11
$\begingroup$

Outside of a specially crafts environment, could liquid water exist on Mars? Not some time in the distant past, but within the last few years. Mars' atmosphere is too thin for water to exist, right?

Perhaps if the water was mixed with salts or other minerals, its sublimation point might be altered enough to exist? Mars can get warm during its summer days, up to about 20 degrees Celsius at the equator. Perhaps, in certain areas and time periods, we may get a kind of liquid water?

Sorry for the speculative nature of this question, but after NASA talked about the possibility of liquid water, it got me wondering if it was possible.

$\endgroup$
9
$\begingroup$

Liquid water can in principle exist at many locations on present-day Mars, but there are a few interesting twists to the story.

  1. At low elevation, the atmospheric pressure is high enough. The triple point pressure of H2O is 611 Pascal, which corresponds to about mid-elevation. At low elevations, such as the northern lowlands, the atmospheric pressure will exceed the triple point pressure, so the liquid phase of H2O becomes possible.

  2. There will be substantial "sublimation cooling" or evaporative cooling. When the partial pressure of H2O is almost as high as the total pressure, vapor rapidly escapes into the atmosphere, which takes a lot of energy. If you put an ice cube at the equator of Mars, it will not melt, because the sublimation cooling is larger than the incoming solar energy, even at noon.

  3. However, if that ice cube is covered with a thin layer of dust, or contains dust that will accumulate on top as the ice sublimates, vapor loss will be reduced. In this circumstance, ice can melt to form liquid water.

  4. That leaves the question of how the ice cube would have gotten there initially. I call it the "source problem". Places on Mars that get hot have already lost all their ice, and places on Mars that have ice do not get warm enough. So, it is difficult to get liquid water on Mars, but it is possible in principle.

  5. With salts, which lower the melting point, everything becomes much easier, although supplies of ice/frost and salt are needed. When water with salt evaporates, the salt concentration increases, so the solution is driven toward the eutectic point, where the melting point suppression is the strongest.

$\endgroup$
  • $\begingroup$ Perhaps wind could disperse ice from colder climates to warmer ones? $\endgroup$ – Johnny Dec 28 '17 at 9:37
5
$\begingroup$

Had you asked a couple of months ago the answer would be "probably yes", now it is more "maybe".

Pure water could not exist in the Martian Environment. The pressure is too low, so the water would turn to water vapour.

Yes, if you produce hyper-concentrated brine it can remain in liquid form for a while in some martian environments. Solutions of perchlorates have been shown to keep liquids from freezing even when conditions are as cold as minus 70 Celsius. This was thought to be the cause of recurring slope lineae (ie lines that repeatedly appear on slopes around the equator). The actual model of how water (or brine) could get there was challenging.

Further modelling this year suggests that the features appear to be "granular flows" ie sand, not water. They don't exist on shallow slopes. Water will flow downhill on any slope, but sand needs a steep slope to flow. Since the lines only appear on steeper slopes, they look more like sand flows not water.

However, water is involved, as hydrated salts have been detected on slopes with the lines. There is no clear mechanism for causing sand to start to slide, on some slopes and not others.

$\endgroup$
0
$\begingroup$

It's not just the temperature that causes problems for liquid water, but also the atmosoheric pressure.

On Earth, water boils at ~96°C because the atmosoheric pressure is around 1 bar. If you lower the pressure, the boiling point for water is loser as well.

On Mars, the atmospheric pressures range is from 30 Pa on Olympus mon (highest mountain) to 1.155 Pa in the depths of hellas planitia.

If you look at this chart for Vapor Pressure you will see, that 1.155 Pa (0.001155 kPa) is not enough pressure for liquid water when the ice on Mars is melting. It would turn instantly into gas.

In addition you could look at the Triple Point of water and the chart at this site, and you see when water will be a solid, liquid or gas.

$\endgroup$
  • 3
    $\begingroup$ The boiling point of pure water at 1 bar is 100C at 1 bar $\endgroup$ – James K Dec 27 '17 at 9:22
  • 1
    $\begingroup$ @JamesK, well, kinda - that is generally a good assumption, but if you want to get the exact normal boiling point temperature of water, its best to use saturation steam tables (such as this NIST table) which shows that more accurately, the normal boiling point of pure water at 1 bar is 99.60593°C $\endgroup$ – Taylor Scott Dec 27 '17 at 15:04
  • 1
    $\begingroup$ @TaylorScott To be fair, 100°C is basically 99.6°C. Makes a lot more sense to say that than ~96°C. $\endgroup$ – JMac Dec 27 '17 at 15:47
  • 1
    $\begingroup$ @JMac, that's fair statement, and is absolutely correct, but isn't exactly what I was trying to point out - which is that the value for the NBP of water or any liquid is not going to be some clean, nice value, but rather that it is entirely dependent on system pressure - now that said, it is further complicated by composition dissolution of any solute into the system is likely to lead to both freezing point depression and boiling point elevation $\endgroup$ – Taylor Scott Dec 27 '17 at 17:55
  • $\begingroup$ @TaylorScott I don't know if anyone was disputing that. It's more the use of "~96°C at 1 bar" part that seems to be the only concern. Just a really random value to pull out considering there are cleaner looking alternatives that would be closer to correct. $\endgroup$ – JMac Dec 27 '17 at 17:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.