Astrogeography English>Chinese
Thread poster: Phil Hand

Phil Hand  Identity Verified
China
Local time: 06:42
Chinese to English
Jan 6, 2015

http://www.nature.com/nature/journal/v517/n7532/full/nature14088.html

Higher-than-predicted saltation threshold wind speeds on Titan

Titan, the largest satellite of Saturn, exhibits extensive aeolian, that is, wind-formed, dunes1, 2, features previously identified exclusively on Earth, Mars and Venus. Wind tunnel data collected under ambient and planetary-analogue conditions inform our models of aeolian processes on the terrestrial planets3, 4. However, the accuracy of these widely used formulations in predicting the threshold wind speeds required to move sand by saltation, or by short bounces, has not been tested under conditions relevant for non-terrestrial planets. Here we derive saltation threshold wind speeds under the thick-atmosphere, low-gravity and low-sediment-density conditions on Titan, using a high-pressure wind tunnel5 refurbished to simulate the appropriate kinematic viscosity for the near-surface atmosphere of Titan. The experimentally derived saltation threshold wind speeds are higher than those predicted by models based on terrestrial-analogue experiments6, 7, indicating the limitations of these models for such extreme conditions. The models can be reconciled with the experimental results by inclusion of the extremely low ratio of particle density to fluid density8 on Titan. Whereas the density ratio term enables accurate modelling of aeolian entrainment in thick atmospheres, such as those inferred for some extrasolar planets, our results also indicate that for environments with high density ratios, such as in jets on icy satellites or in tenuous atmospheres or exospheres, the correction for low-density-ratio conditions is not required.

土卫六气层中启动风沙跃移运动的最低风速高于模型预期

在土星的最大卫星——土卫六(泰坦)的地面上已确认存在着大量的原本专属地球、火星、水星三个行星的风成沙丘。对于类地行星的风沙过程,以往的研究者已在地球和模仿外星等多种条件下进行风洞试验,根据试验结果形成风沙过程模拟模型,并加以广泛应用。虽然根据模拟模型可计算推动沙粒跃移运动的最低风速,但模拟模型在非类地行星的环境下能否保持准确尚未验证。本文在一台经改造的高压风洞里模仿土卫六近地气层的运动黏度条件下做实验,进而得出能在土卫六的气体浓、引力低、沉淀物密度低的环境中产生风沙跃移运动的最低风速。与原有的基于类地行星环境实验的模拟模型相比,这次实验结果得出的风沙跃移运动最低风速更快,说明原有模拟模型不宜应用于像土卫六的极端环境。为了让原有模拟模型能够符合实验结果,可以把土卫六特别低的沙粒密度/流体密度比值纳入模拟模型中。引用沙粒密度/流体密度比值变量使得原有模拟模型对沙粒启动的描述能准确地反映浓气层行星环境中的实际情况,包括部分太阳系外行星,但对于多冰卫星、稀气层或散逸层等沙粒密度/流体密度比值高的环境,本次实验结果显示,不必对原有模拟模型加以沙粒密度/流体密度比值底相关的调整。

Any comments welcome. I do these just for fun, so let me know what you think.


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