Questions: Impact-Induced Outgassing and Atmospheric Loss

Question 1

A planet is struck by thousands of small asteroids over millions of years versus one giant impactor delivering the same total kinetic energy. How do the atmospheric outcomes likely differ?

• A. Both scenarios deliver the same net volatiles because total kinetic energy is identical
• B. The giant impact strips more atmosphere; many small impacts are net volatile contributors
• C. Many small impacts strip more atmosphere because cumulative shock waves penetrate deeper
• D. Giant impacts always add more volatiles because large impactors contain more water ice

Answer: (B)

The key insight is that atmospheric erosion requires an impactor large enough to accelerate overlying atmosphere to escape velocity — small impacts deliver volatiles without achieving this threshold. Many small impacts therefore tend to be net atmospheric contributors. A single giant impact, concentrating enormous energy, can blast the entire atmospheric column into space. The same total energy distributed among many small events has very different consequences than concentrated in one catastrophic event.

Question 2

Earth's current atmosphere is described as a 'secondary atmosphere.' What does this imply about the Moon-forming impact?

• A. The Moon-forming impact delivered Earth's initial hydrogen and nitrogen from the impactor
• B. The Moon-forming impact stripped Earth's primordial atmosphere, requiring a complete rebuild from volcanic outgassing and later impacts
• C. The Moon-forming impact was too small to affect Earth's atmosphere significantly
• D. Earth's secondary atmosphere formed because the primary atmosphere slowly leaked into space via thermal escape

Answer: (B)

A 'secondary atmosphere' is one rebuilt after the original was lost — the term implies the primary atmosphere was stripped. The Moon-forming impact was so energetic it reset Earth's atmospheric composition entirely, removing the primordial hydrogen-rich atmosphere. Earth's current atmosphere accumulated subsequently through volcanic outgassing and volatile delivery from later, smaller impactors. This is direct evidence that giant impacts can be catastrophic atmospheric erasers, not just contributors.

Question 3

Mars lost much of its early atmosphere to impacts while Earth retained most of its inventory because Mars is less massive.

• T. True
• F. False

Answer: True

This is correct. For a given impact energy, smaller planets with weaker gravity have a lower escape velocity, so impacts can accelerate atmospheric gas to escape velocity more easily. Mars's lower escape velocity means a larger fraction of its atmosphere was lost per impact event. Earth's stronger gravity made atmospheric retention more favorable under the same bombardment conditions.

Question 4

Because comets and carbonaceous asteroids are volatile-rich, most large cometary impacts are net contributors to a planet's atmosphere.

• T. True
• F. False

Answer: False

This is false. While comets and carbonaceous asteroids do carry substantial volatiles (comets are roughly half water ice), a sufficiently large impact can erode far more atmosphere than it delivers. The delivered volatile mass and the eroded atmospheric mass scale differently with impactor energy. Very large impacts can be net destroyers of atmosphere even when the impactor is volatile-rich. The balance depends on impact energy, impactor composition, planet size, and impact angle.

Question 5

Why is Earth's current atmosphere called a 'secondary atmosphere,' and what does that tell us about the net atmospheric effect of the early bombardment history?

Model answer: Earth's atmosphere is 'secondary' because the Moon-forming giant impact stripped its original primordial atmosphere. The current atmosphere was rebuilt from volcanic outgassing and volatile delivery from subsequent smaller impactors. This implies that early bombardment had mixed effects: many smaller impactors delivered volatiles that built the secondary atmosphere, but at least one giant impact caused catastrophic atmospheric loss.

The distinction matters for planetary habitability: a planet that retains its primordial atmosphere has a different chemical history than one that was reset by a giant impact. For Earth, this reset may have actually been beneficial by removing a hydrogen-dominated reducing atmosphere and enabling the buildup of a nitrogen-oxygen-carbon dioxide secondary atmosphere more conducive to complex chemistry and eventual life.