Justin Cowart's Research Projects

This page describes my research work in plain language -- the goal is accessibility of understanding!

Martian Bedrock Plains

The Martian cratered highlands cover the planet's entire southern hemisphere and extend into its northern hemisphere. Most of the cratered highlands are heavily pockmarked by craters, telling us that this region is incredibly ancient. The craters in this region exist at various levels of preservation, ranging from pristine young craters to ancient craters that have been eroded to oblivion. Much of this erosion was driven by an geological and hydrological system that was most active during roughly the first billion years of the planet's history. The active movement of water weathered the rocks exposed on the surface, carved drainage networks into the landscape, and carried sediments into craters (many of which likely hosted lakes). In addition, volcanic activity produced prodigious amounts of ash and lava. To get a picture of what this early environment looked like, first we need to find where these rocks are exposed at the surface before we can explore them in more detail.

However, this task is easier said than done. Many of the tools for studying these rocks, such as spectroscopy, can only see the surface of these rocks. Even a thin coating of dust can hide what we want to measure from view. For a lot of the extremely dusty surface of Mars, places where it might look like rocks are exposed are in practice totally inaccessible to researchers. In addition, as ancient rocks are broken down by new impact craters and constant abrasion by wind, they create loose sediments called regolith. Regolith rarely provides a good picture of what the local rock may look like. First, regolith is often a mixture of several rock types, as small craters often penetrate 10s to 100s of meters into the subsurface. In addtion, minerals behave differently from one another when shattered by an impact - some have crystal structures that promote easy shattering, while others are more study. Finally, as wind moves the smallest grains around, small minerals may be ground into dust while harder grains stay intact. Most places on the Martian surface where bare rock looks like it is exposed contains some degree of regolith cover.

To find places where a considerable amount of rock is exposed, we use a property called thermal inertia. This property measures how quickly a surface changes temperature, and for rocky materials, is related to their density and porosity. An everyday example of this can be seen by watching the ground as it starts to snow. The snow first starts to stick on loose dirt, then sidewalks. This is because loose dirt has a low thermal inertia, which allows it to change temperature very quickly and become cold enough for snow to stick. Sidewalks have a high thermal inertia, which causes them to take a while to be cold enough for snow to stick. On the Martian surface, we expect dusty surfaces to have the highest thermal inertia, followed by regolith-covered surfaces, sedimentary and pyroclastic rocks, and then lavas. To date, we have flown three instruments capable of measuring surface temperature from orbit, the Viking Infrared Thermal Mapper (Viking IRTM), the Mars Global Surveyor Thermal Emission Spectrometer (TES), and the Mars Odyssey Thermal Emission Imaging System (THEMIS). Using these instruments, researchers have produced global maps of thermal inertia.

These maps show patchy regions (some only a few hundred sq km in size, others more than a hundred thousand sq km) where the thermal inertia is a lot higher than the average Martian surface, which tells us that there is less dust and regolith cover on these surfaces relative to other areas.