Water Ice On Mars 364
cathector sends along a story from SpaceWeather.com on the discovery of water ice on Mars.
"Scientists have figured out the mysterious white substance unearthed by NASA's Phoenix lander on Mars. It's frozen water. The breakthrough came last week when Phoenix's stereo camera caught the substance in the act of disappearing. Bathed in martian sunlight for four days, the white substance sublimated — i.e., it transformed from solid to gas without passing through the liquid state. This is how water behaves on Mars.... Some readers have asked, how do we know the white substance is not frozen CO2 (dry ice) instead of frozen water? Answer: Phoenix's landing site is too warm for dry ice. The average daily temperature is about -70 F while dry ice requires temperatures lower than about -109 F." The animated GIF showing the ice sublimating is pretty nice too.
Dupe from Thursday (Score:5, Informative)
Better picture (Score:5, Informative)
Re:Wind? (Score:5, Informative)
It was at the bottom of a trench. Plus, wind doesn't selecticely blow white rocks away while letting the rest of the scene untouched. Plus, you can also see some white areas at the end of the trench getting smaller.
It's ice. Definitely.
Re:Stupid terraforming.. (Score:5, Informative)
Re:Water sublimating (Score:5, Informative)
Doesn't really need to be under low air pressure, if ice is in the presence of low vapor-pressure it will sublimate (see icecube tray in your freezer).
Re:Water sublimating (Score:4, Informative)
Indeed it does, and it's probably better explained using a triple point diagram:
http://www.chemistrydaily.com/chemistry/Image:Phase-diag.png [chemistrydaily.com]
On earth (at higher pressures), increasing temperature goes from the solid, to liquid, and then to gas phases (the triple-point in the middle is at zero degress celcius)
The lower atmospheric pressure on Mars (~1% of sea-level earth pressure) means that you go straight from solid to gas. In fact, the liquid part is actually impossible (IANAChemist) unless you increase the pressure sufficiently.
Re:This is a dumb question, but... (Score:2, Informative)
Re:What about the pressure? (Score:2, Informative)
Re:What about the pressure? (Score:4, Informative)
You are also correct to assume that Martian pressure is nowhere near what is required for room-temperature dry ice. In fact it's about 1% that of earth's atmosphere. More reading here. [wikipedia.org]
One Problem: (Score:5, Informative)
Here's the problem: We still don't know conclusively. Yes, we have observations which are highly suggestive, but we don't have a chemical composition of the substance, so we don't know for sure.
Science is a hard mistress; she demands proof before making such claims.
Re:Water sublimating (Score:2, Informative)
CO2 sublimes on earth of course, and many other substances do under different conditions. Per the summary, we know it could not be frozen CO2.
Re:Wind? (Score:5, Informative)
Re:Um ...Dumb Question. (Score:2, Informative)
It's 1% of Earth's atmospheric pressure. At that point there is no water, only solid and gas. And the sublimation point is a lot lower because of the lower pressure. (Less pressure = less molecules keeping the other molecules tightly packed)
Re:Water sublimating (Score:5, Informative)
water sublimation doesn't need to be exotic; it happens in your freezer all the time.
you know how ice cubes gradually lose their sharp edges and finally become just little puddle-shaped lumps in the bottom of the ice try ? that's sublimation too.
phase diagram (Score:3, Informative)
Here [wikipedia.org]
How to read them [wikipedia.org]
I feel that a great disservice was done to a lot of us early on with a simplistic view of the usual three phases of matter.
And yes, you're right. That is part of the explanation.
Re:Water sublimating (Score:5, Informative)
To quote wikipedia: This can occur if the atmospheric pressure exerted on the substance is too low to stop the molecules from escaping from the solid state.
Atmospheric pressure is not as important as the partial pressure of the substance at its surface. That is, in this case, the vapour pressure of water which is practically zero on Mars. Therefore water, if it is not locked down in crystalline form, cannot exist in liquid form because it cannot form an equilibrium with its surroundings to form a 'triple point' (solid/liquid/vapour phase temperature).
It also depends, as far as I understand, on the interaction between molecules of the substance. If it is too weak, the range of temperatures at which the substance can be liquid is narrow (or practically zero). It's a fairly wide range for water, though.
I didn't study the topic beyond that and it was years ago.
PS. Iodine is another substance that sublimates.
Re:Stupid terraforming.. (Score:3, Informative)
But you're right, either way the dissociated hydrogen is way lighter.
Re:could be CO2? (Score:4, Informative)
Yes, still not cold enough as far as I can tell given this phase diagram [fsnet.co.uk] and these temperatures and pressures [wikipedia.org].
Standards.... (Score:5, Informative)
U.S needs to upgrade it's standards. A good start would to move from Fahrenheit to Celsius. After that you can move over to the metric system.
-70 F is -56 C
-109 F is -78 C
Conversion done with Google.
Seems to me... (Score:3, Informative)
Re:average daily temperature (Score:5, Informative)
Calculate your own conversion to attempt-by-the-French-to-regain-relevance-on-the-world-stage units.
Metric is the global standard. Get over it.
Re:Wow (Score:5, Informative)
"Just like dry ice does here on Earth, water ice goes from solid to gas when the pressure is below 6.1 millibars and it gets heated (like it does in the Martian sun). It can also go straight from solid to gas above 6.1 millibars when the vapor pressure (amount of water vapor in the air) is low enough. This is because the molecules of water in solid form and gas form are not at equilibrium."
http://blog.wired.com/wiredscience/2008/06/answering-mars.html [wired.com]
Re:How does the ice exist this close to the surfac (Score:3, Informative)
PI'm just flabbergasted that the ice could exist so close to the surface without sublimating purely from solar warming of the soil.
Re:Water sublimating (Score:3, Informative)
Re:This is a dumb question, but... (Score:3, Informative)
If I understand correctly, the water is blowing away.. just not as crystals. It is blowing away as discreet water molecules much like evaporation. The crystals gain energy from the sun and a little from the impact of the atmospheric gases and then the water molecules lift from the crystal lattice and suspend in the atmospheric gas matrix.
If you visualize everything as tiny versions of the colored balls in a child's play pit, you will notice that each type of ball (atom) has a different weight and tends to stratify. With enough energy added to the ball, it will then de-stratify and lift into the active matrix of the atmosphere.
Re:average daily temperature (Score:2, Informative)
The Phoenix lander is an international project including instruments from many non-US contributors. There is little justification for using Fahrenheit in this article, because these numbers do not relate to human intuition -- fucking cold is fucking cold in Celsius and Fahrenheit.
Re:Water sublimating (Score:4, Informative)
All materials sublimate. The liquid phase doesn't exist beneath a substance's triple point, so at pressures beneath that level temperature increases cause the material to go directly from solid to gaseous (sublimate). http://en.wikipedia.org/wiki/Image:Phase-diag2.svga [wikipedia.org] has a good picture of what we're thinking about.
Re:average daily temperature (Score:2, Informative)
Re:Stupid terraforming.. (Score:1, Informative)
I'm sorry, but this really annoys me. What does the escape velocity have to do with the rms temperature of vapor? Assuming a Maxwell-Boltzmann distribution (which we certainly can at these temperatures) we have that the rms speed is (3RT/M)^1/2, M being the molar weight. If this were to be the 5*10^6m/s you're claiming it is, it would mean that T=1.8*10^10 K. Clearly, that's not the temperature of Mars.
Then again, I guess that fancy words sound smarter, and more +5 Informative.