Stratigraphic and Isotopic Evolution of the Martian Polar Caps From Paleo‐Climate Models. Issue 3 (21st March 2022)

• Record Type: Journal Article
• Authors: Vos, E.; Aharonson, O.; Schörghofer, N.; Forget, F.; Millour, E.; Rossi, L.; Vals, M.; Montmessin, F.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-21
• Subjects: Mars -- atmosphere -- polar caps -- global climate model -- ice -- stratigraphy; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2021JE007115
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: Exposed scarps images and ice‐penetrating radar measurements in the North Polar Layered Deposits (NPLD) of Mars show alternating layers that provide an archive of past climate oscillations, that are thought to be linked to orbital variations, akin to Milankovitch cycles on Earth. We use the Laboratoire de Météorologie Dynamique Martian Global Climate Model to study paleoclimate states to enable a better interpretation of the NPLD physical and chemical stratigraphy. When a tropical ice reservoir is present, water vapor transport from the tropics to the poles at low obliquity is modulated by the intensity of summer. At times of low and relatively constant obliquity, the flux still varies due to other orbital elements, promoting polar layer formation. Ice migrates from the tropics toward the poles in two stages. First, when surface ice is present in the tropics, and second, when the equatorial deposit is exhausted, from ice that was previously deposited in mid‐high latitudes. The polar accumulation rate is significantly higher when tropical ice is available, forming thicker layers per orbital cycle. However, the majority of the NPLD is sourced from ice that temporary resided in the mid‐high latitudes and the layers become thinner as the source location moves poleward. The migration stages imprint different D/H ratios in different sections in the PLDs. The NPLD is isotopically depleted compared to the South Polar Layered Deposits in all simulations. Thus we predict the D/H ratio of the atmosphere in contact with NPLD upper layers is biased relative to the average global ice reservoirs. Plain Language Summary: In this work we run simulations of a Global Climate Model for Mars with a broad range of orbital elements, and ice initially placed in the tropics, we calculate the growth rate and hydrogen isotopic composition of the polar caps. These simulations help to understand the migration of ice from the tropical region to the polar caps, as believed to have occurred in the recent past. Ice transport to the poles occurs at two stages. The first is when ice is present in the tropics. The second is after the tropical reservoir has been exhausted, and the source of vapor that reaches the poles is from ice accumulated in mid‐high latitudes during the first stage. The polar caps growth rate during the first stage is larger and results in thicker layers. We find that both physical and chemical records are expected in the polar caps, controlled by the orbital elements and the surface ice distribution. The chemical record also depends on the source enrichment. These results should help to interpret ice records in order to decode past climate variations, and suggest the current hydrogen isotopic composition of the atmosphere is not representative of the total ice reservoirs on Mars. Key Points: The North Polar Layered Deposits growth rate strongly depends on perihelion position, in late stages accumulation can be of order a meter over a precession cycle The isotopic stratigraphic signal in the polar caps experiences secular evolution, in addition to oscillations due to orbital elements Northern ice deposits are depleted in deuterium compared to the south, biasing the isotopic composition of the present‐day atmosphere