Buried impact crater on Mercury

This week’s guess the planet image comes from Mercury, and was imaged by the messenger spacecraft in 2011. Unfortunately the messenger team haven’t provided scale information to go with this image in their gallery, although we can infer that it covers quite a large area.

The most obvious feature of this image is the heavily cratered plane, however there are some other features of note as well. Craters are depressions, so they have what we call “negative relief”, they go down into the surface of the plane. However there are also some positive relief features in this image, narrow ridges which stick up out of the surface of the plane. The main thing I want to talk about this week is how to distinguish between the two types of feature in images like this. 

When you first looked at this picture, many of you may have seen the craters as domes, sticking up out of the image. This is quite a common optical illusion. Because we are looking down on the area, all we have to tell us which are positive features and which are a negative are the shadows. But it can be hard to figure out where the sun is coming from. Luckily we know what craters are and can easily identify them. Since they have to be negative relief features we can work out that the sun is coming from the bottom left corner of the image, and casting shadows over the crater floors. Knowing this doesn’t necessarily make it easier to see them as depressions, if you have got stuck seeing them as domes. One good way to trick you mind into seeing the image correctly is actually to turn it upside down. 

Once we can distinguish between negative and positive relief features we can interpret the image. We can see that this cratered plane is crisscrossed by ridges, and that they form a large circular feature on the left hand side of the image. These are called “wrinkle ridges”, which form as a result of the contraction of cooling lava. These features are not unique to Mercury, but are also common on the moon, and have been observed in some locations on Mars as well.   These ridges are interesting as they appear to be following the rim of a large, buried impact crater.  

This crater would once have been an obvious feature of the landscape. However it has been completely buried when this region, part of Mercury’s volcanic planes, was inundated with lava. As this lava cooled contraction occurred, causing the development of the wrinkle ridges. The presence of the buried crater had a controlling effect on the development of these ridges causing them to form in a circle. The result is that we can still see where the crater once was. It’s amazing what you can figure out from a few shadows!

Image Credit:

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington