Mars volcanoes erupted water
31 July 2009
Ascraeus Mons, one of the biggest volcanoes on Mars, expelled boiling water instead of lava during its last eruptions one billion years ago. The water was powerful enough to carve sinuous channels on the volcano slope that ended in large mudflows.
Artist's impression of the Mars Express spacecraft.
By Earth's standards, Ascraeus Mons is a giant: 400 km in diameter and rising up to 18 km in altitude - twice the height of Mount Everest.
Dr John Murray, from the Open University, led an international team to analyse images of the volcano sent by Mars Express since the spacecraft started orbiting the red planet in 2004. The resolution of the photos is super-high and allows the researchers to identify structures on the surface as small as two metres across.
The photos revealed a complex landscape surrounding Ascraeus Mons, including cones, lava flows and sinuous channels, one to two km wide and 20-50 km long. Murray was hoping the photos would help to figure out the origins of the unusual network of channels around the volcano.
'We calculated the amount of lava needed to carve the channels on the surface,' says Murray. 'The amount is enormous,' he says, more than 200 times the volume of long lava flows on Mars. 'Our model shows that the channels were not carved by lava, so it has to be water.'
Photos from the Mars Express orbiter show volcanic cones, lava flows and sinuous channels (click to enlarge).
'This is very different from what we see on Earth, but not so surprising on Mars,' says Murray, considering that the top six kilometres of the crust of Mars are extremely rich in frozen water.
Murray explains: 'as the magma rises and lava erupts at the surface, the volcano builds up and melts the water within the crust.' The molten water is then expelled outward, as the weight of the volcano pushes the whole edifice downwards.
Since Mars has lower atmospheric pressure than Earth, the water boils very quickly as it erupts from the sides of the volcano. If the water discharge rate is high enough, the surges of boiling water are capable of carving channels down the slope, creating gigantic mudflows.
The sinuous channels and mudflows cut across all other structures visible on the images. This means that the water eruptions probably occurred in of the last stages of the volcano's activity.
Some of the channels may have been carved beneath the surface, as the water flowed underground, and were then exposed when the roof of the channel collapsed. The hypothesis of an underground network of channels raises an intriguing possibility: 'if we had water flowing below the surface, we have a warm and wet environment, protected from cosmic radiation. This is a great place for life to develop,' suggests Murray.
Although the idea of underground water channels might seem odd, Murray found an example on Earth. The Piton de la Fournaise volcano, in Réunion Island in the south Indian Ocean, is surrounded by a network of channels very similar to the Martian channels surrounding Ascraeus Mons. The Réunion channels are formed when rainwater penetrates the porous volcanic rocks and flows down hill through an underground network.
Murray thinks that comparing Martian structures with terrestrial examples is extremely useful. 'It gives us a lot of ideas and brings us "down to Earth", so to speak,' he says. 'It helps us to ground our interpretations of Mars on mechanisms that we understand and know how to work on Earth.'
The results, published this week in the journal Earth and Planetary Science Letters, help to reconstruct the history of Ascraeus Mons and 'provide us with a framework to interpret the structures around other volcanoes in Mars,' says Murray.
John B. Murray, B. van Wyk de Vries, Alvaro Marquez, David A. Williams, Paul Byrne, Jan-Peter Muller, Jung-Rack Kim. Late-stage water eruptions from Ascraeus Mons volcano, Mars: Implications for its structure and history. Earth and Planetary Science Letters.
Available online 25 July 2009, doi:10.1016/j.epsl.2009.06.020
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