Earth has retained its nitrogen-oxygen atmosphere over billions of years but lost most of its primordial hydrogen. What is the primary reason for this difference?
AHydrogen was chemically incorporated into water and rocks before it could escape
BAt Earth's exospheric temperature, lighter hydrogen molecules move fast enough that a significant fraction exceeds escape velocity, while heavier N₂ and O₂ molecules do not
CThe solar wind selectively strips hydrogen because protons interact more strongly with the magnetosphere
DHydrogen reacts with ozone in the upper atmosphere and is destroyed before it can escape
This is Jeans escape in action. At a given temperature, lighter molecules have higher average velocities (v_rms = √(3kT/m)). Hydrogen (m ≈ 2 amu) moves ~3.7× faster than nitrogen (m ≈ 28 amu) at the same exospheric temperature (~1,000 K). Enough hydrogen molecules populate the high-velocity tail of the Maxwell-Boltzmann distribution to exceed Earth's 11.2 km/s escape velocity over geologic time. Nitrogen and oxygen molecules are far too heavy to escape thermally at that temperature. Option A is a true fact but explains the form of hydrogen (water), not why the remaining free hydrogen escaped.
Question 2 Multiple Choice
Mars has lost most of its original atmosphere over billions of years. Which combination of factors best explains this outcome?
AMars is too cold for atmospheric chemistry and all gas molecules froze out over time
BMars has both lower escape velocity than Earth AND lacks a global magnetic field, making it vulnerable to both Jeans escape and solar wind ion stripping
CMars lost its atmosphere because it is farther from the Sun and thus receives too little solar energy to maintain atmospheric pressure
DVolcanic outgassing on Mars was insufficient to replenish the atmosphere after early impact erosion
Mars suffers from two compounding vulnerabilities. Its lower gravity (escape velocity 5 km/s vs Earth's 11.2 km/s) means more molecules can escape thermally. And without a global magnetic field (Mars lost its dynamo ~4 billion years ago), the solar wind interacts directly with the upper atmosphere, ionizing molecules and sweeping them away. The MAVEN spacecraft measured this ion escape directly. Being farther from the Sun (option C) actually reduces solar wind intensity slightly — it works against this explanation, not for it.
Question 3 True / False
A planet with a strong global magnetic field is mostly protected from atmospheric escape.
TTrue
FFalse
Answer: False
A strong magnetic field deflects solar wind, substantially reducing direct ion stripping of the atmosphere. However, it does not eliminate escape entirely. First, the polar wind — ions accelerated outward along open magnetic field lines at the poles — allows a continuous stream of escaping ions (mostly H⁺ and O⁺) even on magnetically active planets like Earth. Second, Jeans escape (thermal escape) operates regardless of the magnetic field — light molecules in the upper atmosphere simply fly off if they exceed escape velocity. A magnetosphere dramatically *reduces* escape rates but cannot reduce them to zero.
Question 4 True / False
Atmospheric escape rates on most planets were higher during the early solar system than they are today.
TTrue
FFalse
Answer: True
Young stars emit far more extreme ultraviolet (EUV) and X-ray radiation than they do in maturity. The early Sun emitted roughly 10–100× more EUV flux than it does today. This intense radiation drives all non-thermal escape mechanisms more aggressively: more photodissociation (photochemical escape), stronger photoionization and solar wind interaction (ion escape), and higher exospheric temperatures (enhanced Jeans escape). The first ~500 million years of the solar system saw dramatically higher loss rates — this early intense escape period shaped the bulk atmospheric compositions we observe today and is central to understanding why Venus, Earth, and Mars diverged so dramatically.
Question 5 Short Answer
Explain why Jeans escape is far more effective at removing hydrogen from a planetary atmosphere than removing nitrogen or oxygen, using the physics of the Maxwell-Boltzmann distribution.
Think about your answer, then reveal below.
Model answer: Jeans escape occurs when molecules at the exobase (the level where the atmosphere becomes collisionless) happen to have velocities exceeding the planetary escape velocity. The Maxwell-Boltzmann distribution gives the probability that a molecule has a given speed; the fraction exceeding escape velocity depends critically on molecular mass. At a given temperature, the root-mean-square speed is v_rms = √(3kT/m), so lighter molecules move faster. Hydrogen (m ≈ 2 amu) has an rms speed roughly 3.7 times higher than nitrogen (m ≈ 28 amu) at the same temperature. This means a far larger fraction of hydrogen molecules populate the high-velocity tail of the distribution and exceed escape velocity. For nitrogen and oxygen, whose masses are 14–16 times larger than hydrogen, escape velocity is so far into the tail of the distribution that thermal loss is negligible over planetary timescales. This mass dependence explains why planets systematically lose light gases (H, He) while retaining heavier species.