Martian Air Pressure, How much variance is there? Options

[nprev]

Just thinking when looking at the Victoria pics...how much does the average air pressure increase with respect to the surrounding area at the bottom of the crater? Also, where is the lowest point on Mars (which also presumably has the highest air pressure)? I can't remember if it's in Hellas or Coprates Chasma.

Rationale here is that, given MRO's astonishing capabilities, these low areas should be given priority for long-term monitoring; if transient surface water exists anywhere, it's probably way down in a crater or a canyon.

[ngunn]

Just thinking when looking at the Victoria pics...how much does the average air pressure increase with respect to the surrounding area at the bottom of the crater? Also, where is the lowest point on Mars (which also presumably has the highest air pressure)? I can't remember if it's in Hellas or Coprates Chasma.

Rationale here is that, given MRO's astonishing capabilities, these low areas should be given priority for long-term monitoring; if transient surface water exists anywhere, it's probably way down in a crater or a canyon.

A very quick and simplistic response: Negligible difference over depth of Victoria. Globally? The atmospheric scale height is about 11km, of the same order as the height difference between Hellas and Olympus Mons, so a pressure and density ratio of about 2 or 3. I think that liquid water would be VERY transient, even in the most favoured spots.

[JRehling]

A very quick and simplistic response: Negligible difference over depth of Victoria. Globally? The atmospheric scale height is about 11km, of the same order as the height difference between Hellas and Olympus Mons, so a pressure and density ratio of about 2 or 3. I think that liquid water would be VERY transient, even in the most favoured spots.

Olympus Mons is 25 km tall relative to the baseline, and Hellas is 7 km deep, so the range of 32 km is nearly triple the scale height, meaning a variation in pressure of about 3 factors of e = a factor of 18 (!) in pressure between the top of Olympus and the bottom of Hellas. Of course, there are also seasonal and diurnal pressure cycles, so the overall ratio of the maximum and minimum pressure on the surface of Mars is simply tremendous. But most of that is on the "low" side associated with the tall volcanoes.

To quote from
http://science.nasa.gov/headlines/y2000/ast29jun_1m.htm

"I used the model to look for regions that meet the minimum requirements for liquid water -- above the triple point and below the boiling point," explained Haberle. "According to the model, the highest surface pressure, 12.4 millibars, occurs at the bottom of the Hellas Basin (a low-lying area created by an ancient asteroid strike). The problem is that the boiling temperature there is only +10 °C. It can't get very hot or the water will boil away."

It seems like a niche to look for would be places where the temperature never gets about 10C, but sometimes get above 0C. But if tempertures HAVE gotten above 10C over geological time, then subsurface ice stores may have been depleted relatively rapidly.

I'd say MRO should be surveying the heck out of Hellas. A small fraction of Mars's surface may turn out to be wildly more interesting than the other 99% of the planet if the water condition works out there.

---

A couple of less-authoritiative cites that sound not obviously wrong:

http://www.nfinity.com/~exile/marsweather.htm

"It may possibly be the last place where liquid water ever existed on the surface of Mars. In mid summer, it is one of the "garden spots" of the whole planet, with atmospheric pressure of over 10 millibars, more than twice what it is at the mean planetary horizon. Since the vapor pressure of water is lower than 10 millibars if the temperature is less than 60 deg. F (16 deg. C)(4), this may be one of the few places on the planet where any water on the surface will not immediately boil away."

And the Wikipedia article on Hellas states:

"The altitude difference between the rim and the bottom is 9 km (5.6 Miles). The depth of the crater (4 km (2.4 Miles) below the topographic datum, or "sea level" of Mars) explains the atmospheric pressure at the bottom: 840 Pa (8.4 mbar) (.25 InHG). This is 38% higher than the pressure at the topographical datum (610 Pa, or 6.1 mbar or .18 InHG). The pressure is high enough that water is speculated to be present in its liquid phase at temperatures slightly above 0 °C (32 F)."

[ngunn]

Olympus Mons is 25 km tall relative to the baseline, and Hellas is 7 km deep, so the range of 32 km is nearly triple the scale height, meaning a variation in pressure of about 3 factors of e = a factor of 18 (!) in pressure between the top of Olympus and the bottom of Hellas. Of course, there are also seasonal and diurnal pressure cycles, so the overall ratio of the maximum and minimum pressure on the surface of Mars is simply tremendous. ---

Thanks for correcting my minor innacuracy(!) Do we have any good temperature measurements for the bottom of the Hellas basin in summer? If I haven't got this completely wrong too the Sun's elevation at noon should be about 80 degrees. I still think liquid water is unlikely, though, unless there is a process that releases it rather quickly, because warmish air near the Martian tropics would have very low relative humidity. If there were any active water-producing process in Hellas today I think we would have observed it in the form of anomalous cloud formations. Even so I agree it would be an exceptionally interesting place to visit as it must have had a unique watery history.