The existence of water on Mars is a contentious subject. We know there used to be water on the surface of the planet, though it’s long gone now. We know there’s frozen water underground in the world, and we know there’s water vapour in the air. But life needs liquid water.

Could there be liquid water on Mars?

A new study shows how salty water could emerge from the atmosphere onto Mars’ surface under the right conditions.

The research is based on hygroscopic salts. They have an affinity for water and can absorb it from the atmosphere. The process is called deliquescence, and while previous research and lab experiments show deliquescence takes place on Mars, there are gaps in our understanding of how, when, and where it might happen.

This new paper helps to fill in some of the gaps.

The paper’s title is “Deliquescence probability maps of Mars and key limiting factors using GCM model calculations.” The authors are Bernadett Pál and Ákos Kereszturi, planetary scientists at Konkoly Observatory in Budapest, Hungary. The paper is published in the journal Icarus.

Liquid water faces some hurdles on Mars. There’s frozen water, but it doesn’t form liquid water if it thaws. The temperature and the atmospheric pressure are so low that water sublimates from ice to vapour. Stable fresh liquid water can’t form on Mars’ surface.

But salty water remains liquid at much lower temperatures than freshwater.

Calcium perchlorate is a hygroscopic salt that’s common on Mars. MSL Curiosity found it there, and so did the Phoenix Lander. Research shows that calcium perchlorate accounts for up to 1% of Martian soil by weight.

NASA’s MSL Curiosity found evidence of perchlorates in Martian soil with its Sample Analysis at Mars (SAM) instrument. Image Credit: NASA/JPL

Calcium perchlorate can both absorb and adsorb water. In chemistry, absorption means the diffusion of particles of gas or liquid into liquid or solid materials. Adsorption is different. Adsorption is the adherence of particles to the surface of the adsorbent. Deliquescence is when, under the right conditions, a hygroscopic salt absorbs and adsorbs enough water to form a brine.

Deliquescence is a well-studied phenomenon here on Earth, where it’s important in everything from agriculture to food science. But we have no in-situ knowledge of how it works on Mars. Our understanding of deliquescence on Mars is based on powerful computer simulations.

The authors modelled two salts in this study: calcium perchlorate and magnesium perchlorate. Brines made of both of these salts can remain liquid down to -70C (-90F, 203 K) as long as the relative humidity is high enough.

Their models covered an entire Martian year and the planet’s entire surface. The authors found that both hemispheres have conditions that promote calcium perchlorate deliquescence. But ideal conditions are between local spring through late summer. Inside that, the best conditions are from late night until early morning.

This figure from the paper shows the probability of deliquescence for calcium perchlorate salts and formation of liquid water at 23 hours local Martian time, averaged over an entire Martian year. Labels mark the largest surface features and most important landing sites on the planet. Credit: Pál & Kereszturi 2022, Icarus

The study shows that the entire planetary surface likely doesn’t