QUESTION: We are designing a Mars colony and need to know about surface. Does NASA know anything about the sand on Mars? We have heard that it might contain magnetic compounds. If so, could you elaborate on this? What exactly is the sand is composed of? It is an extreme problem and may be one of our most difficult tasks to master on how to eliminate it as a problem, since it is worse than lunar dust. Do you think a magnetic device would attract it? ANSWER from Nathan Bridges on July 26, 1999: You ask a very important and fundamental question. First of all, it is important to understand what sand is. It is not just any soil particle, but rather refers to a specific size ranging from 62 to 2000 µm. It generally forms by fluvial erosion of rock. Because water is usually involved in its formation, the identification of abundant sand on another planet such as Mars has important implications. We do not know for sure if sand exists on Mars. Of course, we know that Mars has soil, but whether this is made up of small dust-size particles, conglomerations of small grains, sand, or a combination of these is not known. The cameras on the Viking Landers and Pathfinder/Sojourner had insufficient resolution to distinquish individual soil grains. However, there is strong circumstantial evidence for sand on Mars. Dunes, located in vast fields, within depressions, and even at the Pathfinder landing site, exist on Mars. Dunes on Earth generally form by the saltation of particles. Saltation is a process whereby wind transports sand-size particles in a skipping action, forming dunes. Particles smaller than sand are suspended in the atmosphere and those that are larger can only move slowly by rolling along the surface. So, the presence of dunes strongly indicates large concentrations of sand that has been moved by the wind. The second piece of evidence is the finding of rocks at the Pathfinder site that appear to have been abraded by sand carried in the wind. These type of rocks, called ventifacts, are found in sandy, arid regions on Earth. By analogy with our planet, they may have formed by sand abraion on Mars. We know the composition of soils on Mars, but, again, the size of the grains making up the soil is unknown. The soils are iron rich and have a composition similar to weathered basalt. Some soil grains are indeed magnetic and, as you propose, magnets were used on Viking and Pathfinder to attract these particles. It is likely that the composition of sand on its own is different than that of soil (which is probably of mixture of sand- and dust-sized material). One possibility is that sand on Mars, if present, is composed of pyroxene, one of the dominant minerals that makes up basalt. This makes sense because a lot of basalt probably exists on Mars' surface. Another possibility is that the sand is quartz. On Earth most sand making up dunes and beach materials is quartz. There are two reasons for this. First of all, quartz-rich rocks, such as granite, are fairly common on Earth. Second, quartz is very resistant to erosion, so it lasts a long time whereas other minerals weather away. The Sojourner APXS instrument indicated that the rocks at the Pathfinder landing site have an andesitic composition. If true, then quartz could be fairly abundant in Martian rocks and the erosion of these rocks would then produce quartz particles that could be collected into dunes. The question of the presence of sand on Mars will hopefully be resolved soon. In December, the Mars Polar Lander will land on the southern polar layered terrain on Mars. One of the instruments is a high resolution camera that will examine soil in fine detail. The 2001 Mars lander currently under development will actually have a microscope to look at particles. So, expect to see Mars *very* close up in the near future!