Computer Science – Sound
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p41b1379h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P41B-1379
Computer Science
Sound
0343 Planetary Atmospheres (5210, 5405, 5704), 0758 Remote Sensing, 0764 Energy Balance, 0768 Thermal Regime, 3300 Atmospheric Processes
Scientific paper
Using radiometric observations from the Mars Climate Sounder (MCS) during southern winter, we report evidence that polar cold spots (Tb < 148 K) at Mars' south pole are correlated with tropospheric, optically thick clouds. In limb observations coincident with nadir measurements of low brightness temperature regions, clouds appear as radiation sources at typical altitudes of 20 - 30 km. Using statistical methods, we confirm that transient cold spots appearing during polar night are correlated with the presence of clouds above the surface, which show spectral features consistent with CO2 ice. Areas where cold spots persist over the entire winter season are also characterized by persistent clouds, the thickness of which is inversely correlated with nadir brightness temperature. We model the Mars south polar night atmosphere with a δ-Eddington radiative transfer code, including spherical geometry for limb and nadir viewing. By forward-modeling a multi-layered atmosphere including clouds, we attempt to reproduce the MCS radiance data. Emission, absorption, and scattering by gas, dust, and ice (CO2 and H2O) clouds are all included in the model. Surface emissivity, as well as cloud optical thickness, composition, and altitude are constrained by comparison of the MCS data with the model output. Theories on Cold Spot Origins: Three primary explanations for "cold spots" have been explored previously: (1) Localized depletion of atmospheric CO2, (2) Non-unit emissivity of surface CO2, and (3) Carbon dioxide clouds. Hess (JGR, 1979) demonstrated that (1) is feasible only under special circumstances due to dynamical instability. The spectral properties of CO2 frost and clouds are similar such that it has been difficult to discriminate between (2) and (3) as viable hypotheses. Our novel approach uses radiance measurements of both the atmospheric limb and nadir to better constrain the origin of polar cold spots. Observations of Cold Spots: The Mars Climate Sounder onboard the Mars Reconnaissance Orbiter records nadir and limb radiance measurements in nine spectral channels, from ~1-40 μm wavelength. The observations used in this study spanned a range of solar longitude, Ls = 111-148°, i.e., southern winter. Radiance is converted to equivalent brightness temperature using the radiometric response of each filter and the Planck function. Observations are binned by latitude and longitude (1-5° bins) and Ls, and nadir brightness temperatures are compared to measured limb brightness temperature profiles. MCS Results: We find a statistical correlation (R ~0.55, p < 10-14) between cold spots and clouds (defined by their high T22). Spectral features in the infrared, specifically absorption at the wings of the 15-μm (667 cm-1) CO2 bending fundamental and a broad feature from 20 - 35 μm (300 - 500 cm-1) attributable to scattering by CO2 ice grains, suggest the clouds are composed of carbon dioxide ice. Modeling: Monochromatic fluxes are calculated using a custom δ-Eddington code for an N- layer atmosphere and convolved to the MCS filter response. Cloud height and composition are varied to produce an emission spectrum comparable to the MCS observations. We forward-model the polar atmosphere iteratively to constrain cloud properties, surface emissivity, and dust content. Our results may have important implications for Mars' polar heat balance and climate.
Hayne Paul
Paige David A.
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