How the Moon produces its own water

Oct 15, 2009
These are the Chandrayaan-1 SARA measurements of hydrogen flux recorded on the Moon on February 6, 2009. Credit: Elsevier 2009 (Wieser et al.), ESA-ISRO SARA data

(PhysOrg.com) -- The Moon is a big sponge that absorbs electrically charged particles given out by the Sun. These particles interact with the oxygen present in some dust grains on the lunar surface, producing water. This discovery, made by the ESA-ISRO instrument SARA onboard the Indian Chandrayaan-1 lunar orbiter, confirms how water is likely being created on the lunar surface.

It also gives scientists an ingenious new way to take images of the Moon and any other airless body in the .

The is a loose collection of irregular dust grains, known as regolith. Incoming particles should be trapped in the spaces between the grains and absorbed. When this happens to protons they are expected to interact with the oxygen in the lunar regolith to produce hydroxyl and water. The signature for these molecules was recently found and reported by Chandrayaan-1's Moon Mineralogy Mapper (M3) instrument team.

The SARA results confirm that solar nuclei are indeed being absorbed by the lunar regolith but also highlight a mystery: not every is absorbed. One out of every five rebounds into space. In the process, the proton joins with an electron to become an atom of hydrogen. "We didn't expect to see this at all," says Stas Barabash, Swedish Institute of Space Physics, who is the European Principal Investigator for the Sub-keV Atom Reflecting Analyzer (SARA) instrument, which made the discovery.

Although Barabash and his colleagues do not know what is causing the reflections, the discovery paves the way for a new type of image to be made. The hydrogen shoots off with speeds of around 200 km/s and escapes without being deflected by the Moon's weak gravity. Hydrogen is also electrically neutral, and is not diverted by the magnetic fields in space. So the atoms fly in straight lines, just like photons of light. In principle, each atom can be traced back to its origin and an image of the surface can be made. The areas that emit most hydrogen will show up the brightest.

Whilst the Moon does not generate a global magnetic field, some lunar rocks are magnetised. Barabash and his team are currently making images, to look for such 'magnetic anomalies' in lunar rocks. These generate magnetic bubbles that deflect incoming protons away into surrounding regions making magnetic rocks appear dark in a hydrogen image.

This artist's concept shows the Indian lunar orbiter Chandrayaan-1. The spacecraft will carry two European experiments on board which are direct descendents of ESA's SMART-1 - the infrared spectrometer, SIR2, and the X-ray spectrometer, C1XS, to study the mineralogy and the chemical composition of the lunar surface. The third European instrument on board is the SARA Sub-kiloelectronvolt Atom Reflecting Analyser, that will study the interaction between the lunar surface and the solar wind. Credits: ISRO

The incoming protons are part of the , a constant stream of particles given off by the Sun. They collide with every celestial object in the Solar System but are usually stopped by the body's atmosphere. On bodies without such a natural shield, for example asteroids or the planet Mercury, the solar wind reaches the ground. The SARA team expects that these objects too will reflect many of the incoming protons back into space as hydrogen atoms.

This knowledge provides timely advice for the scientists and engineers who are readying ESA's BepiColombo mission to Mercury. The spacecraft will be carrying two similar instruments to SARA and may find that the inner-most planet is reflecting more hydrogen than the because the solar wind is more concentrated closer to the Sun.

More information: This article reflects findings presented in ‘Extremely high reflection of solar wind protons as neutral hydrogen atoms from regolith in space’, by M. Wieser, S. Barabash, Y. Futaana, M. Holmström, A. Bhardwaj, R. Sridharan, M.B. Dhanya, P. Wurz, A. Schaufelberger and K. Asamura, in press, Planetary and Space Science, 2009.

Source: European Space Agency (news : web)

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User comments : 6

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holoman
1 / 5 (1) Oct 15, 2009
Should be easy to trace as hydrogen and oxygen production produces alot of heat.
probes
5 / 5 (3) Oct 15, 2009
be careful though holoman - we must not jump to conlusions, since heat can be produced by a lot of reactions. For example, the production of cheese is generally mildly exothermic.
LKD
3.7 / 5 (3) Oct 15, 2009
This is absolutely stunning. Am I reading this right? The sun is making atoms and molecules on the moon constantly? Wow that is such a huge detail that seems to be glossed over. Please forgive me if my ignorance is showing as I have never heard of such a thing before.
antialias
5 / 5 (1) Oct 15, 2009
The sun is making atoms and molecules on the moon constantly?


The quantities are likely rather small. So don't expect any puddles forming from 'solar manufactured molecules'
gmurphy
3 / 5 (1) Oct 15, 2009
tiny quantities indeed, what sort of influx volume does the solar wind provide?, surely after eons of buffeting by the solar wind, the moon should have a strong positive charge?
nkalanaga
5 / 5 (1) Oct 15, 2009
No, because the Sun also releases electrons. Otherwise it would have a strong negative charge, and the protons couldn't escape. They don't recombine is space because they're moving too fats and are too far apart. On the Moon, an electron can create a negative charge in a molecule, and then next proton will neutralize it. In the process, the proton and electron combine, producing the hydrogen atom.

If the negative ion happens to be oxygen, the two can combine to form to form OH, which is very reactive, stealing another O from nearby, and becoming a water molecule. This tends to stick to the regolith rather than escape.

Either way, the Moon doesn't acquire a net charge, although local areas, especially near sunrise, can have the upper surface charged. Apollo astronauts and some satellites have detected electrostatically levitated dust early in the Lunar day.