Guest post by Sebastien Besse, Research Fellow at ESTEC
On 5 September 2008, ESA’s Rosetta comet chaser completed its first encounter with a main belt asteroid when it flew past Steins, which revealed itself as a diamond-shaped object with dimensions of 6.67 × 5.81 × 4.47 km.
At the time, I was a PhD student working with the OSIRIS imaging team in France studying the craters on Steins. With the asteroid’s diamond shape for inspiration, I began to think about naming the craters after different gemstones. I tried to select names that would be familiar or appealing to the general public, and easy to pronounce.
As is the case with the official place names given to all worlds in the Solar System, they have to be approved by the International Astronomical Union (IAU). We had quite some discussions because we had to choose names of gems or precious minerals/stones that would not be confusing for studying the true mineralogy of the asteroid’s surface. In the end, the OSIRIS imaging team agreed that the names of 23 gemstones or precious minerals would be recommended to the IAU.
To date, about 40 craters have been identified on Steins, but only the most prominent of them have been given names. In some cases we tried to name craters geographically close to each other with names from the same gemstone family. So for example, Aquamarine and Emerald are both from the Beryl family of gemstones, and Agate, Amethyst and Citrine are all types of quartz. Another example is that the crater Alexandrite sits inside the crater named Chrysoberyl – I named them like this because Alexandrite is a type of Chrysoberyl.
The most obvious crater is an impact feature near the south pole (top in the images) – now known as Diamond crater – which measures 2.1 km in diameter and is 300 m deep. The relatively large size and depth of this crater compared with the asteroid itself indicates that Steins was probably shattered by the force of the collision, meaning much of its interior resembles a rubble pile rather than solid rock.
A circular crater 650 m wide and 80 m deep named Topaz sits in the centre of the asteroid. Topaz is the easiest feature to see in this region and so is used to mark the asteroid’s centre of longitude.
Another noticeable feature is the chain of craters that stretches from the asteroid’s north pole (the pointed area at the bottom of the images) all the way to Diamond crater. This feature may be linked to the shattering impact that created Diamond crater at the south pole. At least some of the craters in the chain may have been formed by loose material sinking into a subsurface fracture. Several of these craters are named after familiar gemstones such as Opal and Jade. I included Citrine, since it sounds like the French word for lemon: citron.
As a geologist it feels good to have promoted the names of these gemstones out in space! But as is the case with the naming of geological features on any world, it also provides a solid framework for discussions about certain features. Rather than having to say “you know that crater next to the funny shaped one…”, we can refer to named features, which makes discussions and writing papers a lot easier!
Read more in Sebastien’s paper Identification and physical properties of craters on Asteroid (2867) Steins.