There are currently (June 2018) two new papers (links may be paywalled):
"Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars", Eigenbrode et al., Science 360, 1096–1101 (2018)
"Background levels of methane in Mars’ atmosphere show strong seasonal variations", Webster et al., Science 360, 1093–1096 (2018)
The first is looking for organics in a drilled sample of rock; the second is looking at the long history of atmospheric methane.
First, the mudstone paper. It's abstract is, well, abstract:
Establishing the presence and state of organic matter, including its possible biosignatures, in martian materials has been an elusive quest, despite limited reports of the existence of organic matter on Mars. We report the in situ detection of organic matter preserved in lacustrine mudstones at the base of the ~3.5-billion-year-old Murray formation at Pahrump Hills, Gale crater, by the Sample Analysis at Mars instrument suite onboard the Curiosity rover. Diverse pyrolysis products, including thiophenic, aromatic, and aliphatic compounds released at high temperatures (500° to 820°C), were directly detected by evolved gas analysis. Thiophenes were also observed by gas chromatography–mass spectrometry. Their presence suggests that sulfurization aided organic matter preservation. At least 50 nanomoles of organic carbon persists, probably as macromolecules containing 5% carbon as organic sulfur molecules.
Curiosity took samples, heated them up, and then observed the molecular weight (really, the mass/charge ratio, but close enough) as a function of temperature. What you get when doing that is plots like this (lots of plots like this):
The peaks allow various molecular components to be partially identified. The interesting things about this are that (a) much more complicated components have been found here than earlier work and (b) there are (comparatively) large amount of nitrogen- and sulfur-carrying components.
The body of the paper discusses:
The measurement process
Component identification procedure
What they found
Comparison of various sample locations
Geology of the locations, to try to identify how the samples (which are really old) may have been modified or mineralized
The conclusion is three paragraphs:
SAM’s molecular observations do not clearly reveal the source of the organic matter in the Murray formation. Biological, geological, and meteoritic sources are all possible. Certainly, if ancient life was the organic source, then despite sulfur incorporation, the material has been altered sufficiently, such as by diagenesis or ionizing radiation (23), to obscure original molecular features more consistent with life (e.g., a greater diversity of molecules or patterns of limited structural variation within compound classes, such as hydrocarbon chains), or an insufficient amount of organic matter was deposited to allow detection by pyrolysis–GC-MS.
Past habitability interpreted for the Sheepbed lacustrine mudstones focused on chemolithoautotrophy (8, 30), but observations of geologically refractory organic matter in Murray lacustrine mudstones opens the door for past and present habitability for heterotrophy as well. Organic matter can directly or indirectly fuel both energy and carbon metabolisms and in doing so can support carbon cycling at the microbial community level.
Our results suggest that it is likely that organic matter from various sources may be widely distributed in the martian rock record. Even if life was not a key contributor, meteoritic and igneous or hydrothermal sources have a strong potential to be broadly emplaced. Our detection of organic matter at the martian surface, where ionizing and oxidizing conditions are extreme, suggests that better-preserved molecular records may be present below the surface, where the effects of radiation are small, or in materials exposed in the last sev- eral thousand years.
I would paraphrase this as
- We're not sure this is past or present life. It could be, but it could be other things too.
- If it was ancient life like ours, then the aging has modified it (because it doesn't look like current Earth-life organics; too much sulfur)
- But it could be past or present life food
- If we could find it here, it could be everywhere; let's keep looking.
The atmospheric methane paper's abstract is more direct:
Variable levels of methane in the martian atmosphere have eluded explanation partly because the measurements are not repeatable in time or location. We report in situ measurements at Gale crater made over a 5-year period by the Tunable Laser Spectrometer on the Curiosity rover. The background levels of methane have a mean value 0.41 ± 0.16 parts per billion by volume (ppbv) (95% confidence interval) and exhibit a strong, repeatable seasonal variation (0.24 to 0.65 ppbv). This variation is greater than that predicted from either ultraviolet degradation of impact-delivered organics on the surface or from the annual surface pressure cycle. The large seasonal variation in the background and occurrences of higher temporary spikes (~7 ppbv) are consistent with small localized sources of methane released from martian surface or subsurface reservoirs.
The previous data on methane in Mars' atmosphere is confusing, and the spend the 1st page going through the "there it is, not it isn't, yes it is, wow look at that! where'd it go?" history. There seem to be huge peaks, and long times of very little.
Curiosity used two forms of atmospheric sampling into a laser spectrometer to measure the methane concentration. Using about two Martian years of measurements, they found two components:
- Big puffs
- And a slowly-varying constant level
Much of the paper is spent confronting possible causes for the two components. For example:
Existing models including atmospheric transport and circulation (23–26) are unable to reproduce the reported high concentrations of methane and its spatial and temporal variability, even when including possible clathrate release (9), surface/regolith adsorption/desorption (10), seasonally variable production from UV breakdown of surface organics (6, 7), or proposed mechanisms of rapid loss (27, 28). Analysis of all methane measurements up to 2016 (29) provides little evidence for any correlation between meteor streams and methane plumes as previously suggested (30).
I.e. "The puffs are too sharp to be explained by geological processes, chemical processes or even Mars being hit by methane-carrying meteors"
The yearly cycle, measured at one location, is perhaps more interesting as people have argued that's a clear sign of current life. (It's measured in one spot, so seasons matter, etc)
The problem is that this is varying, but it's a Really Small concentration. Earth's pre-industrial value was about 640 ppb, roughly $10^3$ larger, in a much more dense atmosphere. Mar's methane only amounts to grams per cubic kilometer of atmosphere. If it's life, it's not much of a life. And at that level, there are other possibilities. The conclusion paragraphs argue against life as an explanation:
With ancient atmospheric pressures of several hundred millibars (40), large amounts of methane may be stored in the cold martian subsurface as clathrates in a stability zone several times thicker than that of Earth (41–43). Although the seasonal signature of the TLS-SAM measurements is not consistent with direct clathrate release, clathrates may provide a source of surface microseepage (diffuse exhalations without any specific morphological structure that may vent from outcropping of rocks or river or lake beds) (43–45). On Mars, such seepage would occur preferentially through permeable pathways, such as faults, fractures, or in breaches in sealing lithologies; this would not require identifiable geomorphological structures on the surface. Weak microseepage exhalations could explain back- ground and plume methane anomalies observed on Mars (43), perhaps near the dichotomy boundary and at Gale crater, where there is fractured sedimentary rock. Microseepage flux may vary over time, depending on variations of gas pressures along the subsurface migration pathway or on seasonal changes in the soil, or even where microbial activity may consume methane.
Regardless of the subsurface origin, methane that finds its way to surface layers over long time periods (42, 43) may be expected to show seasonal variation. We consider a process that retains methane at the surface temporarily before releasing it through a process linked to the surface temperature. That process could be adsorption on a surface with a high surface area– to-volume ratio, such as dust or soil. Although mineral dust cannot serve as a methane sink, it can moderate the release (11, 12). Adopting an energy barrier of ~20 to 35 kJ/mol—which is somewhat higher than that reported for the physical adsorption of methane into clays (46), zeolites (47), and Mars analog soil (12)—we found that large seasonal variations are expected (fig. S41). Plausible correlations of the background methane values with atmospheric water vapor and with surface temperatures point to physical or chemical surface (or dust) processes, or micro- seepage release. The amplitude of the seasonal cycle indicates that there remain unknown atmospheric or surface processes occurring in present-day Mars.
The last sentence of each paragraph holds out a bit of hope that there's "microbial activity" as an "unknown .. surface process()" that's causing it, but it's one of several possibilities.