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 für 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." 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.
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 'pāhoehoe', or 'ropy', lava, which is familiar from large volcanoes on Hawaii and Iceland," says the lead author of the study, Petr Brož 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."
Major impact of low atmospheric pressure
The key reason for the flow behaviour of the mud is the very thin atmosphere of Mars. The pressure is 150 times lower than the pressure at sea level on Earth. This difference has a major impact. Under such conditions, liquid water on the Martian surface is not stable and begins to boil and evaporate. This process absorbs latent heat in the vapor and cools the remaining mud, which then freezes at its surface, forming a crust. In a phase transition, such as during a freezing or thawing process, latent heat is released or absorbed by a material without changing its temperature. “Of course, we already know that liquid water begins to boil sooner under low pressure – that is why, for example, it takes longer to cook pasta on a camping stove on high mountains on Earth," explains Hauber. "However, the impact of this familiar effect on mud has never been investigated in an experiment before. Once again, it turns out that different physical conditions must always be taken into account when looking at apparently simple surface features on other planets. We now know that we need to consider both mud and lava when analysing certain flow phenomena," adds Hauber.
The team of researchers were able to show in detail that the mud flows in the experiment behaved like pāhoehoe lava, with liquid mud spilling from ruptures in the frozen crust, and then refreezing to form a new flow lobe. When mud escapes onto the Martian surface, it is able to flow for some time before it solidifies due to the low temperatures. However, the morphology – the shape of the mud flows – is different from those found on Earth. The research work that is currently being carried out is important for investigations of other planetary bodies, because similar processes may also play a role in cryovolcanic eruptions, in which liquid water comes to the surface, instead of magma or mud, such as on icy moons in the outer Solar System.