Ice in Micro Cold Traps on Mercury: Implications for Age and Origin

Evidence in radar, reflectance, and visible imagery indicates that surface and subsurface water ice is present inside permanently shadowed regions in the north polar region of Mercury. The origin of this ice and the time at which it was delivered to the planet are both unknown. Finding the smallest, most easily eroded ice deposits on Mercury can help answer these questions. Here we present evidence for volatiles trapped in cold traps of scales ∼ 1–10 m. We consider two possible delivery methods for these deposits: a gradual, slow accumulation by micrometeorites or solar wind implantation and an episodic deposition, either primordial or by a recent comet impact. We conclude that the mechanism that best explains the presence of volatiles in these micro cold traps is a comet impact that most likely occurred in the last ∼ 100 Ma. Plain Language Summary Craters near the north pole of Mercury, one of the warmest planets in the solar system, cast persistent shadows that are so cold they can trap water ice for billions of years. Although many observations show that ice is present inside these polar cold traps, its age and the way it was delivered to the planet are not well known. Here we present evidence for the presence of ice deposits on scales of 1–10 m, the smallest discovered so far, near the north pole of Mercury. Due to the relative shallowness of these micro cold traps, an episodic deposition, such as a comet impact, is a more probable delivery mechanism than an ongoing slow accumulation by micrometeorites. Using previously estimated surface erosion rates, we find that the age of this ice is most likely lower than 100 million years. Key Points Using MLA data and a thermal illumination model, we find evidence for ice trapped in micro cold traps of scales < 10 m and thickness < 1 m Surface micro cold traps occupy 1%‐2% of the polar region, compared to ∼7% occupied by the larger cold traps A recent comet impact is more likely to explain our findings than continuous delivery by, for example, solar wind implantation or micrometeorites