2014 was marked in the world of planetary geology by the publication of two new geologic maps, portraying the geology of Mars and Vespa, the largest asteroid in the asteroid belt. The maps, which describe the geologic histories and structural features of the two bodies, were both the culmination of many years of work.
On Earth, a geologic map is typically made by combining field observations made by a geologist with data collected from remote sensing instruments. Once a map is complete, a geologist will often spot-check certain areas on the map in order to ensure that the interpretation of the remote sensing data in light of limited field activities is sufficiently accurate.
Obviously, geologic mapping is much different on other planets. All data (excluding detailed data collected over very small areas by landers) must be collected remotely.
In the case of Mars, data collection has been ongoing since the late 1960’s. A series of increasingly sophisticated orbiters have mapped the entire surface using a variety of instruments. By combining many types of data and an understanding of geologic processes, geologists can come up with a geologic map.
Let’s take a look at some of the types of data geologists can use to interpret geology from space (take a look at the technical report for more details):
Visual Range Imagery: Basically, the imagery produced using a very expensive and accurate digital camera. Visual range imagery has several weaknesses, including increased likelihood of atmospheric interference and the inability to differentiate types of materials. However, visual imagery often makes the most intuitive sense to humans, and is useful as a base layer.
Laser altimetry: Laser altimetry measures the topography of the surface by measuring the time it takes for a laser pulse to reflect back from the surface at the speed of light. This method, which has long been used on Earth, provides scientists with important data about the shapes of surface features such as crater rims, faults, and canyons. Over 600,000,000 laser altimetry measurements were used in the creation of the geologic map of mars.
Infrared Imagery: Infrared imagery is used to measure the temperature and emissivity of surface features. It can be used to help differentiate rock units made up of different types of material, as well as the surface textures of the units.
Once all the imagery has been collected and integrated, geologists attempt to interpret surface features using what they know about geomorphology, volcanology, and other geologic processes.