Mars Express image shows the boundary between the planet’s hemispheres

Mars is very much a world of two halves – as highlighted by this new image from ESA’s Mars Express, which shows where the planet’s dramatically different hemispheres come together as one.
Mars is very much a world of two halves — as highlighted by this new image from ESA’s Mars Express, which shows where the planet’s dramatically different hemispheres come together as one. ESA/DLR/FU Berlin

The European Space Agency (ESA)’s Mars Express orbiter has captured this image of the Martian surface, showing the boundary between the planet’s north and south hemispheres. The two hemispheres are strikingly different. The northern hemisphere of Mars is generally smooth and flat, consisting of low plains with few impact craters. The southern hemisphere, however, is mountainous and covered in craters, with many areas of past volcanic activity.

To see the boundary between the two more clearly, the ESA also released this colored version of the image which represents elevation. The purple and blue areas are low, while the yellow and reds have higher altitudes. The bottom half of the image shows a high ridge, with a boundary beyond which lower plains lie.

This colour-coded topographic image shows a region of Mars’ surface named Nilosyrtis Mensae, based on data gathered by the Mars Express High Resolution Stereo Camera on 29 September 2019 during orbit 19908.
This color-coded topographic image shows a region of Mars’ surface named Nilosyrtis Mensae, based on data gathered by the Mars Express High Resolution Stereo Camera on September 29, 2019 during orbit 19908. ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

The area which separates the two hemispheres is called a “dichotomy boundary.” In this area, there are broken sections of ground called “fretted terrain,” and in this particular image, the region of fretted terrain is called  Nilosyrtis Mensae.

“Nilosyrtis Mensae has a labyrinthian appearance, with numerous channels and valleys carving through the terrain,” the ESA explained in a blog post. “Water, wind and ice have been strongly affecting this region, dissecting and eroding the terrain, along with changes in martian geology: valleys have formed over time and sliced across the region, and once-defined impact craters have slowly degraded, their walls and features gradually wearing away.”

The area is of interest not only because it is a boundary region, but also because it may contain clues to the history of water on Mars. There are ridges and grooves on the surface in the area which seem to have been made by a flowing material such as ice. At a distant time in the planet’s history, ice glaciers may have flowed across the surface of the planet, leaving impressions in the rock which are still visible today.

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