Rose here –
One of the famous first stories of modern geology involves the publishing of a geologic map of England by William Smith in 1815. This was one of the first geologic maps made by a geologist doing fieldwork, which often involves camping out in an area for a few days, weeks, or months to find out as much as possible about the area to be mapped. Geologists walk around the area to be mapped and take measurements of what types of rocks are there, how thick each layer is, whether they are tilted or faulted, etc. They may also take samples of the rocks to do chemical tests or look at them under a microscope. Field geologists look at the morphology (shape) of the landscape in order to map the locations of ridges, depressions, and other features and determine the processes that formed them.
My experiences in geology as an undergraduate major were largely field-based, going on many field trips to various places and taking notes and measurements at different locations. But how do you make a map when your field area is on average 140 million miles (225 million km) away from Earth? I had never considered studying geology in a field area anywhere other than Earth, but shortly after starting my master’s degree I had the opportunity to work with a team of collaborators to create a geologic map of a small region on Mars. Geologists can now create maps of planets, moons, and asteroids using high-resolution images from spacecraft orbiting Mars, Mercury, the Moon, and many other bodies in our solar system. I was excited to begin this project, but first I had to learn a whole new set of skills than what I had used in field camp as an undergrad.
There are several software programs scientists can use to make maps using images and other types of geospatial data. These software programs are collectively called Geographic Information Systems (GIS). GIS software is used in many different fields for different kinds of projects and analyses. For example, biologists might use GIS to make maps of where certain species of animals live in relation to cities, lakes, highways, etc. Geologists might use GIS to produce maps showing the location of certain types of rocks or geologic features.
For my master’s project, I used a mosaic (several images digitally “stitched” together) of images from the Mars Reconnaissance Orbiter’s (MRO) Context Camera (CTX). To identify a feature in these spacecraft images, it needs to be big enough to have at least two pixels across it each way (so a minimum 2×2 grid). CTX images of Mars have a resolution of 6 meters (m) per pixel, which means they can be used to find features about the size of a large room. When I upload these images into my GIS program, I can zoom in and out to see features better. When I find a feature that looks interesting, I can mark its location and shape by making a new “layer” and drawing on the image. I use different layers for different types of features, and each layer can be turned on and off so I can see where different features are in relation to each other.
My first step in mapping was actually not mapping, but reading lots of previously published papers about the geology of my study area and about the particular type of feature I wanted to map. I am mapping a type of ridge on Mars called a wrinkle ridge. This ridge is formed by tectonic contraction and is found in layered igneous or sedimentary rock units. Once I had read as many papers as I could find on wrinkle ridges and made several tables summarizing the various types of information on them, I could finally start mapping. It took quite a while for my eyes to get used to looking at these images and to pick out the features I was looking for. However, wrinkle ridges have several common distinguishing characteristics, mentioned in many published papers, that I used to double-check my visual identification. When I had gone over my whole study area several times and marked any feature I thought could possibly be what I was looking for, I went over it again and narrowed down the number of features using my list of common characteristics. Learning to identify wrinkle ridges and other features visually is a good skill and I spent a great deal of time trying to do so. However, it is also important to make my results understandable and reproducible by other scientists. Thus I need to be able to clearly show how I identified a feature as either a wrinkle ridge or not. With my list of common characteristics, I decided how many of them would be required to determine if a feature is a wrinkle ridge, and within those determined to be wrinkle ridges I further divided them by how many characteristics they had into certainty levels: Certain, Probable, and Possible. This process allows my work to be reproduced or at least easily followed by any future scientists studying the same type of features.
I’ve been working on this project for about two years now and while it’s been a lot of hard work and tired eyes, it so rewarding to see my map finally coming together. While I’ve been mapping one type of feature, other scientists in my research group have been mapping different types of features and we are about to put them all together and make one complete map. When we have all our mapping together on one map, it will be published as an official United States Geological Survey (USGS) geologic map. Stay tuned!