Just this week, NASA announced that it had discovered one of the youngest, possibly the youngest major volcano on Mars to date.
Arsia Mons – a gigantic, 110-kilometer-long (68-mile-long) caldera, which is also more than twice the height of Everest – had been known about before, but new satellite observations were used to get a better timeline for its extensive lava flows. The youngest is likely to be around 50 million years old.
Although this was extensively reported as being a volcano that died alongside the dinosaurs, in reality, this meant it handsomely outlived the lumbering beasties by around 16 million years. Weirdly, this volcano only tended to erupt once every 1 to 3 million years, whereas shield volcanoes on Earth erupt almost continuously. Hawaii’s Kilauea, for instance, hasn’t stopped spewing lava since 1983.
You might have also noticed that Earth is covered in volcanoes, whereas Mars, although pockmarked by volcanic provinces, doesn’t appear to have any active volcanoes left on it. So what’s the deal? Why is Mars volcanically defective, but our own pale blue dot is alive with the sound of explosions?
First, let’s go through what you need to make a volcano. Essentially, all that’s required is a heat source – and as far as we know, there are three of them powerful enough to make a difference.
When rocky planets form via the violent accumulation of space debris, they get incredibly hot. By the time they reach a certain size, the intense gravitational pressure of the celestial body ensures the heart of the planet is at an unfathomably high temperature. This massive, ancient cache of thermal energy is known as “primordial heat.”
As the world begins to cool and different parts of the sphere differentiate into a crust, a mantle and a core, a temperature and pressure gradient is set up, with both variables dropping off dramatically as you get closer to the surface. In order to balance out the gradient, this primordial heat escapes to the surface, and this heat – in many different, complex ways – is needed to melt rock, produce magma, and fuel volcanoes.
This isn’t the only source of heat, however. Plenty of elements within rocky worlds are radioactive, and over time, they decay, which also releases heat. On Earth, this “radiogenic heat” provides up to 50 percent of the planet’s internal thermal energy – on Pluto, where liquid lakes of nitrogen exist, radiogenic heat is thought to be entirely responsible for that dwarf planet’s internal pyre.
If you look at Io, the volcanic moon of Jupiter, you’ll spot more volcanoes there than perhaps anywhere else in the Solar System. This hellish domain’s volcanism is driven by something called “tidal heating,” where the immense gravitational pull of Jupiter – and the amplification provided by the orbits of nearby moons Europa, Ganymede and Calisto – rips apart Io’s insides. This generates frictional heating, which melts rock and provides the moon’s plentiful volcanoes with a near-perpetual supply of magma.
So, those are our three heat engines. Earth, being quite sizeable and full of radioactive elements, is still plenty hot on the inside. Consequently, we have a huge range of volcanoes, from hotspot monsters powered by upwelling fountains of fire from the outer core-mantle boundary, to complex ones that form when one tectonic plate descends beneath another.
Mars, however, is just over half the size of Earth, and as a result, it’s already cooled down so much that there isn’t enough primordial heat left to keep magma molten anymore. There clearly isn’t enough radioactivity inside it to keep the furnace going either, and there’s nothing massive enough nearby to generate significant tidal heating. So today, it’s a dead planet.
In fact, Mars was perhaps always a failed volcanic planet in some respects. Plate tectonics is required to create “evolved” magma compositions and a variety of volcanoes – and there’s no strong evidence that it was ever in operation on our planetary neighbor. Without the movement of continents, the chemistry of the planet becomes fairly dull and your volcanic repertoire is sorely limited.
So what kind of volcanoes were active on Mars? Mainly, you could find shield volcanoes – unbelievably large mountains that are far wider than they are tall, powered by colossal mantle plumes. Take Olympus Mons, the largest volcano ever discovered, for example: this titan is 25 kilometers (16 miles) high but it’s also a mind-blowing 624 kilometers (374 miles) long. It is about the same size as Arizona.
Shield volcanoes on Earth are so big because they are fueled by mantle plumes for many millennia. The lava that erupts is basic, unevolved and extremely fluid, which means it spreads out over long distances. Give these shields enough time, and they grow to immense sizes.
Mars is no exception, but all its shield volcanoes are far more sizeable than our own. Why is that?
Well, on Earth, our tectonic plates gradually and continuously move around, but the mantle plumes underneath them stay still. So in 100,000 years or so, Hawaii’s Kilauea will be extinct, and the baby volcano growing off its shore, Loihi, will become the primary source of volcanism in the region.
Mars, as aforementioned, doesn’t have plate tectonics, which means that the underlying mantle plumes just kept on melting rock beneath the same spot for millions of years. The shield volcanoes there got so large that if they were any more massive, they’d break the Martian crust and sink back into the mantle.
There are a few other types of volcanic activity on Mars too. In the past, there was a lot more water (and ice) available in certain regions of the planet. Under certain mixing conditions, the interaction of water and magma causes a powerful explosion and a very rapid release of heat into the surrounding environment.
When magma is involved, this is known as a phreatomagmatic blast – when it’s not, and hot rock is the trigger, it’s known as a hydrothermal blast. Based on characteristically glassy deposits spotted by orbiting satellites, scientists know that both types occurred frequently on Mars in the distant past.
Mostly, however, Mars was once dominated by lava flows, some reaching the sizes of some of Earth’s continents. Nowadays, it’s dead quiet there, but it was once a far more impressive world.
In fact, around 3.5 billion years ago, a gargantuan eruption produced so much lava for so long that it effectively emptied out part of the mantle onto the top of the crust. This had the effect of tipping Mars over by 20°, which would be like moving the North Pole to Paris or Lisbon to the Sahara Desert.
Arsia Mons represented the last significant period of volcanic activity on the Red Planet that we know of. The youngest lava flow anywhere on Mars, though, is no less than 10 million years old – the last gasp of a tiny Martian magma chamber, and the last light to go out.