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NearABE t1_iwvrzgv wrote

Reply to comment by MidnightPlatinum in Mars was once covered by 300-meter deep oceans, study shows by magenta_placenta

UV light splits water into hydrogen and hydroxyl. Atomic hydrogen would quickly escape.

Earth does not have enough gravity to retain helium.

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LaunchTransient t1_iwwlwb3 wrote

>Earth does not have enough gravity to retain helium

Nor hydrogen, which is lighter still. Your mechanism is problematic, as the Earth is both 2/3rds the distance from the sun as Mars is (and so has 2 times the UV intensity) but it also has a relatively consistent quantity of water over the last two billion years despite being glared at by those same rays.

A more likely driver of ocean loss on Mars is pressure-driven boil off. With no magnetosphere to protect it, the atmosphere gets stripped away by solar winds, causing the oceans to evaporate at an increasing rate.

The one issue I have with this hypothesis of deep oceans that have evaporated away is the apparent lack of surface evaporite deposits. Where's the salt flats?Just look at anywhere on Earth where there has been an endorheic basin - The Utah salt flats, Lake Karum in Ethiopia, Salar de Arizaro in Argentina, etc. Even under the Mediterranean sea there are layers of salt 3 kilometres thick from the era of the Messianic salt crisis. The absence of evaporite deposits just doesn't add up.

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NearABE t1_iwwpge8 wrote

On Earth water condenses and forms clouds. There is very little of it at the top of the atmosphere. Instead we see atomic oxygen and ozone.

>The one issue I have with this hypothesis of deep oceans that have evaporated away is the apparent lack of surface evaporite deposits. Where's the salt flats?

https://en.wikipedia.org/wiki/Vastitas_Borealis

>"Results published in the journal Science after the Phoenix mission ended reported that chloride, bicarbonate, magnesium, sodium, potassium, calcium, and possibly sulfate were detected in the samples"

Not too far off from "salt flat".

Mars has extensive dunes and dust storms. The frost reworks water soluble material. It should not look exactly the same as Earth's salt flats.

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LurkerInSpace t1_iwwxjcv wrote

To nitpick; Earth is about 70% the distance Mars is - it gets roughly twice as much sunlight rather than four times as much. Venus in turn gets about twice as much as Earth and four times as much as Mars.

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LaunchTransient t1_iwwz0vf wrote

edited. I am tired, so my memory for orbital radii gets kinda crap. I should have checked.

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StrangeTangerine1525 t1_iwzw4m4 wrote

Earth has an ozone layer that causes a temperature inversion. Water can't readily reach the upper atmosphere like it can on Mars and Venus. Once Earth's temperature inversion in the middle atmosphere is gone, hydrogen and water vapor will readily escape like it does on Venus, magnetic field or not.

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