低于预期的火星压力及以下的跳跃阈值
A lower-than-expected saltation threshold at Martian pressure and below
摘要：
在火星和其他地外环境中也观察到风成沉积物的迁移，产生与地球上一样的涟漪和沙丘。寻找行星床形的陆地类似物，以及能够在行星条件下再现其形成的环境模拟实验，是质疑我们对不寻常环境条件的地貌过程的理解的有力途径。在这里，我们进行沉积物运输实验室实验中放置在一个真空室中的闭路风洞，并在极低的压力下运行，以表明火星的条件属于以前未探索的跃移制度。起动跃移所需的临界风速仅由最先进的模型定量预测，直到谷物和空气之间的密度比为$4 \times 10^5$，但对于更高的密度比，却意外地下降到低得多的值。与此相反，冲击波纹，其出现是连续观察到的颗粒床在整个压力范围内的调查，显示的特征波长和传播速度基本上独立于压力。这些研究结果与现有模型的比较表明，在低雷诺数，但高颗粒与流体的密度比的泥沙输运可能占主导地位的集体效应与颗粒碰撞层中的颗粒惯性。
英文摘要：
Aeolian sediment transport is observed to occur on Mars as well as other extraterrestrial environments, generating ripples and dunes as on Earth. The search for terrestrial analogues of planetary bedforms, as well as environmental simulation experiments able to reproduce their formation in planetary conditions, are powerful ways to question our understanding of geomorphological processes towards unusual environmental conditions. Here, we perform sediment transport laboratory experiments in a closed-circuit wind tunnel placed in a vacuum chamber and operated at extremely low pressures to show that Martian conditions belong to a previously unexplored saltation regime. The threshold wind speed required to initiate saltation is only quantitatively predicted by state-of-the art models up to a density ratio between grain and air of $4 \times 10^5$, but unexpectedly falls to much lower values for higher density ratios. In contrast, impact ripples, whose emergence is continuously observed on the granular bed over the whole pressure range investigated, display a characteristic wavelength and propagation velocity essentially independent of pressure. A comparison of these findings with existing models suggests that sediment transport at low Reynolds number but high grain-to-fluid density ratio may be dominated by collective effects associated with grain inertia in the granular collisional layer.
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