It is the poor relation of the Solar System. Small, charred, riddled with craters, Mercury has long been considered the most boring star in our cosmic neighborhood. A simple ball of inert rock, baked by the Sun, resembling an uneventful Moon. But this reputation as a “dead” star has just been shattered. Careful analysis of thousands of images reveals that the planet is still geologically active, releasing gases buried in its bowels via mechanisms we are only beginning to understand.
A survey of 100,000 photos
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Scientific dogma established for decades described Mercury as a geologically extinct planet, whose internal activity had stopped billions of years ago. To challenge such certainty, solid evidence was needed. This is what an international team of researchers, led by Valentin Bickel from the University of Bern, undertook.
They did not return to the site, but delved into the archives of NASA’s MESSENGER probe. This probe, which orbited the planet between 2011 and 2015, left a colossal legacy of data. The team combed through about 100,000 high-resolution images of the Mercurian surface. This painstaking work paid off: they identified nearly 400 visual anomalies that had escaped previous analyses.
“Tears” of dust and gas
What the researchers discovered were “lineae”. In geological terms, these are distinct streaks or streaks that mark the slopes of craters and landforms across the planet. Until now, only a handful were known, attributed to classic landslides. But the systematic census carried out by Bickel’s team changes the situation in terms of its scale and location.
These traces are not simple avalanches of stones. Scientists believe that they are the visual signature of an active outgassing phenomenon. Picture the scene: beneath Mercury’s hot crust, volatile materials (like sulfur or other light elements) are trapped. Under the effect of heat or tectonic movements, these gases seek an exit. They end up escaping through cracks or onto the walls of craters, carrying dust with them and changing the color of the ground, creating these famous streaks.
The Sun as a driving force
The study reported in Nature Communications Earth & Environment highlights a fascinating detail: these lineae do not form just anywhere. They appear mainly on the slopes of the most recent craters which are directly exposed to the Sun.
This suggests a complex interaction between the planet’s interior and its unforgiving space environment. It is not only the internal geology that works, but the intense solar radiation which, by heating the surface, weakens the crust and facilitates the expulsion of these volatile elements. Mercury is therefore not a passive pebble which experiences the heat of its star; it reacts, it “sweats” and is still changing today. As Valentin Bickel points out, this paints a “totally different and dynamic” portrait of a planet that we thought was frozen in time.
BepiColombo to the rescue
This discovery could not have come at a better time. If MESSENGER has paved the way, the next generation is already on the way. The BepiColombo mission, a titanic joint project between the European Space Agency (ESA) and the Japanese agency (JAXA), is about to enter the scene.
The two probes of the mission must go into orbit around Mercury in a few months to begin their scientific mission in 2027. They are equipped with much more precise instruments than those of their predecessor. One of the priority objectives will now be to focus the cameras on these famous “cavities” and streaks to confirm the degassing hypothesis. We may be on the cusp of a complete rewriting of the geological history of the Solar System’s first planet.
