A parametric study of Io鈥檚 thermophysical surface properties and subsequent numerical atmospheric simulations based on the best fit parameters

详细信息	查看全文 | 推荐本文 |

• 作者：Andrew C. Walker^a ; ^{andrew.walker@mail.utexas.edu} ; Chris H. Moore^c ; David B. Goldstein^a ; Philip L. Varghese^a ; Laurence M. Trafton^b
• 关键词：Io ; Atmospheres ; Dynamics ; Atmospheres ; Structure ; Jupiter ; Satellites ; Satellites ; Atmospheres
• 刊名：Icarus
• 出版年：2012
• 期刊代码：90_00191035
• 类别：ph
• 出版时间：July, 2012
• 卷：220
• 期：1
• 页码：225-253
• 文件大小：8323 K

摘要

Io鈥檚 sublimation atmosphere is inextricably linked to the SO₂ surface frost temperature distribution which is poorly constrained by observations. We constrain Io鈥檚 surface thermal distribution by a parametric study of its thermophysical properties in an attempt to better model the morphology of Io鈥檚 sublimation atmosphere. Io鈥檚 surface thermal distribution is represented by three thermal units: sulfur dioxide (SO₂) frosts/ices, non-frosts (probably sulfur allotropes and/or pyroclastic dusts), and hot spots. The hot spots included in our thermal model are static high temperature surfaces with areas and temperatures based on Keck infrared observations. Elsewhere, over frosts and non-frosts, our thermal model solves the one-dimensional heat conduction equation in depth into Io鈥檚 surface and includes the effects of eclipse by Jupiter, radiation from Jupiter, and latent heat of sublimation and condensation. The best fit parameters for the SO₂ frost and non-frost units are found by using a least-squares method and fitting to observations of the Hubble Space Telescope鈥檚 Space Telescope Imaging Spectrograph (HST STIS) mid- to near-UV reflectance spectra and Galileo PPR brightness temperature. The thermophysical parameters are the frost Bond albedo, 伪_F, and thermal inertia, 螕_F, as well as the non-frost surface Bond albedo, 伪_NF, and thermal inertia, 螕_NF. The best fit parameters are found to be 伪_F 鈮?#xA0;0.55 卤 0.02 and 螕_F 鈮?#xA0;200 卤 50 J m^鈭? K^鈭? s^鈭?/2 for the SO₂ frost surface and 伪_NF 鈮?#xA0;0.49 卤 0.02 and 螕_NF 鈮?#xA0;20 卤 10 J m^鈭? K^鈭? s^鈭?/2 for the non-frost surface.

These surface thermophysical parameters are then used as boundary conditions in global atmospheric simulations of Io鈥檚 sublimation-driven atmosphere using the direct simulation Monte Carlo (DSMC) method. These simulations are unsteady, three-dimensional, parallelized across 360 processors, and include the following physical effects: inhomogeneous surface frosts, plasma heating, and a temperature-dependent residence time on the non-frost surface. The DSMC simulations show that the sub-jovian hemisphere is significantly affected by the daily solar eclipse. The simulated SO₂ surface frost temperature is found to drop only 鈭? K during eclipse due to the high thermal inertia of SO₂ surface frosts but the SO₂ gas column density falls by a factor of 20 compared to the pre-eclipse column due to the exponential dependence of the SO₂ vapor pressure on the SO₂ surface frost temperature. Supersonic winds exist prior to eclipse but become subsonic during eclipse because the collapse of the atmosphere significantly decreases the day-to-night pressure gradient that drives the winds. Prior to eclipse, the supersonic winds condense on and near the cold nightside and form a highly non-equilibrium oblique shock near the dawn terminator. In eclipse, no shock exists since the gas is subsonic and the shock only reestablishes itself an hour or more after egress from eclipse. Furthermore, the excess gas that condenses on the non-frost surface during eclipse leads to an enhancement of the atmosphere near dawn. The dawn atmospheric enhancement drives winds that oppose those that are driven away from the peak pressure region above the warmest area of the SO₂ frost surface. These opposing winds meet and are collisional enough to form stagnation point flow.

The simulations are compared to Lyman-伪 observations in an attempt to explain the asymmetry between the dayside atmospheres of the anti-jovian and sub-jovian hemispheres. Lyman-伪 observations indicate that the anti-jovian hemisphere has higher column densities than the sub-jovian hemisphere and also has a larger latitudinal extent. A composite 鈥渁verage dayside atmosphere鈥?is formed from a collisionless simulation of Io鈥檚 atmosphere throughout an entire orbit. This composite 鈥渁verage dayside鈥?atmosphere without the effect of global winds indicates that the sub-jovian hemisphere has lower average column densities than the anti-jovian hemisphere (with the strongest effect at the sub-jovian point) due primarily to the diurnally averaged effect of eclipse. This is in qualitative agreement with the sub-jovian/anti-jovian asymmetry in the Lyman-伪 observations which were alternatively explained by the bias of volcanic centers on the anti-jovian hemisphere. Lastly, the column densities in the simulated average dayside atmosphere agree with those inferred from Lyman-伪 observations despite the thermophysical parameters being constrained by mid- to near UV observations which show much higher instantaneous SO₂ gas column densities. This may resolve the apparent discrepancy between the lower 鈥渁verage dayside鈥?column densities observed in the Lyman-伪 and the higher instantaneous column densities observed in the mid- to near UV.