Evidence for diurnally varying enrichment of heavy oxygen in Mars atmosphere

Icarus 335 (2020) 113387

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Evidence for diurnally varying enrichment of heavy oxygen in Mars atmosphere

T

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Timothy A. Livengooda, ,1, Theodor Kostiukb,1, Tilak Hewagamac,1, Ramsey L. Smithd,1, Kelly E. Faste,1, John N. Annenf,1, Juan D. Delgadog,1 a

CRESST, University of Maryland at Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, United States of America Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, United States of America c University of Maryland at Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, United States of America d Geaux Strategic and Technologies, LLC, Washington, DC, United States of America e Planetary Science Division, NASA Headquarters, Washington, DC, United States of America f Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, United States of America g Vrije University, Amsterdam, the Netherlands b

A R T I C LE I N FO

A B S T R A C T

Keywords: Terrestrial planets Isotope ratios Atmosphere

Terrestrial groundbased spectroscopy of Mars at subsolar latitude in October 2007 (southern summer, Mars year 28) reveals variability in the depth of a spectral absorption feature from a carbon dioxide isotopologue, 16 12 18 O C O (‘628 CO2’), which correlates with increasing surface temperature in the early afternoon of the Martian sol. The correlation suggests a fractionation process that depletes isotopically heavy CO2 from the atmosphere by surface adsorption at night and restores atmospheric enrichment during the day through thermal desorption. Measurements reported here yield a depleted condition at noon, with 18O depleted to −92 ± 23‰ relative to a terrestrial isotopic standard (VSMOW, Vienna Standard Mean Ocean Water), increasing to +71 ± 18‰ at 13:19 LST, over a temperature increase from 266.9 K to 275.4 K. The average is +9 ± 14‰, consistent with results from landers and remote spectroscopy that averaged data collected over a broad range of daylight local times to obtain an average close to the terrestrial standard. Past measurements of CO2 isotopologues have been inconsistent with each other and in some cases inconsistent with predicted enrichment in heavy isotopes, with some measurements even slightly depleted in heavy isotopes. Local solar time of a measurement thus may skew this important constraint on estimating the density of the primordial atmosphere, although seasonal polar temperature variation also may have a substantial influence.

1. Introduction Numerous lines of geophysical and geochemical evidence suggest that Mars once had a much more dense atmosphere than at present, capable of supporting liquid water long enough to develop flow channels, streambeds, sediment deposits, and evaporite deposits. The modern atmosphere of Mars is less than 1% terrestrial atmospheric pressure (Gómez-Elvira et al., 2014), composed of 95.7% CO2, 2.1% Ar, 2.0% N2, and 0.2% O2 by volume (Franz et al., 2017). Atmospheric loss processes typically result in the escape of light isotopes more rapidly than heavy counterparts, leaving the remnant atmosphere enriched in the heavier isotopes (Haberle et al., 1994; Hunten, 1993; Jakosky, 1991). The Mars Atmosphere and Volatile Evolution (MAVEN) mission

currently is measuring abundances and loss rates for various species in the upper atmosphere, recently reporting evidence that supports the deduced loss of Mars' primordial atmosphere based on differential enrichment of argon isotopes (Jakosky et al., 2017). Heavy-isotope enrichment has been measured for nitrogen and hydrogen by the Mariner 9 orbiter and by the Viking, Phoenix, and Curiosity landed missions, supplemented with measurements of various species by Earth-based remote sensing (McElroy and Yung, 1976; Maguire, 1977; Owen, 1982; Owen, 1992; Niles et al., 2010; Webster et al., 2013; Schrey et al., 1986; Krasnopolsky and Feldman, 2001; Krasnopolsky et al., 2007; Villanueva et al., 2015). Measured isotope ratios for atmospheric carbon dioxide prior to Phoenix and Curiosity were inconsistent with other species and with heavy-isotope enrichment by atmospheric loss, finding carbon and

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Corresponding author at: Code 693, CRESST/UMD, NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States of America. E-mail address: [email protected] (T.A. Livengood). 1 Visiting Astronomer, NASA Infrared Telescope Facility, operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration https://doi.org/10.1016/j.icarus.2019.113387 Received 17 January 2019; Received in revised form 27 June 2019; Accepted 23 July 2019 Available online 01 August 2019 0019-1035/ © 2019 Elsevier Inc. All rights reserved.

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0.1

Radiance (erg/s/cm2/cm-1/sr)

a

b

952.8808 cm–1

70

952.8629 cm–1

Best fit MGS-TES (initial)

Non-LTE

60 1.0

18OCO

50

271K

CO2

237K 7.6 mbar

40 200

400

600

800

1000

1200

Δν (MHz from LO)

1400

150

186

223

T (K)

240K 5.6 mbar 259

295

290K

Fig. 1. Mars measured spectrum, time average to clarify spectroscopic features. (a) The overall spectrum envelope is due to the wing of a telluric CO2 line at rest at zero frequency difference from the CO2 laser local oscillator (LO). A CO2 absorption feature from Mars' troposphere is Doppler-shifted from rest frequency, with nonLTE core emission arising in the mesosphere. A feature of 18OCO forms in the lower troposphere. (b) Thermal profile and surface temperature retrieved from MGSTES data (blue) and modified profile and surface temperature (red) retrieved from fitting our observed spectrum, with cooler surface (dots), warmer boundary layer, and reduced surface pressure (horizontal lines). The blue model spectrum in (a) corresponds to the blue MGS-TES profile, while the red model spectrum results from the red best-fit temperature profile. These model spectra use VSMOW isotope ratios. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

which furnishes a beam diameter of ~0.9 arcsec diameter (FWHM) at 10–12 μm wavelength under seeing conditions typical for the summit of Mauna Kea. Mars' apparent diameter of 11.4 arcsec greatly exceeds the beam diameter and the beam was far from the limb, facilitating accurate radiative-transfer modeling. The beam's projected width at disc center was 535 km, covering a projected surface area of 22.5 × 104 km2. For comparison, Gale Crater, the site of the Mars Science Laboratory Curiosity rover investigation, is 154 km diameter and 1.9 × 104 km2 in area. Mars was at orbital longitude LS = 335°, in late southern hemisphere summer. Eight spectra were obtained between 10:50 and 15:30 UT on two targeted offsets from disc center, at subsolar latitude. Parameters describing the individual target positions are tabulated in Table 1, and the individual spectra are displayed in Fig. 2. The target regions were at 10.3°S (subsolar latitude), offset 34.75° in longitude from the sub-Earth meridian to coincide with the subsolar point at local solar time 12:00, and a position offset by 15° in longitude from the sub-Earth meridian to coincide with local solar time 13:19 on Mars. The areographic regions beneath the telescope beam were in the range 24°–88° East longitude. This is a band north of Hellas Basin at altitude ~4–6 km above the mean surface, implying a variation of up to about 10% in pressure between scans with a scale height in CO2 of 12.2 km at the retrieved boundary layer temperature of 240 K. HIPWAC uses the infrared heterodyne technique to optically combine the telescope beam with a laser local oscillator (LO), resulting in a spectrum of beat frequency (difference frequency) in a high speed photomixer detector at the focus of the combined beams (Kostiuk, 1994). An earlier measurement by Schrey et al. (1986) also used the IR heterodyne technique with coarser spectral resolution, centered on the illuminated disc at a single local time. Infrared heterodyne spectroscopy is comparable to sub-mm and radio spectroscopy methods, using radiofrequency electronics to analyze the spectral energy distribution of the difference frequency spectrum at resolving power orders of magnitude greater than conventional dispersive methods. The present measurements were collected using a CO2 gas laser LO at frequency 952.8808 cm−1 (10.49449 μm wavelength), with resolving power

oxygen ratios similar to terrestrial and thus posing a challenge to interpret the history of the major species in the modern atmosphere. Phoenix, at high northern latitude, found 18O enriched but not 13C, while Curiosity, near the equator, found both 18O and 13C enriched relative to terrestrial to a similar degree. The narrow uncertainties of the Phoenix and Curiosity measurements do not overlap (Niles et al., 2010; Webster et al., 2013; Franz et al., 2015). The measurements reported here were acquired using infrared heterodyne spectroscopy (IRHS) to obtain extremely high resolution spectra that resolves signature transitions of CO2 isotopologue gases. The detailed lineshapes in the measured spectra are compared with radiative-transfer models for the emergent spectrum from Mars to constrain atmospheric and surface parameters. We report remote measurements of the 16O12C18O (‘628’) isotopologue of carbon dioxide in Mars in comparison to the 16O12C16O (‘626’, or ‘normal isotope’) isotopologue in the same observations (Fig. 1). These measurements provide evidence for variable enrichment of a stable oxygen heavy isotope that appears to correlate with the ground temperature on Mars in the early local afternoon. We interpret this variability as resulting from exchange between the atmosphere and a surface reservoir through a sequestration process controlled by the ground temperature, which could account for inconsistency among prior measurements. Re-examining published values from some previous results on CO2 isotope ratios suggests the action of mass-dependent fractionation that could account for diurnally and seasonally variable quantities of the 18O isotopologue of CO2 as well as other isotopologues.

2. Observations We measured the infrared thermal emission spectrum of Mars at 10.494 μm wavelength (952.9 cm−1; Fig. 1) on 22 October 2007 (UT) from the NASA Infrared Telescope Facility (IRTF), using the Goddard Space Flight Center's Heterodyne Instrument for Planetary Winds and Composition (HIPWAC; Schmülling et al., 1999). HIPWAC was mounted at the Cassegrain focus of the IRTF 3.1 m-diameter telescope, 2

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Table 1 Identity, location, and other properties of spectra acquired from Mars. Spec

Earth UT

A B C D E F G H

10:55 11:17 11:40 15:17 13:35 13:57 14:29 14:53

Earth AM

Vtopo km/s

WCML

Offset

Wobs

Loc time

Mars AM

1.80 1.59 1.44 1.01 1.06 1.03 1.01 1.00

−11.25 −11.23 −11.20 −10.86 −11.04 −11.00 −10.95 −10.90

237° 243° 248° 301° 276° 281° 289° 295°

35°W 35°W 35°W 35°W 15°W 15°W 15°W 15°W

272° 278° 283° 336° 291° 296° 304° 310°

12:00 12:00 12:00 12:00 13:19 13:19 13:19 13:19

1.279 1.279 1.279 1.279 1.084 1.084 1.084 1.084

Spec = individual spectrum identification. Earth UT = Earth received time, midpoint of integration. Earth AM = airmass of observation, path length through atmosphere relative to zenith. Vtopo = topocentric velocity of Mars center. WCML = West Central Meridian Longitude at time of observation. Offset = offset to planetary West, in degrees longitude toward approaching limb. Wobs = West longitude of observed site on Mars at integration midpoint. Loc Time = local time of observed position on Mars. Mars AM = airmass in Mars atmosphere at offset position from Mars disc center.

R = λ/Δλ = 3.8 × 107 over a bandwidth of ± 1537 MHz ( ± 0.051 cm−1 or ± 0.00057 μm) with an acousto-optic spectrometer having 0.751 MHz-width channels (2.5 × 10−5 cm−1) for the radiofrequency analysis. The center of the 626 absorption feature on Mars is occupied by a solar-pumped non-thermal (non-LTE) 626 emission that forms in the lower mesosphere (Mumma et al., 1981). An absorption from 628 CO2 appears at ~540 MHz frequency difference from the 626 transition (Figs. 1 and 2). Spectral intervals with strong transitions of 627 (16O12C17O) and 636 (16O13C16O) carbon dioxide were not accessible due to the particular value of Doppler shift from line-of-sight orbital motion at the time of these observations. The CO2 transition used as the LO is at rest frequency and highly pressure-broadened in Earth's atmosphere, creating a transmittance profile from the wings of the telluric absorption feature. The corresponding feature in Mars' atmosphere was shifted by the radial velocity of 10.9 km/s approaching, shifting the frequency by ~1100 MHz above the LO frequency. The Doppler shift further varied over the course of the observing night due to the combined projection of velocity from Earth's rotation, Mars' rotation at the offset pointing position, and changes in the relative orbital velocity of Earth and Mars. The spectrum includes two weak features, a ‘hot band’ transition of 626 CO2 at ~1240 MHz and a transition of the 16O12C17O (‘627’) isotopologue from the opposite sideband (frequencies less than the LO) at difference frequency similar to the 626 principal transition. These weak transitions are sensitive close to the surface at the greatest atmospheric pressure. The principal line has such high opacity that it saturates above the altitude probed by the hot band line and mostly masks it. The 627 transition is less than a quarter the strength of the 628 line and is completely hidden by the non-LTE emission. These weak lines are incorporated into the modeled emergent spectrum without deviation from Vienna Standard Mean Ocean Water (VSMOW) isotopic abundances.

Quoted uncertainties represent multivariate 68.3% (1-sigma) confidence limits. As shown in Fig. 2, the spectra can be modeled with high precision. The modeled parameters relevant to this work, the surface temperature and the abundance of 628 CO2 relative to VSMOW abundance, are reported in Table 2. We determined a single temperature-pressure profile to use in representing Mars' atmosphere for all eight spectra. The T-P profile was constrained by matching the global average of the measured spectra exclusive of spectral intervals dominated by the 628 transition and the non-LTE emission core (Fig. 1). The spectral models for Mars are generated using the Composition and Data Analysis Tools (CODAT) software, described by Hewagama et al. (2008). Retrievals by Smith (2004) provided an initial T-P profile based on the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), selected appropriate to the latitude, longitude, and season of our observations. The MGS temperature profile differs from the circumstances of our measured spectra, which were acquired at earlier local time than the 2 PM Sun-synchronous orbit of MGS. We derive a T-P profile that is decreased in surface air pressure relative to the MGS profile, from 7.6 mbar to 5.6 mbar, and increased in boundary-layer temperature, from 237 K to 240 K. These parameters primarily affect the wings of the major 626 absorption feature, where the optical depth in the pressure-broadened transition is lesser and thus probes the near-surface atmosphere. Recent observations in 2018 have confirmed that a weather phenomenon such as a dust storm results in substantial changes to the emergent spectrum, particularly by decreasing the pressure at the effective bottom of the temperature profile. The result would be easily distinguishable in the observed spectra (Fig. 2) as a significant narrowing and shallowing of the 626 transition. No such phenomenon is observed. Fig. 2 shows empirically that the eight measured spectra are modeled well by a single atmospheric temperature profile. If not, the wings of the 626 line would deviate between the model and data, resulting in broad structures within the residuals. The fitted profile deviates from the initial profile by only a few Kelvin (Fig. 1), illustrating how small a deviation would be discernible. The dominant normal-isotope 626 transition increases in strength by a greater factor than the 628 isotopologue transition in response to an increase in temperature. Deviations of the 628 line in response to an erroneous model for the thermal profile thus should be weak, as this transition is less sensitive to the relevant temperature range. The majority of variability between the measured spectra is in the continuum radiance due to surface temperature and in the narrow optically thin features, the 628 CO2 absorption and the 626 core emission. The residual discrepancy between the observed and fitted spectra is minimal and has no significant features associated with the

3. Analysis The measured spectra constrain parameters describing the thermal structure and composition of the Mars atmosphere by comparing them with model spectra simulated by radiative-transfer modeling for the atmosphere of Mars and Earth. The transmittance spectrum for Earth atmosphere uses the GENLN2 code (Edwards, 1992) adjusted for the current abundance of CO2 in Earth's atmosphere. Parameters that describe the surface continuum emission and the atmosphere of Mars are adjusted iteratively to achieve a fit to the measurements. We estimate uncertainties by exploring the range of deviations in parameter values that are permissible within the measurement noise in the spectra. 3

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120

a

b

c

d

e

f

g

h

Fig. 2. Mars measured spectra (black) with models (red line), showing CO2 in absorption and emission in Mars atmosphere. The fitted spectra accurately model the measurements, as shown by the small residuals plotted below each spectrum. The model spectra use the thermal profile shown in Fig. 1b, with surface temperature fitted individually. The labels a–h correspond to Tables 1 and 2. The abundance of 628 CO2 is scaled relative to VSMOW to fit the feature at ~540 MHz. Text within each figure describes parameters fitting the non-LTE core emission feature in each case. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

100 80 60 40 20

120 100

Radiance (erg/s/cm2/cm-1/sr)

80 60 40 20

120 100 80 60 40 20

120 100 80 60 40 20

200 400 600 800 1000 1200 1400

200 400 600 800 1000 1200 1400

Δν (MHz from LO)

Δν (MHz from LO) depth of the 628 and 626 absorption features in these spectra have to be due to variations in the actual abundance of the isotopologue species, since the 626 line fits the spectrum so precisely in all our spectra, thus assuring that the atmospheric temperature profile has been determined correctly. The modeled thermal continuum emission assumes a single representative temperature in the field of view and treats Mars surface as having unit emissivity, a reliable assumption since solid-phase spectral features are far too broad to be discernible over our narrow bandwidth. The radiative-transfer calculation from surface emission through to space is conducted at sampling resolution finer than the measurement, using line-by-line radiative-transfer with pressure-broadened lineshapes and self-emission. Molecular parameters to model atmospheric opacity are from the HITRAN2012 database (Rothman et al., 2013), including strengths for the CO2 isotopologue lines assuming VSMOW isotope ratios. Fitting the 628 transition is decoupled from the 626 abundance, so that the 628 line is scaled relative to the VSMOW

wings of the 626 line, which would be the most sensitive to differences in surface air pressure or boundary-layer temperature between the spectra (Fig. 2). Numerical experiments indicate that varying the surface pressure in the modeled atmosphere, equivalent to adjusting for topography by half an atmospheric scale height, does not significantly affect the retrieved isotope ratios. We can discount unrecognized gas temperature variations as a source of relative variability in the 628 and 626 features. The lower state energy of the 628 line in our spectrum is 1478.6 cm−1, while the lower-state energy of the 626 line is 1431.1 cm−1. These energies differ by only about 3.3%. A change in the gas temperature by 10 K (240–250 K) would result in changing each line strength by about 30%, but the ratio between them varies by only 1.1%. The effect of temperature on line strength is incorporated into the radiative transfer modeling and provides the sensitivity to temperature in the 626 line that constrains the low-altitude part of the thermal profile to be 3 K warmer than in the nominal MGS/TES profile. Variations in the relative 4

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Table 2 Surface temperature and

18

O/16O ratio retrieved from measured spectra.

Spec

Earth UT

Wobs

Eobs

Loc time

A B C D E F G H

10:55 11:17 11:40 15:17 13:35 13:57 14:29 14:53

272° 278° 283° 336° 291° 296° 304° 310°

88° 82° 77° 24° 69° 64° 56° 50°

12:00 12:00 12:00 12:00 13:19 13:19 13:19 13:19

Tsurf K 265.7 ± 0.1 266.8 ± 0.1 266.5 ± 0.1 268.4 ± 0.1 275.3 ± 0.04 273.9 ± 0.04 273.9 ± 0.04 278.5 ± 0.04

M/E

0.89 0.85 0.84 1.00 1.15 1.03 1.00 1.09

± ± ± ± ± ± ± ±

0.05 0.05 0.05 0.04 0.04 0.04 0.04 0.03

δ18O ‰ −110 −150 −160 0 150 30 0 90

± ± ± ± ± ± ± ±

50 50 50 40 40 40 40 30

Spec = individual spectrum identification. Earth UT = Earth received time, midpoint of integration. Wobs = West longitude of observed site on Mars at integration midpoint. Eobs = East longitude of observed site on Mars at integration midpoint. Loc Time = local time of observed position on Mars. Tsurf = fitted surface temperature, in K. M/E = ratio of 628 fractional abundance in Mars CO2 compared to Earth. δ18O = 18O enrichment in Mars relative to Earth, in units per mil = 1000·(M/E − 1).

fractional abundance of 18O. HIPWAC measurements use an internal blackbody source as a calibrator, interspersing astronomical measurements with occasional measurements on the blackbody. Comparisons to previous IR heterodyne measurements on astronomical targets suggest that the absolute intensity calibration currently is not certain. Although the derived temperatures may not be accurate, the relative variability of the measured radiance is well determined. HIPWAC measurements targeting Gale Crater have been obtained and will be used in future work to refine a calibration factor for HIPWAC sensitivity using ground temperature measurements at the Curiosity rover. Uncertainty in the absolute intensity calibration at the current level has no effect on the retrieved atmospheric temperature profile or other atmospheric parameters on Mars, which are constrained by the detailed lineshapes within the measured spectrum. Properties of 626 lasing transitions are known to extremely high accuracy due to the value of CO2 gas lasers in metrology and the importance of CO2 in telluric radiative transfer. Minor isotope spectroscopic parameters may be more uncertain. The retrieved δ18O value depends on the accuracy of the line strength. A one-percent uncertainty in the line strength translates to 1% systematic uncertainty in retrieved 18 O fraction. This uncertainty would appear as an added or subtracted value applied uniformly to all the retrievals. Modeling with a line strength 1% greater than we have used would uniformly decrease the retrieved 18O fraction by 1%, and the reverse if the line strength were 1% weaker. The core emission of 626 is due to solar pumping of the normalisotope line in the mesosphere (Mumma et al., 1981). This emission is incorporated into the model as a Gaussian feature added to the emergent thermal-emission spectrum. The parameters for this feature are its amplitude, Doppler-broadened width due to local temperature, and center frequency to account for wind shear relative to the troposphere (Sonnabend et al., 2006). While the non-LTE feature is not directly relevant to modeling the 628 feature, it is useful in accurately modeling the shape of the 626 feature that constrains the thermal profile. The Mars spectrum is Doppler-shifted for line of sight velocity and transferred through telluric opacity using transmittance modeled by the GENLN2 code (Edwards, 1992). Spectra in the upper and lower sidebands of the model spectrum are rebinned according to the difference frequency relative to the local oscillator rest frequency and co-added to model the double-sideband measured spectrum. Parameters describing the source environment and the Doppler shift are iteratively adjusted to achieve the best fit to the measured spectra. The telluric transmittance is controlled primarily by the column abundance of carbon dioxide in Earth's atmosphere at the time of observation, which is available from

various public resources. Telluric transmittance also can be checked by comparing with the measured thermal continuum from the Moon and is reproduced well at IR heterodyne resolution. 4. Results and interpretation The individual spectra are modeled for the thermal emission continuum from Mars' surface, the 628 abundance relative to VSMOW, the non-LTE emission line, and the overall Doppler shift of the Mars CO2 spectrum, making six parameters in all. The relevant fit parameters of surface temperature and 18O enrichment are reported with estimated uncertainty in Table 2. Fig. 2 shows the quality of the fits to the individual spectra, with no significant systematic defects in residuals of the fit. The surface radiance increases noticeably from the four spectra at local noon (spectra A–D) and the four spectra at 13:19 LST (E–H) due to increased surface temperature in the afternoon. There are smaller quantitative differences in retrieved temperature between the areographic regions over which the spectra were measured. The apparent enrichment or depletion of 18O is expressed in Table 2 and in Fig. 3 in parts per thousand, or parts per mil (‰), as δ18O = 1000 • (M/E – 1), in which M/E is Mars fractional abundance relative to Earth. The retrieved δ18O is displayed in Fig. 3a at the observed longitude of each measurement, colour-coded by the target offset position for each spectral measurement and compared to earlier published retrievals. The uncertainty-weighted average combining all eight individual retrievals is δ18O = +9 ± 14‰, similar to Viking and previous remote spectroscopy and indistinguishable from the terrestrial VSMOW standard. This value is less enriched than retrievals from the Phoenix and Curiosity in situ measurements shown in Fig. 3 and tabulated in Table 3. The dispersion in retrieved isotope ratios is substantial, and it is apparent that the noon and 13:19 LST sets fall into separate groups. The uncertainty-weighted mean value retrieved from the spectra acquired at Mars local noon is δ18O = −92 ± 23‰, depleted relative to VSMOW and comparable to the 18O/16O isotope ratio measured for the solar wind (McKeegan et al., 2011). No prior retrieval for Mars has reported depletion in 18O this great. The mean value retrieved from the spectra acquired at 13:19 LST is δ18O = +71 ± 18‰, enriched relative to VSMOW. No prior retrieval for Mars has reported enrichment in 18O this great. These two sets of retrieved values each differ from VSMOW by about 4σ and are separated from each other by more than 5σ, suggesting that the difference between them is systematic rather than a stochastic measurement uncertainty. Two obvious systematic differences distinguish the measurement sets: the slant angle through Mars atmosphere to the targeted position, 5

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a

b

Spectroscopy

Curiosity Phoenix

Viking

Spectroscopy

Curiosity Phoenix

Viking

Genesis (solar wind)

Genesis (solar wind)

Fig. 3. Mars δ18O deviation from VSMOW. Oxygen-18 abundance correlates with surface temperature. Measurements at subsolar (12:00 local) highlighted orange; measurements at 13:19 local highlighted yellow. Orange band at 0 ± 50‰ from Viking landers (7–8); red band at 18 ± 18‰ from groundbased spectroscopy (Krasnopolsky and Feldman, 2001); blue band at 31.0 ± 5.7‰ from Phoenix lander (Niles et al., 2010); blue band at 48 ± 5‰ from Curiosity (Webster et al., 2013). Unenriched solar wind abundance at −102.3 ± 3.3‰ (McKeegan et al., 2011). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

artifact of the fitting process. We conclude that the enrichment of 18O in the Mars atmosphere increases as the surface temperature increases. This suggests a possible cause-and-effect relationship in which an increase in surface temperature causes reservoirs in the surface to release 628 carbon dioxide into the near-surface atmosphere.

and the local solar time on Mars and thus the surface temperature and radiance. The inset in Fig. 3 includes a graphic depicting the targeted regions on Mars, showing that the earlier the local time on Mars, the closer the measured region is to the Western limb on the sky and the greater the slant angle through the atmosphere. The noon measurements were acquired at greater slant angle than the 13:19 LST spectra, a path length of 1.279 times greater than at disc center compared to an enhancement by a factor of 1.084 at 13:19. The noon measurements thus conceivably might exhibit a depth increase of ~18% (180‰) in the optically thin 628 feature compared to the early afternoon measurement, which is the opposite of the observed pattern. The radiativetransfer analysis of course compensates for the atmospheric path length. These slant angles are not extreme and are well within the capacity to be handled by standard techniques. The other systematic distinction between the two sets of retrievals is the surface temperature. Fig. 3b displays δ18O for all eight spectra in comparison to the retrieved surface temperature (not air temperature, which is uniform in the models), showing that the apparent large random dispersion in δ18O is correlated with the surface temperature. The temperature retrieval is constrained by the background continuum in the modeled spectra, while the isotopic line strength is fitted by comparison to the local continuum, regardless of the value of the continuum radiance. These features are not strongly covariant as an

5. Comparison with other measurements Previous remote spectroscopy of Mars has been on disc center at a single local time near noon to measure a time-average spectrum that averages over a range of surfaces and surface temperature while accessing only a fixed range of local solar time (e.g., Krasnopolsky et al., 2007; Schrey et al., 1986). In situ measurements similarly have averaged over multiple samples without regard to local solar time. Recent in situ data have the benefit of investigating multiple isotopologue species on multiple occasions, offering a different way to investigate for evidence of fractionation processes in Mars' atmosphere. Table 3 tabulates measurements of δ18O in this work and others, as well as δ13C (not measured here). The 1976 Viking lander mass spectrometers, at temperate northern latitudes, found 18O and 13C fractions consistent with VSMOW, with broad uncertainty (Owen, 1982). The actual measurements do not appear to have been archived in any accessible location, so it is not possible to inspect the original data for

Table 3 Measured δ18O from this work compared to previous δ18O and δ13C measurements. Source

HIPWAC HIPWAC HIPWAC Genesisa Vikingb Schrey et al.c Krasnopolsky et al.d Phoenixe Curiosityf Curiosityg a b c d e f g

Method

IRHS IRHS IRHS Solar wind sample Mass spec IRHS IR FTS TEGA TLS QMS

Loc time

12:00 (266.9 K) 13:19 (275.4 K) – – – ~12:00 ~12:00 Morning Dusk-to-midnight Dusk-to-midnight

McKeegan et al. (2011). Owen (1982). Schrey et al. (1986). Krasnopolsky et al. (2007). Niles et al. (2010). Webster et al. (2013). Mahaffy et al. (2013). 6

Latitude

δ13C ‰ VPDB

δ18O ‰ VSMOW

10°S, subsolar 10°S, subsolar 10°S, subsolar – 22.5°N (V1) 48°N (V2) – ~Equator 68°N 4.6°S 4.6°S

– – – – −11 ± 55

−92 ± 23 +71 ± 18 +9 ± 14 −102.3 ± 3.3 −2 ± 50

−73 ± 58 −22 ± 20 −2.5 ± 4.3 +46 ± 4 +45 ± 12

−37 ± 121 +18 ± 18 +31.0 ± 5.7 +48 ± 5 –

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consistent with the slope of a line fitted to the Phoenix measurements and display similar dispersion. Neither Niles et al. (2010) nor Webster et al. (2013) claim evidence for mass-dependent fractionation, and Niles has suggested that the observed correlation may be an instrumental artifact (P. Niles, personal communication, 2013). The fact that the Phoenix data are stochastically distributed around a line of correlation rather than piling up on a saturation limit, and the fact that the line fitted to the Phoenix measurements agrees with the Curiosity measurements using an entirely different methodology, encourages us to conclude that these experiments have detected actual variability in the isotopic fractionation of CO2 in Mars' atmosphere on short timescales. We rule out an association of isotopic enrichment or depletion with specific locations on Mars, due to the special conditions that would have to be assumed in explaining the present data through such an association. The regions observed in each spectrum would need to have properties that constrain the surface temperature but which only coincidentally correlate with localized regions of unusual isotope ratio. The topography would need to restrict horizontal diffusion to prevent dilution of isotopically peculiar gas by the global atmosphere, or there would need to be localized deposits of isotopically unusual volatiles that are in the process of sublimating but are not yet depleted, creating a localized region of distinctive isotope ratio. These unusual properties would need to map coincidentally onto the pointings used in acquiring the observed spectra with the ~535 km beam diameter, including spectrum D, which was acquired at a considerably different longitude than the other local-noon spectra (Tables 1 and 2, Fig. 3). No distribution of conveniently located depressions or plateaus of this dimension appears in topographic maps of Mars at the targeted latitude. The near-surface heterogeneity required to explain our observations through location-specific properties would invalidate the global relevance of isotope ratio measurements from any single landed location. Spectroscopic and in situ measurements of atmospheric chemistry have not appeared to be particularly heterogeneous in this patchy way, so variability in chemistry and isotope ratios would have to be decoupled. This is an insupportable number of assumed properties in the absence of further relevant data.

fresh insights. IR heterodyne spectroscopy in the 1980's probed the 628 and 636 isotopologues at disc center and retrieved near-terrestrial isotope ratios, with marginal evidence (~1σ) for depletion of 13C (Schrey et al., 1986). Remote Fourier Transform Spectroscopy (FTS) measured CO2 isotopologue bands at near-infrared wavelengths in a broad beam on disc center (Krasnopolsky et al., 2007), finding marginal enrichment in 18O and similarly marginal depletion in 13C relative to VSMOW. The Thermal Evolved Gas Analyzer (TEGA) mass spectrometer on the Phoenix lander, at high latitude on Mars, found significant 18 O enrichment in atmospheric CO2 but no enrichment of 13C (Niles et al., 2010). Reconsideration of the Phoenix TEGA data have concluded that the lack of measured enrichment in 13C may be a measurement artifact, but differential measurements relative to the mean remain useful (C. Webster, personal communication, 2018). The tunable laser spectrometer (TLS) and quadrupole mass spectrometer (QMS) instruments on the Curiosity rover report enriched 18O as well as enriched 13C in atmospheric CO2 within Gale Crater, near the equator (Webster et al., 2013). Both Phoenix TEGA and Curiosity TLS individual measurements were reported in online supplements for the respective publications (Niles et al., 2010; Mahaffy et al., 2013; Webster et al., 2013). Fig. 4 plots these measurements on a scale chosen to emphasize the joint variation of the measured isotope ratios, ± 30‰. Both missions measured both δ18O and δ13C in individual gas samples. TEGA collected multiple samples on each of four sols during both day and night (P. Niles, personal communication, 2017). The published TEGA data do not report the local solar time of the measurements or the ground temperature. Mahaffy et al. (2013) tabulate physical parameters for measurements by Curiosity, including surface temperature and local solar time, at the time that many of the samples measured by TLS were ingested. The set of samples reporting all relevant parameters is too small to investigate correlations with temperature or local solar time. Curiosity acquired gas samples in late afternoon through shortly after midnight on various occasions during the first months of the mission but did not acquire measurements consecutively in any single sol or in consecutive sols. A comparison between Phoenix measured δ18O and δ13C shows a clear positive correlation, suggesting that the observed dispersion in retrieved values is actual variability consistent with mass-dependent fractionation. An increase in 636 abundance, at mass number 45, is associated with a proportional increase in 628 abundance, at mass number 46. The Curiosity measurements are fewer and are not sufficient to establish a correlation, although the Curiosity measurements are

a

6. Thermal modulation of isotope ratios We propose a thermally controlled exchange of isotopologues between the atmosphere and surface reservoirs that would result in diurnal variability of isotopic enrichment in the atmosphere due to

b

Phoenix TEGA

Curiosity TLS

Sol 09

Sol 28

Sol 11

Sol 53

Sol 12

Sol 73

Sol 16

Sol 79 Sol 106

Slope = 1.3

Slope = 1.3

Fig. 4. Evidence for varying fractionation in carbon dioxide measured in situ on Mars. (a) Correlation between δ18O and δ13C differential measurements relative to the mean from the Phoenix lander TEGA (Niles et al., 2010). A line of slope 1.3 is drawn through the data, with correlation coefficient 0.74 and negligible probability of false correlation. Data are colour-coded by the mission sol on which the sample was collected and analyzed. (b) Correlation between δ18O (VSMOW) and δ13C (VPDB) from the Curiosity rover TLS (Webster et al., 2013). A line of slope 1.3 shows consistency with the correlation observed in Phoenix measurements. 7

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If temperature is the dominant property controlling the fractionation of isotopically heavy CO2, then it is likely that there will be an additional variability due to seasonal temperature changes in polar and circumpolar regions. Heavy isotope enrichment is an important constraint on the loss of planetary atmosphere, with enrichment increasing due to differential loss to space of lighter versus heavier isotopes (Hunten, 1993; McElroy and Yung, 1976). If isotopically heavy gas is exchanged between the surface and the near-surface atmosphere, as postulated in this work and by Villanueva et al. (2015), then it will be underrepresented in the upper atmosphere that can be sampled by an orbital mission like MAVEN (Jakosky et al., 2017). Diurnal and seasonal sampling near the surface by landed missions or by remote measurements to cover the surface of Mars is necessary to explore the exchange between surface and atmosphere and to establish the true constraints on Mars' volatile isotope ratios. The greatest enrichment of 18O measured in the current work, from averaging the four measurements acquired over warmer surfaces, is +71 ± 18‰. This enrichment is about 1.2σ greater than the measurement by Curiosity TLS. A line fitted to the variation of δ18O with temperature predicts an enrichment of +119 ± 25‰ at the warmest surface temperature measured here, 278.5 K, 2.8σ greater than the measurement by Curiosity TLS. Absolute calibration of these retrievals is uncertain due to uncertainty in the transition strength, creating an additive uncertainty on our retrieved values. The fact that our average agrees with near-noon averages from other measurements (Table 3) suggests that our measurements are reasonably accurate but still subject to revision as improved spectroscopic parameters become available. Our retrievals may be reconciled with the Curiosity or Phoenix measurements by adding or subtracting an appropriate quantity to our retrieved average, the maximum, or the minimum, but the magnitude of the measured variability and its correlation with surface temperature will remain. The range of variability that we see within just one sol is such that diurnal variability convolved with seasonal variability and variability that may be controlled by sampled altitude and latitude, as with the D/ H measurements by Villanueva et al. (2015), is so great that it can reconcile the entire wide range of measurements that have been acquired by various methods and at various times and locations on Mars. It is clear that atmospheric constituents cyclically condense and sublimate at the polar caps with season. Our work and the work by Villanueva et al. (2015) show that temperature cycles in the surface and atmosphere of Mars can change measurable isotope ratios to a significant extent. Characterizing the process may require a network of measurements distributed over the surface of Mars. An important contribution, that does not require launching an entire new mission, would be to locate the original Viking mass spectrometer data that were acquired over a period of years. These data appear to have been lost, but may exist in some researcher's private collection. The Viking results were published with extremely broad uncertainty limits that encompass all of the measured values retrieved prior to the present work. Published analyses (e.g., Owen, 1982) do not address details of the actual dispersion in the data. Few of the Viking researchers survive to report their memories, and even fewer may recall the nuts and bolts of the numerical analysis. The original data could resolve whether the broad uncertainty quoted for Viking comes from intrinsic precision, or from the dispersion of actual measurements that could be correlated between species or with other atmospheric and surface properties.

diurnal variability of surface temperature. The data presented here do not constrain or require seasonal variability, but it is reasonable to infer its existence due to the thermal contrast between summer and winter at the poles (Smith, 2004). Spectroscopic remote sensing by Villanueva et al. (2015) has demonstrated seasonal and regional variability in the deuterium-to‑hydrogen ratio (D/H) in water vapor in the Mars atmosphere, with deuterium-enriched gas evolving off a warming polar cap in spring. The thermal control that we suggest thus is not unique to carbon dioxide in the Mars surface environment. As a model for the thermal correlation that we observe, we postulate that CO2 molecules are trapped onto regolith grains during the cold Martian night (~170–200 K, Smith, 2004), preferentially capturing the heavier isotopologues by surface adsorption (e.g., Rahn and Eiler, 2001), in water frosts as clathrate hydrates, or by some other mechanism. Sunlight warming the cold regolith after dawn thermally desorbs the heavy-isotope enriched population, increasing the heavyisotope enrichment in the near-surface atmosphere as trapped gas volatilizes and enters the optical path between Earth and the Martian surface. The isotopologue species do not need to be well mixed to be detectable spectroscopically, they only need to be a significant fraction of the total column abundance in the line of sight. Although we observe a substantial variation in the 628 CO2 fraction that correlates with temperature, the physical mechanism(s) that could quantitatively account for such fractionation are not obvious. Rahn and Eiler (2001) investigate CO2 adsorption on Mars-relevant mineral surfaces and find that adsorption depletes isotopically heavy CO2 to a modest extent. A large fraction of the total atmospheric abundance of CO2 would have to be trapped onto the surface at night in order that modest differential adsorption could substantially change isotope ratios in the residual atmosphere. Eiler et al. (2000) show that CO2 frost formation, as opposed to surface adsorption, can deplete 628 CO2 from the residual gas but does not deplete 636 and thus is not consistent with the mass-dependent fractionation that we suggest describes the Phoenix and Curiosity results (Fig. 4). The regolith of Mars is a chemically and topologically complex system that may not be adequately modeled by current laboratory simulations. The 628 fraction that we measure is not likely to be in equilibrium with the surface reservoir at the instantaneous surface temperature. Phase lags associated with surface and subsurface heating earlier in the day, and with finite time for gaseous diffusion, would result in considerable hysteresis. The total column fraction of 18O will decrease monotonically from an afternoon peak until dawn as the heavy isotopologue diffuses toward sinks in the soil. The greatest 18O depletion (negative δ18O) would occur near dawn, while the greatest 18O enrichment (positive δ18O) would occur in the early to middle afternoon, following peak surface temperature (Vasavada et al., 2017). 7. Conclusions We have measured the spectrum of infrared emission from Mars at 10.494 μm wavelength, targeting a lasing transition of normal-isotope (626) carbon dioxide accompanied by a distinct absorption feature due to CO2 substituted with one atom of 18O isotope (628). Measurements were obtained at noon local solar time on Mars and at 13:19 LST. Variations in continuum radiance indicate variability in surface temperature correlated with the Martian local time as well as smaller variations due to surface properties. Each of the measured spectra was modeled using a single atmospheric temperature-pressure profile overlying the ground, and separately fitted parameters for the 628 abundance and for the ground temperature to represent the emergent spectrum. Comparing the retrieved 628 fraction and ground temperature demonstrates a correlation at mid-day on Mars between increasing ground temperature and increasing abundance of CO2 with the heavy isotope 18O. We postulate a cyclic process in which isotopically heavy CO2 is preferentially trapped in or on the cold regolith during the night and thermally desorbed over the day as ground temperature increases.

Acknowledgments The authors would like to thank Dr. Regina Cody (NASA GSFC emeritus) for pointing out the strong temperature dependence in laboratory adsorbents that led to considering plausible mechanisms to interpret the observed data. The authors thank the NASA Infrared Telescope Facility, operated by the University of Hawaii, for hosting 8

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