Volcanic Slopes and Plains

An ancient voice grumbles through the air and the ground. A tall tower rules above the desert. Its fiery breath melts its snow-capped peak. The waters dribble down the talus to end up in a pathetic but vital stream. A meagre gift by the underworld gods to the inhabitants of these harsh sands.

One of the bigger surprises our geologists faced on Mars was that many of the volcanoes are very much still active. The surprisingly frequent eruptions might very well be what still keeps this planet alive, as without their gases much of the atmosphere might already have been carried away by the unrelenting solar rays. Volcanoes form such an important part of the Mars system because the planet has long ago lost any semblance of plate tectonics. On Earth, the mechanic movements of the plates are the main function by which the molten interior of the planet gives off its heat and energy. On Mars, volcanoes have instead become the main outlet, often growing to titanic sizes, as the massive amounts of magma fizzle out for millions of years in the same spot with no tectonic plate moving over their hot spot. Imagine all the islands of Hawaii being stacked on top of each other instead of standing in a line. This is how you get something like Olympus Mons, the tallest mountain in the entire solar system, which is not only nearly 22 km tall, but also has a surface area the size of Poland.

The vast majority of Martian volcanoes are such Hawaiian-type shield volcanoes, though they still differ in many ways from the shield volcanoes on Earth. The low gravity causes a greater accumulation of gas bubbles in the upwelling lava, which means that Martian shield volcanoes can erupt explosively, with massive clouds of ash and even pyroclastic flows, something more typical of Plinian eruptions on Earth. Due to the gravity, the magma also becomes less buoyant, meaning that the magma chambers feeding the volcanoes lie much deeper underground and are much bigger. Due to the depth, this means only the biggest of the big magma chambers actually reach the surface to cause eruptions. Mars therefore has a lower rate of eruptions than Earth, but when an eruption does happen it usually results in a titanic catastrophe with much wider and longer-lived lava flows on the surface. Such events have thankfully not happened yet during our stay, but several mountains, especially Elysium Mons, are under close observation.

The two major volcanic provinces on Mars are the Tharsis Plateau and Elysium Planitia. Tharsis consists of the aforementioned Olympus Mons, Alba Mons to its north-east and a chain of three shield volcanoes, Tharsis Montes, to its south-east, all arranged in a suspiciously straight line and consisting of Ascraeus Mons, Pavonis Mons and Arsia Mons. Between these five mountains exist various series of smaller volcanic cones, formerly called either tholi or paterae. Today it is recognized that these are not a separate type of volcano, as formerly believed, but that tholi and paterae are instead simply the top cones of much older shield volcanoes, whose bases have been buried by billions of years of lava flows. Life on much of Tharsis is harsh if not downright impossible. The height makes the air extraordinarily thin and the temperatures predictably low. Nearly the whole region is below the 0°-isotherm and almost all the area of Olympus, Alba and the Tharsis Montes is covered in large ice sheaths that are connected to the major glaciers of the South Pole. Only the most extremophilic microbes can cling to these environments. Conditions become only marginally better in the lower plateau between Alba Mons and the Tharsis Montes, where temperatures rise above freezing for only 67 sols (out of 668 per year). On the eastern slopes of Alba Mons leading down into the northern lowlands, conditions become dramatically better, with 268 sols spent above melting point. In those faint summers, the permafrost thaws and the rims of the glaciers above start to melt. The mountain valleys thus become inundated in stagnant bogs and ponds and the flora springs back to life from its deep winter sleep. The bogs are often anoxic, while the melting permafrost releases sulphuric molecules from the volcanic soil, making this environment ideal for anaerobic life. Primitive arephytes are predictably abundant in these Martian Alps.

The other major volcanic province, Elysium Planitia, lies at the edge of the eastern hemisphere and consists of Elysium Mons, Albor Tholus and Hecates Mons. Together they form a somewhat circular feature which stands out of the otherwise flat northern hemisphere like an island. Elysium is not as tall as Tharsis and is not wholly covered by an ice sheath. Conditions are thus generally more hospitable, but still precarious. Our geologists suggest that only a few tens of thousands of years ago an eruption had occurred on the west of Elysium Planitia that may have devastated an area the size of the United Kingdom. An explosive eruption first seems to have thawed all the permafrost of the mountain’s peak, leading to a catastrophic flash flood, followed by pyroclastic flows, streams of lava and massive ash fall. Time will tell if such an event might occur again in our lifetimes.

Apart from these two regions, there are smaller, solitary volcanoes, some active, some dormant, spread all throughout Mars. Hadriacus Mons near the Hellas Basin is one example, as is Eden Patera in Arabia Terra.