科研之友
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摘要
Recent observations of the Martian hydrogen corona in the UV H Ly-alpha emission by the Hubble Space Telescope (HST) (Clarke et al., 2014) and the SPICAM UV spectrometer on Mars Express (Chaffin et al., 2014) reported its rapid change by an order of magnitude over a short few months period in 2007 (MY28), which is inconsistent with the existing models. One proposed explanation of the observed increase of the coronal emission is that during the global dust storm water vapor from the lower atmosphere can be transported to higher altitudes, where its photodissociation rate by near-UV sunlight increases, providing an additional source of hydrogen for the upper atmosphere.& para;& para;In this work we study the water vapor vertical distribution in the middle atmosphere of Mars during the 2007 global dust storm based on solar occultation measurements by the SPICAM IR spectrometer onboard the Mars-Express spacecraft. The vertical profiles of H2O density and mixing ratio have been obtained for solar longitudes Ls = 255 degrees-300 degrees in MY28. In the Northern hemisphere from Ls = 268 degrees to Ls = 285 degrees the H2O density at altitudes of 60-80 km increased by an order of magnitude. During the dust storm the profiles extended up to 80 km, with an H2O density exceeding 10(10) molecules/cm(3) (mixing ratio >= 200 ppm). Two maxima of the H2O density were detected. The largest H2O densities observed at latitudes higher than 60 degrees N, over Ls = 269 degrees-275 degrees, do not directly correlate with the aerosol loading and likely relate to the downwelling branch of the meridional circulation that was intensified during the dust storm, and transported water from the Southern hemisphere to high northern latitudes. The second smaller maximum coincides with the high dust loading at middle northern latitudes. The comparison with geographically close observations in the quiet Mars year MY32, when the H2O content in the Northern hemisphere did not exceed 2 x 10(10) molecules/cm(3) and 50 ppm at 60 km, showed that the global dust storm was a unique event. The situation was different in the Southern hemisphere. During the dust storm the water density at 50-80 km increased by a factor of 4-5 with a mixing ratio >100 ppm, well correlated with the aerosol vertical extension. A somewhat weaker increase of the H2O density by a factor of 2-3 with a mixing ratio >100 ppm was also observed during MY32 starting from Ls = 260 degrees, suggesting a seasonal repeatability.& para;& para;The observed amount of water at high altitudes in both hemispheres can produce a large increase in the H escape rate on a timescale of weeks, as was shown in the photochemical modeling by Chaffin et al. (2017). Future modeling would be necessary to separate the seasonal and the dust storm contributions to the hydrogen escape.
- 出版日期2018-1-15
