New near-IR observations of mesospheric CO₂and H₂O clouds on Mars

Abstract

[1] Carbon dioxide clouds, which are speculated by models on solar and extra-solar planets, have been recently observed near the equator of Mars. The most comprehensive identification of Martian CO 2 ice clouds has been obtained by the near-IR imaging spectrometer OMEGA. CRISM, a similar instrument with a higher spatial resolution, cannot detect these clouds with the same method due to its shorter wavelength range. Here we present a new method to detect CO 2 clouds using near-IR data based on the comparison of H 2 O and CO 2 ice spectral properties. The spatial and seasonal distributions of 54 CRISM observations containing CO 2 clouds are reported, in addition to 17 new OMEGA observations. CRISM CO 2 clouds are characterized by grain size in the 0.5-2 mm range and optical depths lower than 0.3. The distributions of CO 2 clouds inferred from OMEGA and CRISM are consistent with each other and match at first order the distribution of high altitude (>60 km) clouds derived from previous studies. At second order, discrepancies are observed. We report the identification of H 2 O clouds extending up to 80 km altitude, which could explain part of these discrepancies: both CO 2 and H 2 O clouds can exist at high, mesospheric altitudes. CRISM observations of afternoon CO 2 clouds display morphologies resembling terrestrial cirrus, which generalizes a previous result to the whole equatorial clouds season. Finally, we show that morning OMEGA observations have been previously misinterpreted as evidence for cumuliform, and hence potentially convective, CO 2 clouds.

Martian Water Ice Clouds During the 2018 Global Dust Storm as Observed by the ACS‐MIR Channel Onboard the Trace Gas Orbiter

“…Figure 11 shows the vertical profiles of at 3.4 m (i.e., the opacity per km). As 3.4 m is on the edge of the 3‐ m water ice band (Vincendon et al, ), it provides an estimator of the global atmospheric opacity, whereas the opacity at 3.2 m is strongly increased by the presence of water ice clouds. We observe that the water ice cloud opacity at 3.2 m (vertically integrated along the cloud) typically goes from 0.01 to 0.05 before the GDS but falls to a few for the mesospheric clouds during the GDS, with around 90 km (and ).…”

“…These high‐altitude water ice clouds are observed only at the beginning of the GDS. High‐altitude water ice clouds can also be seen during non‐dust GDS years, when they occur during the storm season (Clancy et al, ; Vincendon et al, ). However, high‐cloud altitudes are nearly systematic in our observations during the peak phase of the GDS (cf.…”

The vapor pressure over nano-crystalline ice

“…For instance, water ice clouds are frequently observed in the middle atmosphere of Mars (Guzewich et al, 2013;Vincendon et al, 2011) with temperatures commonly falling below 160 K (Maltagliati et al, 2011). In the terrestrial atmosphere, noctilucent clouds form at the high-latitude summer mesopause (Rapp and Lübken, 2004) with temperatures falling to 120 K on average (Lübken et al, 2009) with extremes down to 100 K (Rapp et al, 2002).…”

Mars’ water vapor mapping by the SPICAM IR spectrometer: Five martian years of observations

“…The number of discrete ordinates and Legendre coefficients has been chosen as a compromise between calculation time and accuracy of the model (8 for zenith angles and 16 for azimuth angles). The optically thin CO 2 clouds are formed in the cold mesospheric conditions mostly at low latitudes near the northern spring and summer (Vincendon et al, 2011). The observed optical depth of the clouds is below 0.2 and they were not included in our radiative-transfer model.…”