Terrestrial Analogues

This week’s guess the planet image comes from Earth. It shows the Barringer Impact Crater, also known as Meteor Crater in Arizona. This image comes from Google Earth, and appears to be an aerial photograph taken by Google. The crater is approximately 1.186 kilometres across. Roads and tracks can clearly be seen to the north of the crater, which I had to crop out of the original image in Monday’s post.

We find impact craters like this all across the solar system, the surfaces of numerous planets, moons and asteroids are covered with them. Only a handful can be found on Earth. Most are badly weathered and eroded by centuries of exposure to the hostile conditions produced by our planet’s thick atmosphere and prevalence of life. What makes this crater interesting to planetary scientists is how easy it is to get to. Earth has the most geologists of any planet in the solar system, making it much easier, and cheaper to study than any other world.

Luckily for us Earth can tell us a lot about the other planets as well. What I want to talk about this week is the importance of studying “terrestrial analogues” these are places where we can compare features on Earth to those we find on other planets.

One of the main principles of comparative planetary science is that the same fundamental processes shape every planet. They might function slightly differently from one world to another, but the principles, and frequently the results are similar. For example, in previous posts I’ve talked about cryovolcanoes, where the environment is so cold that water ice behaves like rock and liquid water like lava. We don’t have cryovolcanic environments on Earth, but we can compare these landscape on Titan or Europa with a silicate volcano on Earth to form theories about how the alien landscape has formed. This allows us to move beyond describing what a landscape looks like, and start to understand the geomorphic processes which resulted in its formation. Processes which likely have implications for the environment of the planet we are studying.

Some landforms develop through virtually identical processes on different planets, and so have fairly obvious analogues. In other cases it is far more difficult to figure out what the nearest terrestrial equivalent might be. One complication to studying the landscapes of other planets in this manner is a phenomenon called “Equifinality”. This means that multiple processes result in similar looking end results. For example a crater could be formed by a meteor impact, or by volcanic collapse. Both have a similar shape, but there are usually subtle clues with which we can distinguish between the two.

On Earth it is relatively easy to go to a site and make “In Situ” observations. Such measurements can let us limit the variety of processes which could be responsible. Quickly figuring out how something was produced. For example the area around our crater doesn’t have any evidence of volcanism, so it is more likely to be an impact crater than a volcanic one. If we only have satellite data from another planet, then it can be harder to make such a determination.

This is where our analogue studies prove useful. We can identify a range of possible analogues and then study all of them to see which are the best morphological match for the features we are seeing on our other planet. We likely have limited information to work with, so we have to take our terrestrial data and examine it as though we were looking at the other planet, seeing which features would be obvious in our satellite images, and which wouldn’t.

In this way we can identify characteristic features to look for in the planetary data, and get closer to determining which processes shaped the surface of our other world.