Centauri Dreams rarely looks at Mercury, the operative method being generally to focus on the outer Solar System and beyond. But a new paper out of the Planetary Science Institute in Tucson (AZ) raises the eyebrows in suggesting that parts of Mercury may once have been able to shelter prebiotic chemistry and perhaps, according to the authors, even primitive life forms. Such a finding might thus extend our ideas of ‘habitable zones’ much closer to parent stars than previously assumed.

It seems a long shot, given surface temperatures reaching 430? in the daytime and -180? at night, but the PSI work turns up interesting possibilities in some subsurface regions of Mercury. The heart of this research is found in the datasets returned by the MESSENGER (MErcury Surface Space ENvironment GEochemistry and Ranging) spacecraft. The Mercury orbiter identified numerous volatile-bearing surfaces on Mercury, with high abundances of sulfur, chlorine and potassium, and polar ice in permanently shadowed craters at the planet’s poles.

Key to the research are Mercury’s ‘chaotic’ terrains, hilly and fractured areas first seen in 1974 in the flybys of Mariner 10. The planet’s spectacular Caloris Basin is a crater about 1525 kilometers across ringed by mile-high mountains. Interestingly, numerous chaotic terrains are found directly on the other side of the planet from the Caloris Basin, leading to the theory that the impact produced them. The new paper finds that idea unconvincing. Instead, the chaotic terrains seem to have formed through gradual developments of a non-catastrophic nature. From the paper:

We attribute the immense volume losses, which we infer to have occurred during chaotic terrain formation… to widespread collapse associated with the devolatilization of hundreds of meters to a few kilometers of upper crustal materials. In the context of this hypothesis, we define “collapse” as encompassing elevation losses due to (1) mass wasting associated with the sublimation of surface/near-surface volatiles and (2) gravity-driven terrain disintegration over zones of deep volatile evacuation.

Image: Extent of a vast chaotic terrain (white outline) at the antipode of the Caloris basin (~5 x 105 km2). Credit: Rodriguez et al.

In other words, no impact needed. The authors note that the chaotic terrain is not geographically limited to the Caloris antipode, suggesting that the volatile-rich crust may have been global. They also find that the antipodal area experienced an active phase about 1.8 billion years ago for reasons unknown. The paper identifies large areas of surface elevation losses within the antipodal chaotic terrains, a finding the authors interpret as the result of crustal volatiles turning into gas and escaping from Mercury’s upper crust over an area of about 500,000 square kilometers.

Daniel Berman (Planetary Science Institute) is a co-author of the paper:

“The deep valleys and enormous mountains that now characterize the chaotic terrains were once part of volatile-rich geologic deposits a few kilometers deep, and do not consist of ancient cratered surfaces that were seismically disturbed due to the formation of Mercury’s Caloris impact basin on the opposite side of the planet, as some scientists had speculated. A key to the discovery was the finding that the development of the chaotic terrains persisted until approximately 1.8 billion years ago, 2 billion years after the Caloris basin formed.”

Image: Zoom in showing variable magnitudes of collapse, which includes a relatively unmodified rim section that is smooth but not broken into knobs (arrow 1). This area adjoins another part of the rim that has been almost entirely removed (arrow 2). The adjacent intercrater regions also exhibit deep and abrupt relief losses (arrows 3 & 4). Credit: Rodriguez et al.

Where did this volatile-rich crust come from? One possibility is impacts from outer Solar System objects or perhaps main belt asteroids. Another is outgassing of volatiles from the interior. PSI’s Jeff Kargel, likewise a co-author, makes this interesting point:

“We also observe evidence of surficial devolatilization, probably caused by solar heating. If so, we have an opportunity to infer the range of Mercury’s volatile properties and compositions… While not all volatiles make for habitability, water ice can if temperatures are right. Some of Mercury’s other volatiles may have added to the characteristics of a former aqueous niche. Even if habitable conditions existed only briefly, relics of prebiotic chemistry or rudimentary life still might exist in the chaotic terrains.”

Image: Context view showing the location and extent of the chaotic terrain antipodal to the Caloris basin (outlined in white) relative to the ray systems of the Kuiper and Debussy impact craters. (B) Close-up view of panel A that provides the context and locations for panels C and D. The numbers 1-9 identify individual rays within the region’s view. (C, D) Close-up view showing crater rays that extend over the chaotic terrain (green lines 6, 8, 9) and other crater rays that appear truncated over the chaotic terrain (red lines 1-5, 7). We provide the location of the hollow hosting crater Dario in panel C. Credit: Rodriguez et al.

That volatile losses may not all be ancient is inferred from the fact that crater ejecta rays are not found in extensive areas of the chaotic terrain, which the authors interpret as indicating more recent activity. Mercury’s ‘hollows’ — small depressions resembling melt pits in terrestrial permafrost — have likewise been investigated as signs of near-surface volatile losses, although the matter is still under investigation. Whatever the case, it’s apparent that MESSENGER has returned enough information to offer up a first round of potential landing sites to investigate the planet’s volatile-rich crust and conceivably study its unexplored potential for astrobiology.

The paper is Rodriguez et al., “The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System,” Scientific Reports 10, Article number: 4737 (2020). Full text.