Coevolution of Mars’s atmosphere and massive south polar CO2 ice deposit

Abstract

A massive CO2 ice deposit overlies1 part of Mars’s primarily H2O ice2,3,4 south polar cap5. This deposit rivals the mass of Mars’s current, 96% CO2, atmosphere6. Its release could substantially alter Mars’s pressure and climate1. The deposit consists of alternating CO2 and H2O ice layers to a depth of up to approximately 1 km (refs. 1,7,8). The top layer is an enigmatic9,10,11 1–10 m covering of perennial surface CO2 ice12 called the residual south polar cap. Typical explanations of the layering invoke orbital cycles1,7. Up to now, models assumed that the H2O ice layers insulate and seal in the CO2, allowing it to survive high-obliquity periods7,13. However, these models do not quantitatively predict the deposit’s stratigraphy or explain the residual south polar cap’s existence. Here we present a model in which the deposit’s near-surface CO2 can instead exchange with the atmosphere through permeable H2O ice layers. Using currently observed albedo14,15 and emissivity16 properties of the Martian polar CO2 ice deposits, our model predicts that the present massive CO2 ice deposit is a remnant of larger CO2 ice deposits laid down during periods of decreasing obliquity that are ablated, liberating a residual lag layer of H2O ice, when obliquity increases. Fractions of previous CO2 deposits remain as layers because the amplitudes of the obliquity maxima have been mostly decreasing during the past ~510 kyr (ref. 17). Our model simultaneously explains the observed massive CO2 ice deposit stratigraphy, the residual south polar cap’s existence and the presence of a massive CO2 ice deposit only in the south. We use our model to calculate Mars’s pressure history and determine that the massive CO2 ice deposit is 510 kyr old.

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Fig. 1: Overview of geologic context and representative model outputs.
Fig. 2: Schematic of the onset of lag layer formation during increasing mean annual absorbed polar insolation, labelled by southern season.
Fig. 3: Model input and results.
Fig. 4: Long-term model-predicted pressure evolution.

Data availability

The radar and image datasets that support the findings of this study are publicly available from the NASA Planetary Data System (https://pds.nasa.gov). Source data for Fig. 3 are provided with the paper.

Code availability

The code that produces the figures and numerical results stated in the text is available from the corresponding author on reasonable request.

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Acknowledgements

P.B.B. was supported by NASA Earth and Space Sciences Fellowship NNX16AP38H and the NASA Postdoctoral Program. Early work by B.L.E., P.B.B. and A.P.I. was partially supported by the NASA Mars Fundamental Research grant NNX14AG54G to B.L.E. and C. Pilorget. Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Government support acknowledged.

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Contributions

P.B.B. conceived of the study, performed the numerical modelling and data analysis, and wrote the paper. Substantial discussions with A.P.I., S.P., B.L.E. and P.O.H. refined the results of the study and the manuscript presentation.

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Correspondence to P. B. Buhler.

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Peer review information Nature Astronomy thanks Isaac Smith and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Energy balance model schematic and model output comparison to data.

Left. Schematic of the energy balance model (Equation 1). Right. One-year model outputs with incoming flux (orange), outgoing flux (red dashed), and modelled accumulated CO2 (purple line), compared to observed accumulated CO2 (purple points; Kelly et al. (2006)43). LS is solar longitude. Apparent line-width of incoming flux is due to daily insolation oscillations, which are not resolved at plot resolution. The small fluctuations in the incoming flux curve are due to seasonal variations in the atmospheric opacity (Extended Data Fig. 3). Model runs for current orbit (obliquity = 25.19°, eccentricity = 0. 0934, longitude of perihelion = 251°). H2O layer thickness = 15 m. A. Latitude = 89.5°S, pressure = 1.0 × Peq,0, elevation = 4750 m, \({\it{\epsilon }}_{CO_2}\)= 0.8. B. Same, but with pressure = 0.95 × Peq,0, \({\it{\epsilon }}_{H_2O}\) = 1.0, \(A_{H_2O}\) = 0.4. C. Latitude = 89.5° N, pressure = 1.00 × Peq,0, elevation = −2000 m, \({\it{\epsilon }}_{CO_2}\)= 0.485, \({\it{\epsilon }}_{H_2O}\) = 0.55, \(A_{H_2O}\) = 0.4.

Extended Data Fig. 2 Examples of metre- to 10-metre-scale polygonal patterning (white arrows) on the H2O ice layer overlying the MCID, adjacent to and beneath mesas of RSPC CO2 (black arrows).

Locations of panels are indicated in Extended Data Fig. 5A. HiRISE images A. ESP_014179_0930 B. ESP_058543_0930 C. PSP_003716_0930 D. ESP_058527_0940 E. ESP_058896_0910 F. ESP_058119_0940 G. ESP_041120_0920 H. ESP_014182_0960.

Extended Data Fig. 3

Opacity data from Montabone et al. (2015) from 87–90° S, -90-0° E from Mars Years 27–33, with mean (black) and median (grey) values.

Extended Data Fig. 4 Effect of varying obliquity on absorbed insolation due to albedo dependence on insolation.

(Blue) Total annual incident insolation at 89.5°S normalized to maximum annual incident insolation (at 90° obliquity); (Orange) total absorbed insolation, normalized to maximum annual absorbed insolation (at 36° obliquity); (Green) equivalent mean annual albedo. The equivalent mean annual albedo is calculated from one minus the ratio of the sum of the absorbed insolation at each 30-minute time step throughout one Mars year divided by the sum of the incident insolation at each 30-minute time step throughout one Mars year. At each time step the albedo is calculated from Equation 4 at 89.5° S and zero eccentricity. Nonzero eccentricities slightly modify the obliquity at which peak annual absorbed insolation occurs and are taken into account when we calculate the instantaneous albedo at each time step in our model.

Extended Data Fig. 5 Regional context of textures in topmost H2O ice layer, timing of seasonal exposure of topmost H2O ice layer, and topmost H2O ice layer seasonal thermal profile.

Top Left. CTX mosaic context for top right panel (black rectangle) and panels in Extended Data Fig. 2 (blue dots labelled A–H). Top Right. Regions of dark H2O layer are exposed by LS 297 (HiRISE image ESP_013775_0931). The regions correspond to kilometre-scale depressions in the H2O layer1. Bottom panels show time progression of modelled thermal profile for a 20 m thick H2O layer overlying a semi-infinite CO2 layer that becomes exposed at LS 301. The basal temperature of the H2O reaches the CO2 sublimation temperature between LS 335 and LS 183 the following year. LS 300: last day of seasonal CO2 coverage. LS 301: first LS bare H2O is exposed. LS 316: maximum surface temperature reached. LS 335: basal temperature of H2O layer reaches CO2 sublimation temperature. LS 355: surface CO2 condenses. LS 90: winter solstice; thermal wave decaying. LS 183: last LS basal temperature of H2O layer is at CO2 sublimation temperature. LS 270: summer solstice; basal temperature of H2O layer ~1 K below CO2 sublimation temperature.

Source data

Source Data Fig. 3

Model pressure history for Fig. 3b.

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Buhler, P.B., Ingersoll, A.P., Piqueux, S. et al. Coevolution of Mars’s atmosphere and massive south polar CO2 ice deposit. Nat Astron 4, 364–371 (2020). https://doi.org/10.1038/s41550-019-0976-8

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