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Play it again, SAM

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Science  23 Jan 2015:
Vol. 347, Issue 6220, pp. 370-371
DOI: 10.1126/science.aaa3687

Some discoveries are new, others old. Here, we consider one of each from NASA's Curiosity rover. The new discovery, reported by Mahaffy et al. (1) on page 412 of this issue, is a remarkable measurement of the deuterium-hydrogen (D/H) ratio in a Gale crater mudstone from 3 billion years ago. On page 415, Webster et al. (2) report on the latest chapter in the muddy matter of methane on Mars. What links them is that both were made using the tunable laser spectrometer (TLS), part of the SAM (Sample Analysis at Mars) package on the rover.

Martian water is enriched in deuterium (D), hydrogen's heavier isotope, compared with most water in the solar system. The cause of the enrichment is the preferential escape to space of H. The D/H ratio in Mars's air is now about 6 × SMOW (standard mean ocean water; that is, Earth) and was roughly the same 180 million years ago, as recorded by water trapped in martian meteorites (3). Curiosity has found in a single sample that D/H was only 3 × SMOW when a mudstone formed in a cold little pond at the bottom of Gale crater some 3 billion years ago (1).

Mars today holds the equivalent of 30 m of water in polar ice (4)—this is how deep the water would be if it were spread uniformly over the surface. Climate modelers think that all the polar water is exchangeable on 10-million-year time scales in response to Mars's Milankovich cycles (5). When D escape is taken into account, the implication is that Mars had at least ~90 m of exchangeable water at the time of the pond.

Looking back at you.

A selfie taken by NASA's Curiosity rover. Whether the detected methane is from the rover itself or from the martian surface is not so clear.

PHOTO: NASA/JPL-CALTECH/MSSS

All this water will make many Mars scientists happy, but there are consequences. Today, H escape is equivalent to the loss of ~2 m of water per billion years, which is a lot less than ~60 m in 3 billion years. Faster H escape in the past, although not unexpected, needs to be balanced by an oxygen sink (6). Extreme oxidation is seen at the surface of Mars (for example, perchlorates), but there is no evidence that oxidation is deep enough and pervasive enough to accommodate all the oxygen from 60 m of water in the past 3 billion years. Apparently most of the oxygen escaped with the hydrogen (6).

The saga of methane on Mars begins with its first discovery in 1969. That announcement, based on a spectrum obtained 48 hours earlier by Mariner 7, was greeted by a front-page story in the New York Times (7). The team soon realized that they had actually seen a forbidden band in frozen CO2. But the fascination with methane—the simplest, most stable, and most abundant organic molecule in the cosmos—has not gone away. Methane does not have a known chemical source in an atmosphere like Mars's, and its lifetime [standard photochemistry predicts 300 years (8)] is short enough that its presence in the atmosphere almost demands an exciting source. Pursuit has been vigorous, and there have been many subsequent discoveries of variable credibility and consistency (2). The reports describe an ephemeral gas with a lifetime of weeks or months rather than the expected 300 years, and the best of them describes phenomena seen only during the winter of 2003 (9). The reality of ephemeral methane has been contested nearly as vigorously (10, 11).

The TLS/SAM experiment was intended to resolve the matter by looking for a distinctive pattern of spectral lines that uniquely identifies methane. The TLS looks at two slivers, one for the isotopes of H, C, and O, and the other for CH4. The good news is that a lot of methane is seen. The bad news is that little of the methane is martian. Most is in the antechamber to the sample cell and comes from several sources, known and unknown, in the rover itself. Martian methane, when present, would be seen in the small difference between the signal obtained when there is martian air in the cell and the signal obtained when the cell is empty.

Figure 1 of Webster et al. chronicles the differences (2). At first, no martian methane was seen, both while the rover was awash in stowaway Florida air and then after the rover was evacuated. But later, as methane slowly built up again inside the rover [see column I, table S2, of the SM in (2)], methane appeared in five of six samples of Mars's air at levels on the order of 7 parts per billion by volume (ppbv). The statistics are marginal, but the measurements are self-consistent. After the fifth sighting, TLS/SAM performed the first of two higher-sensitivity enrichment experiments, only to find the methane nearly gone. A second enrichment experiment done 3 months later gives a low but nonzero CH4 abundance of 0.9 ppbv with better statistics. Although 0.9 ppbv may not seem like much, it is probably more than can be supplied in steady state by the degradation of incoming exogenic matter.

It is an intriguing story. William of Ockham (12) would warn us to be wary of peekaboo methane when a known source—the rover—is so nearby. Because the concentration of methane inside the rover is approximately 1000 times as high as that in the martian air, it would not take much. But it would be a mistake to be too confident that the methane was never there. We know very little about Mars. The synchronous report that O2 is seasonally variable [from Chemcam, another Curiosity instrument (2)] sends a clear warning that theory may be missing something fundamental about Mars's atmosphere. Methane could be caught up in this.

There is no denying that Mars has been something of a disappointment (13), at least if one had been hoping for a second Earth. The disappointment of today may turn to the wonder of tomorrow when we come to see Mars as the unique world that it is.

References and Notes

  1. I have contested in print previous claims that methane was detected on Mars.
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