Although Mercury is the closest planet to the sun, some of its craters hide water ice in their shaded depths - but how much ice is there, and where exactly did it come from?
A new study has part of the answer, suggesting that the ice is around 50 meters thick on average, but there is considerable uncertainty with that measurement. What kind of comets deposited the water there in the first place, however, is a mystery.
The researchers based their information on the Mercury Laser Altimeter instrument on the now-complete NASA MESSENGER mission, which crashed into Mercury as planned in 2015. Previously, a combination of Earth-based radar and neutron data suggested water ice deposits that were at least one to two meters thick. The new data suggests an ice thickness of 50 meters, possibly up to 85 meters.
But what's possibly most intriguing is figuring out the source of this ice. The researchers aren't quite sure if the ice came from long-period Halley-type comets (which originate from the Oort Cloud, a collection of icy objects far out in the solar system) or short-period Jupiter-family comets (which originate from the Kuiper Belt, an icy group of objects beyond Neptune's orbit).
"If we had found a lower limit for the ice depth that was significantly greater than zero, which we didn't quite, then we would have been able to rule out the possibility of Halley-type comets or asteroids being responsible for delivering Mercury's water," said Vincent Eke, a reader in the department of physics at Durham University, U.K., in an email to Seeker. Eke is the lead author of a paper published in the journal Icarus.
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"The models for the micrometeorite flux reaching Mercury are quite uncertain," Eke Some of the more modern models could manage to bring 50 meters of ice to the polar craters, as could Jupiter-family comets. Also, if the ice depth is as thick as 50 meters in all the polar craters, then it would rule out the possibility that the ice was all brought by a single recent impact. One other study did consider that the impactor that created Hokusai crater might have been responsible for the polar water ice deposits."