Surface Rocks and Regolith
By Mike Mellon
Explanation
of the theme.
A
wide range of geologic processes operate on the surface of Mars today and throughout
its history. Erosion by wind and water, impacts, volcanism, and chemical
weathering all act to break bedrock down into soils and dust. Wind and water
are also effective agents at sorting soil particles into deposits of distinct
sizes. Sand dunes are an excellent example. The nature of these soil surfaces
tells us much of the history of the planet. From orbit we can attempt to map
the distribution of soil types and particle sizes though thermal mapping. Daily
heating and cooling of the soil by the sun results in distinctly different
temperatures depending on the soil, much the same way beach sand becomes
scalding in the sun and rocks retain heat at night. However, some soils
surfaces with different mixtures of rocks and sand can exhibit the same temperature
characteristics. Images of these surfaces can help separate soil types by
observing rock distributions, slopes, and erosional patterns.
Major
science questions for this theme.
What
soil forming processes act on Mars over time? The nature of soils and the size
of rocks tell us something of the history of the soil surface and the nature of
geologic processes that formed the soils. Are rocks primarily formed by impacts
or mass wasting? Both processes produce rocks in different ways and the rock size
distribution will differ. How common are desert pavements? Desert pavements are
lag deposits of small rocks and pebbles that armor the soil against erosion.
They commonly occur on Earth in arid environments including Antarctic polar
deserts. What locations on Mars exhibit cohesive soils, indicating cementation?
Cemented soils on Mars, called duricrust, have been observed at every landing
site thus far, but how globally common are duricrusts is yet unknown.
Relationship
to other science themes.
The
history of soils on Mars relates to many HiRISE science themes. Images of Mars
are primarily images of a soil surface. While this soil reflects much of the
bedrock beneath, the soil itself is reworked, eroded, and deposited by a
variety of geologic processes. Aeolian transport moves sand and dust in the wind to redeposit them as dunes,
sand sheets, and dust mantles. Volcanic ash erupts onto the Martian surface forming loose or cemented
deposits. Mass wasting can
result in landslide deposits of jumbled rocks, sand, and dust. Impacts generate large quantities of rocks and fine debris
scattered in an ejecta blanket.
Features
of interest potentially visible at HiRISE scale.
Rocks
on Mars come in a variety of sizes. However, rocks typically become more
abundant as they reach smaller sizes due to natural weathering processes - one
large rock breaks into many small rocks. Large boulders the size of small cars
and large houses are commonly observed in Mars Global Surveyor images. Smaller
rocks have yet to be observed except at landing sites. HiRISE allows us to see
rocks in a new size range and determine their distribution over the planet's
surface. Determining the size distribution of smaller rocks also provides clues
about their formation.
Return
to HiRISE Learning and Activities Page