Water on Dry Worlds

Updated 09/24/2009 12:03 PM MST based on info from this morning's press conference:

The internet is abuzz this evening regarding the possible discovery of wide-spread "water" or hydroxyl molecules of the surface of Earth's moon, a discovery made by spectrometers on three different spacecraft: M³ on Chandrayaan 1, VIMS on Cassini, and Deep Impact.  The papers, if I understand correctly, will be published later today in this week's issue of the journal Science and I have not had a chance to look at them.  There will also be a press conference later today at 2pm EDT (11am MST) discussing these results.

Why do I bring these results up here on this blog?  Well, according the few reports I have been able to find online, like this one here from Universe Today, from Bad Astronomy, and from NASA Watch, this discovery was made by finding a weak absorption band near 3 microns, associated with water and the hydroxyl ion (OH^-), concentrated mostly near the moon's high latitude.  The absorption band found on the Moon is very weak, suggesting a very low concentration of water or OH^- in the moon's soil.  The M³ instrument team suggests a concentration of as much as 770 ppm has been observed on the sunlit side of the Moon, according to the NASA Watch posting.  While the discovery isn't quite Moon-shattering, previously water ice (or hydrogen anyway) had only been observed within cold-traps in permanently-shadowed craters near the poles.

A similar absorption band was found on Io using ground-based spectroscopy (Salama et al. 1990) and Galileo NIMS (Carlson et al. 1997 and Cataldo 1999) observations.  In these measurements, a weak absorption band in Io's near-infrared spectrum at 3.15 microns was observed to be ubiquitous across its surface with a concentration of 4 ppm according to Carlson et al. 1997 and 1000 ppm according to Salama et al. 1990, on the order with what has been observed on the Moon.  This absorption band is associated with the O-H stretch transition.  Such an atomic bond between an oxygen and hydrogen atom would be found in water, hydrated minerals, or the hydroxyl ion (OH^-).  Small concentrations of this band have also been observed.  An absorption band near 3 microns, attributed to water ice crystals or hydrates mixed with sulfur dioxide frosts, was seen to the north and west of Gish Bar Patera by the Galileo NIMS instrument during the October 2001 I32 encounter (Douté et al. 2004).  The low spectral resolution of NIMS at the time (12 spectral measurements spread out between 1 and 5 microns) makes this result a bit tenuous, but if true would indicate that concentrations of possible water ice on top of the low background levels exist on the surface of Io.

So where does the "water" come from on these two, supposedly dry worlds?  For the Moon, two possible mechanisms are likely.  The first would be recent cometary impacts, which would bring their water to the Moon's surface near the site of these impacts.  Concentrations within the ejecta blankets of several small craters on the moon provide further evidence for this hypothesis, but the pattern of the hydroxyl absorption within the ejecta seems to be more consistent with material from the target body rather than material from the impactor.  The widespread distribution of water or hydroxyl ions across the moon's sunlit surface suggests another explanation.  In this scenario, charged particles, in the form of hydrogen ions and transported from the Sun by the solar wind, impact the Moon's unprotected surface (remember that the Moon is outside Earth's magnetic field most of the time).  These hydrogen ions split oxygen atoms from silicate molecules in the Moon's soil, and combine with newly freed oxygen ions to form hydroxyl ions or water.  As the day progresses and the Moon's surface heats up, these new molecules themselves split up, freeing the hydrogen to space and returning the oxygen to the soil.  The process of water formation from the combination of hydrogen from the solar wind and oxygen in the lunar soil kicks back up the surface starts to cool down in the late afternoon and evening.  Alternatively, the water molecules may become excited and be transported to Moon's polar regions, where they are deposited within those aforementioned cold-traps.

For Io, the solar wind can't reach its surface due to Jupiter's strong magnetic field.  So where does its water come from?  Again, oblique cometary impacts could be a source of water for Io.  The two recent cometary impacts on Jupiter in 1994 and again this year would suggest that Io could be hit by water-rich cometary bodies on a regular basis.  This could certainly be the source for the concentration found near Gish Bar Patera.  For the global ubiquitous concentrations of water or hydroxyl ions, another mechanism maybe necessary.  For example, low concentrations of water might be present in Io's magma, like here on Earth.  Water vapor would then be released during volcanic eruptions and water ice would be deposited on the surface, however, no water vapor has ever been observed within Io's plumes.  Another possibility could be that hydrogen ions from Jupiter's magnetic field break off oxygen from sulfur dioxide and silicate compounds on the surface then combine with them to form OH^- or water, akin to the preferred scenario for the Moon.

This discovery of OH molecules on the Moon is certainly interesting, and just goes to show everyone that water is quite common place in the solar system, even in the driest of places.

Graphic above by University of Maryland/F. Merlin/McREL.

Link: Water on the Moon...?  Yep. It's Real. [blogs.discovermagazine.com]