One of the questions I’m most interested in seeing answered in my life is this: Did Mars once support life? If it still has life today, that is going to create a lot of international chaos as billionaires and governments argue the ethics of sending humans to a world that has life our germs may kill. I’m all for fossil hunting on Mars, but I’m kind of hoping there isn’t current life we need to worry about with our rovers and future people.

In trying to sort if Mars did or does have life, scientists are working hard to understand just what is needed to get atoms to go from having simple chemical reactions to having biological interactions. The one thing everyone seems to agree on is that life benefits from water and a lack of radiation. Today, no liquid water remains hanging out on Mars’s surface, and its lack of a modern magnetic field allows problems with radiation to persist.

These two issues – lack of water and excess of radiation – are both related to the same thing: Mars is small. Planets with low gravity struggle to hold onto water. When water vapor rises through the atmosphere, the molecules can get broken apart, and it is super easy for hydrogen atoms to get accelerated to escape velocity during collisions. As atmospheric pressure drops, water goes to vapor more easily, more gas is lost, and the cycle continues until you end up with a thin atmosphere. 

Aiding in the destruction of the atmosphere is the lack of a magnetic field. Internally hot planets have liquid layers with charged particles that can drive magnetic fields and volcanism. Small planets – like Mars – cool faster than larger worlds like Earth, and while our world still has liquid layers, Mars doesn’t really. It may have pockets of magma, but that’s not the same.  A protective magnetic field stops high-energy particles from reaching deep into the atmosphere and to the surface and stops much of the atmosphere from escaping.  Even if we could somehow make Mars a water-rich world with a thick atmosphere, that lack of a liquid inner core combined with its low mass would make terraforming only be temporary.

And in a new article in the Proceedings of the National Academy of Sciences, researchers led by Kun Wang use Mars as an example of the tipping point between worlds that can and can’t have long-term life. Wang explains: Mars’ fate was decided from the beginning. There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics, with mass exceeding that of Mars.

To trace the history of water on Mars, they looked at the abundance of potassium, which is linked to the amount of easily lost materials – volatile elements – that are on planets. According to Wang: It’s indisputable that there used to be liquid water on the surface of Mars, but how much water in total Mars once had is hard to quantify through remote sensing and rover studies alone. … Martian meteorites are the only samples available to us to study the chemical makeup of the bulk Mars. Those Martian meteorites have ages varying from several hundred millions to 4 billion years and recorded Mars’ volatile evolution history. Through measuring the isotopes of moderately volatile elements, such as potassium, we can infer the degree of volatile depletion of bulk planets and make comparisons between different solar system bodies.

The depletion of potassium that they measured indicates Mars was wet but not as wet as life might have appreciated. While putting a damper on my whole desire to go fossil hunting on the red planet, this research also means we need to change what we look for when we look for habitable worlds. Co-author Klaus Mezger states: This study emphasizes that there is a very limited size range for planets to have just enough but not too much water to develop a habitable surface environment. These results will guide astronomers in their search for habitable exoplanets in other solar systems.

For better or worse, tiny worlds are hard to find, so at least we know we’re more likely to find the potentially habitable worlds first. I think that is good news?

More Information

Washington University in St. Louis press release

“Potassium isotope composition of Mars reveals a mechanism of planetary volatile retention,”  Zhen Tian et al., 2021 September 28, PNAS