26 October 2008

The Science of Chandrayaan: Part 2

Posted by Ryan Anderson

Last time I described the Indian-made instruments on the Chandrayaan-1 spacecraft. But the mission is a huge international collaboration, and there are six more instruments to talk about made by countries around the world. Let’s take a look:

An X-ray image of the moon.

Chandrayaan-1 X-Ray Spectrometer (C1XS) – “Wait a second!” I hear you saying. “Didn’t you talk about an x-ray spectrometer last time?” Well, yes, but x-rays have a wide range of energies. The HEX instrument was for high-energy x-rays, but C1XS is for the more “gentle” 1-10 keV (1.2 nm – 0.12 nm) range. This range is perfect for x-ray flourescence, which is the “glow” of lower energy x-rays that are emitted when materials are bombarded by higher energy x-rays. C1XS will allow scientists to detect the fluorescence of common elements like Mg, Si and Al. Also, during solar flares, when the moon is really getting blasted with x-rays, other elements on the surface such as Fe, Ti and Ca will glow too. C1XS was made at the Rutherford Appleton Laboratory, UK and modified by the Indian Space Agency (ISRO).

Near-Infrared Spectrometer (SIR-2) – Yes, another spectrometer. This one looks at sunlight reflected off of the lunar surface in the near infrared range (0.93 – 2.4 microns). This range has a lot of features diagnostic of different minerals in the surface. One scientific objective of SIR-2 that I found interesting is the study of “space weathering”: the process that causes the lighter-toned ejecta from craters to gradually get darker and fade away. It is thought that this is caused by the solar wind hitting the surface and forming tiny iron grains. Iron has a very strong near-IR signature, so SIR-2 should be able to shed some light on this. SIR-2 was made at the Max Plank Institute for Solar System Science in Germany.

Sub-keV Atom Reflecting Analyser (SARA) – Particles from the solar wind hit the moon’s surface and blast other particles into space in a process called “sputtering”. By analyzing the sputtered particles, SARA should be able to tell us what the surface is made of. The ISRO site says that this instrument will take images of the surface and of the solar wind’s interaction with the surface. I’m not exactly clear how this works. As I understand it, sputtered particles enter the instrument, get ionized, and then get separated by mass and analyzed. If you know the mass of an atom, then you know what element it is. That makes sense, but I don’t know how they will make images. SARA’s goals are to study the surface composition, including a look inside the permanently shadowed craters at the poles to see if there are volatile materials like ice. SARA will also be able to study magnetic anomalies, presumably because the magnetic fields will change how the solar wind interacts with the surface. SARA was made by the Swedish Institute of Space Physics.

Radiation Dose Monitor Experiment (RADOM) – This is a pretty simple instrument (to the extent that anything on a space mission is “simple”). The whole goal is for this thing to get bombarded with radiation and see how much there is, what range of energies the particles have, and figure out how that dose might change for different locations on the moon. The Apollo astronauts were only out of Earth’s protective magnetic field for a few days, but for colonists spending months or years out there, it’s important to know how much radiation shielding they will need, and what type of radiation is the most dangerous. RADOM was made by the Bulgarian Academy of Sciences.

An Earth-based radar view of the Moon’s south pole.

Miniature Synthetic Aperture Radar (Mini-SAR) – Synthetic Aperture Radar is the technique of bouncing radio waves of the surface of a planet and making an image out of them. It’s the radio wavelength equivalent of shining a bright light on something and then taking it’s picture. Since Mini-SAR provides its own “illumination” it will be able to easily look inside the permanently shadowed craters on the moon. Even better, water ice reflects radio waves in a unique way, so Mni-SAR will be able to tell if there is any ice on the moon. Even better, radio waves penetrate a few meters into the surface! Mini-SAR was made by the Applied Physics Laboratory, Johns Hopkins University and the Naval Air Warfare Centre, USA through NASA.

Moon Mineralogy Mapper (M3) – The final instrument on Chandrayaan’s impressive payload, M3 is an infrared mapping spectrometer. That means that it takes images of the moons surface with a resolution of 70m per pixel at hundreds of wavelengths of infrared light. This gives you a spectrum for every pixel in the image, which can be used to make maps of minerals on the moon (hence the instrument’s name). This is very similar to the HySI instrument which is also on Chandrayaan. I suspect there are subtle differences that make having both instruments onboard worthwhile. In any case, it’s clear that Chandrayaan is going to be doing a lot of mineralogy mapping! M3 was made by Brown University and NASA’s Jet Propulsion Laboratory.

That wraps up my summary of the Chandrayaan instruments. It’s quite a powerful suite, and I suspect we’re going to be overwhelmed with data about the moon’s composition pretty soon. It will be especially interesting to see what Chandrayaan can discover about the potential ice deposits at the poles. Those deposits have been a major rallying point for human exploration, but nobody is really sure how much ice is there. We’ll find out soon enough!