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In 2016 a large ice deposit has been found within the Utopia Planitia region on Mars by the SHARAD instrument on board of the Mars Reconnaissance Orbiter.
The thickness of the deposit ranges from about 80 meters to about 170 meters with a composition that's 50 to 85 percent water ice, mixed with dust or rocky particles.
It is shielded from the atmosphere by a soil covering estimated to be about 1 to 10 meters thick.

The volume of the ice in the deposit is estimated to be about 14,300 cubic km, so the mass will be about 14 teratonnes, the mass of the atmosphere of Mars being 25 teratonnes.

Now, if bulldozers could remove the soil covering in summertime, at daytime the ice would sublimate into the atmosphere and the water vapour would be spread out by the winds.
Of course, at night the vapour would deposit on the surface again, and when all the ice of the deposit would be spread out evenly in this way about the whole planet, it's thickness would be about 10 cm.
In practice however, it's likely that the various winds will gather the ice at certain places.
Because at any time only a small part of the surface of Mars will be heated enough by the Sun to sublimate the deposited ice, the global pressure of the atmosphere would not raise much.

But will not at daytime high-pressure area form at places where the ice has sublimated and could not locally, if only for a few hours, the pressure raise significantly by the water vapour ?

If so, the low regions of Hellas Planitia could have transient liquid water on the surface and that would have a positive feedback on the climate in that region.

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    $\begingroup$ What is the use of doubling a very small atmospheric pressure on Mars? It is very small before doubling, but is very small after doubling too. $\endgroup$
    – Uwe
    Feb 21, 2018 at 20:36
  • $\begingroup$ Why was this edited to not be a question anymore? This is a Question and Answer site. $\endgroup$
    – Erin Anne
    Feb 24, 2018 at 6:56
  • $\begingroup$ @ErinAnne To be honest, at that time i thought the question was not useful anymore. Right now i think it's still important for me if i ask for more specific questions. .So i ask the main question again, although i would rather have changed it, to be more specific. $\endgroup$
    – Cornelis
    Feb 24, 2018 at 9:54
  • $\begingroup$ @Uwe But if pressure doubles on Mars, lakes can form on Hellas Planitia. $\endgroup$
    – Cornelis
    Feb 25, 2018 at 22:37

3 Answers 3

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Well, let's see how we can try to calculate that. First of, we need to find out what is the mass of the column of air on Mars and then find out how much mass the sublimation of ice will add to it and what the effect on atmospheric pressure will be. Also, we will need to study a bit the water phase diagram.

But we will start by evaluating the possibility of sublimating all this ice.

Can we even sublimate all this ice?

At any moment in time, Mars receives between 492W/m² and 715W/m² of solar radiation (in space, before being filtered by its atmosphere).

This received energy is absorbed in part by the atmosphere, another bigger part by the ground and the last part is sent back to space (Mars Bond albedo is 0.25). This means that for every hour, on a square meter, the received energy from the Sun (if the atmosphere would not absorb energy) is at most 1.931MJ (in the summer, on the most favorable latitude).

To sublimate the ice, it's necessary to add 2 590 600J per kilogram of ice. So, every hour, you would vaporize 745 grams of it. Our 10 cm layer of ice weighs in at 100kg, so to sublimate it completely, it would take you around 135 hours, which is much more than 24h39m (the length of a martian sol). And whatever you'd sublimate during the day would just transform back to ice during the night.

It's thus impossible to sublimate all this ice.

But hey, let's check if it can have an impact on the atmospheric pressure anyway.

So, let's start by studying the phase diagram for water!

Water phase diagram

What's important to note here is the location of the Solid/Liquid/Vapour triple point. It's set at 273.16K and 611.657Pa.

Also, very importantly, if we want liquid water, we will have to be at a temperature above 273.16K, almost regardless of the atmospheric pressure. (The lowest temperature at which we can have liquid water is 251.165K and 209.9MPa).

Regarding what we just learned, it's unlikely having a huge variation in atmospheric pressure will lead to a climate change with a positive impact regarding the availability of liquid water.

What would matter most would be a temperature change.

Let's also have a quick look in the meantime at Mars climate.

We can see that in summer, the temperature regularly go above 0°C with maximums at 30°C recorded. However, since the atmosphere is so thin, there is very few dampening, so the day/night variation is quite high, in the range of a 100K (so -80°C at night, and 20°C during the day).

If you want you can stop reading there: the day/night variation is too high to keep any water liquid through the martian night. So yes, there probably is some water in a liquid state during some days around the equator, however, it's not flowing long enough to have a measurable impact on the climate.

Now, let's see how much pressure the sublimated ice will add to the air.

Little reminder, the average atmospheric pressure is basically the weight of the column of air (martian air throughout this answer) over the surface.

The average atmospheric pressure on Mars sits at 600 pascals, but raises to 1155 pascals down at the bottom of Hellas Planitia. 1155 pascals is 1155 N/m² so this means the total weight of the air column over a square meter in Hellas Planitia is 1155N.

Interestingly, this is above the water triple point. This means there could already be liquid water in some places on Mars without needing to sublimate the underground ice.

Mars surface gravity being 3.711 m/s², the mass of this column of air is then 311.24kg.

Let's say the ice is spread out evenly over Mars to a 10cm layer of ice. This would be 100kg of ice on this square meter. If it's fully sublimated, this layer would be in the atmosphere, so the mass of the column of air will be 411.24kg, which on Mars is a weight of 1526N, so a pressure of 1526 pascals on the ground down at the bottom of Hella Planitia (an addition of 371Pa, or an overall average atmospheric pressure of 971Pa).

The problem we have is that the range of temperature at which the water stays liquid at this pressure is quite low, about 0-10°C.

So, in average over Mars, sublimating all of the ice would not lead to a very significant change in atmospheric pressure regarding the formation of water.

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  • $\begingroup$ Thank you for your answer. I have been so bold to edit your answer. I have added that the pressure of 1526 pascals will be at Hellas Planitia. Adding the pressure of 371 pascal to the average on Mars will lead to 971 pascal. Isn't that a significant change in pressure ? And why concentrate all the ice inside the Hellas crater if the pressure you mention cannot last for long !? I'm looking forward to your 'will have a look afterwards '. $\endgroup$
    – Cornelis
    Feb 26, 2018 at 16:11
  • $\begingroup$ This is a significant change, however not significant enough to have a meaningful change on the ability of water to stay liquid and not sublimate right away (the pressure would need to be much much higher). However, all of this does not matter, since there is not enough energy to sublimate all this ice anyway. $\endgroup$
    – gromain
    Feb 26, 2018 at 17:09
  • $\begingroup$ Also, concentrating all of the ice was just as an exercise to see what kind of pressure we could locally get to. Still not enough to keep water flowing for long. $\endgroup$
    – gromain
    Feb 26, 2018 at 17:10
  • $\begingroup$ A very clear turning point that most of the ice wil not be melted. So uncoveriing the deposit will make the climate even worse. Thank you very much ! $\endgroup$
    – Cornelis
    Feb 26, 2018 at 20:39
  • $\begingroup$ Maybe we could try to still figure out what would happen if we force the ice to melt (like if we use huge nuclear reactors to force heat in it to melt it), but at that point, the terraforming effort will be so huge that in any case you could use that power to also heat the atmosphere (once you have melt the ice). $\endgroup$
    – gromain
    Feb 27, 2018 at 8:23
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The answer above from @Gromain states that during the martian day a maximum of a 0.7 mm thick layer of ice of the deposit could be sublimated in one hour.
Since part of the Martian year the temperature at the ice deposit will be too low for sublimating, and estimating 2 mm ice evaporating in summer days, that could result in a 20 m thick layer evaporating within a 100 Earth years.

But the main question is : What will happen to the atmosphere of Mars ?

At the ice deposit, the water vapour will be spread out by the winds and will deposit as soon as the temperature drops below zero. In this way, more and more water vapour from the ice deposit will gradually accumulate at places where the ice barely sublimates, like at the poles.

It's thus possible to sublimate all this ice !

Conclusion: the whole deposit could sublimate within a 1000 years, but the climate of Mars would hardly change !

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Mars has no magnetic field surrounding it. It did have one a very long time ago, but it has lost it. So, Mars's atmosphere is subject to the Sun's strong solar winds. The solar winds have over the course of time, stripped down Mars's atmosphere which is why it has a very thin atmosphere today.

So, if all that water vapour was released into Mars's atmosphere, it may change the climate temporarily, because it would do the work of a greenhouse gas. But it too, would be stripped into outer space by the solar winds. In fact, Mars loses about a 100 grams of its atmosphere every second for the same reason.

Also, Mars has a much weaker gravity than Earth, so it may not be able to hold on to the water vapour for long.

ref:

https://www.forbes.com/sites/startswithabang/2017/03/03/nasa-discovers-how-mars-lost-its-atmosphere/#27566b7e326a

atmosphere :https://www.space.com/36277-mars-thick-atmosphere-lost-in-space.html

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    $\begingroup$ @UrvishRamaiya Yes, it's too bad that so much of the atmosphere is being lost. But if i calculated it the right way, a 100 grams every second corresponds to about 3300 tonnes a year. That's a lot, but compared to the 14 teratonnes of the deposit it means not that much ! $\endgroup$
    – Cornelis
    Feb 21, 2018 at 18:12
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    $\begingroup$ -1 for replacing "lack of magnetosphere allows atmospheric stripping on geologic timescales" with an implicit "... on human timescales". $\endgroup$
    – Nathan Tuggy
    Feb 21, 2018 at 18:21
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    $\begingroup$ 100 grams every second are 3155.78 tonnes a year of 365.25 days. $\endgroup$
    – Uwe
    Feb 21, 2018 at 20:31
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    $\begingroup$ @UrvishRamaiya We only have to remove the soil covering and let the sun do the work ! Yes, it will take years and years to remove hundreds of cubic km soil .But if only one percent of the deposit could be sublimated, that woud be 140,000,000,000 tonnes compared to the loss of 3156 tonnes a year. In this way, your 'short term future ' will be at least a million years ! $\endgroup$
    – Cornelis
    Feb 22, 2018 at 10:44
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    $\begingroup$ @Philipp As Nathan Tuggy already stated, this is about human timescales, not geological or astrophysical timescales. $\endgroup$
    – Cornelis
    Feb 22, 2018 at 12:44

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