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Earth is no longer the only planet with confirmed geological activity

Earth just gets less and less special by the day. First it was the heliocentric model robbing the Earth of its central place in the universe. Then came models of planetary formation, showing that our home formed the exact same way as every other lifeless rock we can see. And now, brand new insights into Mercury have confirmed Earth isn’t even the only geologically active planet. It’s not surprising that planets in general would have this attribute, but for Mercury scientists it is a surprise. It also opens the door to all new insights into the ways planets form, cool, and die.

You might be wondering how this could possibly be a first — we’re constantly hearing about moonquakes and big space volcanoes, aren’t we? The answer is that we are almost exclusively hearing about moons, from our own satellite to those around Jupiter and Saturn. There have been strong but unconfirmed indications of volcanic activity on Venus, but this is the first direct observation.

The insight comes from new images collected by the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) probe, showing jagged fissures in the crust called “scarps.” By looking at these scarps, geologists can confirm not only the presence of geological activity, but also that it has occurred fairly recently. The scarps are small enough to lead the study’s authors to conclude that Mercury is in fact shrinking and that the crust of the planet is cracking and resettling as a result.

If that sounds a bit familiar, it’s because it’s the same basic claim that was made about the Earth’s moon, back when similar scarps were discovered on its surface. The conclusion that the moon is slowly cooling, and thus slowly shrinking, despite its small size led to a new hypothesis: Perhaps Mercury is undergoing the same process. These findings, first collected on a flyby in 2014, confirm the independent MESSENGER finding that Mercury has a weak magnetic field. Both of these facts imply the interior of Mercury is still at least somewhat active.

The question is: Why? Mercury should have been cool for a good billion years at this point, according to existing theory. Mercury is only slightly larger than our moon, which implies scientists’ understanding of the impact of mass on planetary evolution could do with some work. Thankfully, we’ve got the Moon and Mercury right there, just begging to be studied.

The assumption among planet scientists, supported by quite a bit of evidence, is there is nothing special about the Earth’s geological activity. The Earth is in a uniquely unhelpful solar system, from this perspective. Aside from Mercury, our nearest neighbor planets are Venus, which is shrouded in cloaking greenhouse gasses that make it extremely difficult to observe the surface, and Mars, which has long been thought to be at least mostly geologically dead. The others are all gas giants, the interiors of which we know little about for certain.

It’s interesting that this breakthrough came from a simple observation on the Moon, falsifying an assumption and prompting further tests of that assumption. There’s no telling just what this might mean going forward, but the better we understand the dynamics of planet formation, the better we’ll be able to predict which distant planets might be able to host life.

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