Space Probe Reveals Novel Geology on Mercury With Abundant "Hollows"

Mercury impact crater "de Graft"  showing numerous 'hollows'  -  shallow depressions that form via a process of volatile loss.

Relatively unknown to most planetary aficionados (but not planetary astronomers) are recent results of the MESSENGER ('Mercury Surface Space Environment, Geochemistry and Ranging') mission.  Among the new results has been the discovery of thousands of 'shallows': shallow, fresh -looking depressions up to a few kilometers wide- scattered over the planet's surface.

Having said that, the nature of these depressions remains a mystery and only one model has evidently come close to explaining their origin. Thus, while several lines of evidence indicate formation as a result of the loss of volatile materials (easily dissipated) in surface rocks, the specific process for this remains unknown.

A breakthrough has arrived via unprecedented, high resolution MESSNGER craft imagery.  From these, Blewett et al have proposed a new model for the growth and formation of the hollows. See e.g.

https://doi.org/10.1002/2016JE005070


Blewett's team used measurements of shadow lengths to calculate the depths of more than 2,500 hollows and found that the depressions' average depth was only 24 m. This is substantially less than the typical thickness of the layer of dark, volatile-rich material in which the features are most frequently found. This mismatch suggests an alternative process at work.  This alternate process had to account for the depressions not attaining the depth of the surrounding volatile materials. 

The key material to fill the role is known as low reflectance material or LRM. Generally, this is kilometers thick or much greater than the computed depths of the hollows.  The Blewett team interpreted this to mean the shapes of the shallow depressions aren't controlled by the thickness of the host LRM.  Hence, the depressions are halted before they reach the bottom of the LRM layer.

The Blewett team has argued that a volatile-depleted "lag" material protects the underlying (LRM) material once it becomes sufficiently thick. A fair indication the hypothesis is accurate is the observation that hollows often occur on the walls and central peaks of impact craters. These are formations too steep for lag to develop so consistent with the proposed model.

From this the team concludes the hollows' formation and growth can be traced to the volatilization of carbon. (Evidence suggests that carbon is an important constituent of Mercury's crust). Again, bear in mind that "volatilization" is an analogous process to vaporization. Thus, a substance can often be separated from another by volatilization and then be recovered by condensation of the vapor.  One of the mechanisms proposed for carbon volatilization is the conversion of graphite to methane (CH4) via proton bombardment,

E.g. because graphite is an allotrope of carbon, we write in succinct form:

C _graphite  +  H1 + H1 + H1 + H1   ->   CH4

It is also noteworthy that the Blewett et all team was able to estimate the lower limit for the rate of hollow horizontal growth, on account of the presence of hollows within several impact craters with ray systems. See e.g. the image of the impact crater de Graft. Therein, the dark blue central peak complex is presumed composed of LRM which may include graphite bearing crust material. 

The pace of horizontal growth is likely via the retreat of scarps that form the hollows' walls.  The authors estimate the rate of retreat at 1 centimeter per 10,000 years - not exactly blinding speed.  Of course, this rate itself places further constraints on the formation and history of Mercury's land forms. That is, we cannot expect the rate of  formation of Mercury's other land forms  to significantly outpace the hollows' horizontal growth rate.