Unambiguous Detection Of Mesospheric CO2 Clouds On Mars Using 2.7 μm Absorption Band From The ACS/TGO Solar Occultations

By Keith Cowing
Status Report
June 4, 2024
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Unambiguous Detection Of Mesospheric CO2 Clouds On Mars Using 2.7 μm Absorption Band From The ACS/TGO Solar Occultations
Map of the CO2 clouds detections from the 2.7 μm absorption band from the ACS instrument. Top: Seasonal and latitudinal distribution of solar occultation observations. Circles and triangles correspond to MIR and TIRVIM observations. No CO2 clouds detection is shown in grey, detection at the morning (evening) terminator is in blue (red). Bottom: Latitudinallongitudinal map of the CO2 clouds detections from the ACS instrument with the solar longitude given by the color code. Squares and triangles represent detections made at the morning and at the evening terminators, respectively. Contour lines represent the Mars Orbiter Laser Altimeter (MOLA) altimetry data. Black lines correspond to negative values taken with 1 km step, red lines — positive values with 3 km step. — astro-ph.EP

Mesospheric CO2 clouds are one of two types of carbon dioxide clouds known on Mars. We present observations of mesospheric CO2 clouds made by Atmospheric Chemistry Suit (ACS) onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO).

We analyzed 1663 solar occultation sessions of Thermal InfraRed (TIRVIM) and Middle InfraRed (MIR) channels of ACS covering more than two Martian years that contain spectra of 2.7 μm carbon dioxide ice absorption band. That allowed us to unambiguously discriminate carbon dioxide ice aerosols from mineral dust and water ice aerosols, not relying on the information of atmospheric thermal conditions.

CO2 clouds were detected in eleven solar occultation observations at altitudes from 39 km to 90 km. In five cases, there were two or three layers of CO2 clouds that were vertically separated by 5-15 km gaps. Effective radius of CO2 aerosol particles is in the range of 0.1−2.2 μm.

Spectra produced by the smallest particles indicate a need for a better resolved CO2 ice refractive index. Nadir optical depth of CO2 clouds is in the range 5×10-4 –4×10-2 at both 2.7 μm and 0.8 μm. Asymmetrical diurnal distribution of detections observed by ACS is potentially due to local time variations of temperature induced by thermal tides.

Two out of five cases of carbon dioxide cloud detections made by the TIRVIM instrument reveal the simultaneous presence of CO2 ice and H2O ice aerosols.

Temperature profiles measured by the Near InfraRed (NIR) channel of ACS are used to calculate CO2 saturation ratio S at locations of carbon dioxide clouds. Supersaturation S > 1 is detected in only 5 out of 19 cases of CO2 cloud layers; extremely low values of S < 0.1 are found in 9 out of 19 cases.

Illustration of an alternative aerosol extinction model for the ingress occultation #14501. Panel a: A standard for solar occultation spherically symmetric model. Three CO2 cloud layers A, B and C (shown as orange frames) are located at different altitudes and are uniform in latitude and longitude. Panel b: vertical profiles of aerosol extinction for standard model (black), alternative model (red) and layers II (blue) and III (green) of the alternative model. Panel c: Alternative model. Three CO2 cloud layers I, II, and III (shown as orange frames) are located at the same altitudes, but different latitudes/longitudes. — astro-ph.EP

M. Luginin, A. Trokhimovskiy, A. Fedorova, D. Belyaev, N. Ignatiev, O. Korablev, F. Montmessin, A. Grigoriev

Comments: 29 pages, 10 figures. Submitted to Icarus
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2406.01515 [astro-ph.EP] (or arXiv:2406.01515v1 [astro-ph.EP] for this version)
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Submission history
From: Mikhail Luginin
[v1] Mon, 3 Jun 2024 16:46:43 UTC (3,694 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻