New images captured by ESA’s Mars Express space probe’s High Resolution Stereo Camera (HRSC) show Greeley Crater on Mars. Its name was officially approved in 2015 by the International Astronomical Union (IAU) to honor geologist Ronald Greeley, who passed away in 2011 after having worked on various Mars missions, also being part of the team that manages the HRSC instrument.
Greeley Crater is located in the region called Noachis Terra in the Noachis quadrangle between two huge impact basins, Argyre and Hellas Planitia. Greeley Crater is very large as well with a diameter close to 450 kilometers making it the largest of that type in the southern Martian hemisphere. It’s a very old crater that over time suffered considerable erosion that completely erased some parts of its rims. This makes it difficult to measure its size accurately and in any case hasn’t had a circular shape for a long time. It was filled with sediments with the consequence that it’s no more than 1.5 kilometers deep.
Noachis Terra is one of the oldest regions of Mars and bears traces of many impacts that happened over four billion years. Greeley Crater shows the signs of age also because in turn it contains impact craters that are younger and therefore much more visible due to their greater depth and rims still easy to recognize. Many of them are really small and are called secondary craters because they’re generated by materials ejected when other larger objects hit the planet’s surface.
Some of the inner craters are crossed by small and narrow gullies and are the traces of an ancient water flow. Other wider channels suggest that once ice mixed with sediments slid down their slopes. Different compositions of materials in Greeley Crater’s bottom are shown by different colors with lighter materials exposed to the weather through the water and darker surfaces covered by volcanic sands. Basically, more traces of water flows but also the consequences of an ancient volcanic activity.
All these geological features make Greeley Crater a concentrate of Martian history. The images were produced creating a mosaic of photos taken by the HRSC camera during 16 orbits of the Mars Express space probe. The study of the data collected contributes to the information on Mars’ history, their publication also serves to honor a pioneer of planetary geology that from the 1960s until his death contributed to that kind of research.