You can be forgiven for thinking that the Solar System’s innermost planet is a perennial disappointment. Mercury has always appeared a dead, crater-scarred ball of rock with little of interest for the discerning astronomer.
And yet, last week the planet surprised everybody by presenting planetary geologists with a conundrum. The MESSENGER spacecraft, currently in orbit about Mercury, sent back images of strange features on the surface that defy explanation.
The pictures have generated more than a little fuss. The hollows are irregularly shaped, shallow, rimless depressions which have have formed in craters, indicating that they must be relatively recent additions to the landscape. They are also strangely bright, indicating that they are covered in a highly reflective material.
That means the surface of Mercury must be active in some way. The question is: how?
Today, Marvin Herndon, an independent scientist based in San Diego, gives us his idea. He points out that if, during the planet’s formation, the pressures and temperatures were high enough for iron to become liquid, then it ought to have absorbed large amounts of hydrogen.
As the liquid solidifies, this hydrogen would be released and escape through the surface of the planet. The result would be hydrogen geysers that erupt from time to time on the planet’s surface.
These hydrogen geysers could certainly have caused the rimless depressions that MESSENGER sees, says Herndon, a self-proclaimed maverick in the world of planetary geology.
What’s more, hydrogen would react with various substances as it passes through the planet’s crust. One possible reaction is the reduction of iron sulphide to metallic iron. Iron sulphide is common on the surface of Mercury.
It is this light dusting of metallic iron that gives the depressions their high reflectivity, says Herndon.
: Explanation for Observed Evidence of Geologically Recent Volatile-Related Activity on Mercury’s Surface