Rabu, 04 Juli 2012

Rocky outcrops on ancient mountain range on Mars could offer a window into planet's mysterious history

Rocky outcrops on ancient mountain range on Mars could offer a window into planet's mysterious history

  • Rocky outcrops excavated from deep beneath surface by asteroid impacts
  • Outcrops detected by Mars Express and Mars Reconnaissance orbiters
  • Hints that water flowed deep beneath the surface in Mars's past

By Rob Waugh

PUBLISHED: 09:43 EST, 4 July 2012 | UPDATED: 09:43 EST, 4 July 2012


Water once flowed beneath the surface of Mars - and the warmer, wetter ancient planet could have been considerably more hospitable for life.

Scans by spectrometers on board the Mars Express and Mars Reconnaissance Orbiters have revealed 175 outcrops in Mars's Tyrrhena Terra mountains that are unmistakably 'weathered' by water.

The rocks, found in craters, seem to have been 'carved' out from beneath the crust by asteroid impacts - and offer an insight into the planet's history, and what lies beneath the surface.

The find proves water flowed beneath the surface of Mars for a 'long period', say the researchers.

Scans by spectrometers on board the Mars Express and Mars Reconnaissance Orbiters have revealed 175 outcrops in Mars's Tyrrhena Terra mountains that are unmistakably 'weathered' by water

Scans by spectrometers on board the Mars Express and Mars Reconnaissance Orbiters have revealed 175 outcrops in Mars's Tyrrhena Terra mountains that are unmistakably 'weathered' by water

The find of 175 outcrops with weathered minerals in a region of ancient highlands, Tyrrhena Terra, could reveal more about the planet's mysterious history.

The find of 175 outcrops with weathered minerals in a region of ancient highlands, Tyrrhena Terra, could reveal more about the planet's mysterious history

Today, atmospheric pressure is so low on Mars that surface water cannot exist on its surface.

However, studies made by orbiting spacecraft and surface rovers show that Mars was once much warmer and wetter than it is today.

The find of 175 outcrops with weathered minerals in a region of ancient highlands, Tyrrhena Terra, could reveal more about the planet's mysterious history.

The spacecraft observations revealed hydrated silicate-bearing outcrops at 175 different sites in Tyrrhena Terra, almost all of which are associated with ecrater walls and rims, and central uplifts inside craters.

Although the region is dissected by numerous valley networks, rocks altered by water are rarely visible between the craters in Tyrrhena Terra.

This implies that the chemical weathering revealed by the craters was not associated with weathering by liquid water at the surface.

‘These minerals are associated with fresh craters that happened after activity caused by water,’ said Damien Loizeau, an ESA Research Fellow and lead author of the study. ‘They probably represent ancient hydrated terrains that have been excavated by the crater-forming impacts. These impacts have revealed the composition of the Noachian crust, which was chemically weathered more than 3.7 billion years ago.’

Their preferred explanation is that the hydrated minerals were formed underground in the presence of ground water.

The rocks, found in craters, seem to have been 'carved' out from beneath the crust by asteroid impacts - and offer an insight into the planet's history, and what lies beneath the surface.

The rocks, found in craters, seem to have been 'carved' out from beneath the crust by asteroid impacts - and offer an insight into the planet's history, and what lies beneath the surface.

The buried hydrated silicate-bearing rocks were then exposed and exhumed by later impacts. These impacts transported the hydrated minerals mechanically, without significant mineralogical transformations.

The presence of a few phyllosilicate-bearing outcrops on a few buttes in the region, far from visible craters, also reinforces the idea that the crust has been broadly altered, not only around craters.

Phyllosilicates are hydrous minerals formed by interaction between rock and liquid water. This important group of minerals includes the micas, chlorite, serpentine, talc and clays - primary products of chemical weathering of rocks.

The minerals form at different temperatures and seem to be located at different depths, consistent with temperatures of up to 300 degrees Celsius a few kilometres beneath the surface.

The presence of hydrated silicates on larger craters of the smooth plains can be explained by excavation: the lava plain i s thin enough for larger craters to excavate rocks underneath, from the buried Noachian unit. Hence, hydrated silicate-bearing craters in volcanic plains illustrate the variation in plain thickness and excavation by impacts of buried hydrated rocks.

‘The large range of crater sizes studied, from less than 1 km to 84 km wide, indicates that these hydrated silicates are were excavated from depths of tens of metres to kilometres,’ said Damien Loizeau. ‘The composition of the rocks is such that underground water must have been present there here for a long period of time in order to have altered their chemistry.’

‘Water circulation occurred several kilometres deep in the crust some 3.7 billion years ago, before the majority of craters formed herein this region,’ said co-author Nicolas Mangold, from the Laboratoire de Planétologie et Géodynamique, Université de Nantes, France. ‘The water generated resulted in a wide range of chemical chang es in the rocks that reflect low temperatures near the surface to high temperatures at depth, but without any a direct  relationship to the surface conditions at the that time.’

‘The role of liquid water on Mars is of great importance for its habitability and this study using data from Mars Express describes a very large zone where groundwater was present for a long time,’ said Olivier Witasse, ESA's Mars Express project scientist.

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