Lifting Dust on Mars by Wind and Light Induced Forces

• Applicant: Professor Dr. Gerhard Wurm
• Subject Area: Astrophysics and Astronomy
• Term: from 2012 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 225970202

• Final Report Year: 2017

Final Report Abstract

Dust is highly abundant on the Martian surface. It gets entrained into the atmosphere during dust storms or in dust devils. However, the Martian atmosphere is rather thin and winds in the category of storms are essential to provide enough force to lift particles. This is supposedly in contradiction to observed lift at lower wind speeds. Curiously though, a low ambient pressure, which leads to reduced dynamic pressure is beneficial for particle lift in other ways. At low pressure the physics of gases changes. Gas can move from cold to warm regions along walls known as thermal creep. A temperature difference applied to a small capillary can therefore act as a gas pump. During the project, we found that the dusty surface of Mars acts in the same way and works like a gas pump. The pore space within the soil just acts like a collection of capillaries. If the top soil is illuminated by the sun, the lower levels stay cool. The gas therefore flows from underground to the top and into the atmosphere. It was a surprise to pin down this mechanism as it suggests that the Martian soil is a giant planet wide gas pump. This effect strongly depends on pressure and therefore is not observed in nature on Earth. Mars is the only planet in the solar system with this effect as he has just the right atmospheric pressure of a few mbar. This finding got into a number of news articles on Mars as giant gas pump (e.g. Spiegel, 02.12.2013). As far as the original problem of dust lifting is concerned, the gas flow induces an overpressure somewhat below the surface which is strong enough to lift particles on its own if the illumination is strong enough. The sun does not shine that intense on Mars but it could be measured in wind tunnel experiments that the illumination of the surface reduces the necessary wind speeds by several tens of percent. It therefore supports particle lift significantly and solves or at least reduces the problem of explaining dust lifting on Mars.

Publications

• Photophoresis on Polydisperse Basalt Microparticles under Microgravity, Journal of Aerosol Science, 76:126-137, 2014
  M. Küpper, C. de Beule, G. Wurm, L. S. Matthews, J. B. Kimery, T, and W. Hyde
  
• Propulsion of Porous Plates in Thin Atmospheres by Temperature Fields, Microgravity Science and Technology, 25:311-318, 2014
  M. Kuepper, C. Duermann, C. de Beule, and G. Wurm
  
• The Martian Soil as a Planetary Gas Pump, Nature Physics, 10:17-20, 2014
  C. de Beule, G. Wurm, T. Kelling, M. Küpper, T. Jankowski, and J. Teiser
  
• Thermal Creep Assisted Dust Lifting on Mars: Wind Tunnel Experiments for the Entrainment Threshold Velocity, Journal of Geophysical Research – Planets, 120:1346-1356, 2015
  M. Küpper and G. Wurm
  
• Amplification of Dust Loading in Martian Dust Devils by Self-Shadowing, Icarus, 274:249-252, 2016
  M. Küpper and G. Wurm
  
• Particle Lifting Processes in Dust Devils, Space Science Reviews, 203:347-376, 2016
  L. D. V. Neakrase, M. R. Balme, F. Esposito, T. Kelling, M. Klose, J. F. Kok, B. Marticorena, J. Merrison, M. Patel, and G. Wurm
  
• Photophoretic Force on Aggregate Grains, Monthly Notice of the Royal Astronomical Society, 455:2582-2591, 2016
  L. S. Matthews, J. B. Kimery, G. Wurm, C. de Beule, M. Küpper, and T. Hyde