Counting craters
Craters, craters, and yet more craters: this snapshot from ESA’s Mars Express is packed full of them, each as fascinating as the last.
This view of the Red Planet – taken by Mars Express’s High Resolution Stereo Camera – shows a slice of Arabia Terra, a large plain in Mars’s ancient highlands. This part of Mars is known for being heavily pockmarked with craters, each formed as a space rock hurtled inwards to collide with the planet.
Ancient ground
The glut of craters seen here is no surprise. Arabia Terra is truly ancient. As a result, it’s had lots of time to add to its impressive crater collection – between 3.7 and 4.1 billion years, in fact.
The main image above shows just a few of these. Some are filled with strikingly dark material, others are home to lighter sands and rippling dunes, while yet others show signs of collapsing walls and worn-away rims.
The most prominent crater seen in the image, extending out of frame to the bottom-right, is Trouvelot Crater. This crater is around 130 km across, and shows signs of being very old: it has a rim that has long since started to crumble away, uneven interior ‘terraced’ walls that have collapsed under their own weight over time, and a number of smaller overlapping and overlaid craters that have formed since the creation of Trouvelot Crater itself.
These, and other, features are all clearly labelled if you click on the image. Be sure to take a look to easily find features of interest and explore this intriguing landscape in detail.
Dark and volcanic
To the left of Trouvelot Crater sits another basin that appears to be even older and more eroded, with a wall that’s almost completely worn away. Trouvelot cuts through this crater, further indicating that this more deteriorated crater companion was there first.
The floor of this more ancient crater is nearly entirely covered in dark rock, which is rich in minerals such as magnesium, iron, pyroxine and olivine (known as ‘mafic’ rock, and often created by volcanism). These volcanic rocks may have been thrown up by crater-forming impacts, and later moved around as winds swept across the terrain and gravity pulled material down crater walls.
The other large craters seen here – and across Arabia Terra, beyond the edges of this frame – have similar dark deposits on their floors or walls, indicating that these processes are widespread across this part of Mars.
In Trouvelot Crater, the dark material has been shaped by wind into rippling dunes known as ‘barchan’ dunes. These are characteristically sickle- or crescent-shaped, and created when winds predominantly blow in one direction. Mars Express has spotted barchan dunes on Mars before, such as in the planet's north polar region and near the large volcanic province of Tharsis.
From dark to light
Sitting amid the dark material in Trouvelot Crater is a sign that other processes have been at play here: a light-toned mound around 20 km long and covered in ridges and grooves.
Such mounds have been spotted elsewhere on Mars – in the nearby Becquerel crater, for example, as seen by Mars Express in 2013 and 2014. They typically show signs of minerals that have come into contact with, or formed in the presence of, water, and are usually far lighter than their surroundings.
Water is thought to play a key role in how the mounds themselves form, too, but this is still a topic of debate. The mounds may have formed in a lake or sea in Mars’s past. Alternatively, layers of light-toned rock may have gradually built up as water in and below the martian surface (‘groundwater’) swelled upwards to mix with wind-swept sediments on the crater floor.
Decades of Mars exploration
This image was captured by one of eight instruments aboard Mars Express: the High Resolution Stereo Camera. The Mars orbiter has been exploring Mars’s many landscapes since it launched in 2003. It has mapped the planet’s surface at unprecedented resolution, in colour, and in three dimensions for over two decades, returning insights that have drastically changed our understanding of our planetary neighbour (read more about Mars Express and its findings here).
The Mars Express High Resolution Stereo Camera (HRSC) was developed and is operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). The systematic processing of the camera data took place at the DLR Institute of Space Research in Berlin-Adlershof. The working group of Planetary Science and Remote Sensing at Freie Universität Berlin used the data to create the image products shown here.
