Big SAM hits Mars: Search for organic compounds is poised
- Published: Sep 1, 2012
- Author: Steve Down
- Channels: Base Peak
Big SAM flexes muscles
The SAM module
Image: courtesy NASA GSFC
The current mission to Mars landed the Curiosity rover on the surface of the planet on August 5th this year at a site within Gale Crater that NASA has named Bradbury Landing in honour of the writer Ray Bradbury. The rover is packed with instrumentation designed to test the soil, rocks and atmosphere for signs that Mars does support life, or might have done at one time.
The ChemMin module, described in spectroscopyNOW.com, will examine Martian rocks using a powder X-ray diffraction (XRD) instrument which also has X-ray fluorescence capabilities. It is accompanied by nine other instruments, including the Sample Analysis at Mars (SAM) module which will search for traces of organic compounds and measure the isotopic compositions of other elements that are important to life, like oxygen, nitrogen, sulphur and hydrogen. It contains a gas chromatograph (GC), a mass spectrometer (MS) and a tunable laser spectrometer (TLS).
While SAM is the biggest of the instrument packages on Curiosity, it is far smaller than the typical lab-based kit on Earth, taking up the space of a microwave oven. However, it packs a punch. The combination of GC and MS can detect organic compounds down to ppb levels while the TLS measures low concentrations of atmospheric methane, carbon dioxide and water vapour and their isotopic variants.
The Curiosity mission is based on the principle of “follow the carbon” which is an extension of previous Martian missions that were designed to “follow the water.” Over the last decade, the Spirit and Opportunity rovers on Mars have found convincing evidence of the effects of water by discovering minerals formed by aqueous alteration and spotting water trails in sedimentary layers of rocks. These have been supported by orbiting telescopes and imaging spectrometers.
The Martian landscape pictured from the Curiosity rover
Now, it is a natural step to see if there are any organic compounds on Mars, although they are not necessarily evidence of life. The original Viking Mars landers of 1976 found no organic compounds but Curiosity has several key advantages over those earlier missions. The landing site has been chosen because it contains clays and other minerals which are good at trapping organic compounds and the mobility of Curiosity will allow many locations to be examined compared with the Viking landers, which were static.
In addition, Sam has a greater sensitivity and the capability to heat samples to drive off trapped organic compounds. The instrumentation also carries stocks of reagents that will allow for derivatisation reactions to take place, which extends the range of organic compounds which can be detected.
The search for organic compounds
So, how will the search for organic compounds be carried out? Finely sieved samples of rock and soil collected by the rover’s arm will be dropped into one of 59 sample cups which are moved into an oven. Here, they will be heated directly and the volatiles that are emitted will be direct towards the GC and the TLS. Nine more sample cups contain the solvents and reagents that will be used for the derivatisation reactions, while six contain solids that will be used to calibrate the instrument.
A total of six complementary columns have been fitted in the GC for the separation of a range of analytes, specifically C5-C15 organics, >C15 organics, C1-C4 organics containing nitrogen and sulphur, chiral compounds, and permanent gases.
Once separated in the GC, the compounds will be transferred to the quadrupole mass spectrometer which has a m/z range up to 535, allowing for the detection of compounds of medium molecular weight.
The number of experiments is not limited to the number of sample cups, because they can be baked in the oven for cleaning before reuse. Each cup is about one-sixth of a teaspoon in volume (0.78 cm3), which is sufficient for these experiments.
The whole of Curiosity is powered by a nuclear battery which converts heat into electricity and the instrumentation is designed for minimal power consumption. The GC oven can heat samples to 1000°C using maximum power of just 40 W.
It will take about four weeks to check that all of the Curiosity systems are working correctly within the Martian environment and its gravitational field after the trauma of landing and, so far, things are looking good. Paul Mahaffy, principal investigator for SAM based at NASA Goddard Space Flight Center told spectroscopyNOW.com “We have carried out three engineering tests of the SAM health and all is great so far. We hope to secure atmospheric samples in a few sols and solid samples several weeks down the road.”
Several weeks is not too far away in a mission that is expected to last for at least one Martian year, which is 98 Earth weeks. In that time the rover will be able to drive at least 20 km to find many different locations for testing, aided by the onboard cameras.
If organic compounds are eventually found, the scientists must still practice caution as they do not automatically point to life on Mars. They might have originated from meteorites or even from contamination on Curiosity that travelled all the way from Earth. This possibility can be checked onboard by analysing organic check bricks consisting of a silicon dioxide ceramic material laced with fluorinated organic compounds that are not found naturally on Earth and are not expected on Mars.
The safe and successful landing of Curiosity on Mars has generated a wave of excitement around the world. Now we have to wait a few weeks for the experiments to begin. As NASA administrator Charles Bolden said in a message sent to the rover and beamed back to Earth “The knowledge we hope to gain from our observation and analysis of Gale Crater will tell us much about the possibility of life on Mars as well as the past and future possibilities for our own planet."
Article by Steve Down
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.