The Red Planet of Dust
by John MooresJuly 08, 2005 -
I hope to use this space to share with you some of the aspects of Mars Research. In that spirit today I'm going to talk a little bit about dust and soil on Mars and how scientists simulate them on Earth.
If you look at any picture of the surface of Mars something that jumps out at you right away is that everything appears to be roughly the same reddish-brown color. The reason for this is that everything on the surface of mars is covered by a thin layer of very fine dust. It is this dust that gives Mars its reddish hue when seen from space and even gives the atmosphere its red tint (without the dust content, the atmosphere of Mars would be a very dark blue due to the carbon dioxide it contains). This dust is spread around globally by winds and suctioned off the surface by dust devils which sometimes fuel huge storms which obscure the entire planet.
Where did all this dust come from? On the earth, fine grained rock is usually formed by the action of water or ice as the processes of erosion break down the rocky landscape producing a fine material called silt. When the water dries up this silt can be lofted away by winds and can accumulate in dry areas. This is an enticing argument for water on the surface of Mars, however, it is misleading. Erosion and fine silt can also be the product of wind action or by the surface being impacted from space (the thin atmosphere of Mars makes the planet susceptible to small impactors which hit the ground, grinding up the surface instead of burning up in the atmosphere as they do here on earth ). These processes, while much less efficient then the action of water can produce similar results given enough time. This is one explanation for dust deposits we see on Mars.
In preparation for the Phoenix Lander's arrival in 2008 the science team is conducting experiments which require simulating this surface. It is important to replicate not only the dust but also the angular rock and soils we see in surface images. This material is referred to as Regolith, a broad planetary science term which refers to a broken up crust of material on a planetary surface.
Our best information of what this regolith is made of comes from the Viking landers which found a surface made up mainly of silicates (43wt%) and iron oxides (18.1wt%) with smaller amounts of aluminum oxides (7.1wt%), sulfates (7.3wt%), magnesium oxides (6wt%) and Calcium Oxides (5.8wt%) along with other trace constituents. This material is unlike any on earth, however, a class of rust-colored rocks called palagonites comes closest. Palagonites are a type of volcanic rock, typically somewhat dissagregated, which form in the presence of large amounts of water. This oxidizes the minerals found within the rock to produce the rust-like color.
At right you can see a selection of palagonites which have been used in Phoenix Experiments. The center pile shows one of the most popular simulants called JSC Mars-1 after the Johnson Space Center where it is prepared. The tubes contain sifted palagonites and from left to right are 2-8mm sieved, unsieved, >1mm-sized grains, <0.5mm sized grains, <50 micron sized grains, Hawaiian Mauna Kea Palagonite 101 (HWMK 101) and HWMK 103. These samples are all somewhat similar to Martian rocks with one major exception. Terrestrial Palagonites typically contain about 22wt% water (with some Sulfur Dioxide) which becomes adsorbed on the rock. Any palagonite left out on anything but the driest day here on earth will soak up water directly from the air (this is the principle behind silica drying pellets) therefore they have to be carefully baked prior to use. On mars this level of water content is only seen in close proximity to the poles where it is stabilized as ice.
Special Thanks to Carlton C. Allen for the sample of JSC Mars-1 and to Andrew Schuerger for the palagonite tube samples.