Planetary Geology: Formation and Features of Planets

Origin of Terrestrial Atmospheres

Venus, Earth, and Mars received their atmospheres through outgassing. Chemical reactions caused CO₂ on Earth to dissolve in oceans and go into carbonate rocks, like limestone. Mars lost much of its atmosphere through impacts.

Formation of the Maria

• The Moon once had a molten interior.
• Several large impacts made huge crater basins.
• This “runny” lava filled in the basins.

Mercury: A Dead Planet

• No atmosphere.
• Has no maria, but small lava plains.
• Has fewer craters than the Moon.
• Craters are shallower than the Moon's due to higher gravity on Mercury.
• Evidence for tectonic processes.
• Evidence for ice at the North Pole.

Mars: Mountains and Canyons

• Mountains and canyons.
• Volcanoes.
• Thin atmosphere.
• No plate tectonics; volcanoes are higher.
• Evidence for water erosion.
• Olympus Mons, the largest volcano in our solar system.
• Mars has a rotation period and axis tilt almost identical to Earth’s, implying that Mars has seasons.

Venus: A Cloudy World

• Has a thick, cloudy atmosphere; you cannot visually see the surface.
• Smooth plains with few mountain ranges.
• Few craters.
• Many volcanoes and domes of lava (corona).
• Venus is very active with tectonics and volcanism.
• Searing heat, heavy pressure, clouds of sulfuric acid, frequent volcanic eruptions.
• Impact craters are evenly spread over the Venusian surface, implying that the planet’s entire surface is the same age.
• No erosion features are seen on Venus (so far), meaning no wind, rain, or ice on the surface.

Earth: The Most Active Geology

• Volcanoes and tectonics.
• Ongoing plate tectonics.
• Moderate atmosphere.
• Life.

Formation of Jovian Planets

Outside the frost line, ices condense. More solid material is available for accretion, leading to more massive cores, which can capture and hold hydrogen and helium.

Composition of the Jovian Planets

Almost entirely H, He – but not all gaseous!

Jovian Planet Formation

The Jovian cores are very similar, with a mass of 10 Earths. The differences between the giant planets lie largely in the amount of H/He gas accumulated.

Differences in Jovian Planet Formation

• Timing: The planet that forms earliest captures the most hydrogen and helium gas. Capture ceases after the first solar wind blows the leftover gas away.
• Location: The planet that forms in a denser part of the nebula forms its core first.

Why is Jupiter Much More Massive than Saturn?

It is larger but also much denser. It became so large that piling on more material caused compaction of the existing material, allowing it to gain mass without gaining much size.

Magnetic Fields

• Strong fields, especially Jupiter (20,000 times as strong as Earth’s field!).
• Jupiter and Saturn have fields created by a layer of metallic hydrogen.
• Uranus and Neptune have weaker fields caused by core materials.

Jovian Weather

• Cloud covered.
• Different colors indicate different compositions (e.g., Jupiter – water, ammonium hydrosulfide, ammonia).
• Tremendous winds due to the Coriolis effect.
• Giant storms.

Coriolis Effect

Causes deflection of objects moving in a rotating frame. For example, weather systems rotate counterclockwise in the Northern Hemisphere.