# Just how huge is Mars' cloud of hydrogen?

NASA Goddard's July 2018 press release NASA’s MAVEN Spacecraft Finds That “Stolen” Electrons Enable Unusual Aurora on Mars says:

But two puzzles make this type of aurora seem impossible at first glance: how did these protons get past the planet’s “bow shock,” a magnetic obstacle which normally diverts the solar wind’s charged particles around the planet? And how could the protons give off light, since atoms need electrons to do so?

“The answer was thievery,” said Justin Deighan, of the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder, lead author of a paper on this research appearing July 23 in Nature Astronomy. “As they approach Mars, the protons coming in with the solar wind transform into neutral atoms by stealing electrons from the outer edge of the huge cloud of hydrogen surrounding the planet. The bow shock can only divert charged particles, so these neutral atoms continue right on through.” When those high-speed incoming atoms hit the atmosphere, some of their energy was emitted as ultraviolet light, which is invisible to the human eye but detectable to instruments like the IUVS on MAVEN. In fact, one incoming atom can collide with molecules in the atmosphere hundreds of times before it slows down, giving off a slew of ultraviolet photons.

It's an interesting story; some of the protons from the sun become neutrals at the bow shock, then "tunnel through" to areas of increased density where they collisionally decelerate producing UV auroral light.

But I'd never heard of this huge cloud of hydrogen, likely produced from sublimation of near-surface ice on Mars. It must have a long residence time since the sublimation rate must be quite low these days.

Question(s):

• Just how huge is this cloud of hydrogen around Mars? Is there something like a scale height, and a total mass or at least a partial pressure at a given altitude?
• If it's really the decelerating, recently neutralized protons that emit the UV light as described in the quote, then what role does this huge cloud of Hydrogen play?

oh, the answer to the second part is the charge exchange in the p-H2 collisions.

NASA GIF from the same article:

This animation shows a proton aurora at Mars. First, a solar wind proton approaches Mars at high speed and encounters a cloud of hydrogen surrounding the planet. The proton steals an electron from a Martian hydrogen atom, thereby becoming a neutral atom. The atom passes through the bowshock, a magnetic obstacle surrounding Mars, because neutral particles are not affected by magnetic fields. Finally, the hydrogen atom enters Mars' atmosphere and collides with gas molecules, causing the atom to emit ultraviolet light. Credits: NASA/MAVEN/Goddard Space Flight Center/Dan Gallagher

According to Michael Chaffin, a scientist at the University of Colorado Boulder working on the Mars Atmosphere and Volatiles EvolutioN (MAVEN) mission that discovered this hydrogen cloud, the hydrogen gets as far as ten Martian radii or 34,000 km above the surface[1].

It isn't really a uniform cloud, but "clumps and streamers" of hydrogen atoms which, true, are derived from breaking up water molecules. This clumpy nature does not lend itself to defining a density or partial pressure profile as a function of height. Total water vapor content in the atmosphere, from which the hydrogen is derived, is reported at $$10$$ to $$20$$ grams per square meter of surface, with a maximum of $$70$$ grams per square meter above the north polar ice cap [2,3].

Reference:

1. "Hydrogen Cloud Blows Off Mars", by Alexandra Witze, https://www.nature.com/news/hydrogen-cloud-blows-off-mars-1.16156 (October 2014).

2. Trokhimovskiy, Alexander; Fedorova, Anna; Korablev, Oleg; Montmessin, Franck; Bertaux, Jean-Loup; Rodin, Alexander; Smith, Michael D. (1 May 2015). "Mars' water vapor mapping by the SPICAM IR spectrometer: Five martian years of observations". Icarus. Dynamic Mars. 251: 50–64. Bibcode:2015Icar..251...50T. doi:10.1016/j.icarus.2014.10.007. ISSN 0019-1035.

3. "Scientists 'map' water vapor in Martian atmosphere". ScienceDaily. Retrieved 8 June 2019.

• Thanks! After having a look at these, I'm going to meditate on the orbital dynamics of "clumps and streamers" and then probably ask separately about them. Unfortunately the images in the Nature news item are UV, otherwise they could be added as a supplemental answer to Examples of gamma ray or X-ray imaging of small planets, moons, or asteroids, or at least how to calculate intensity thereof?
– uhoh
Dec 18 '19 at 6:50
• – uhoh
Dec 24 '19 at 0:47
• I would not be surprised if they were really the same as clouds forming on Earth. Earth's water vapor/clouds, however, are (for the time being) protected from the ultraviolet radiation that would decompose them. Dec 24 '19 at 1:13