Mercury may have deep underground salt glaciers that encircle the whole planet and could possibly create the conditions necessary for life
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| Geological formations called hollows on the Mercury taken by NASA's Orbital Messenger Spacecraft (Credit: NASA/JHU APL/CIW) |
Mercury might have salt glaciers that could create the conditions necessary for life kilometres underground. These rock salts may encircle the entire planet, and could transform our image of Mercury as a barren wasteland.
In space, easy-to-vaporise compounds such as water and salts are called volatiles. Historically, scientists have believed that Mercury largely lacked volatiles because of the extreme temperatures that result from its proximity to the sun and lack of atmosphere. However, in recent decades, they have discovered some types of terrain on Mercury that indicate there may be volatiles on the surface sublimating away into space.
Alexis Rodriguez at the Planetary Science Institute in Arizona and his colleagues used data from the Messenger spacecraft, which orbited Mercury between 2011 and 2015, to examine the area around the planet’s north pole. They found several areas pocked with hollows and collapsed ravines that indicate some sort of glacial activity kilometres underground. These were partially exposed by meteorite impacts and weathering.
Glacial activity doesn’t necessarily mean ice, though. “What makes this a glacier is essentially the flow downhill, driven by gravity,” says Rodriguez. “It’s not just the composition that makes it a glacier.” In this case, the glaciers are most likely made of halite – otherwise known as table salt – or other forms of salt.
This layer of salt is roughly 2 kilometres deep, and it may encircle the entire planet. The most likely explanation for such a thick global layer is the collapse of a primordial atmosphere, says Rodriguez. “Imagine, the first thing you would notice is atmospheric pressure dropping very fast, like you’re going up Everest,” he says. “Around you, you would have basically snowfall accumulating – the atmosphere turns into ice and accumulates at the surface.” This layer of former atmosphere would get buried deep underground over time and after bombardment from asteroids.
These salt glaciers are particularly intriguing because on Earth, life can persist in hydrated lumps of salt. Deep beneath Mercury’s surface, there could be small habitable pockets of salty slush that have persisted for a million years.
“When we have Earth in our solar system where we have so much life, thinking about these patchy niches of habitability so close to the sun sort of seems irrelevant, until you think of the galaxy,” says Rodriguez. “I think that Mercury is most likely very sterile, but it could have habitable zones underground – and then think about 100 million Mercuries, all with habitable zones.” This discovery could drastically increase the number of worlds that scientists consider potentially habitable across the cosmos.
Journal reference:
The Planetary Science Journal DOI: 10.3847/PSJ/acf219