Greeley Crater – major crater pays tribute to a leading planetary scientist

Greeley Impact Crater is located in Noachis Terra, an area of the southern highlands between the Argyre and Hellas Planitia impact basins and, with a diameter of 427 kilometres, is one of the largest impact craters on Mars. This ancient crater, which has already been heavily eroded and filled with sediment, is difficult to make out against the Martian landscape due to its relatively shallow depth of only 1.5 kilometres – indeed, the crater rim has disappeared altogether in places. An astronaut standing in the middle of the crater would not be able to see the crater rim on the horizon at all due to the curvature of Mars, which is greater than Earth's. The crater outline is most visible on the colour-coded digital terrain model (image 4).

With an age of up to four billion years, Noachis Terra is one of Mars' oldest regions. Countless impact craters of different sizes were formed and later eroded here over the course of billions of years. Through these erosion processes, impact craters that had once been trough-shaped were worn away over time by wind, water or ice into shallow depressions devoid of relief. The lack of ejected material, the low and partially non-existent crater rim, the flat crater floor and the numerous impact craters within it indicate that Greeley Crater is very old. A closer look reveals a myriad of tiny craters here and there. Known as secondary craters, these are a result of the impact of material ejected when other objects hit the planet’s surface. Small, narrow gullies cross some of the larger inner craters, indicating that water once flowed within the crater. Other wider channels suggest that masses of ice loaded with sediment once crept down the slopes. The different colouration of the material on the crater floor is due to the differences in composition – the lighter material was weathered through water and the darker surfaces covered by volcanic sands.

An early HRSC team member

Ronald Greeley's passion was Mars science. Not only was he a co-investigator for the Mars Express HRSC, but he was also involved in other Mars missions, such as Mariner, Viking, Pathfinder, Mars Global Surveyor and the Mars Exploration Rovers Spirit and Opportunity. His career in planetary research began in 1967 at NASA AMES Research Center, where he studied volcanic landforms and lava tubes on Earth and the Moon. He later worked with planetary mission data acquired by Galileo for Jupiter, Magellan for Venus, and Voyager 2 for Uranus and Neptune. He was also interested in surface features and processes that occur due to the effects of wind on other planets. In 1977 he became Professor of the School of Earth and Space Exploration at Arizona State University, where he created the Planetary Aeolian Laboratory, which is still running to this day. Atmospheric processes on planets were researched in closer detail here using a wind tunnel and a device that could generate vortex motion. Ronald Greeley passed away on 27 October 2011 as an internationally renowned and distinguished scientist. In 2015 the International Astronomical Union (IAU) named this Martian crater in his honour.

• Image processing
  
  The mosaic consists of 16 orbit strips (0430, 1910, 1932, 2412, 2467, 2478, 4306, 4317, 4328, 6556, 8613, 8620, 8708, 12835, 14719, 16778) and covers the area from 2 degrees west to 9 degrees east and 31.5 degrees north to 43.5 degrees south. The image resolution is approximately 100 metres per pixel. The colour mosaic was generated from the colour channels of the HRSC and the nadir channels of the individual image strips directed vertically at the surface of Mars. The oblique perspective view was generated by the stereo channels of the HRSC. The aerial view, encoded in rainbow colours, is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived. The reference unit for the terrain model is an equipotential surface of Mars (Areoid).