PerspectivePlanetary Science

Organic molecules on Mars

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Science  08 Jun 2018:
Vol. 360, Issue 6393, pp. 1068-1069
DOI: 10.1126/science.aat2662

A low-angle self-portrait of NASA's Curiosity Mars rover. SAM is safely hidden inside the rover, ready to analyze when samples are delivered from the top.

PHOTO: NASA/JPL-CALTECH/MSSS

On 6 August 2012, the Sample Analysis at Mars (SAM) instrument suite (1) arrived on Mars onboard the Curiosity rover. SAM's main aim was to search for organic molecules on the martian surface. On page 1096 of this issue, Eigenbrode et al. (2) report SAM data that provide conclusive evidence for the presence of organic compounds—thiophenic, aromatic, and aliphatic compounds—in drill samples from Mars' Gale crater. In a related paper on page 1093, Webster et al. (3) report a strong seasonal variation in atmospheric methane, the simplest organic molecule, in the martian atmosphere. Both these finding are breakthroughs in astrobiology.

To appreciate the importance of these detections, we must go back to NASA's 1976 Viking mission and its search for life on Mars. Viking 1 and 2 were two stationary landers that studied the atmosphere and surface of their local environment with a range of instruments, including a gas chromatograph mass spectrometer (GCMS) dedicated to the detection of organic compounds. However, neither signs of life nor organic compounds were detected in the regolith samples analyzed during this mission (4). It is arguable whether not finding signs of life was surprising, but finding no evidence for organic molecules was unexpected. What makes organic compounds so special that we are still searching for them on Mars, more than 40 years later?

Nearly all molecules containing carbon are organic compounds, apart from a few such as CO and CO2. Many organic molecules are not produced by living organisms. Organic molecules on Mars may have been formed abiotically on the martian surface, delivered from space, or produced by past or present martian life. Space missions to Mars are carefully cleaned to prevent accidental delivery of terrestrial organic molecules to the planet (5).

Throughout the Universe, organic compounds are produced abiotically (6) and delivered to planetary surfaces through impacts of comets, asteroids, meteorites, and interplanetary dust particles (7). They are therefore expected to exist on the martian surface. More speculative is the possibility of past or even present life on Mars. Life on Earth uses and produces four major types of organic compounds: carbohydrates, lipids, proteins, and nucleic acids. Each of these types is constructed from smaller organic molecules, such as sugars, amino acids, and nucleobases. Based on the assumption that hypothetical martian life would not greatly differ from terrestrial life, the search for martian life focuses on these building blocks.

The current influx of abiotically produced organic molecules to the martian surface, estimated from scaling the terrestrial influx to a martian scenario, combined with measurements of the martian atmosphere (8, 9), is 100 to 300 metric tons per year. Most of the martian surface is billions of years old (10), so organic molecules should be abundant. Why did the Viking landers not detect any? Are organic molecules degraded on the martian surface, particularly by ultraviolet radiation (which has much shorter wavelength on Mars than on Earth) (11), ionizing radiation (12), or oxidizing compounds (13)? All such processes may eradicate organic molecules from the upper few centimeters or even meters of the surface (12).

But even if all abiotic organic molecules on the martian surface were degraded, their degradation products should still be detectable. Moreover, minerals such as sulfates and clay minerals that are present on Mars may store organic molecules in their crystal structure, protecting them from the destructive environment (13). It thus remained unclear how representative the regolith samples, and the analyses Viking performed on them, were of the organic inventory of Mars.

Clearly, there were still ample reasons to justify a second mission to Mars with an instrument dedicated to the search for organic molecules. SAM was inspired by Viking's GCMS. In the instrument, martian regolith samples are heated so that gases trapped in the samples, organic compounds adsorbed onto the samples, and compounds released by thermal breakdown of minerals are released. The gases are analyzed on a GCMS and a tunable laser spectrometer (1).

In 2015, the first analyses of SAM hinted at the presence of organic molecules on Mars (14), but those measurements were hampered by the presence of perchlorate salts. These salts, present in martian regolith, break down upon heating within the SAM instruments to temperatures of 200°C. The oxygen and chlorine hereby released react with organic molecules. Leakage of reactive agents presented another challenge. Eigenbrode et al. overcame both challenges by only analyzing the gases released above 400°C. They can be certain that these gases are not a result of leaking reagent or reaction with perchlorate. The authors meticulously show all data obtained on Mars by the SAM instrument since its first measurements in 2013 and have thoroughly analyzed all potential contaminants and other signals that might have influenced the actual measurements. They thereby carefully avoid any bias toward hypotheses developed over the past decades. The results convincingly show the long-awaited detection of organic compounds on Mars.

As Webster et al. show, methane has also been conclusively detected in the martian atmosphere (3). During 5 years of analysis, SAM has found not only a stable methane background, but also local seasonal peaks. It may be that the gas is released from a large subsurface reservoir, but neither the source of that methane nor the driving force of its release is understood. Although many geological processes produce methane, its possible link with biological processes warrants further study to fully understand the martian methane cycle.

The detection of organic molecules and methane on Mars has far-ranging implications in light of potential past life on Mars. Curiosity has shown that Gale crater was habitable around 3.5 billion years ago (15), with conditions comparable to those on the early Earth, where life evolved around that time. The question of whether life might have originated or existed on Mars is a lot more opportune now that we know that organic molecules were present on its surface at that time.

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