Weather update: Looking good! Low wind, partly cloudy.
This morning Cosmoboy realized that one important piece of equipment hadn’t been covered in the “Countdown” series: the Alpha Particle X-ray Spectrometer (APXS). This is Canada’s contribution to Curiosity’s instrument suite! APXS was built in Richmond B.C. by MacDonald, Dettwiler and Associates Ltd and the Principle Investigator is Dr Ralph Gellert at the University of Guelph. Dr Gellert actually designed a previous version of the instrument for Spirit and Opportunity, so he’s a bit of a star of the Canadian space instrumentation scene!
OK, so what does APXS actually do? Basically, APXS is an instrument for determining precisely what elements are present in a given sample. To do this APXS will rely on two techniques, Particle-Induced X-ray Emission (PIXE) and X-ray Fluorescence (XRF). PIXE relies upon the fact that fast moving alpha particles can knock electrons from the lowest energy levels of atoms right out of the atom itself. This leaves an atom with an unstable configuration of electrons, and one of the electrons in the high energy levels of the atom will now drop down to the low level one, emitting an X-ray as it does so. APXS has a detector to capture these X-rays and we can determine the elements in the sample by looking at the energy of the X-rays – each elements emits X-rays with very specific energies, an energy signature if you like. PIXE is good for detecting lighter elements, essentially sodium through to calcium.
APXS contains Curium 244 as a source of alpha particles. But it also decays into Plutonium 240 (one of the tracer elements people have looked for in tracking Fukushima emissions) which emits X-rays that can in turn excite X-ray emission in other atoms. This is called X-ray Fluorescence and the idea is pretty much the same as PIXE, accept instead of alpha particles causing the excitation, this time it is incoming X-rays. The two methods turn out to be very complementary though as XRF is good for detecting heavier elements, calcium through to zirconium.
OK, so that’s the science behind APXS. For it to work effectively you have to be able to get close to the sample. So the emitter and detector part of APXS sits on the end of Curiosity’s robotic arm while the main electronics are back in side the body of the rover. The APXS on Curiosity is also 3 to 6 times better than the ones on Spirit or Opportunity. It will take data much more rapidly, and quick tests for a few specific elements can be done in as little as 10 minutes – it used to take several hours. It can even take data during the day as well, because a special active cooler has been added.
The main science goals of APXS are similar to that of CheMin – to understand the elemental composition of Martian geology, what elements are present and in what concentrations? The high precision of APXS will also allow it to look for any local anomalies in elemental abundances, which will help pinpoint interesting areas for further sampling with SAM. There is also a possibility of combining results with CheMin to find X-ray invisible compounds like bound water or carbonates.
How expensive was APXS to build? I’ll present this in a way that puts the investment in context. Curiosity’s overall budget is about 2.5 billion dollars. Canada’s gross domestic product is about 1/10th that of the US. So if we collaborate on a project with the US, you would anticipate that we might contribute about 10% of the overall cost, or around $250 million. Well, we didn’t do that, so about half that, say $100 million? Nope, I may as well cut to the chase, we spent $17.8 million. OK, so that’s still a good sum of money, but from an international perspective it’s a pretty minimal investment: 0.6% of the overall mission cost. I’ve written at length about the funding for the Canadian Space Agency and how low it is, and that’s true regardless of which party is in power. Although things are about to get notably worse as the CSA budget is to be cut by almost 25% beginning in 2013. But at least we’re involved, it could be worse.
APXS is a great instrument, and you can be sure that once it starts taking data on Mars there’s going to be a lot of interesting new discoveries!
T-minus one hour and counting…. Go Curiosity!