images, acquired by the High Resolution Stereo Camera (HRSC), show part of the Nereidum Montes region. Wind, water, ice and tectonic forces have created a highly diverse landscape. The HRSC has been mapping the Red Planet with unprecedented resolution, in three dimensions and in colour, since 2004 as part of ESA's Mars Express mission. It was developed at the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR) and is operated by the DLR Institute of Planetary Research.

Mars Express delivers new data about the geology, mineralogy and atmosphere of Mars to shed light on the climate history of the Red Planet and clarify the role of water and determine the location of whatever water is left.

The Martian 'Alps'
Nereidum Montes is part of the northern rim of Argyre Basin. With a diameter of 1800 kilometres and a depth of up to five kilometres, Argyre is the second-largest impact basin on Mars (the biggest being Hellas Planitia). Similar to the Alps, the Nereidum Montes mountain chain extends for over 1100 kilometres in an elongated arc, running parallel to the edge of the basin. Similar to the mountainous areas of Europe, there are individual massifs that are up to 4000 metres high.

However, the Alps were formed by a very different process to the one that created the ring mountains of Nereidum Montes. The latter were originally the result of an extremely large asteroid impact. The impact was so enormous that it not only formed a bowl-shaped basin several kilometres deep, but also created several concentric rings of mountains at the basin edge. These have acquired a terrace-like structure due to tectonic landslide processes, which moved entire blocks of the surface.

By contrast, the Alps were formed when the African continental plate collided with the Eurasian plate, crumpling the collision zone into fold mountains almost 5000 metres high. These are still growing by approximately one centimetre per year, although they are being eroded at the same time.

The 'Silver Plain' and the 'Nereid Mountains'
Before Mars could be closely examined by spacecraft, even Earth's most powerful telescopes could only discern major geological formations. These included the Argyre Basin, but scientists were unable to draw any conclusions about its topography on the basis of their observations by telescope.

Chryse and Argyre were two mythical islands that Pliny the Elder (23-79 AD) described as being located where the Indus flows into the Indian Ocean, and which were said to harbour large deposits of gold (in Greek, chrysos) and silver (argyros). These names were incorporated into the earliest cartographic representations of Mars. Giovanni Schiaparelli (1835-1910) made the most of Mars' proximity to Earth in 1877 to conduct extensive observations and map Mars in detail for the first time.

At that point, the 'Golden Plain', or Chryse Planitia, further north was also included in the nomenclature. Nereidum Montes, or 'Mountains of the Nereids' (named after the 50 daughters of Nereus, a sea god in Greek mythology, and his consort Doris, the daughter of Oceanos and Tethys), received its name only in the space age, when it became possible to identify smaller regional structures.

The images show a region characterised by an array of geological processes that occurred after the formation of the Argyre Basin four billion years ago. Water, ice (both on and beneath the surface) and, more recently, wind have all contributed to the erosion that has taken place here. Argyre was originally deeper than it is today. Eroded rock was transported into the basin by glaciers and flowing water, so that it gradually became partially filled.

Indications of ice beneath the surface
A striking network of small, branching valleys on the right-hand side of Image 1 attest to water having flowed over the surface from the edge of Argyre and into the basin. This came either from rainfall in the early days of Mars or from melted glacial ice. To this day, these drainage networks document the planet's water-rich past.

There has been a long and intensive debate as to whether there was actually a significantly warmer climate (warm and wet) in the early days of Mars. This would have enabled a water cycle with precipitation and a network of bodies of water on the surface, either over a longer period or perhaps just episodically.

Alternatively, given that the thin atmosphere offered little in the way of a greenhouse effect, the conditions may have been cold and wet, leading to ice age processes with, at most, precipitation of snow and the associated glacial phenomena. Sporadic volcanic activity or impacts events would have triggered thawing processes that melted the ice on the surface and subsurface, thus causing water to flow.

The latest findings show that the many dry river valleys on Mars were formed by at least four different run-off processes: surface run-off due to precipitation, melting glaciers, subglacial run-off beneath glacial ice masses and escaping groundwater (which in turn may have been formed by melting ground ice). Nereidum Montes is considered to be a key region for thoroughly testing these hypotheses. Due to its morphology, which is thought to be of glacial origin, it is likely that its features were formed by melting glacial ice.

Most of the impact craters in this region are filled with a material that has a striking concentric pattern on its surface. Such structures indicate the former presence of glaciers covered by rock debris, known as rock glaciers. The resulting landforms can also be seen as large-scale deposits in valleys between mountain ranges. It is thought that these landforms still contain water ice deep under a layer of debris that prevents sublimation - the vaporisation of ice in the low-pressure Martian atmosphere.

The power of the wind
Last but not least, the dark dune fields on the left-hand side of Image 1 testify to the power of the wind to transport grains of sand over large distances and around obstacles. The greyish-black dune sands that are common on Mars are of volcanic origin. They consist primarily of old, once-buried volcanic ash, which was often brought up from beneath the Martian surface by impact events. These sands often contain fragments of crushed lava rock and volcanic glass. As they were covered by Martian surface materials for a time, they were not transformed into lighter minerals by the action of water.

Based on the dune types, the direction of the wind that has formed the various dunes can easily be determined. In this example, some individual, crescent-shaped dunes, known as barchans, can be seen. These have grown together to form a barchanoid dune field at the end of the valley. In this case, the wind came from the southeast (bottom left in Image 1), blew into the valley, drove the sands before it and deposited most of the material at the foot of the mountains.

A high resolution slide deck of the images in this report are available here

Related Links
Mars Express at DLR
Mars News and Information at MarsDaily.com
Lunar Dreams and more

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