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Spatial variability, composition and thickness of the seasonal north polar cap of Mars in mid-spring

Giuranna, M. and Hansen, G. and Formisano, V. and Zasova, L. and Maturilli, A. and Grassi, D. and Ignatiev, N. (2007) Spatial variability, composition and thickness of the seasonal north polar cap of Mars in mid-spring. Planetary and Space Science, 55 (10), pp. 1328-1345. Elsevier. DOI: 10.1016/j.pss.2007.03.006

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Abstract

The planetary fourier spectrometer (PFS) for the Mars express mission (MEX) is an infrared spectrometer operating in the wavelength range from 1.2 to 45 μm by means of two spectral channels, called SWC (short wavelength channel) and LWC (long wavelength channel), covering, respectively, 1.2–5.5 and 5.5–45 μm. The middle-spring Martian north polar cap (Lsnot, vert, similar40°) has been observed by PFS/MEX in illuminated conditions during orbit 452. The SWC spectra are here used to study the cap composition in terms of CO<sub>2</sub> ice, H<sub>2</sub>O ice and dust content. Significant spectral variation is noted in the cap interior, and regions of varying CO<sub>2</sub> ice grain sizes, water frost abundance, CO<sub>2</sub> ice cover and dust contamination can be distinguished. In addition, we correlate the infrared spectra with an image acquired during the same orbit by the OMEGA imaging spectrometer and with the altimetry from MOLA data. Many of the spectra variations correlate with heterogeneities noted in the image, although significant spectral variations are not discernible in the visible. The data have been divided into five regions with different latitude ranges and strong similarities in the spectra, and then averaged. Bi-directional reflectance models have been run with the appropriate lighting geometry and used to fit the observed data, allowing for CO<sub>2</sub> ice and H<sub>2</sub>O ice grain sizes, dust and H<sub>2</sub>O ice contaminations in the form of intimate granular mixtures and spatial mixtures. A wide annulus of dusty water ice surrounds the recessing CO<sub>2</sub> seasonal cap. The inner cap exhibits a layered structure with a thin CO<sub>2</sub> layer with varying concentrations of dark dust, on top of an H<sub>2</sub>O ice underneath ground. In the best-fits, the ices beneath the top layer have been considered as spatial mixtures. The results are still very good everywhere in the spectral range, except where the CO<sub>2</sub> ice absorption coefficients are such that even a thin layer is enough to totally absorb the incoming radiation (i.e. the band is saturated). This only happens around 3800 cm<sup>−1</sup>, inside the strong 2.7-μm CO<sub>2</sub> ice absorption band. The effect of finite snow depth has been investigated through a layered albedo model. The thickness of the CO<sub>2</sub> ice deposits increases with latitude, ranging from 0.5–1 g cm<sup>−2</sup> within region II to 60–80 g cm<sup>−2</sup> within the highest-latitude (up to 84°N) region V. Region I is at the cap edge and extends from 65°N to 72°N latitude. No CO<sub>2</sub> ice is present in this region, which consists of relatively large grains of water ice (20 μm), highly contaminated by dust (0.15 wt%). The adjacent region II is a narrow region [76–79°N] right at the edge of the north residual polar cap. This region is very distinct in the OMEGA image, where it appears to surround the whole residual cap. The CO<sub>2</sub> ice features are barely visible in these spectra, except for the strong saturated 2.7 μm band. It basically consists of a thin layer of 5mm CO<sub>2</sub> ice on top of an H<sub>2</sub>O ice layer with the same composition as region I. A third interesting region III is found all along the shoulder of the residual cap [79–81°N]. It extends over 1.5 km in altitude and over only 2° of latitude and consists of CO<sub>2</sub> ice with a large dust content. It is an admixture of CO<sub>2</sub> ice (3–4 mm), with several tens of ppm by mass of water ice and more than 2 ppt by mass of dust. The surface temperatures have been retrieved from the LWC spectra for each observation. We found an increase in the surface temperature in this region, indicating a spatial mixture of cold CO<sub>2</sub> ice and warmer dust/H<sub>2</sub>O ice. Region IV is close to the top of the residual cap [81–84°N]; it is much brighter than region III, with a dust content 10 times lower than the latter. The CO<sub>2</sub> grain size is 3 mm and strong CO<sub>2</sub> ice features are present in the data, indicating a thicker CO<sub>2</sub> ice layer than in region II (1–2 g cm<sup>−2</sup>). The final region V is right at the top of the residual cap (greater-or-equal, slanted 84°N). It is “pure” CO<sub>2</sub> ice (no dust) of 5 mm grain sizes, with 30 ppm by weight of water ice. The CO<sub>2</sub> ice features are very pronounced and the 2.7 μm band is saturated. The optical thickness is close to the semi-infinite limit (30–40 g cm<sup>−2</sup>). Assuming a snowpack density of 0.5 g cm<sup>−3</sup>, we get a minimum thickness of 1–2 cm for the top-layer of regions II and III, 4–10 cm for region IV, and greater-or-equal, slanted60–80 cm thickness for region V. These values are in close agreement with several recent results for the south seasonal polar cap.

Item URL in elib:https://elib.dlr.de/52480/
Document Type:Article
Title:Spatial variability, composition and thickness of the seasonal north polar cap of Mars in mid-spring
Authors:
AuthorsInstitution or Email of AuthorsAuthors ORCID iD
Giuranna, M.Istituto di Fisica dello Spazio Interplanetario CNR (IFSI), Roma, Italy and CISAS, University of Padova, ItalyUNSPECIFIED
Hansen, G.Department of Earth and Space Science, University of Washington, Seattle, WA 98195, USAUNSPECIFIED
Formisano, V.Istituto di Fisica dello Spazio Interplanetario CNR (IFSI), Roma, ItalyUNSPECIFIED
Zasova, L.Space Research Institute of Russian Academy of Sciences (IKI), Moscow, Russia and Istituto di Fisica dello Spazio Interplanetario CNR (IFSI), Roma, ItalyUNSPECIFIED
Maturilli, A.UNSPECIFIEDUNSPECIFIED
Grassi, D.Istituto di Fisica dello Spazio Interplanetario CNR (IFSI), Roma, ItalyUNSPECIFIED
Ignatiev, N.Space Research Institute of Russian Academy of Sciences (IKI), Moscow, Russia and Istituto di Fisica dello Spazio Interplanetario CNR (IFSI), Roma, ItalyUNSPECIFIED
Date:1 April 2007
Journal or Publication Title:Planetary and Space Science
Refereed publication:Yes
Open Access:No
Gold Open Access:No
In SCOPUS:No
In ISI Web of Science:Yes
Volume:55
DOI :10.1016/j.pss.2007.03.006
Page Range:pp. 1328-1345
Publisher:Elsevier
Status:Published
Keywords:Mars; North polar cap; Composition; Thickness
HGF - Research field:Aeronautics, Space and Transport (old)
HGF - Program:Space (old)
HGF - Program Themes:W EW - Erforschung des Weltraums
DLR - Research area:Space
DLR - Program:W EW - Erforschung des Weltraums
DLR - Research theme (Project):W - Vorhaben Vergleichende Planetologie (old)
Location: Berlin-Adlershof
Institutes and Institutions:Institute of Planetary Research
Deposited By: Musiol, Stefanie
Deposited On:15 Jan 2008
Last Modified:27 Apr 2009 14:36

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