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Wondering if there's any evidence to date of the existence of maar craters on the Moon? If not, how probable is it that the moon ever supported the necessary conditions for maars to form? A report prior to the Apollo 15 mission suggested that the Hadley C crater might be a maar (a depression produced by exploding steam when rising magma comes into contact with sub-surface water). No doubt the crater's location atop a volcanic feature - the Hadley sinuous rille - factored into its speculated non-impact genesis.

But why was this even considered a possibility at the time? I understand that water on the Moon may have come from a variety of sources, like hydrogen and oxygen reactions triggered by solar winds, water ions from Earth hitting the Moon and turning into permafrost etc. But could enough sub-surface water have accumulated in this way for a powerful enough crater-forming steam explosion to occur?

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  • $\begingroup$ Initially I would have thought not, given ground water is required to produce maars & also because for ground water to exist as a liquid an atmosphere is required. However, it is possible that within a crater, of whatever type, in a volcanically active region sufficient water ice may have accumulated that could have reacted with rising magma/lava to create a maar. $\endgroup$
    – Fred
    Commented Mar 3, 2023 at 18:19
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    $\begingroup$ Interestingly, Hadley C crater (5.5km) is about the size of the largest maar on Earth - Devil Mountain Lake in Alaska. Maars anywhere near this size are thought to have formed from the collision of magma and permafrost (ice between sediment grains), not liquid water. Now, widespread ice has been detected in the lunar polar regions, but I'm not aware of any maar discoveries there. And is it even possible for there to have been sufficient ice stores at the latitude of Hadley C? Sub-freezing temperatures would need to persist below the surface to prevent much of the ice from vaporizing. $\endgroup$
    – samiant
    Commented Mar 4, 2023 at 9:06
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    $\begingroup$ The other possibility is if some maars were formed they could have been completed inundated with lava from an eruption, either during the creation of the maar or later and all we see now is a large area of lava flow, not knowing what is underneath it. $\endgroup$
    – Fred
    Commented Mar 4, 2023 at 10:26
  • $\begingroup$ There's no definitive evidence for the existence of maars on the Moon. While there have been suggestions that some lunar craters could be maars, these ideas have not been widely accepted. $\endgroup$
    – W Kristianto
    Commented Mar 21, 2023 at 13:16

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