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Isotope Ratios of H, C, and O in CO2 and H2O of the Martian Atmosphere

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Science  19 Jul 2013:
Vol. 341, Issue 6143, pp. 260-263
DOI: 10.1126/science.1237961

Mars' Atmosphere from Curiosity

The Sample Analysis at Mars (SAM) instrument on the Curiosity rover that landed on Mars in August last year is designed to study the chemical and isotopic composition of the martian atmosphere. Mahaffy et al. (p. 263) present volume-mixing ratios of Mars' five major atmospheric constituents (CO2, Ar, N2, O2, and CO) and isotope measurements of 40Ar/36Ar and C and O in CO2, based on data from one of SAM's instruments, obtained between 31 August and 21 November 2012. Webster et al. (p. 260) used data from another of SAM's instruments obtained around the same period to determine isotope ratios of H, C, and O in atmospheric CO2 and H2O. Agreement between the isotopic ratios measured by SAM with those of martian meteorites, measured in laboratories on Earth, confirms the origin of these meteorites and implies that the current atmospheric reservoirs of CO2 and H2O were largely established after the period of early atmospheric loss some 4 billion years ago.

Abstract

Stable isotope ratios of H, C, and O are powerful indicators of a wide variety of planetary geophysical processes, and for Mars they reveal the record of loss of its atmosphere and subsequent interactions with its surface such as carbonate formation. We report in situ measurements of the isotopic ratios of D/H and 18O/16O in water and 13C/12C, 18O/16O, 17O/16O, and 13C18O/12C16O in carbon dioxide, made in the martian atmosphere at Gale Crater from the Curiosity rover using the Sample Analysis at Mars (SAM)’s tunable laser spectrometer (TLS). Comparison between our measurements in the modern atmosphere and those of martian meteorites such as ALH 84001 implies that the martian reservoirs of CO2 and H2O were largely established ~4 billion years ago, but that atmospheric loss or surface interaction may be still ongoing.

The Sample Analysis at Mars (SAM) suite (1) on the Curiosity rover that landed in August 2012 is conducting a search for organic compounds and volatiles in rocks and soils and characterizing the chemical and isotopic composition of the modern atmosphere. Atmospheric characterization is one of the exploration goals of the Mars Science Laboratory (MSL) mission (2), and it is accomplished using SAM’s tunable laser spectrometer (TLS) and its quadrupole mass spectrometer (QMS). Here we focus on TLS measurements; a companion paper (3) focuses on those from the QMS. Results for nondetection by TLS of atmospheric methane are reported elsewhere (4).

Previous measurements of isotopes of H, N, and noble gases in the martian atmosphere to date (5) have indicated enrichment in the heavier isotopes, consistent with the idea of atmospheric loss to space of the lighter isotopes (6, 7). Although meteoritic analyses of δ13C and δ18O (8) in shergottite, nakhlite, and chassigny (SNC)–class meteorites are made at higher precision than the atmospheric measurements to date, they are challenged to correctly account for possible terrestrial contamination (9). Measurements of CO2 isotopes at Mars and in particular δ13C values have not been consistent with atmospheric loss (10). Viking (11) measured δ13C and δ18O values of 23 ± 43 per mil (‰) and 7 ± 44‰. Earth-based spectroscopy has suggested depleted values for δ13C of –22 ± 20‰ and δ18O of 18 ± 18‰ (9). The recent Phoenix lander measured δ13C and δ18O values for CO2 in the martian atmosphere of –2.5 ± 4.3‰ and 31 ± 5.7‰, respectively (12). Although uncertainties in these earlier atmospheric measurements of δ13C and δ18O overlap (Table 1), their δ13C values are in marked contrast to measurements of trapped CO2 in martian meteorite EETA 79001, generally considered to be closest to the true martian atmosphere and which yielded a δ13C of 36 ± 10‰ (8).

Table 1 Carbon dioxide isotope ratios ‰ ± 2 SEM (standard error of the mean).

*, not measured.

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For D/H in water, the difference in ground-state energies of HDO and its parent HHO are large enough to cause large changes in δD in equilibrium and nonequilibrium (kinetic) processes (13, 14), especially where condensation or freezing occurs. For this reason, D/H has become a universally important ratio to identify planetary origin and history (7, 15). The 1988 telescopic observation of D/H values in the martian atmosphere that were ~6 times that of Earth (7) were pivotal in the idea of atmospheric loss to space from a dense, warm, ancient atmosphere. Initial measurements in meteorites (16) gave a wide range of D/H values that may have included terrestrial contributions. A more recent analysis (17) of the ancient meteorite ALH84001 (~4 billion years old) and young meteorite Shergotty (0.17 billion years old) produced δD values of 3000 and 4600, respectively. These results have been interpreted (17) as evidence for a two-stage evolution for Martian water—a significant early loss of water to space [before 3.9 billion years ago (Ga)], followed by only modest loss to space during the past 4 billion years. Until Curiosity landed, there had been no in situ measurements of the water isotopic species HDO and H218O.

Oxygen isotopes in carbonates and sulfates from martian meteorites do not show any enrichment in δ18O and therefore have not been used as indicators of atmospheric escape (18, 19). It has been suggested that they are buffered by interaction with a larger O reservoir such as the silicates in the crust, or crustal ice deposits (20), although this is complicated by evidence for disequilibrium between the crust and the atmosphere (21). Oxygen isotopes in CO2 and H2O are therefore likely indicators of more complex interactions between the large reservoir of O in the hydrosphere, lithosphere, and atmosphere of Mars.

TLS is a two-channel tunable laser spectrometer that uses direct and second harmonic detection of infrared (IR) laser light absorbed after multipassing a sample cell (1). One laser source is a near-IR tunable diode laser at 2.78 μm that can scan two spectral regions containing CO2 and H2O isotopic lines; the second laser source is an interband cascade laser at 3.27 μm used for methane detection alone (4). The near-IR laser makes 43 passes of a 20-cm-long sample (Herriott) cell that is evacuated with a turbomolecular pump for background scans, then filled to 0.7 mbar using volume expansion of Mars air originally at ~7 mbar. TLS scans over individual rovibrational lines in two spectral regions near 2.78 μm; one centered at 3590 cm−1 for CO2 isotopes and a second centered at 3594 cm−1 for both CO2 and H2O isotopes (Figs. 1 and 2). The lines used in both regions have no significant interferences. In the 3594 cm−1 region, the CO2 and H2O lines we used interleave across the spectrum without interference, allowing the determination of accurate isotope ratios across widely varying CO2 and H2O abundances in both atmospheric and evolved gas experiments. The laser scans every second through the target spectral regions. Each 1-s spectrum is then co-added on board in 2-min periods, and the averaged spectra are then downlinked as raw data during a given run, typically of ~30 min duration. Data reported here were collected from 6 days (martian sols 28, 53, 73, 79, 81, and 106). During data collection, the Herriott cell and other optics are kept at 47° ± 3°C using a ramped heater that also serves to increase the signal-to-noise ratio in spectra by reducing the effect of interference fringes occurring during the 2-min sample period. The measured background amounts (empty cell) of both CO2 and H2O are negligible and also reflect an insignificant contribution to the signal from the instrument foreoptics. TLS is calibrated using certified isotopic standards (22) that improve the accuracy of isotope ratios over using the more uncertain HITRAN (high-resolution transmission molecular absorption) database spectral parameters.

Fig. 1 Spectral scan regions used by the TLS instrument.

Calculated spectra from the HITRAN database (36) for measuring CO2 (A and B) and H2O isotope ratios (B). The HDO line intensity has been increased by a factor of 6 to better represent the martian environment.

Fig. 2 Observed versus calculated spectra.

A single spectrum (middle section) downloaded from Curiosity (black), showing observed enrichment in 13CO2 and 18OCO compared to the calculated HITRAN spectrum (red) based on terrestrial (VPDB and VSMOW) isotope ratios (36). Both spectra are normalized in depth to the 16O12C16O line near 3590.1 cm−1 (Fig. 1). Ringing to the left side of the lines is explained in (22).

Our CO2 isotope ratios (Table 1, table S1, and Fig. 3) are given relative to Vienna Pee Dee belmnite (VPDB) for δ13C and relative to Vienna standard mean ocean water (VSMOW) for all oxygen isotopes (13). The measured value of δ13C18O agrees within uncertainty to the sum of the individual δ13C and δ18O measurements, providing a valuable check-sum on our results. Also, our measured value for δ17O is half that of δ18O, as predicted from mass-dependent fractionation (δ17O = 0.528 × δ18O) and consistent with previous SNC meteorite analysis. The independent SAM QMS result for δ13C of 45 ± 12‰ (3) agrees well with that from TLS at 46 ± 4‰, both values notably disagreeing with the much lower Phoenix lander result (12) of –2.5 ± 4.3‰. The sol-by-sol data plotted in Fig. 3 is not over a sufficiently long period to assess possible seasonal variation in δ13C or δ18O.

Fig. 3 Sol-by-sol mean values for CO2 isotope ratios.

The mean values for all sols combined (dashed lines) are given in Table 1. See (22) for values and uncertainties of the individual sol data plotted.

Our measured water abundances of up to 1% by volume in our Herriott cell after atmospheric intake exceed those expected (~150 parts per milion by volume) in martian air, and allowed us to retrieve a value for atmospheric δD, although with high uncertainty. Because our measured highly enriched δD values (Table 2 and table S2) are clearly martian and not terrestrial, we attribute the high water mixing ratios to either high near-surface humidity (natural or from enhanced temperatures in the vicinity of the rover) or to water entrained from frozen or liquid sources on or near the heated inlet valve. Also, in evolved gas experiments from pyrolysis of Rocknest fines (23), water was seen coming off at relatively low temperatures that we here identify as representative of the δD and δ18O values of the martian atmosphere. The TLS measurement of δD agrees well with observations from ground-based telescopes (24), but the contribution from expected seasonal cycling (25) is unknown. The enriched atmospheric values contrast with the low primordial D/H values postulated for the martian mantle (26) and are higher than those from our Rocknest higher-temperature studies (23).

Table 2 Water isotope ratios ‰ ± 2 SEM.

*, not measured.

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Modeling estimates of escape processes and atmospheric stability during Mars’ initial history point to catastrophic loss of atmospheric mass, and suggest that many atmospheric species carrying records of early isotopic evolution did not survive beyond approximately 3.7 to 4 Ga (27, 28). Carbonates in the ALH 84001 meteorite derived from an alteration event that occurred at ~3.9 Ga (29) preserve our best record of these events. Measurements of ALH 84001 carbonates show enriched isotopic values of δ13C = +27 to +64‰ (30, 31), δD values of ~3000‰ (16, 17), and low δ18O values (32). These values are similar to the composition of the modern martian atmosphere, suggesting that the δ13C, δD, and δ18O of the martian atmosphere were enriched early and have not changed much over ~4 billion years. Our higher values of δD and δ18O measured in the atmosphere suggest that escape processes may have also continued since 4.0 Ga, in accordance with a two-stage evolutionary process (17) described above.

We observe large enrichments of δ18O in atmospheric water vapor and CO2. The δ18O values of the water vapor are much larger than the δ18O observed in carbonates and sulfates in martian meteorites and suggest that the oxygen in water vapor in the martian atmosphere is not in equilibrium with the crust (33, 34) and could have been enriched in heavy isotopes through atmospheric loss. Another possibility is that the elevated oxygen isotope values in the more abundant martian CO2 are being transferred to the water vapor through photochemical reactions in the atmosphere. However, δ18O values of CO2 in Earth’s atmosphere are similarly elevated because of low-temperature equilibration between CO2 and H2O, and this process could also be operative on Mars (12).

In addition to atmospheric loss, other processes such as volcanic degassing and weathering might act to change the isotopic composition of the atmosphere through time. Estimates for the magnitude of these two contributions over the ~4-billion-year history of Mars vary widely (30, 34, 35), yet could have a strong impact on the isotopic composition of the atmosphere and challenge the status quo model described above.

Supplementary Materials

www.sciencemag.org/cgi/content/full/341/6143/260/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S3

Tables S1 to S4

Reference (37)

MSL Science Team Authors and Affiliations

References and Notes

  1. See the supplementary materials on Science Online.
  2. Acknowledgments: The research described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.
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