Washington: Scientists have long suspected that the 'fire-breathing' volcanoes that spread large quantities of flowing lava over Mars were not the only kind. The numerous mountain cones in the northern hemisphere of the Red Planet may be the result of mud volcanoes.
However, until now, researchers have lacked knowledge about the behaviour of water-rich mud on the surface of Mars. An unusual laboratory experiment involving the German Aerospace Center (Deutsches Zentrum fuer Luft- und Raumfahrt; DLR) has now been able to show how mud flows at very low temperatures and under reduced atmospheric pressure. It behaves in a similar way to very specific lava flows on Earth.
The results, which have now been published in the journal Nature Geoscience, add important details to the existing knowledge of Mars and its history, which has been shaped by volcanic activity.
"We have long been aware that in the early history of Mars, several billion years ago, large amounts of water were released over a short period of time, eroding very large valleys in the landscape, which have long since dried up," explains Ernst Hauber of the DLR Institute of Planetary Research in Berlin-Adlershof, who was involved in the study.
"Extensively eroded masses of fragmented rock were transported through these outflow channels and into the northern lowlands of the planet, where they were quickly deposited. Later, these rocky masses were covered by younger sediments and volcanic rocks," Hauber says.
Some Mars researchers had previously suspected that these underground, water-rich sediments could have become liquefied under certain circumstances and been pushed back up to the surface under pressure. In reference to the similar rise of magma, this process, which is well documented in many sedimentary basins on Earth, is referred to as sedimentary volcanism or mud volcanism.
Are small volcanic cones the result of mud extrusions? Tens of thousands of conical hills populate the northern highlands of Mars, often with a small crater at their summit. These may be the result of mud volcanism. However, the evidence for this is not easy to acquire. This is due to the fact that little is known about the behaviour of low-viscosity mud under the environmental conditions on the Martian surface.
To fill this knowledge gap, a group of European scientists carried out a series of experiments in a cylindrical low-pressure vessel 90 centimetres in diameter and 1.8 metres long, in which water-rich mud was poured over a cold sandy surface. Apart from the gravity on Mars, which could not be simulated, this experimental setup was somewhat reminiscent of building a large sandcastle under Mars-like conditions.
How does watery-mud move on Mars? Exploration of Mars has revealed the presence of large outflow channels which have been interpreted as the products of catastrophic flood events during which a large quantity of water was released from the subsurface. The rapid burial of water-rich sediments following such flooding may have promoted an ideal setting to trigger sedimentary volcanism, in which mixtures of rock fragments and water erupt to the surface in the form of mud.
The aim of these unusual experiments was to find out how the changed physical parameters influence the water component of the mud and thus alter its flow behaviour. The results came as a surprise.
"Under the low atmospheric pressure of Mars, the mud flows behave in much the same way as 'pahoehoe', or 'ropy', lava, which is familiar from large volcanoes on Hawaii and Iceland," says the lead author of the study, Petr Broz of the Czech Academy of Sciences.
These findings were somewhat unexpected, as comparable geological processes on other bodies in the Solar System are thought to occur in a similar way to conventional volcanic processes on Earth.
"Our experiments show that even a process as apparently simple as the flow of mud - something that many of us have experienced for ourselves since we were children - would be very different on Mars," Broz adds.
Did water-rich sediments also reach the Martian surface? Water that flowed over the surface of Mars billions of years ago transported large quantities of sediments to the northern lowlands, where they were later covered by younger sediments and volcanic rocks. Some Mars researchers suspect that these water-rich sediments became liquefied underground and rose back to the surface under pressure -- similar to the hot 'mud spring' at Bakhar in Azerbaijan (diameter approximately 1.5 metres).
Experiments in a low-pressure chamber, in which DLR scientist Ernst Hauber was involved, have now shown that the flow behaviour is similar to that of what is referred to as 'ropy lava' (or, according to the Hawaiian term for smooth, unbroken lava, also known as ' pahoehoe' lava), which is at a temperature of several hundred degrees Celsius.
This implies that mud flows on Mars take a completely different course than those on Earth. This observation could support the assumption that many of the conical hills with central craters discovered in the north of Mars are also mud volcanoes.
