Población y procedimiento muestral

This is the fun part of the meeting when the science team should throw out their ideas about the data. Freewheeling ideas are the order of the day.

11.4 Roving on Mars

Curiosity Rover on Mars Science Lab was launched on November 11, 2011, and landed safely on Mars August 6, 2012. It’s been roving every day since then without a hitch, far exceeding NASA’s original goals for its lifetime. The mission goal is not to look directly for signs of life, but rather to determine whether or not Mars was ever able to support microbial life. The Rover has examined sedimen- tary deposits, visited the bottom of possible lake beds, and scratched away at the surface oxidised, thick dust to sample more interesting rocks beneath.

Jennifer Eigenbrode, my former postdoctoral fellow, used her organic geochemistry training to focus on data coming from SAM. In a recent paper

(Eigenbrode et al., 2018), she and colleagues, including Andrew Steele, detailed

results from evolved gas analysis (EGA) of 3 billion year old sedimentary rocks from the bottom of Mars’ Gale Crater. They found complex sulphur compounds, known as thiophenes, that could only have come from the complexation of sulphide with organic matter during diagenesis. Thiophenes were released at temperatures between 550 and 820 °C in the EGA system, in which solid material is heated gradually and the evolved gases are swept into a simple

mass spectrometer system that monitors certain masses: mass 45 for CO2;

mass 60 for COS; and mass 84 for C4H4S, a thiophene fragment (Fig. 11.10).

For this work, they were able to take a cut of the volatile gases and direct them into SAM’s GC-MS system, which confirmed the presence of thiophene, 2- methylthiophene, and 3-methylthiophene, as well as various aromatic compounds. Because of the diversity of molecular structures, the nature of this organic material is consistent with complex synthesis, whether from meteoritic or geological interactions.

Another interesting observation from many of the SAM analyses is that the chemical structures of the organic compounds are variable between samples. These rocks were lacustrine mudstones formed when Mars had liquid water on its surface. If SAM were only measuring background contamination, we would expect a more uniform composition. Organic matter, which has been found to be widespread among martian sedimentary rocks, could fuel living organisms, if they occurred, and might support a biological carbon cycle. The authors stopped short of declaring that they found evidence of past life on Mars. They concluded, however, that given that sediments found near the surface contain detectable amounts of complex organic material, drill cores from greater depths might provide even more definitive samples of organic matter, traceable to that formed by living organisms. Ultimately, Mars sample return remains the single most important goal for confirming if martian organic matter was formed as the result of biological processes.

Figure 11.10 Evolved gas analysis (EGA) of volatile gas profiles from the Mars Mojave site. Profiles for thiophenes (a), thiols and sulphides (b and c), other volatiles (d), and O2 and CO2 (e) are shown. Symbols mark correlations between panels in peak maxima within an error of ±25 °C due to signal smoothing: squares, 625 °C; circles, 750 °C; and triangles, 790 to 820 °C. Axes and the placement of symbols relative to the temperature are the same in Fig. 2 and Figs. S1 to S6. The x axis is scaled linearly relative to the run time, and the corresponding sample temperature is shown. The y-axis scale bar in counts per second (cps) is for all panels. Profiles in (a) are shifted along the y axis to show peaks clearly. From Eigenbrode et al. (2018) with permission from The American Association for the Advancement of Science.

Steele and colleagues (Steele et al., 2018) followed up on Eigenbrode

et al.’s (2018) paper, once again taking full advantage of the martian meteorites that we have on hand that can be fully analysed by sophisticated instrumenta- tion. Using confocal Raman imaging spectroscopy and transmission electron microscopy, they were able to examine the intimate details of organic matter formation. Their findings, consistent with Mars Curiosity measurements, show that it is the interaction of brine-fluids with sulphides and spinel minerals by an electrochemical mechanism that results in the deposition of MMC in the

martian samples. They conclude “The hypothesis developed from our observations

on martian meteorites has profound implications for our understanding of other martian phenomena, including the presence of methane in the atmosphere and the origin of the refractory organic material in ancient sedimentary rocks found in situ by the SAM instrument.” Steele and his colleagues who participated in AMASE (Eigenbrode, Benning, Fries, Siljesrtom, Conrad, and McCubbin) “cut their teeth” on the samples from Sverrefjell volcano, demonstrating the power of collaborative inspiration often related by field investigations.

AMASE team member Ashley Stroupe sometimes “drives” Curiosity from Mission Control at JPL. She posted this blog on August 13, 2018:

“In today’s plan, Curiosity begins analysis of the long-awaited Pettegrove Point drill sample at the “Stoer” target, which was successfully collected last week (and I had the pleasure of helping to sequence as a Rover Planner). Our main activity is the drop-off of sample to CheMin, based on the characterization of the drop-off portion size done in the weekend plan. There is still a good bit of wind, so the drop-off is around noon, during the calmest time. Overnight, CheMin will be busy analyzing the sample; we’ll have the results down late Wednesday, which can then inform decisions about drop- ping off sample to SAM as early as this weekend’s plan for analysis early next week. On both sols of the plan, we’re continuing our atmospheric observations to monitor the dust storm as it continues to abate, with dust devil surveys, and zenith and horizon opacity imaging. We’ll be collecting additional ChemCam and Mastcam images of the drill hole, to look for vertical variability, and of the tailings, for change detection.” Mars Science Lab remains a very active mission, daily engaging many of AMASE’s scientists and engineers.