Most of Mars appears to be an endless expanse of alien desert, with no river or lake in sight. However, liquid water certainly existed in the planet’s distant past. A new paper has also suggested that it’s also possible that small amounts of water are still present in places that otherwise seem barren.
Before the Chinese rover Zhurong (aka Phoenix) went into hibernation last May, researchers at the National Astronomical Observatories and the Institute of Atmospheric Physics at the Chinese Academy of Sciences discovered something unexpected. Zhurong was exploring the Utopia Planitia region, which is located near the planet’s equator. It was thought that liquid water did not exist at those latitudes. But when the rover sent back data from its multispectral camera (MSCam), navigation and terrain camera (NaTeCam) and Mars Surface Composition Detector (MarSCoDe), there may have been evidence that liquid water was present less than half a million years ago.
“[Our findings] to suggest [features] associated with the activity of salt water, suggesting the existence of a water process at the low latitude of Mars,” the researchers said in a study recently published in Science Advances.
Dry with a soaked past
Zhurong is part of China’s Tianwen-1 Mars mission, which has helped advance our understanding of the Martian environment. But evidence suggesting there was liquid water recently (at least in geological terms) is unexpected. Because Mars has lost most of its atmosphere and is exposed to intense radiation and solar wind, it was previously thought that water could not exist there as a liquid. Anything that has formed must quickly freeze or evaporate due to the extremely low pressure and lack of water vapor.
It’s especially dry in the lower latitudes where there are no glaciers, but Zhurong found features on dune surfaces that made the researchers, led by geologist Xiaoguang Qin, suspicious. These include cracks and crusts that must have been left behind when liquid water evaporated from the reddish soil. Further investigation revealed that the surfaces of these dunes hid hydrated silica and sulfates, minerals that contain water molecules, along with certain iron oxides and what appear to be chlorides.
Both the presence of these substances and the surface features observed by Zhurong most likely indicate that frost or snow had once fallen, melted and seeped into the top layer of the soil. It formed a brine after interacting with salt in the dunes and formed something like cement when combined with grains of sand. These cements become crusts after they evaporate.
How did that get here?
But if there really was water in the lower latitudes, no more than 1.4 million and only 400,000 years ago, how did it get there?
Mars, like Earth, has gone through different eras. The Amazon period began about 2.9 billion years ago and extends to the present. After the transition from the Hesperian to the Amazonian period, Mars was no longer besieged by asteroids, while volcanic activity (some of which was caused by those collisions) decreased significantly. Although most of the atmosphere had disappeared by then and the climate was drying out, there were still hot and humid periods.
Qin and his team think it was during these periods that water vapor from the frozen poles spread to the warmer equator. This vapor would solidify into snow or frost in cooler weather and fall to the ground. It would then melt and evaporate as the temperature rose, leaving salty crusts behind.
This discovery may have implications for past or present habitability on Mars. As the climate evolved, so did the planet’s potential to host life (although it remains a mystery whether that ever was the case). Prospective rovers can look for signs of life in previously overlooked areas, especially where there are crusts, cracks and depressions that could be telltale signs of water once present.
“Since salt water once existed at various latitudes on the surface of Mars,” the researchers said, “priority should be given to salt-tolerant microbes in future missions looking for existing life on Mars.”
Science Advances, 2023. DOI: 10.1126/sciadv.add886 (About DOIs).
Elizabeth Rayne is a creature that writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. When she’s not writing, she’s transforming, drawing, or cosplaying as a character she’s never heard of before. Follow her on Twitter @quothravenrayne.
