Why are Craters Useful?

This week’s guess the planet picture is of Herschel Crater on Mimas, one of Saturn’s moons. This image was captured by the Cassini spacecraft. Mimas is a small and heavily cratered moon, with an icy composition. The leading hemisphere of Mimas is dominated by the Herschel Crater. This large impact basin displays many of the features which are common in impact structures across the solar system. In Monday’s post, I drew attention to the prominent central peak. This is very obvious in this glancing view of Mimas. You can clearly see the topographic profile across the crater, with steep walls on both the inside and outside of the rim, and then an irregular central uplift. The bowl-shaped interior of the crater is not entirely flat, but is covered with hummocks and irregularities.

NASA have a much more detailed description of Herschel crater and what it tells us about Mimas, which can be read at this link. They draw attention to the thick blanket of “ejecta” which was thrown out onto the surrounding plains by the impact. They also notes that some of the smaller craters surrounding the Herschel Basin are “secondary craters” produced by ejected material impacting the surface further away. This image shows a head on view of the crater, and gives us a better look at its surrounding terrain. Mimas has been humorously described as looking like the Death Star from Star Wars, and in this view you can see why.

Craters are found on numerous bodies across the solar system, they can be very useful for interpreting the surfaces into which they impact. This is because the number of craters on a surface can tell us approximately how old it is. Any new surface, such as a recent lava flow will clearly be uncratered. As time goes by more and more impacts will mar that pristine surface. Resulting in the heavily cratered terrains common on worlds like Mercury and the Moon. Many planetary scientists spend a lot of time counting craters in order to estimate the age of a geological unit.

This method relies on the fairly basic principle that if one geological feature overlies another then it must have formed later. The surface into which the craters impact had to have formed before the impact occurred. This allows us to determine what is called a relative date for the surface. Determining which terrains are the oldest, and what order the events occurred in.

Relative dates are useful, but frequently we need more information. It we want to compare several regions which don’t overlap then we need absolute dates, which give us a lot more information to work with than just establishing the order of events for a specific site. However, in order to get a we would need to calibrate our relative dates, by figuring out how many craters impacted into surfaces of known age. On Earth, we can get absolute dates quite easily here, using the decay of radioactive elements to work out how recently a material formed. Unfortunately, Earth doesn’t have many surviving impact craters. Our planet is “resurfaced” much more rapidly than those of many other solar system bodies, wiping away the signs of meteorite impacts. We don’t have the heavily cratered surfaces we would need to calibrate our crater counts.

Luckily our nearest neighbour has very little resurfacing, and is largely covered in cratered terrain. We can get absolute dates for surfaces on the moon in the locations where samples were collected by the Apollo astronauts, and compare these to crater counts to connect the two dating systems.

The result is an estimate of the cratering rates throughout the moon’s history, which can be applied to surfaces which have never been visited or sampled. Statistical methods allow us to apply these cratering rates to other solar system bodies as well, although there is some debate as to how accurate the results are. In particular, secondary craters can throw off the crater count, since they formed as a result of an existing impact, not a new meteorite. Determining which craters are due to direct meteorite impacts and which are secondary can be a difficult exercise.

As with many methods in planetary science, crater counting is very valuable, but has to be used carefully. We have to understand the uncertainty in the observations and measurements we make. By doing so we can use techniques like this, and get useful information out of them, even if they do not give us a perfect result. We will never know the exact date of a surface unless we go there to take samples. Even then laboratory dating methods only give a range of probable ages. However by combining all of this information we are able to develop a strong theory for how the surface of a body evolved, and can tell a lot about the processes at work there.

 Image Credits:

Mondays image: NASA/JPL-Caltech/Space Science Institute https://saturn.jpl.nasa.gov/raw_images/400528/

Larger image of Mimas: NASA/JPL-Caltech/Space Science Institute http://photojournal.jpl.nasa.gov/catalog/PIA12568