Written byAaron Gronstal
Nov. 1, 2015
Curious about Methane and Organic Molecules on Mars
Curiosity continues to build on the story of organics on Mars.
At the 2014 Fall Meeting of the American Geophysical Union (AGU), scientists announced two important discoveries made by NASA’s Curiosity rover. The findings included Curiosity’s detection of organic molecules in drill samples taken from a mudstone that once sat on the bottom of and ancient martian lake. The second finding was a spike in levels of atmospheric methane at the rover’s research site.
Curiosity is the rover element of NASA’s Mars Science Laboratory (MSL) mission and landed on Mars in 2012. One of the key goals of MSL is to explore the potential for past or present habitable environments on Mars, and part of this objective is to search for organic compounds of either biological or non-biological origins. To accomplish this goal, Curiosity has been examining the martian regolith, drilling into martian rocks and taking samples of the planet’s atmosphere.
Curiosity carries the most powerful tool for studying volatiles (chemicals with low boiling points often found in gaseous form) and organic compounds (molecules containing carbon and hydrogen) ever sent to Mars: the Sample Analysis at Mars (SAM) instrument suite. So far, SAM has made thirty-four measurements from ten different sites on the planet. The first was a scoop of wind-deposited sand and dust at a place called Rocknest. The elemental composition of the samples was similar to soils previously studied by the Mars Exploration Rovers, Opportunity and Spirit. This meant that the windblown materials were likely widespread on Mars and could have been blown into Curiosity’s landing site from further afield. Curiosity also found methane, a carbon-containing molecule composed of one carbon atom and four hydrogen atoms. However, the methane was chlorine-bearing, meaning that one or more of the hydrogen atoms had been replaced by chlorine. The thought was that the chlorine came from sources on Mars, but the carbon came from terrestrial sources.
Curiosity then traveled into a region called Yellowknife Bay and found evidence of an ancient lake of liquid water and a flowing river in the area. The rover drilled two holes (John Klein and Cumberland) in a formation of mudstone rock informally dubbed ‘Sheepbed,’ which formed more than 3.5 billion years ago at the bottom of the lake. Curiosity found that the clay-rich composition of the mudstone was ideal for preserving organic molecules. In addition, chorine-bearing hydrocarbon molecules were present, but in larger quantities than in the windblown materials at Rocknest. The quantity and structure of the molecules suggested that the organic carbon could have come from the mudstone itself, or that it was at least present on Mars before Curiosity arrived.
This discovery is what led researchers to announce that organics had definitely been identified on Mars. This announcement at the AGU conference was followed by a paper, “Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars,” which featured on the cover of the Journal of Geophysical Research: Planets (1). The publication describes how data from Curiosity, combined with the analysis of blank samples and supporting laboratory experiments, provided the necessary evidence to conclude that organic molecules indigenous to the martian sample had been detected.
Since publication, the team has continued their work, delving deeper into the findings at Cumberland (CB) and making sure that they could not be the result of contamination or any other error. Additional molecules indigenous to the CB sample were later discovered in the same set of data, and further publications are now in preparation.
The research team uses SAM to perform a technique known as pyrolysis-GCMS. The samples are pyrolyzed (heated) to ~900 ºC and the gases that evolve from them are flushed into the instrument for separation by gas chromatography (GC) and identification using a mass spectrometer (MS). Researchers can then look at the molecules, specifically identifying some, and for others they are able to figure out the pathways that led to their formation. Information about their chemical properties can also be gleaned, providing a great deal of information about the original/precursor compounds from which they were derived.
The SAM team also developed and used derivatization. In this technique, a reagent reacts with compounds found in the sample, and those compounds are transformed into new ones of similar structure, known as derivatives. Those derivatives are more volatile than their precursors and can be analyzed more easily. This allows researchers to identify bigger molecules such as amino acids or carboxylic acids, which are typically degradation products of proteins or fatty acids.
The samples collected by Curiosity, whether sand or rock, contain a mixture of chemical compounds. Pyrolysis-GCMS and derivatization-GCMS helps scientists separate the different compounds in order to understand the composition of the sample.
“Following the first Cumberland (CB) pyrolysis-GCMS results, we ran a derivatization experiment on a CB sample we kept in SAM for a year,” said Caroline Freissinet, an astrobiologist at NASA’s Goddard Space Flight Center. “There was a leak in one of our derivatization cups, and the derivatization reagent MTBSTFA was spreading in the samples held in SAM.”
Freissinet and her team used the leaking reagent to perform an opportunistic experiment that is helping them better understand the composition of the samples collected by Curiosity. Rather than trying to stop MTBSTFA leaking into samples, they allowed enough time to take advantage of the leak and maximize the amount of MTBSTFA in order to soak the CB sample. Then they performed the derivatization experiment without having to puncture a cup.
“From this opportunistic derivatization experiment, we detected complex molecules that are difficult to identify specifically,” explained Freissinet. “However, the mass spectra of the derivatized molecules detected in CB points towards general structures, such as long-chain carboxylic acid and long-chain alcohol, and at least one functionalized aromatic compound.”
The team is still working on identifying the precursors that led to these complex molecules, but their preliminary results were presented at the 46th Lunar Planetary Science Conference (LPSC) in March (http://www.hou.usra.edu/meetings/lpsc2015/pdf/2934.pdf ) and at the 2015 Astrobiology Science Conference (AbSciCon) in June (http://www.hou.usra.edu/meetings/abscicon2015/pdf/7504.pdf). Ultimately, they believe that some of the molecules are products of reactions that involve complex organic precursors present in the CB sample. Previous work by Benner et al. (2000) provides some clues about the types of molecules that could have led to the formation of the products identified in the pyrolysis and derivitization experiments.
“We cannot be sure about the precursors, but benzocarboxylates, which were predicted to be the most stable end-products of several organics that would be present at Mars’ near-surface by Benner in 2000, are good candidates,” said Freissinet.
Curiosity and the team behind SAM didn’t stop with the Cumberland samples. The rover rolled to a site called Pahrump Hills, located at the base of Mount Sharp, a mountain in the center of Gale crater that towers over the landing site. A new drill sample dubbed Confidence Hills was collected and, with this sample, scientists were able to announce that the detection of organic molecules at CB was indeed definitive evidence of organics that were martian in origin.
Simply finding organic molecules does not mean they are linked to life. These molecules, which are typically built from atoms of carbon, hydrogen and oxygen, can be made in a myriad of ways that do not involve living organisms. However, life is built from these types of molecules and their discovery can help astrobiologists determine whether or not the ingredients for life’s origin were present on ancient Mars when liquid water was available at the surface.
“The origin of this organic matter (biological vs. abiotic) cannot be determined, and both hypotheses are valid,” said Freissinet. “Hence, the organic matter may be exogenous from meteorites, micrometeorites or comets, or indigenous from hydrothermal formation or biological origin.”
Yet, proving that the carbon did not come from Curiosity itself is a big step, and now the question is, “where on Mars did it come from?” As Freissinet mentions, there are a few possible answers to this question. The carbon might be from Mars itself, originating from geological processes on the planet or even biological processes. A second possibility is that it was delivered from exogenous sources, like meteorites, comets or interplanetary dust particles floating through space. Organic compounds have been detected previously in all of these exogenous sources, and if they fell to the surface of Mars they could undergo reactions that would make them stable enough to build up in the martian regolith.
The definitive discovery of organic molecules on Mars provides important context for life’s potential on the planet, and locations where habitable environments may have persisted in the past. These molecules are critical to the origin of life as we know it, and provide clues as to whether or not life could have found a foothold on Mars. They can also help scientists understand what types of signatures future missions could look for in order to identify signs of past or present life. These measurements represent the first detection on Mars of indigenous organic compounds in surface rocks and address a long standing objective of the Mars exploration program. With the whole set of capabilities of the SAM instrument, scientists are now planning experiments to look into the origin of the organic molecules present in Mars’ subsurface.
Burps of Methane on Mars
The second big discovery from Curiosity was the detection of elevated methane levels in the atmosphere local to Gale Crater. Curiosity wasn’t the first mission to get a whiff of martian methane. Earth-based telescopes and orbital missions at Mars had detected the organic chemical in the past. However, when Curiosity first stretched its wheels at Gale Crater, the rover found only a small fraction of methane in the atmosphere. This raised questions about where and how methane signals could be detected on Mars. Then came the 2014 announcement.
The results presented in 2014 showed a ten-fold increase in methane levels local to the Gale Crater site. It was a large increase over previous measurements, but it was also short-lived. The methane signal dropped down to low levels shortly after it appeared.
The methane cannot be taken as a sign of biology because there are plenty of non-biological sources that could have produced it, such as interactions between water and rocks. However, no matter where it came from, the sharp spike in methane could mean that its source is localized to a specific area.
At first, scientists questioned whether or not the methane spike could have been caused by contamination released from Curiosity itself. That was the first question the research team had to address.
“I cannot find any logical argument to convince me that the methane detected by SAM was leaking from the rover,” said Sushil Atreya, a co-author on the study and professor of Climate and Space Sciences and Director of the Planetary Science Laboratory at the University of Michigan. “First, the rover itself does not generate methane. As for any methane that went to Mars with the rover from Earth, there was ample time for it to dissipate. Multiple observations made under different rover orientations and wind fields throughout the year, and before the methane spikes appeared, showed only a low background level of methane.”
If methane had been trapped somewhere in the rover, the likelihood of it remaining trapped for so long without dissipating is low. Keep in mind that the rover had been driving around the surface since 2012. In addition, methane returned to the same low background level after the spikes were detected (2).
“If the rover were leaking methane into the instrument, it would have shown up at other times also, rather than just when the spikes were detected,” added Atreya.
Curiosity used an instrument called the Tunable Laser Spectrometer (TLS) to detect the methane spikes. There is some methane contained in the foreoptics of the TLS, but the amount is miniscule compared to the amount detected in the spikes.
“A conservative estimate is that one thousand times more methane is required to produce a 10-meter diameter cloud 7 ppbv methane around the rover than the entire amount of methane in the foreoptics, not even accounting for the fact that the cloud would need to be replenished continually over the two-month period of the spikes as methane dissipates away,” added Atreya. “In view of the above, I’m confident Curiosity is detecting martian methane at all times, not terrestrial methane hanging around the rover nor methane leaking into or out of the instrument.”
One possible source of the low levels of methane observed by Curiosity could be the breakdown of organics at the surface. As mentioned, organic molecules can originate from sources beyond Mars, such as cosmic dust or meteorites. After being delivered to the surface of the planet, the organics are exposed to ultraviolet (UV) radiation from the Sun and are eventually broken down over time.
“Geology and biology are still the most plausible origins of the methane observed by Curiosity on Mars,” said Atreya. “Although, UV degradation of surface organics delivered by exogenous sources may be sufficient for the low background level of methane, geologic and biotic contributions are not ruled out.”
Although not impossible, this gradual breakdown of organics is not a likely source for the dramatic spikes in methane detected by Curiosity. For exogenous organics to serve as a source of the methane spikes, a specific event would have had to occur in order to release a quick burst of methane.
“Existing SAM data argue for serpentinization and/or methanogenesis as the most likely mechanisms for elevated levels of methane seen in the spikes,” said Atreya. “The methane spikes did not coincide with any specific meteor events on Mars, but we plan to investigate this possibility further in future observations.”
Serpentinization is a geological process that involves water being added to the crystalline structure of minerals found in rocks. It does not involve biology, but often occurs under geological stresses like heat or pressure. Methanogenesis is a biological process in which methane is a byproduct of metabolism. Either of these processes could be responsible for quick bursts of methane being released into the atmosphere, especially if methane was stored in clathrates that can be destabilized by any number of stresses. Right now, there is no indication as to which process is the most likely source of the methane Curiosity sniffed out, or whether or not there might be another explanation for Curiosity’s measurements altogether.
“The burps of methane we observed over a two-month period in 2013-2014 (Ls 55.7 and 81.7 deg) indicate that Mars is presently active,” explains Atreya. “However, we are constantly looking into alternate hypotheses as well.”
To explore other possible sources of methane, Curiosity is continuing to monitor changes in methane levels. The team is also considering other measurements that they could take to better understand the behavior of methane on Mars and how it interacts with other gases in the planet’s atmosphere, such as oxygen.
“Additionally, we plan to do targeted observations of methane to time with any meteor showers on Mars,” says Atreya.
Other missions may also help add to the story of methane on Mars, working with Curiosity to gain a better picture of methane on the planet as a whole. India’s Mars Orbiter Mission (MOM) entered martian orbit in the autumn of 2014. The European Space Agency’s (ESA)\Roscosmos ExoMars Trace Gas Orbiter (TGO) is currently scheduled to launch for Mars in 2016.
“Both MOM and ExoMars TGO carry methane sensors,” said Atreya. “If successful, they have the potential of mapping methane over Mars, something a relatively localized rover cannot do.”
From orbit, MOM and ExoMars TGO will help hunt for hotspots of methane at the martian surface. They will provide maps that show when and where methane appears, and how those levels of methane vary over time.
“Combining those maps with in situ, on-surface data on methane by Curiosity holds the promise of giving new insights into the sources and sinks of methane on Mars,” said Atreya.
The exact origins of organic molecules and atmospheric methane on Mars are still unknown, but the answers get closer with each observation made by the international team of robotic explorers currently operating at the planet.
Astrobiology and SAM
Curiosity’s SAM instrument is the most complex analytical chemistry laboratories ever delivered to another planet. One of SAM’s most important astrobiological goals is to search for evidence of ancient habitable environments in Gale crater that could have supported life in Mars’ past. For more information about astrobiology and the Curiosity rover, visit the Astrobiology Missions page here.
(1) Freissinet et al. (2015) Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars. Journal of Geophysical Research: Planets, doi:10.1002/2014JE004737
(2) Webster et al. (2013) Low Upper Limit to Methane Abundance on Mars. Science. 342(6156), 355-357. http://www.sciencemag.org/content/342/6156/355.short
(3) Benner et al. (2000) The missing organic molecules on Mars. Proceedings of the National Academy of Science, 96(6), 2425–2430. http://www.pnas.org/content/97/6/2425.short