Following a 1-day hiatus because of the launch delay, we’re back on the countdown trail. Last time in this series of blog posts we looked at the CheMin experiment and what it will tell us about the mineralogy of Mars. Today we’ll focus on Curiosity’s Sample Analysis at Mars (SAM) system. In a nutshell, SAM will help us to look for the organic compounds that are necessary for life.
SAM is actually a suite of three main instruments plus some sub-systems that sit within the body of the rover. It has an inlet on the top that is fed by the robotic arm part of the sample acquisition system, but it can also sample the atmosphere directly through a pair on inlet tubes.
The first instrument we’ll talk about is QMS, the Quadrupole Mass Spectrometer. Mass spectrometers work on the principle that the deflection of a particle in an electric field is determined by the ratio of its mass to its electrical charge. Notice that the particles must have an electric charge, which means that the particles in the sample need to be ionized (i.e. have electrons removed from them). The stream of particles is passed through the electric field and only particles of a certain mass to charge ratio then reach the collector. With a little effort the exact masses of the atoms can then be determined. The Quadrupole name of the system comes from the electric field geometry being a quadrupole between four rods.
The TLS instrument (Tunable laser spectrometer, sometimes called TDLAS) is another type of spectrometer that is specially designed for handling gases. This type of spectrometry measures the amount of molecules present by essentially determining how much light of given wavelength is absorbed. The wavelength of the light is determined beforehand to correspond to a specific molecule. If you have more absorption, you have more of that molecule present. The TLS on Curiosity will target measurements of methane (CH4), water (H20) and carbon dioxide (CO2), and will be able to measure their concentrations to levels of parts per billion in some cases.
The last main instrument in SAM, is the GC for gas chromatography. Gas chromatography applies to organic compounds that will evapourate into a gas (in this case Helium) and then rise up through that gas. Different compounds will “rise”, strictly speaking the gas passes through a wound spiral tube so perhaps “travel” is better, at different rates allowing them to be separated. If the rates are known beforehand then the molecules present can be determined. This process can also be used to feed into the QMS to provide combined “CGMS”. GCMS is a very powerful method because it combines the best of GC, i.e. separating different compounds, with the ability to determined the pieces that make up the compound itself (through mass spectroscopy).
So those are instruments, what are the experiments going to address? Direct from the Goddard website, the goals can be summarized as follows:
- Question 1: What does the inventory of carbon compounds near the surface of Mars tell us about its potential habitability?
- Goal 1: Survey carbon compound sources and evaluate their possible mechanism of formation and destruction.
- Goal 2: Search for organic compounds of biotic and prebiotic importance expecially methane.
- Question 2: What are the chemical and isotopic states of the lighter elements in the solids and atmosphere of Mars and what do they tell us about its potential habitability?
- Goal 3: Reveal the chemical and isotopic state of elements (i.e., N, H, O, S and others) that are important for life as we know it.
- Goal 4: Evaluate the habitability of Mars by studying its atmospheric chemistry and the composition of trace species that are evidence of interactions between the atmosphere and soil.
- Question 3: Were past habitability conditions different from today’s?
- Goal 5: Understand atmospheric and climatic evolution through measurements of noble gas and light element isotopes.
SAM is the experiment that will follow up on the questions posed by the Viking landers: even if there isn’t life now, are the building blocks for life there?
Tomorrow: the radiation assessment detector (RAD).