I have been keeping close tabs on the Curiosity rover’s progress on Mars. While I find many things about Mars interesting, I find the thin atmosphere of Mars especially interesting. If you are looking for some reading on the subject, the Wikipedia has a good article . The rover image in Figure 1 shows that Mars looks pretty bleak.
During my reading, I have seen different values listed for the altitudes on Earth that have the same pressure as Mars’ surface. Let’s see if we can understand how these equivalent Earth altitudes are arrived at.
On Earth, we usually talk about pressure at sea level. Mars does not have an ocean that we can use as an altitude reference. The Wikipedia gives two points of reference for the atmospheric pressure on Mars:
This is quite a range of values. The atmospheric pressure on the surface of Mars has dynamic range of 38.5 = 1155/30. To compute the dynamic range of the Earth’s surface pressure, let’s use the following two points:
- Peak of Mount Everest (8,848 meters above sea level): 33,730 pascals
- Dead Sea (423 meters below sea level): 106,200 pascals
This means that the dynamic range of air pressure at the Earth’s surface is only 3.14 =106,200/33,730. We see that the dynamic range of air pressure on Earth is much less than we would encounter on Mars.
Let’s find the altitudes on Earth with the same atmospheric pressures as Olympus Mons and Hellas Planetia.
The quickest (and cheapest) way to find the altitudes we want is to go out to NASA’s web site and download a table. Using this table, we can look up the altitudes that correspond to pressures of 0.3 millibars and 11.5 millibars. Those altitudes are:
- 11.5 millibars ⇒ 30.125 km = 98,35 feet
- 0.3 millibars ⇒ 57.150 km = 187,500 feet
The surface pressure on Mars is equivalent to the range of pressures on Earth at altitudes between ~30 km and ~60 km. That seems like pretty thin atmosphere. Since humans require pressure suits for altitudes above ~19 km (called the Armstrong limit), it looks like people will always be wearing pressure suits while walking about Mars. Too bad — I actually kind of liked the scenario shown in Robinson Crusoe on Mars (Figure 2).
I have to admit it — Robinson Crusoe on Mars is one of my guilty pleasures.