Water Discovered on Mars! (Again.)

It's a bit of a running joke amongst planetary scientists that any research discussing water (in any form) on Mars will get reported in the press with a breathless headline. Year after year, somehow the 'discovery' of water on Mars persists as a topic of great interest. One of our MSc students digs into this phenomenon below. Luckily, she isn't working on astrobiology topics - that often leads to headlines about aliens.
(Image above: Jezero crater, as it might have appeared when filled with water in the past)

By Grace Bischof

Recently, fellow PVL member Charissa Campbell was involved in a JPL press release detailing the Martian clouds she captured in her Cloud Altitude Observation. The images consisted of beautiful clouds drifting across the Martian sky, likely composed of CO2-ice due to their high altitude in the atmosphere. I spent some time reading the replies to the press release on social media, many of which pose questions about the composition of the clouds on Mars. “Clouds means water?” writes one of the commenters. Meanwhile, “What are the clouds made of? [because] there ain’t no water vapour floating around on Mars, right?” inquires another.  

Well, I have some good news for everyone: there is water on Mars! But before you call your local newspaper to report the exciting revelation, I have some explaining to do. The presence of water on Mars has been confirmed since the late 1960s, when water vapour lines were detected in spectra of Mars observed with an Earth-based telescope. Since then, water has been “discovered” on Mars over and over and over again (see: any Daily Mail article about Martian water). So, let’s clear this up. Water exists on Mars in both vapour and ice form. The confusion arises when we consider the way we talk about water in day-to-day life. In regular conversation, we say “water” when we mean liquid H2O, and “ice” when we mean solid H2O. This verbiage doesn’t hold in planetary science settings because “ice” to refers to any solid form of a condensable species: for example, CO2-ice or H2O-ice. Because of this confusion, the lack of liquid water leaves people believing there is no water whatsoever.

So, let’s take a closer look at the water on Mars. Mars’ atmosphere is much thinner and has much less water vapour than Earth’s atmosphere. However, the amount of water vapour is sufficient to condense to form water-ice clouds in the atmosphere. Each Mars Year, when the planet is at its aphelion position (the furthest point from the sun) a belt of water-ice clouds forms in the equatorial region of the planet, called the Aphelion Cloud Belt. These clouds have been observed by orbiters around Mars and cameras on the surface, notably by the Curiosity rover. Water-ice clouds exist in other regions, as well, such as in the Martian Arctic. The Phoenix mission captures images of fluffy clouds drifting past its landing site during the second half of the mission. The Surface Stereo Imager and lidar onboard the lander were used together to find evidence of water-ice fog near the surface of the lander. The Phoenix mission was also the first mission to capture precipitation (snow) falling from the clouds.

The other common place to find water on Mars is as water-ice in the subsurface. The Phoenix lander was equipped with a robotic arm to dig into the Martian soil. They were able to find ice in the area around the lander at many depths, ranging down to 14 cm. At both the north and south poles you can find a large polar cap primarily composed of water-ice covered by a layer of CO2-ice. Areas of the south polar cap resemble Swiss Cheese, where pits in the CO2-ice layer expose water-ice beneath. PVL member, Alex Innanen, has examined the Swiss Cheese pits to determine if the water vapour sublimated from the Swiss Cheese contributes substantially to the overall global water vapour abundance. 

So, what about that coveted liquid water? Well, it’s complicated. Presently, no bodies of liquid water can exist on the surface of Mars. The atmosphere is so thin that exposed water-ice sublimates straight to water vapour when heated and deposits back to ice when cooled. Geomorphic evidence, such as river deltas, show us that Mars had liquid water on the surface in its past. In 2018, radar analysis of Mars’ south pole saw evidence of a subsurface liquid-water lake. It was theorized the lakes would be extremely salty, bringing the freezing temperature down and allowing the water to stay liquid. In 2021, a York U professor, Dr. Isaac Smith, released a paper explaining that the radar observations are better described by hydrated and cold clay-rich deposits (https://doi.org/10.1029/2021GL093618), rather than salty lakes. 

Liquid water on Mars is an exciting notion because it leads to many questions about the habitability of the planet, both past and present. But liquid water is not the only interesting phase of water – it is clear we really love the clouds here at PVL. Nonetheless, if liquid water is ever found, you can sure expect to see a bold headline declaring “Water Discovered on Mars!”, in which case, you can refer back to this blog post.