Water ice on Mars. - Elaborate breakdown of the polar caps.

Josh Cryer · 2002-01-23 00:43:00

There is a lot of confusion going on about the presence of water ice on Mars. Especially about the composition of the ice caps. I'm going to try to show how we can deduce that the north polar residual cap is probably, with a large degree of certainity, composed of water ice.

When I talk about ?residual caps? I'm basically talking about the size of the ice caps when it's summer time (but it always varies; basically residual means the ?smallest? size the caps get).

Following are some nice pictures and links to sites which help show this. All I'm doing is compiling the information for others to understand (INAPS- I'm Not A Professional Scientist). All italic emphasis is mine (bold seemed a bit strong).

"Cottage Cheese" Texture on the Martian North Polar Cap in Summer

full image

This picture shows a cottage cheese-like texture on the surface of a part of the residual--summertime--north polar cap.
The north polar cap surface is mostly covered by pits, cracks, small bumps and knobs. In this image, the cap surface appears bright and the floors of pits look dark. Based upon observations made by the Mariner 9 and Viking orbiters in the 1970s, the north polar residual cap is thought to contain mostly water ice because its summertime temperature is usually near the freezing point of water and water vapor was observed by the Vikings to be coming off the cap during summer. The south residual cap is different---its temperatures in summer remain cold enough to freeze carbon dioxide, and very little to no water vapor has been observed to come off the south cap in summer.
The pits that have developed on the north polar cap surface are closely-spaced relative to the very different depressions in the south polar cap. The pits are estimated from the length of shadows cast in them to be less than about 2 meters (5.5 feet) deep. These pits probably develop slowly over thousands of years of successive spring and summer seasons.

Source: http://mars.jpl.nasa.gov/mgs....ex.html

Observation: The north polar residual cap has a higher temperature than the south in the summer. Also, water vapour was observed by the Viking probes.

We need to dig a little more! That's just a page about some pretty picture. Well, let's do that very thing.

The following is an image of recent temperature profiles of the south polar cap, this image is updated daily with about a week lag due to processing constraints, and it's summer in the southern hemisphere right now, but the data (along with some other data) will serve the purpose for determining the chemical composition of the north polar cap.

vamp0_SP-Day_Temp(!Uo!NC).png

Source: http://emma.la.asu.edu/daily.html

This is a daytime reading of the south polar ice cap, as you can see, it's relatively cold there, around -120 to -90 where the residual part lies, but peaking to -15 near the edges of the region. (It's colder in in the north polar region -120, by comparasion- but remember, it's winter there right now.)

I can't find a summer reading of the north polar cap, and I don't quite know how to read the PDS data related to it (when am I ever going to have time to learn this stuff?), but it's not really required to show that the north polar cap consists primarily of water ice, as I'll show in a momment.

North polar topography of Mars
smith.snow.fig1A.jpg

This image is of the north residual polar cap on Mars. Red denotes the actual residual cap, while blue shows the extent of icey layers (it could be said, with simplicity, that red is the summer cap, and blue is the winter cap). As you can see, the north polar cap is quite large! And it barely receeds during the summer, unlike, as you'll see, the south polar cap.

Sorth polar topography of Mars
smith.snow.fig1B.jpg

Isn't that amazing? The south residual cap is four times smaller than the north, yet its winter-time extent is almost twice that!

Source: http://ltpwww.gsfc.nasa.gov/tharsis/snow_paper.html

Now that we know that the north polar residual cap is made of water ice (or some magical carbon dioxide that somehow manages to not evaporate in the summer), how much is it?

There was a study done in 98, right after the first MOLA scans were done, that wanted to find out just that. In it the scientists came to the conclusion that, ?The shape of the polar cap indicates that it is composed primarily of water ice, with a volume of 300,000 cubic miles (1.2 million cubic kilometers). The cap has an average thickness of 0.64 miles (1.03 kilometers) and covers an area 1.5 times the size of Texas. The estimated volume of the north ice cap is about 10 times less than the minimum volume of an ancient ocean that some scientists believe once existed on Mars.?

Source: http://ltpwww.gsfc.nasa.gov/tharsis/agu_f98.html

We know that the north polar cap does not receed as dramatically as the south during the summer, and that its residual size is similar to its full extent, so it should be clear that the north polar cap is made primarily of water ice. If it was carbon dioxide it would have the same properties as the south polar cap.

I don't know how to read the Viking data, but if I did I would certainly provide evidence of water vapour over the north polar cap during the summer.

RobS · 2002-01-23 12:26:00

Josh, thank you for these data. I wish I could give you a source for this additional information, but it was preweb; probably something I read 1975-77 when I was a graduate student in planetary science at Brown. Or maybe it was in the 1980s. Someone calculated that under the north polar cap, the pressure from the overburden soon gets too high for solid carbon dioxide to exist; it sublimates under pressure unless the temperature gets lower (and with geothermal heat, with increased burial the temperature goes up).

So this makes it impossible for the northern cap to be CO2. But it might be a CO2/H2O "clathrate," which is a combination of the two compounds. There has been some research done on clathrates. One could check the old Icarus abstracts if they are on line, like the new ones are. The article about clathrates on Mars may have been there. I think there have been Icarus articles about clathrates in the Jupiter and Saturn systems, too.

Even if they are clathrates, though, it does not change the total water in the polar cap by an order of magnitude. Maybe by an order of two; but that's still a LOT of water!

-- RobS

Josh Cryer · 2002-01-23 21:18:43

That's interesting RobS, and I would think that since water has a higher freezing point than CO2, the lower layers would be composed of H20. Perhaps CO2 sublimation is the cause of those pits seen in that picture. The snow data shows about a one to two meter change in CO2 on the north polar cap, and I think that could indicate (with some degree of error) how much is actually H2O and not.

The caps have been there long enough for all sublimation to have ceased...

Shaun Barrett · 2002-01-24 01:48:12

This is very interesting stuff, Josh. But I think I may be a little confused over some of it.
In the material you quoted, I noticed this: "The south residual cap is different ... its temperatures in summer remain cold enough to freeze carbon dioxide, and very little to no water vapor has been observed to come off the south cap in summer".
As I understand it, in the course of Mars' elliptical orbit, the southern hemisphere experiences its summer at perihelion (Mars' closest point to the Sun). This means that although the southern summer is shorter than the northern summer, it is considerably warmer. In fact, this is the reason why all the big dust storms originate in the southern hemisphere in spring/summer; because of relatively intense convection heating of the atmosphere.
The converse of this, of course, is that the northern summer, though longer, is cooler.
How is it ,then, that the northern cap appears to lose all its CO2 in its cool summer (and even starts to lose water!), while the southern cap retains a very significant quantity of solid CO2 even at the height of its warmer summer?!
I'm sure there must be a perfectly simple answer to this question, but I'm obviously too simple to think of it!!
Can anybody help me out? ???

Josh Cryer · 2002-01-24 02:57:43

Shaun, I believe that perihelion / aphelion differences don't matter when your atmosphere is only 5-10 milibars. Seasonsal temperature differences are due to obliquity. I lamented that I couldn't find summer temperature data for the north cap, because comparing the two would help us significantly. As far as I know, the north cap is slightly warmer than the south. I provided the souths current temperature map to show that it's freezing there right now, especially in the area where the residual cap is.

I just wrote an email to the one of the TES guys asking him about this. I'll get back to you guys.

RobS · 2002-01-25 00:21:02

An extremely important factor is altitude. The northern cap is built on top of the northern lowlands, the southern cap on top of the cratered highlands. The southern cap is thus at a much higher altitude than the northern and experiences a lower atmospheric pressure. The vapor pressure of CO2 is very sensitive to temperature. According to my CRC, page D-150, the vapor pressure of CO2 at -135C is 1 millimeter of Mercury, but at -110C it is 34 millimeters of Mercury. Thus at high altitude, a CO2 cap will sublimate away; while it is colder at a higher altitude, it is not colder enough.

-- RobS

RobS · 2002-01-25 00:42:18

Well, actually, I think my posting made the problem worse, not better. So now I'm confused, too.

-- RobS

Shaun Barrett · 2002-01-25 21:37:50

Thanks RobS for introducing the obvious (in retrospect!) factor of altitude. I confess I didn't give this point any consideration and I can't believe how stupid I can be!
It seems to me though that you have, in fact, made my problem worse! I note your comment that at -110C, the vapor pressure of solid CO2 is 34mm Hg; which I believe translates to about 44 millibars. At the approximately 5km altitude of the south cap, I doubt there'd be more than about 4 millibars of atmospheric pressure. So, shouldn't the south cap in summer, which appears from the above diagram to be at -110C or likely higher, be subliming furiously into the Martian air?
I still don't know why there is any residual cap left at all by the end of summer ..... unless ... no, it couldn't be largely water ice, could it? Just a thought!

Josh Cryer · 2002-01-25 23:46:36

You guys are forgetting one thing. Sublimation increases atmospheric pressure.

[Vapor pressure volatile model] postulates that the major atmospheric constituent is in equilibrium with a frost on the surface. This means that the atmospheric pressure equals the vapor pressure of the frost at equilibrium (which is temperature dependent). The temperature of the frost is determined by radiative equilibrium. During the summer, incident radiation increases the temperature, increasing the vapor pressure, and causes frost to sublime. This increases the atmospheric pressure. In the winter, the poles cool and excess vapor condenses, given up its latent heat.

Source: http://scienceworld.wolfram.com/astrono....el.html

Josh Cryer · 2002-01-28 10:14:19

Well, Tim had promised to update the TES site by Friday with North / South Polar Winter / Summer comparasions but he must not have had time (I'll update when he does). I just wanted to let you guys know, since I never actually told you I had contacted the TES people and they promised to help us out with this little dilemma- I mean, we can't really have a conclusion until we see some evidence for the claims- I wish I knew how to manipulate and analyze the PDS data.

BTW, the volatile vapor pressure model is extremely important on Mars, because one milibar of pressure difference means serious vapor temperature changes (as RobS has shown). The south pole, during the summer, is going to be extremely ?humid? in that respect. So the pressure goes up considerably when enough CO2 sublimes (and if you look at the images, a whole crapload obviously sublimes during the summer- indeed, the cap almost becomes non-existant); and since the summer there is so short, this could explain why the cap doesn't receed completely!

Shaun Barrett · 2002-01-30 00:42:50

Thank you Josh for taking the trouble to try to look into this problem more thoroughly.
In the meantime, I think your well-meaning comments have left me still floundering in the dark .... but maybe not completely!
First of all, I think I know what you mean about the sublimation of the CO2 raising the atmospheric pressure. But it sounds like you're implying that this will somehow cause a localised pressure increase sufficient to impede the further subliming of CO2 (?).
If RobS's data are correct (and I have no reason to doubt his accuracy), then we are dealing with a vapor pressure of 44 millibars for solid carbon dioxide at the temperatures described. At 5kms above datum, I can't see how the ambient pressure could ever get anywhere near 44 millibars. Even in the depths of Hellas basin, at 6kms below datum, the pressure never exceeds 10 millibars (maybe 10-point- something in exceptional circumstances). If all this is true, I still can't see why the sublimation shouldn't proceed to completion i.e. no residual cap left at all.
EXCEPT .... and here's my second point ... for the possibility that the summer, as you point out Josh, is simply too short for cap to disappear entirely. This, I could live with!
I suppose it IS impossible, isn't it, that the south residual cap could be more H2O than CO2? Or maybe some kind of clathrate which might be more resistant to evaporation or sublimation?
Any help out there? My highschool chemistry isn't what it used to be! (Maybe it never was ... !!)

Josh Cryer · 2002-01-30 02:26:10

Not impeed, but prolong, for sure. It's sublimining away very fast, though. In fact, they say the actual residual cap is sublimining away at 3 meters per year (and it's probably a clathrate too). Eventually there won't be anything there at all.

What I'm seeing happening are patches of localized high pressure zones that slow sublimination down. It's this very reason the cap is there at all, I suspect.

BTW, check out the night time temperature for the south cap.

http://wwwflag.wr.usgs.gov/USGSFla....NC).png

I don't think the logic conflicts, since the south cap is subliming away completely, and the north cap is not (there's a saturation model that claims the north cap can get up to 15 milibars, though... we'll have to look in to this... I hope the guy updates the site soon.)

Shaun Barrett · 2002-02-27 06:21:17

Looking forward to the updated site too, Josh! Incidentally, I have recently looked into atmospheric pressures on Mars and discovered I was mistaken about the Hellas Basin.
Apparently, the lowest part of Hellas can experience up to 12.4 millibars of atmospheric pressure, depending on the time of year.
All this is very interesting stuff. Keep in touch, guys!

Josh Cryer · 2002-03-02 00:52:56

He still hasn't updated. I finally got my email working again, and so I will email him as soon as possible. The least he could do is give me the code to build the necessary graphs.

rhw007 · 2002-05-13 10:34:23

You know, there are LAKES OF LIQUID water on Mars if people, especially NASA/JPL would only look at their own images without a 'political' veil over their eyes.

If THAT ain't a lake, even if currently frozen, then someone is paving the sand over there. :0

For the raw data:

LIQUID LAKE raw data with geometric information

MORE LITTLE LAKES!!!

Let's taunt a Billionaire to Mars ! ! ! Beats Begging.

Think out there act down here... NOW ! ! ! Have YOU ??

Give a High School kid a reason to care...a spacewalk.
Teach 'em a lesson in teamwork.
Teach 'em a lesson in humanity.

Bob... ;-{)

http://members.tripod.com/rhw007/

http://communities.msn.com/JPLMarsExploration

Anton Kuratnik · 2002-05-15 17:58:23

maybe it's just me, but I feel like saying "you're full of shit". Or, maybe, "Those sites are full of shit". No offense, but it's true.

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#1 2002-01-23 00:43:00

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#2 2002-01-23 12:26:00

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#3 2002-01-23 21:18:43

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#4 2002-01-24 01:48:12

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#5 2002-01-24 02:57:43

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#6 2002-01-25 00:21:02

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#7 2002-01-25 00:42:18

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#8 2002-01-25 21:37:50

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#9 2002-01-25 23:46:36

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#10 2002-01-28 10:14:19

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#11 2002-01-30 00:42:50

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#12 2002-01-30 02:26:10

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#13 2002-02-27 06:21:17

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#14 2002-03-02 00:52:56

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#15 2002-05-13 10:34:23

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

#16 2002-05-15 17:58:23

Re: Water ice on Mars. - Elaborate breakdown of the polar caps.

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