Loess on Mars?- we keep looking but loess does not appear. It appears that no mechanism operates at the Martian surface which can produce the silt-sized particles necessary for the formation of a loess deposit. The granitic crust and some vigorous geomorphology ensured that the Earth was well supplied with sand sized quartz particles and silt sized quartz particles; the essential ingredients for sand dunes and loess deposits. Mars appears to have an essentially basaltic crust and we are faced with the difficult problem of finding a particle forming mechanism.
Speculation: the last major particle forming events occurred on Mars at the time of the Late Heavy Bombardment, i.e. about 4 billion years ago. These are old particulates- but maybe old particulates can exist at the Martian surface. If there are no internal lithological controls in Martian crustal rocks then an energetic particle forming event should produce a range of particles. On Earth there are internal controls and this tends to give a marked modality in clastic material (in particular sand and silt). So-very large clasts down to very fine dust. Dust due to crushing and abrasion at the impact events, and lasting for billions of years because no lithological processes converted it into anything else. The dust is very old. Sand sized debris can go to make the Martian dunes. Large & very large clasts can litter the Martian surface, as the mechanical explorers reveal.
This range of impact particle sizes should contain some 'loess' sizes, but probably not enough to form a proper modal deposit. The loess ages on Earth are measured in thousands, perhaps millions of years; the aerosolic fine dust is probably recent material- is it reasonable to expect very old, very fine particulates on Mars? Given the age of the Martian surface one might expect more particulates; if there was essentially one particle forming event a very long time ago this might explain the relative paucity of particulate matter.
Speculation: the last major particle forming events occurred on Mars at the time of the Late Heavy Bombardment, i.e. about 4 billion years ago. These are old particulates- but maybe old particulates can exist at the Martian surface. If there are no internal lithological controls in Martian crustal rocks then an energetic particle forming event should produce a range of particles. On Earth there are internal controls and this tends to give a marked modality in clastic material (in particular sand and silt). So-very large clasts down to very fine dust. Dust due to crushing and abrasion at the impact events, and lasting for billions of years because no lithological processes converted it into anything else. The dust is very old. Sand sized debris can go to make the Martian dunes. Large & very large clasts can litter the Martian surface, as the mechanical explorers reveal.
This range of impact particle sizes should contain some 'loess' sizes, but probably not enough to form a proper modal deposit. The loess ages on Earth are measured in thousands, perhaps millions of years; the aerosolic fine dust is probably recent material- is it reasonable to expect very old, very fine particulates on Mars? Given the age of the Martian surface one might expect more particulates; if there was essentially one particle forming event a very long time ago this might explain the relative paucity of particulate matter.
