INSIGHTS
Human T cell plasticity
p. 371
Debates over e-cigarettes
p. 375
PERSPECTIVES PLANETARY SCIENCE
Downloaded from www.sciencemag.org on January 22, 2015
Play it again, SAM Lasers shine new light on the drying of Mars while reviving the mystery of methane By Kevin Zahnle
Space Science and Astrobiology Division, NASA Ames Research Center, Mofett Field, CA, USA. E-mail: [email protected]
370
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. sciencemag.org SCIENCE
23 JANUARY 2015 • VOL 347 ISSUE 6220
Published by AAAS
PHOTO: NASA/JPL-CALTECH/MSSS
S
ome 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. 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).
“William of Ockham (12) would warn us to be wary of peekaboo methane when a known source—the rover—is so nearby.” 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. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
P. R. Mahaffy et al., Science 347, 412 (2015). C. R. Webster et al., Science 347, 415 (2015). D. Bogard et al., Space Sci. Rev. 96, 425 (2001). P. Christensen, Elements 2, 151 (2006). B. Levrard et al., J. Geophys. Res. 112, E06012 (2007). H. Lammer et al., Int. J. Astrobiol. 2, 195 (2003). W. Sullivan, New York Times, 8 August 1969, p. 1. V. Krasnopolsky, J. Maillard, T. Owen, Icarus 172, 537 (2004). M. J. Mumma et al., Science 323, 1041 (2009). K. Zahnle, R. Freedman, D. Catling, Icarus 212, 493 (2011). I have contested in print previous claims that methane was detected on Mars. 12. C. F. Chyba, Nature 348, 113 (1990). 13. E. Anders, T. Owen, Science 198, 453 (1977). 10.1126/science.aaa3687
SCIENCE sciencemag.org
IMMUNOLOGY
Flexibility for specificity Human T cells show remarkable differentiation plasticity in responding to pathogens By Mark M. Davis
D
ifferent types of T lymphocytes play a key role in many immune responses, such as killing virally infected or cancerous cells directly, inducing high-affinity antibody responses in B cells, and increasing or decreasing responses from other immune cells. This multiplicity of roles may relate to their recognition properties, which are very difficult to evade. Moreover, cells are very diverse— for CD4+ T cells alone, there are at least six distinct subtypes. This raises the question of just how these different T cells are produced. Early evidence indicated that the type of T cell that dominates the response was dependent on the type of pathogen and route of entry (1, 2). However, over the past several years, more and more flexibility has been observed in a T cell’s phenotype (3, 4). On page 400 of this issue, Becattini et al. (5) show that this flexibility is more the rule rather than the exception. Most T cells use antibody-like T cell receptors (TCRs) on their cell surface to recognize degraded bits of proteins (peptides) or lipids that are bound to molecules of the major histocompatibility complex (MHC) expressed on the surface of various cell types. These MHC molecules are much like the people in Hollywood who go through the garbage cans of celebrities, looking for interesting bits of information to display to the tabloid press. But MHC molecules cannot tell whether the fragments they bind are from endogenous proteins of the cell; that is the job of T cells, with their diverse TCRs. The functional beauty of this system, and perhaps the reason for its centrality in so much of adaptive immunity, is that everything pertaining to a cell or a pathogen is eventually degraded, making it much harder for pathogens to evade detection. By contrast, antibodies must recognize intact antigens and thus are often diverted from the most important bits. In particular, MHC molecules bind small peptides—typically 8 to 10 amino acids—just enough information to be fairly unique, making it easier 23 JANUARY 2015 • VOL 347 ISSUE 6220
Published by AAAS
37 1
Play it again, SAM Kevin Zahnle Science 347, 370 (2015); DOI: 10.1126/science.aaa3687
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of January 22, 2015 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/347/6220/370.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/347/6220/370.full.html#related This article cites 11 articles, 3 of which can be accessed free: http://www.sciencemag.org/content/347/6220/370.full.html#ref-list-1 This article appears in the following subject collections: Planetary Science http://www.sciencemag.org/cgi/collection/planet_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
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Human T cell plasticity
p. 371
Debates over e-cigarettes
p. 375
PERSPECTIVES PLANETARY SCIENCE
Downloaded from www.sciencemag.org on January 22, 2015
Play it again, SAM Lasers shine new light on the drying of Mars while reviving the mystery of methane By Kevin Zahnle
Space Science and Astrobiology Division, NASA Ames Research Center, Mofett Field, CA, USA. E-mail: [email protected]
370
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. sciencemag.org SCIENCE
23 JANUARY 2015 • VOL 347 ISSUE 6220
Published by AAAS
PHOTO: NASA/JPL-CALTECH/MSSS
S
ome 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. 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).
“William of Ockham (12) would warn us to be wary of peekaboo methane when a known source—the rover—is so nearby.” 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. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
P. R. Mahaffy et al., Science 347, 412 (2015). C. R. Webster et al., Science 347, 415 (2015). D. Bogard et al., Space Sci. Rev. 96, 425 (2001). P. Christensen, Elements 2, 151 (2006). B. Levrard et al., J. Geophys. Res. 112, E06012 (2007). H. Lammer et al., Int. J. Astrobiol. 2, 195 (2003). W. Sullivan, New York Times, 8 August 1969, p. 1. V. Krasnopolsky, J. Maillard, T. Owen, Icarus 172, 537 (2004). M. J. Mumma et al., Science 323, 1041 (2009). K. Zahnle, R. Freedman, D. Catling, Icarus 212, 493 (2011). I have contested in print previous claims that methane was detected on Mars. 12. C. F. Chyba, Nature 348, 113 (1990). 13. E. Anders, T. Owen, Science 198, 453 (1977). 10.1126/science.aaa3687
SCIENCE sciencemag.org
IMMUNOLOGY
Flexibility for specificity Human T cells show remarkable differentiation plasticity in responding to pathogens By Mark M. Davis
D
ifferent types of T lymphocytes play a key role in many immune responses, such as killing virally infected or cancerous cells directly, inducing high-affinity antibody responses in B cells, and increasing or decreasing responses from other immune cells. This multiplicity of roles may relate to their recognition properties, which are very difficult to evade. Moreover, cells are very diverse— for CD4+ T cells alone, there are at least six distinct subtypes. This raises the question of just how these different T cells are produced. Early evidence indicated that the type of T cell that dominates the response was dependent on the type of pathogen and route of entry (1, 2). However, over the past several years, more and more flexibility has been observed in a T cell’s phenotype (3, 4). On page 400 of this issue, Becattini et al. (5) show that this flexibility is more the rule rather than the exception. Most T cells use antibody-like T cell receptors (TCRs) on their cell surface to recognize degraded bits of proteins (peptides) or lipids that are bound to molecules of the major histocompatibility complex (MHC) expressed on the surface of various cell types. These MHC molecules are much like the people in Hollywood who go through the garbage cans of celebrities, looking for interesting bits of information to display to the tabloid press. But MHC molecules cannot tell whether the fragments they bind are from endogenous proteins of the cell; that is the job of T cells, with their diverse TCRs. The functional beauty of this system, and perhaps the reason for its centrality in so much of adaptive immunity, is that everything pertaining to a cell or a pathogen is eventually degraded, making it much harder for pathogens to evade detection. By contrast, antibodies must recognize intact antigens and thus are often diverted from the most important bits. In particular, MHC molecules bind small peptides—typically 8 to 10 amino acids—just enough information to be fairly unique, making it easier 23 JANUARY 2015 • VOL 347 ISSUE 6220
Published by AAAS
37 1
Play it again, SAM Kevin Zahnle Science 347, 370 (2015); DOI: 10.1126/science.aaa3687
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of January 22, 2015 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/347/6220/370.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/347/6220/370.full.html#related This article cites 11 articles, 3 of which can be accessed free: http://www.sciencemag.org/content/347/6220/370.full.html#ref-list-1 This article appears in the following subject collections: Planetary Science http://www.sciencemag.org/cgi/collection/planet_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on January 22, 2015
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