Questions: Terrestrial Planet Formation and Properties

Question 1

Why are terrestrial planets composed primarily of rock and metal rather than hydrogen, helium, and ice?

• A. They formed closer to the Sun where the solar wind stripped away light gases before planets could form
• B. They formed inside the snow line where temperatures were too high for ices to condense, leaving only silicates and metals
• C. They formed from a chemically distinct region of the solar nebula with different elemental abundances
• D. Rocky material is denser and sank inward under gravity while lighter gases floated outward

Answer: (B)

The snow line marks the temperature boundary where water ice and other volatiles can condense from the solar nebula. Inside this boundary, temperatures were too high for ices to exist in solid form, so only refractory materials — silicates and iron — could become building blocks. Solar wind stripping (option A) did affect atmospheres later, particularly for smaller planets, but is not the primary explanation for the initial rocky bulk composition.

Question 2

Mars is smaller than Earth. Based on the key principle about planet size and long-term evolution, which of the following best follows?

• A. Mars cooled faster, lost its magnetic dynamo and geological activity earlier, and its atmosphere was then stripped by solar wind
• B. Mars formed later than Earth and therefore had less material available for accretion
• C. Mars's greater distance from the Sun is the primary cause of its atmospheric loss
• D. Mars lost its atmosphere because it formed outside the snow line where volatiles were available

Answer: (A)

Planet size is the single most important factor in long-term evolution. Smaller planets have a larger surface-area-to-volume ratio and cool faster. Mars's small size caused its interior to cool quickly, shutting down the magnetic dynamo. Without a magnetic field to deflect solar wind, the solar wind gradually eroded Mars's atmosphere. Distance from the Sun (option C) is secondary; Venus is close to the Sun yet retains a thick atmosphere because of its larger size. Option D is wrong — Mars formed inside the snow line.

Question 3

The terrestrial planets formed through a single giant impact between two protoplanets.

• T. True
• F. False

Answer: False

Terrestrial planet formation was a multi-stage hierarchical process spanning tens of millions of years: dust grains stuck together to form planetesimals, planetesimals grew through runaway accretion into planetary embryos, and embryos then collided in giant impacts to assemble the final planets. Earth's Moon may have formed from one such giant impact, but that is one late event in a much longer assembly sequence, not the whole story.

Question 4

A planet's proximity to the Sun during formation is the single most important factor determining how it evolves over billions of years.

• T. True
• F. False

Answer: False

Planet *size* is identified as the single most important factor for long-term planetary evolution. Larger planets retain internal heat longer, sustaining volcanism, plate tectonics, and magnetic dynamos. Proximity to the Sun strongly shapes initial composition (inside vs. outside the snow line) but does not dominate geological evolution. Venus and Earth are at similar distances from the Sun but have evolved very differently, primarily because of differences in atmosphere and internal dynamics — not distance.

Question 5

Describe the multi-stage process by which terrestrial planets assembled from the solar nebula, and explain why the final stage involved the most violent events.

Model answer: Formation began with dust grains sticking through collisions to form kilometer-sized planetesimals. Once planetesimals became massive enough, gravity dominated: larger bodies swept up smaller ones in runaway accretion, producing Moon-to-Mars-sized planetary embryos. The final stage required these embryos to collide directly. With only a few dozen large bodies remaining, each crossing of orbits was a rare but enormous event — giant impacts that released enough energy to melt entire planets and cause differentiation. Earth's Moon likely formed from debris ejected in one such impact.

Each stage changes the dominant physics: surface chemistry (sticking) gives way to gravity (accretion), then chaotic gravitational interactions among a small number of large bodies (giant impacts). The violence of the final stage reflects the transition from many small bodies to few large ones — energy concentrates into fewer, larger events.