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So it’s pretty much agreed that Mars was once a wet planet, and most of the water was lost to space. However, I am unable to see why that is necessarily the case primarily for the same reason that Earth’s water hasn’t been lost to space (we would have lost our water if we relied only on gravity).

Atmospheres have what is called a “cold trap”, which is a spot in the middle where the atmosphere drops to low temperatures and causes water vapor to slow down/freeze and fall back to Earth. Mars should have had the same mechanism, but one that was even colder/better than the one we have here on Earth due to its distance from the Sun.

So here’s my theory, the water vapor wasn’t lost to space but frozen first. Then over millions of years it has been buried over by the massive planet wide dust storms Mars is known for. This in turn would provide alternative explanations for why the northern hemisphere of the planet is so smooth as well as why only the Southern Hemisphere retains any magnetic rocks (the diamagnetic properties of water prevented the magnetism retention of the northern hemisphere that southern possesses).

I realize this is probably not likely, but what I would like to know is why? Are there any sources that can disprove this theory? I have been unable to locate any.

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    $\begingroup$ This should be astronomy, not space exploration. I like your question, but it might get closed or moved for that reason. Please don't repost it there. Let the moderators make the decision first. $\endgroup$ – userLTK Mar 4 at 21:05
  • $\begingroup$ Adding to @userLTK this actually could find a home on Earth science too. Lots of planetary scientists are Earth scientists by training since the basics of the fields are essentially the same. $\endgroup$ – ben Mar 5 at 1:52
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I like your theory and it's creative thinking, but I suspect it's unlikely for the simple reason that if this was the case, why does Mars have ice caps? Wouldn't they get buried too?

Your cold trap idea is correct, that's why icy bodies exist in the solar system. Titan, though we don't think about it that way because it also has methane lakes and an atmosphere, Titan's surface is mostly what we'd call ice. Pluto, Triton, basically any comet, are basically ice covered. Comets are often described as dirty snowballs.

Volcanic Enceladus is ice covered too, constantly recovering itself.

Closer to the sun you get a dusty covering over an thin icy crust like layer with an ocean underneath but moons or dwarf planets with an undersurface ocean and basically a dry surface like Ceres or Europa. Ice has a hard time forming on Ceres because there's still too much sun, but the density of Ceres suggests it has an ocean below it's surface and probably an icy layer below the surface we can see. Europa and perhaps other Galilean moons probably have an icy crust as well because they're cold enough but their surfaces get both sun light and radiation from Jupiter's radiation belt, so they have no ice on their surface, but not far below their surface.

Mars is probably rock all the way down, partly based on it's density but also, some crustal movement and volcanism. Water is lighter than rock so it doesn't stay buried very easily, though it can dissolve or fills in the cracks so there's probably significant water below Mars' surface but not in the way you describe. Mars may have an entire ocean below it's surface, but spread out, similar to how Earth has about as much water mixed in it's crust and below than it has in it's oceans.

The surface of Mars rises above freezing under sunlight, which should, perhaps slowly but steadily sublimate any ice off it's surface and allow the water to escape into space.

Mars may have been much more ice covered in it's past, but regular sunlight and warm enough temperatures likely caused it to melt and/or sublimate over time. That strikes me as the best explanation for where much of it's water went, following the loss of much of it's atmosphere.

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    $\begingroup$ Only avg temp matters for ice insulated by regolith. Polar caps are periodically redeposited, because although CO2 is heavier than H2O, it is also more volatile. $\endgroup$ – amI Mar 7 at 1:56

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