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The Time article Mars Has Much More Water Than Previously Known—But There's a Catch says:

The greater weight of deuterium causes it to behave differently in the Martian atmosphere. While free hydrogen atoms that were once part of a water molecule escape into space, free deuterium weighs enough to hang around in the air. Over time, as more and more hydrogen drifts away from the planet and more and more deuterium stays behind, the ratio of deuterium to hydrogen (D/H) slowly grows.

“The loss of hydrogen is a sort of constant removal,” says Eva Lingh Scheller, Caltech PhD candidate and the lead author of the paper. “Removing it from the total volume is going to give you a larger D/H ratio.”

But there’s a problem with common measures of Mars’s D/H ratio, Scheller and her colleagues found. Using atmospheric observations by NASA’s MAVEN Mars satellite and the European Space Agency’s Mars Express craft, they concluded that the current ratio is simply too low. If all of Mars’s water had escaped to space, taking its free hydrogen atoms with it, there would be much more deuterium in the modern day Martian sky relative to the remaining free hydrogen—by some measures, more than twice as much. That means that much of the planet’s water never escaped, and the only other place it could have gone was into the soil and rocks—especially into clay, the most abundant mineral on Mars.

That Mars' water may not have all evaporated but instead "gone underground" is pretty important, so I'd like to ask these

Questions:

  1. Which instruments on MAVEN and Mars Express measured the D/H ratio in Mars' atmosphere?
  2. What techniques were used to separately detect the amounts of these two isotopes of hydrogen? Were wavelengths involved? Diffraction gratings? Etalons? Microwaves? Mass Spectrometers?

Potentially helpful:

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  • $\begingroup$ I did not quite understand the initial suggestion: "That Mars' water may not have all evaporated but instead "gone underground". If Earth is a valid comparison to Mars, then, "gone underground" needs to be clarified. On Earth there is more water locked in rocks than in the surface oceans. "The high water storage capacity of minerals in Earth’s mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir" science.org/doi/abs/10.1126/science.1253358 $\endgroup$ Feb 26, 2023 at 4:25
  • $\begingroup$ On the other hand, Distribution of water on Earth Glaciers and other ice: 2.15 percent ------------- Groundwater: 0.61 percent $\endgroup$ Feb 26, 2023 at 4:25
  • $\begingroup$ In the primordial Mars atmosphere there was a lot of evaporation ! Didn't you mean escape to space instead of evaporation ? And the article that you cited says that much of the water never escaped, not all of it. $\endgroup$
    – Cornelis
    Feb 26, 2023 at 15:32
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    $\begingroup$ @Cornelis: Atmospheric escape to space is often loosely referred to as evaporation. $\endgroup$ Mar 1, 2023 at 1:12
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    $\begingroup$ @Cornelis What, do you want me to quote the entire atmospheric escape literature at you? Here we go, ui.adsabs.harvard.edu/abs/2016ApJ...817..107O/abstract e.g. Photoevaporation, a process via which cold gas is heated and accelerated to escape velocity is ubiquitous in protoplanetary discs and hot exoplanets. $\endgroup$ Mar 2, 2023 at 17:05

1 Answer 1

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I don't have access to the Science paper in question (would have to register, boo, that ain't open) so I don't see what Scheller et al cited for their D/H sources (the abstract makes it look like the data collection is not part of their work, only simulation to explain it), but two papers make it look like the instrument on MAVEN must be IUVS, the Imaging Ultraviolet Spectrograph, looking for the Lyman-alpha brightness of either isotope:

The IUVS page above also links to a presentation including requirements and details of the instrument which is very detailed, including slides like a Powerpoint slide explains the "spectral resolution requirement flows from need to minimize intensity in wings of the bright H Lyman alpha line at the position of D Lyman alpha" with associated counts per hour vs. wavelength graph

Looking for Mars Express instruments with objectives to measure the D/H ratio brought me to the Planetary Fourier Spectrometer for Mars Express but I'm not readily finding another supporting source (except good indications that the Planetary Fourier Spectrometer actually flew, not just on Mars Express but also Venus Express as well). Unsurprisingly, spectroscopy also appears to be the proposed D/H ratio measurement method with this instrument:

The 1.87 μm band appears to be the most convenient for H 2O detection, as it is almost free of overlap with the CO2 bands. However, the 2.7 μm band will be used to make the H 2O abundance determination more accurate ... PFS also provides an additional opportunity for detection of the 3.7 μm HDO band. It was observed in the martian spectrum by Owen et al. (1988) and the D/H ratio was found to be considerably higher than on Earth (about 6 times). PFS could provide a new, independent determination of this ratio.

The same paper has an extremely detailed Instrument Description section.

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  • $\begingroup$ wow, it's looking like two-for-two! :-) btw I don't know why I labeled it as open access to the paper in Science (linked at the end of my question), but I did manage to find what seems to be an open access copy now, and updated the question with the link (at the bottom of the post). $\endgroup$
    – uhoh
    Feb 26, 2023 at 3:54
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    $\begingroup$ @uhoh without looking back, I think Science themselves labeled it open, since it was available via free registration instead of charging a fee. Their bad, not yours $\endgroup$
    – Erin Anne
    Feb 26, 2023 at 4:30
  • $\begingroup$ Thank you for this answer. As far as I understand, the limb and nadir observation modes both have the possibility of probing the bulk D/H ratio in the atmosphere, not just a high-altitude layer, which might have a different value. Is this correct, according to your reading of the observation modes? $\endgroup$ Mar 1, 2023 at 1:29
  • $\begingroup$ @AtmosphericPrisonEscape I regret to say that I do not understand these papers well enough to answer a question like that. Remote sensing is not my specialty. $\endgroup$
    – Erin Anne
    Mar 1, 2023 at 1:39
  • $\begingroup$ @uhoh BTW, it wasn't the D/H ratio which suggests most Mars' water didn't escape to space, it was a new model in the paper that simulated volcanic degassing, atmospheric escape and crustal hydration that lead the authors to this conclusion. Before this paper, with the same D/H ratio from the instruments, the long-accepted view was that most of Mars' water did escape to space. $\endgroup$
    – Cornelis
    Mar 3, 2023 at 15:02

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