Why is there no life on Mars?
There is no life on Mars because it is too cold and dry, as it has almost no plate tectonics and thus does not benefit from the greenhouse effect.
There may have been life on Mars in the past, when it was warmer and wetter, and there may still be life in Mars, below the surface. But there is almost certainly no life on the surface of the Red Planet.
The immediate reason for this is that it is too cold and dry. The temperature at the surface ranges from about -207°F to about 98°F, but even when it is above 32°F, water cannot exist as a liquid because the pressure is so low. The Martian atmosphere is 100 times thinner than Earth’s, so any liquid water would instantly boil away into gas.
The distance of Mars from the sun is similar to that of Earth; if it had a thicker atmosphere, like ours, the greenhouse effect might keep it warm enough for liquid water and sustainable life.
A few billion years ago, Mars probably had a thicker atmosphere. The thing is, Mars is much smaller than Earth and has weaker gravity, meaning that its atmosphere probably leaked into space.
The lack of life on the planet could also point to the minimal amount of plate tectonics on Mars. On Earth, plate tectonics help maintain a constant level of carbon dioxide, the most important greenhouse gas, in the atmosphere. When water is around, falling as rain or splashing around in the oceans, carbon dioxide dissolves into the water and chemically reacts with rocks. This reaction forms rocks such as chalk and limestone, and the carbon is locked away. Plate tectonics suck these rocks down into the planet’s hot interior, melt them, and then blast the carbon back out into the atmosphere through volcanoes. Thanks to plate tectonics, there is a carbon cycle.
On Mars, there are almost no plate tectonics, and thus, a minimal carbon cycle. When it was warmer and wetter, carbon dioxide in the Martian atmosphere was locked away in rocks through similar processes to those found on Earth, but then it was locked away forever. The more carbon dioxide that was removed, the less of a greenhouse effect there was and the colder it got. The colder it got, the more water condensed out of the atmosphere as rain, and the more carbon dioxide was removed. It was a runaway reverse greenhouse effect, and the result was a cold, dry, barren planet.
Hot and heavy
Almost exactly the opposite happened on Venus. Venus is almost the same size as Earth, and billions of years ago, it too may have had a gentler, wetter climate. But Venus is just a little bit closer to the sun than Earth, and instead of water falling as rain, it stayed in the atmosphere as steam and triggered a mammoth greenhouse effect. The heat released masses of carbon dioxide from the rocks, and now Venus has a crushingly thick, unbearably hot atmosphere. The surface air pressure is 92 times higher than on Earth, so walking on the surface of Venus would be like walking on Earth’s ocean bottom 1,000 yards down, except that the temperature is nearly 900°F, hot enough to melt lead!
Another reason tectonic activity is important to life on Earth is that life may well have started around a hydrothermal vent, a sort of water volcano on the bottom of the ocean. The best place to look for life elsewhere in the universe may be around similar vents; for instance, there might be some on Europa, the sixth moon of Jupiter.
