Channels, pits and caves in the Hebrus Valles on Mars may have been carved by ancient flowing water
NASA’s Mars Global Surveyor
Caves carved by water that once flowed beneath Mars’s surface could have been ideal for life to thrive, if it once existed on the Red Planet, and they might still preserve traces of it today.
Mars is dotted with holes that look like cave entrances, but these are usually near regions that are suspected to have been volcanically active, which suggests they formed due to processes like underground lava flows, rather than the passage of water.
On Earth, there are thousands of caves formed when water dissoves soluble rock, known as karstic caves. But scientists have yet to find signs of such caves on Mars, despite evidence that the planet was covered in water billions of years ago.
Now, Chunyu Ding at Shenzhen University in China and his colleagues say they have identified eight possible caves that look to have been produced by ancient water flows, rather than volcanic activity. The caves are in the Hebrus Valles, a north-western region containing hundreds of kilometres of valleys and depressions that seem to have been carved out by ancient floods.
These caves have been mapped by previous Mars missions, such as NASA’s Mars Global Surveyor, which orbited Mars from 1997 to 2006. Ding and his team used spectrometry data from that mission to analyse the material around the cave entrances. This shows they are high in carbonate and sulphate minerals, which typically form in the presence of water.
They also found evidence of ancient streams that end near the cave entrances. This is similar to what we see near karstic caves on Earth, says James Baldini at Durham University in the UK. “If you’re looking on a map, you would expect the stream to be on a surface, and then all of a sudden disappear, because the stream water is being pirated by the cave system.”
Daniel Le Corre at the University of Kent, UK, says the mineralogical and geological evidence suggests these could be water caves, but there is nothing about their appearance that looks strikingly different from other caves on Mars. “I’ve spent an inordinate amount of time looking at the Mars global cave catalogue, and these do look very similar to ones that are known to be volcanic in origin,” he says.
If these are water caves, they could be particularly good places to look for life. “In order to have life, you need water and an environment that is sheltered from the intense radioactive bombardment on the surface of Mars,” says Baldini. “Volcanic caves and lava tubes are also reasonable places to look for life, but there’s not necessarily any water involved.”
Martian water caves might also contain stalagmites, the bulbous, protruding columns of rock that are common in karstic caves on Earth, which could act as time capsules for aspects of Mars’s ancient environment, such as its temperature.
But stalagmites can take many thousands of years of constant water flow to form, and even if we manage to send a rover or drone into the caves to sample them, working out when exactly the stalagmites were formed could be extremely difficult, says Baldini.
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