the same side always faces Earth.

The Moon's surface preserves an ancient impact record: dark maria (lava-flooded basins), bright cratered highlands, and a thick regolith of pulverized rock. Most craters formed during heavy bombardment ~3.9 billion years ago.

The leading origin theory is the giant impact hypothesis: ~4.5 Gyr ago, a Mars-sized body grazed the still-molten Earth, ejecting mantle material that coalesced into the Moon — explaining its low iron content and isotopic similarity to Earth's mantle.

The Terrestrial Planets

The four terrestrial planets share a basic layered structure — iron/nickel core, rocky mantle, thin crust — but diverge dramatically in surface, atmosphere, and rotation.

Mercury rotates slowly in a 3:2 spin-orbit resonance with the Sun, has essentially no atmosphere (too hot, too low-gravity), and a surface of impact craters, plains, and long scarps (cliffs) formed as it cooled and shrank. Its unusually large iron core (~75 % of its radius) gives a weak residual magnetic field.

Venus rotates retrograde and extremely slowly (243 Earth days). Its atmosphere is a thick CO₂ blanket at ~92 bar surface pressure that drives a runaway greenhouse effect, holding the surface at ~730 K — hotter than Mercury's despite being twice as far from the Sun. Radar mapping (Magellan) reveals volcanic plains, lava domes, coronae, and few impact craters (the surface was resurfaced relatively recently).

Earth is the outlier: liquid surface water, plate tectonics, a strong active magnetic field, and a biosphere-shaped oxygen-rich atmosphere.

Mars has a thin (~1 % Earth) CO₂ atmosphere and dramatic terrain: the Tharsis bulge, Olympus Mons (the largest volcano in the solar system, ~25 km tall), the canyon Valles Marineris, and ancient runoff channels and deltas pointing to past liquid water. There is water ice at the poles and underground today, and Curiosity has detected seasonal methane fluctuations. Whether Mars hosts (or hosted) microbial life is still an open question.

The atmospheric divergence among the three "twins" — Earth, Venus, Mars — is the key story: all three started with similar volcanic CO₂/H₂O atmospheres but ended up wildly different. Venus's runaway greenhouse, Earth's biology and oceans, and Mars's atmospheric escape illustrate how planetary evolution depends sensitively on initial conditions.