← Back to Unit 1 · Block 3
Reading time ~5 min
Taking notes? Power 1 seeds for today
There are riverbeds on Mars. Not metaphorical ones: branching valley networks, deltas fanning out where rivers once emptied into lakes, rounded pebbles of the kind that only form when water tumbles them downstream. The Perseverance rover is parked on one of those deltas right now. Four billion years ago, Mars had rain, rivers, lakes, possibly a northern ocean, and a sky thick enough to hold all of it in place.
Today the Martian sky pushes down with less than one percent of Earth's air pressure. Spill water on the surface and it boils and freezes at nearly the same time. The planet with riverbeds cannot currently support a river. Something removed almost an entire atmosphere, and the question of what, and in what order, is one of the best-documented detective stories in planetary science.
Here is the chain, step by step.
Step one: Mars is small. About half Earth's diameter, about one-tenth its mass. Small planets lose their internal heat faster, the way a cup of chai cools faster than a pot of it.
Step two: the core froze up, and the magnetic shield died. Earth generates its magnetic field through churning motion in its liquid iron outer core. Early Mars did the same; patches of magnetized ancient crust, detected by the Mars Global Surveyor orbiter, prove the field existed. But as the small planet's core cooled, the churning weakened. Around four billion years ago, the global field collapsed. The magnetized crust is its fossil.
Step three: the solar wind went to work. The solar wind is a continuous spray of charged particles blowing off the Sun. Earth's magnetic field deflects most of it around us. Mars, shieldless, took the spray directly to the face of its atmosphere. Particle by particle, molecule by molecule, the upper atmosphere was knocked loose and carried into space. Slowly. Relentlessly. For hundreds of millions of years.
This isn't guesswork. NASA's MAVEN orbiter has been watching it happen since 2014. Its measurements show Mars losing atmosphere at roughly 100 grams per second in quiet conditions, with the rate jumping during solar storms. In 2025, MAVEN directly observed atmospheric sputtering, one of the specific knock-off processes, for the first time. The past is gone, but the crime is still in progress, and we have it on camera.
Step four: thin air, cold planet. Less atmosphere means less pressure and less greenhouse warming. Liquid water needs both. As the air thinned, the water froze into the ground and the poles or escaped upward.
Step five: the thermostat unplugged itself. On Earth, rain and rock run a slow feedback loop that stabilizes climate: warmer means more rain, more rain weathers more rock, weathering pulls CO2 from the air, and the planet cools back. That loop needs liquid water. When Mars lost its surface water, it lost the loop, and with it any way to regulate what little climate remained. The planet settled into the stable frozen state it occupies today.
Notice the shape of this story. No single dramatic catastrophe. One property (small size) knocked over a chain of dominoes, and each domino was a system: core, magnetic field, atmosphere, water, climate. Mars didn't fail a habitability checklist. Its loops came apart.
For your framework, Mars is the calibration case, and there's a rule worth writing down: "Mars is cold and dry" is a symptom, not an explanation. Any criterion you evaluate against Mars should be classified with the mechanism attached. Mars doesn't fail atmospheric retention because it's cold; it fails because a small core cooled, a field died, and a stripped atmosphere followed. If your framework can tell that story, it can interrogate any planet on your candidate list.
One honest complication to carry with you: Venus has no global magnetic field either, and it keeps the thickest atmosphere of any rocky planet we know. So a magnetic field can't be the whole story of atmospheric survival. Planetary scientists are still arguing about exactly how much protection a field provides. Your framework is allowed to hold that uncertainty; pretending it away would be the un-scientific move.
Sources
- River deltas and lakebed evidence: NASA Mars 2020/Perseverance mission pages on Jezero Crater, mars.nasa.gov [government science agency]
- Ancient crustal magnetism: NASA Mars Global Surveyor magnetometer findings, mars.nasa.gov [government science agency; spacecraft measurements, 1997–2006]
- Loss rate (~100 g/s) and solar-storm spikes: NASA, "NASA Mission Reveals Speed of Solar Wind Stripping Martian Atmosphere" (MAVEN, November 2015), nasa.gov [agency press release summarizing peer-reviewed Science papers; the papers are the stronger source if you need one]
- 2025 sputtering observation: NASA Goddard, "Is This How Mars Lost Its Atmosphere?" (May 2025) [agency video + release: this is the Block 3 Watch video; Read and Watch students end at the same evidence]
- Venus's missing field / thick atmosphere: NASA Venus fact sheet, nssdc.gsfc.nasa.gov [agency data compilation]
Go deeper: the 2015 MAVEN release above: the measurements behind steps three and four, written for the public.