Why Can't We Live on Mars?
The short answer
Mostly because Mars barely has any air. Its surface pressure averages 6.36 millibars, less than 1% of Earth's. Liquid water can't last in air that thin. A glass of water set out on Mars would boil and freeze at the same time.
It wasn't always like this. Long ago, Mars seems to have had thicker air, warm enough for liquid water, with rivers that carved valleys and fed crater lakes.
Why the water boils and freezes
The less air presses down on water, the lower the temperature it boils at. On Mars the air is so thin that water can boil at temperatures as low as 0 °C, the same temperature at which it freezes. Water that emerges at the surface "immediately starts to boil."
As the water boils off, the boiling cools the water left behind until it freezes. NASA's astrobiology team puts it plainly: liquid released on the surface of Mars "will simultaneously freeze and boil."
Where the air went
The Sun never stops blowing out a stream of charged particles called the solar wind. Earth's magnetic field, made by its hot, churning iron core, shields our air from being worn away by it.
Mars had a planet wide magnetic field too. But Mars is about half Earth's width and about a tenth of its mass, and a smaller planet loses its heat faster. As Mars cooled inside, its field all but vanished, about 4 billion years ago.
With no shield, the solar wind carried Mars's air off into space, a little at a time. NASA's MAVEN spacecraft still measures it happening: about 100 g (roughly 1/4 lb) of gas every second. The MAVEN team says losing air to space was likely a major process in changing Mars's climate.
Key numbers
| What | Number | Source |
|---|---|---|
| Average air pressure at the surface of Mars | 6.36 millibars | NASA Mars Fact Sheet |
| Range of Mars's surface pressure through the seasons | 4.0 to 8.7 millibars | NASA Mars Fact Sheet |
| Mars's surface pressure compared with Earth's | less than 1% (about 0.6%) | Our arithmetic from NASA Mars Fact Sheet |
| Lowest temperature at which water can boil on Mars | as low as 0 °C | Open University / Nature Geoscience (Massé et al. 2016), via ScienceDaily |
| Mars's radius compared with Earth's | 0.532 (about half) | NASA Mars Fact Sheet |
| Mars's mass compared with Earth's | about a tenth | NASA Mars Fact Sheet |
| When Mars's planet wide magnetic field all but vanished | about 4 billion years ago | NASA Goddard, Sun to Earth Day |
| Gas the solar wind strips from Mars today | about 100 g (roughly 1/4 lb) per second | LASP, University of Colorado (MAVEN) |
| The same rate over one day | about 8,600 kg | Our arithmetic from LASP, University of Colorado (MAVEN) |
What the film simplifies
- "Half Earth's size" means width. Mars's radius is 0.532 of Earth's, but its mass is only about a tenth of Earth's.
- "Thick air" means thicker than today. Scientists still debate how thick the early Martian air really was.
- Mars's heart didn't freeze. NASA's InSight lander found the core is still at least partly liquid. It cooled enough to stop the churning that makes a planet wide magnetic field.
- The shield isn't the whole story. Gravity also helps a planet hold on to its air, Venus keeps a thick atmosphere with no planet wide magnetic field, and scientists still argue about how much a field protects an atmosphere.
- "First the air left, then the lakes" doesn't mean the wind blew the lakes away. Once the air got thin, water couldn't stay liquid on the surface, so it was lost to space or froze.
- The leaf blower, the sky peeling off in sheets, Mars's face and the heart you can see through a window are cartoon. The real loss is slow, from the top of the atmosphere, over a very long time.
For teachers
Runtime 1 minute 9 seconds · Topics: Mars, Atmospheres, Magnetic fields, Solar wind · Full script below
Ask your class
Why would a glass of water on Mars boil and freeze at the same time?
Answer
Mars's air pressure is less than 1% of Earth's, so water can boil at temperatures as low as 0 °C. The boiling cools the water that's left until it freezes.
Earth and Mars both get the solar wind. Why did Mars lose its air while Earth kept its own?
Answer
Earth's magnetic field, made by its hot, churning iron core, shields its air. Mars is smaller, cooled faster, and its field all but vanished about 4 billion years ago, so the solar wind could carry its air into space (gravity matters too).
MAVEN measures Mars losing about 100 g of gas every second. About how much is that in one day?
Answer
A day has 86,400 seconds, so 100 g × 86,400 is about 8,640,000 g, or roughly 8,600 kg.
The script
Host: On Mars, a glass of water boils and freezes at the same time.
Rocket: Built in slushie machine?! Sold! We're moving to Mars!
Host: It boils because Mars barely has any air. Less than one percent of ours.
Host: But long ago, Mars had thick air, and rivers pouring into lakes.
Rocket: It came with lakes?! Where'd they go?!
Host: First the air left, then the lakes. Blame the Sun's wind: a leaf blower that never shuts off.
Rocket: Hey! It blows on Earth too!
Host: Sure. But Earth has a shield. Our hot, churning iron core makes it.
Host: Mars had one too. Until its heart cooled down and stopped churning.
Host: Mars is half Earth's size, and small things cool fast. Just ask a french fry.
Rocket: Cold fries?! I'm leaving a one star review!
Host: With no shield, the wind peeled the sky off Mars, layer by layer.
Rocket: I bought a planet with no roof?!
Host: So that boiling glass of water? It all comes down to a cold heart.
Rocket: Okay! Cancel the move! Earth, keep that heart beating!
Host: Mars is out. But around the very next star, there's a planet where water might stay liquid. Follow!
Sources
- Liquid water on the surface of Mars boils and freezes at once.NASA Astrobiology
- Mars's air pressure is less than 1% of Earth's.NASA Mars Fact Sheet
- Early Mars seems to have had thick air and liquid water.LASP, University of Colorado (MAVEN)
- Martian craters show signs of old lakes and connecting river channels.NASA, MAVEN
- The solar wind is a constant stream of particles from the Sun.NASA Science
- Earth's magnetic field shields its air from the solar wind.NASA Science, Earth's Magnetosphere
- Mars's core made magnetic field all but disappeared.NASA MAVEN blog, 2013
- Mars is smaller than Earth, so it cooled much faster.NASA Goddard, Sun to Earth Day
- At low pressure, like on the surface of Mars, water can boil and freeze at the same time.Exploratorium, Science Snacks: Boyleing Water
- On Mars, water can boil at temperatures as low as 0 °C, and water that emerges immediately starts to boil.Open University / Nature Geoscience (Massé et al. 2016), via ScienceDaily
- Mars's core is still at least partly liquid.NASA JPL (InSight), 2023