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Earth, Titan and Venus all have large amounts of $\mathrm{N_2}$ in their atmospheres. (In the case of Venus it's a small proportion, but Venus' atmosphere is very thick, and the total mass of $\mathrm{N_2}$ is greater than Earth's.) However, other planets and moons, and Mars in particular, have hardly any. Why is this?

$\mathrm{N_2}$ is a relatively light molecule, so I suppose it could be lost to space from smaller bodies. Did Mars start with a thick nitrogen atmosphere and then lose it? Or alternatively, is there some process that produced lots of $\mathrm{N_2}$ on Earth, Titan and Venus, which didn't occur on Mars or the other outer Solar system moons? If so, what is this process likely to be?

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For Earth, Titan and Venus, I think there are continuing processes that are providing $\ce{N2}$ to the atmosphere of these planets.

Concerning the Earth, there is the well documented Nitrogen cycle based on flora. There are also other significant sources for nitrogen from inorganic processes.

Assuming the rock processes on Earth are also occurring on Titan and Venus, similar inorganic processes may be occurring on other worlds. I think it is a good educated guess even though we really have very very little hard evidence of rock processes other planets.

As for Mars, its thin atmosphere may indicate that is cannot hold onto $\ce{N2}$ and there may be very little $\ce{N2}$ currently being created on Mars.

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    $\begingroup$ The nitrogen cycle doesn't really put nitrogen into Earth's atmosphere - organisms take it out, and then eventually put it back again, but they can't really add any $\ce{N2}$ that wasn't there to start with. It would be interesting to see if there's any specific evidence of present-day $\ce{N2}$ outgassing on Venus or Titan - presumably this could be detected spectroscopically by Cassini in the case of Titan. $\endgroup$ – Nathaniel Dec 28 '14 at 7:34
  • $\begingroup$ "Assuming the rock processes on Earth are also occurring on Titan and Venus, similar inorganic processes may be occurring on other worlds." this assumption cannot be made. We know literally nothing of the geochemistry of Titan and Venus. Also the dominant Nitrogen content of Earth's atmosphere will have more to do with the formation conditions and not geochemistry, seeing as Nitrogen is relatively inert. $\endgroup$ – AtmosphericPrisonEscape May 17 at 14:50
  • $\begingroup$ @AtmosphericPrisonEscape: Actually we do know something important about the geochemistry of Titan, namely that the "rocks" are water ice. We might also consider Pluto, where most of the surface is solid nitrogen: en.wikipedia.org/wiki/Geology_of_Pluto $\endgroup$ – jamesqf May 17 at 17:35
  • $\begingroup$ @jamesqf: Titan yes, that's less than 50% of the surfae area. Pluto: Less than 30%, not most. Read the papers in science. Ammonia and CO ice are dominating. $\endgroup$ – AtmosphericPrisonEscape May 18 at 1:08
  • $\begingroup$ @AtmosphericPrisonEscape: OK, but that doesn't invalidate the point I was trying to make, which is that the surface of the outer planets/moons is entirely different from that of the silicate-based inner planets. $\endgroup$ – jamesqf May 18 at 18:42
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A huge factor affecting a planet's atmospheric composition is the planet's escape velocity. From Wikipedia, we have a table of escape velocities, and here are some sample figures:

  • Earth: 11.2 km/s
  • Mars: 5.0 km/s
  • Jupiter: 59.6 km/s
  • Pluto: 1.2 km/s

The molecules of an atmospheric gas all fly around with different velocities. Turns out, these velocities follow the normal distribution. That is, there is a bell curve with a mean. If the mean velocity of the gas particles is higher than the escape velocity of a given planet, then you probably won't see much of that gas on the planet.

Mars has much less mass than the Earth and so its escape velocity is much lower. If you were to look up the velocity distribution of nitrogen molecules in Mars' atmosphere, you'd probably find that the average velocity is greater than 5.0 km/s.

If this is interesting to you, there's so much more! There's an entire field of physics called statistical physics which is the foundation of so many other fields, like chemistry and thermodynamics. Studying these fields allows you develop physical intuition which can be super helpful for reasoning about all the crazy stuff which happens on this planet.

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    $\begingroup$ As it happens I'm an expert on statistical physics already. (But not on planetary atmospheres.) The tricky thing is that Titan's escape velocity is only 2.6 km/s, about half that of Mars, but Titan has loads of nitrogen. The rate of thermal loss depends on the temperature at the top of the atmosphere as well as on the escape velocity, so it's not trivial to calculate (especially when you don't know what the temperature was in the past when the atmosphere was thicker). It's thought that the thermal loss rate for nitrogen on Mars was never that high, but (...) $\endgroup$ – Nathaniel Dec 28 '14 at 7:24
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    $\begingroup$ (...) but you can also lose molecules due to interactions with the Solar wind, especially if you don't have a magnetic field (which Mars doesn't), since the particles in the Solar wind can collide with molecules in the atmosphere and accelerate them past their escape velocity. Mars has probably lost a lot of its atmosphere that way. But this doesn't fully answer my question, which is about whether this is the primary reason for the lack of $\ce{N2}$ on Mars and its presence on other bodies, or whether there are also other important factors. $\endgroup$ – Nathaniel Dec 28 '14 at 7:28

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