# Astronomy/Mars atmosphere

Question
With the lowest portions of Valas Marinaris being 7 miles deep, what would the estimated atmospheric pressure at the lowest altitudes be?

The scale height on Mars (the change in altitude required to cause a change in atmospheric pressure by a factor of e, or 2.718) is a little less than 7 miles, so the pressure at 7 miles "depth" should be that much higher than on the rim. However, Valles Marineris is not 7 miles deep, but 7 kilometers, and its rim is higher than the average "zero" height for the planet because it runs through the Tharsis Ridge, a region of considerable volcanic uplift, so its bottom is not as "low" as you might expect.

The greatest depth on the planet is at the bottom of Hellas, the immense crater on the southern highlands, which does reach depths of 7 kilometers below "zero" level. As a result, the air at the bottom of that crater is the densest on Mars, with about 1.2% the average pressure at the surface of the Earth (compared to 0.6% for "zero" height regions). This is high enough that the boiling temperature of water is about 35 Fahrenheit degrees, so during a warm (but not too warm) spell, liquid water might exist on the floor of the crater. Above 35 degrees the water would boil away, and of course at 32 degrees or less it would freeze.

For a visual confirmation of these heights, see the MOLA topographic map of Mars, which shows the rim of Valles Marineris in bright red and its bottom in blue-green and light blue, while the rim of Hellas is shown in orange-red (meaning it is not as high as the rim of Valles Marineris) and its bottom is shown in dark purple (meaning it is much lower than the bottom of Valles Marineris). A large version of the map is shown on my website at http://cseligman.com/text/planets/bigmola.htm .
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thank you very much. I knew there were areas that matched the water triple point on Mars, but I didn't know there were areas that could, in theory and a 2 degree window, allow liquid water to pool. If any of the Mars probes ever saw a puddle on Mars, that would change everything!

Astronomy

Volunteer

#### Courtney Seligman

##### Expertise

I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.

##### Experience

I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Publications
Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

Education/Credentials
I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)