We're all familiar with what volcanoes are like on Earth. They come in different sizes and shapes, and eruptions can be different — some are explosive, some have arcing fountains, some just ooze out slowly — but they all have one thing in common: The lava is made of molten rock.
Earth has a thin, solid rocky crust over a very thick mantle, made of extremely hot (but not molten!) rock. Sometimes the mantle near the crust does melt, and this material rises and breaks through, causing volcanic eruptions.
In the outer solar system, from the asteroids and beyond, we also see evidence of volcanism — Saturn's moon Enceladus has geysers, for example, and the protoplanet Ceres has volcanic features as well. Many more such examples can be found. But in this case the "lava" isn't rock, it's water. We call this cryovolcanism. Cold volcanoes.
But there's a third kind. Called ferrovolcanism, these would be volcanic eruptions due to molten iron and other metals. It's never been directly observed before, but iron-rich flows from the El Laco volcano in Chile point toward ferrovolcanism. And there's a mysterious asteroid orbiting the Sun out past Mars called Psyche that may be a huge chunk of iron and nickel, and it may have had ferrovolcanism in its past as well (more on that in a sec).
Because it's never been seen, how ferrovolcanism behaves is mostly theoretical. To make matters a little more concrete empirical, a team of scientists from NCSU used the Syracuse Lava Project facility in New York to simulate a metallic volcanic flow. Using a huge crucible, they melted iron-rich basalt (collected from an ancient volcanic formation in the US Great Lakes region, which is about 50% silicates, 10% iron, and the rest various minerals), then poured it down a metal chute onto a base of sand to investigate how it flowed.
The molten silicate lava has a higher viscosity (it's thicker, like paint or syrup) and flowed slowly, at about 4 centimeters per second, forming a thick sheet with a ropey, rippling surface.
The molten iron lava is denser than the silicates, and with much lower viscosity (thinner consistency). Some spilled out along the top of the silicate and flowed down a channel very rapidly, moving at 41 cm/sec, ten times faster than the silicate.
The bulk of the iron, being denser, flowed through the silicate, and moved about 18 cm/sec (slower than on top, but then it's pushing through the silicate lava). They noted seeing three different pulses of rapid iron flow under the silicate, with the first two being violent and breaking out at the front of the flow, forming long tendrils called dendrites. The third was gentler, and created a pool of iron at the flow front.
When they cooled, the silicate flow was darker, quite black, while the metal flow was grayer. The silicate flow was glassy overall, but near the iron the silicate flow became more crystalline, and there was some mixing between the two substances where they were in contact.
OK, cool. Or, cooling. But what are the implications?
Looking at the equations of how flows occur, they find that this same sort of flow should happen even in lower-gravity environments, like on Mars or on an asteroid. Given the overall shape of the flows, this means it may be possible to identify ferrovolcanism on other astronomical bodies!
And that brings us back to Psyche. It is the ninth most massive object in the main asteroid belt between Mars and Jupiter, and the sixteenth largest overall. It's a misshapen space potato about 290 x 245 x 170 km in dimension.
It's been known for some time to be highly metallic, and recent measurements indicate its density is about 4 times that of water (water has a density of 1 gram per cubic centimeter, making it a standard unit); rocky asteroids have a density half that. Iron has a density of 8, so it's likely that Psyche is a mix of iron and rock. Also, spectra of Psyche match a mix of rock and metal best as well.
Psyche may have once been the core of a much larger asteroid. After it formed it was molten, and heavier material like metals sank to the center, while lighter rocky material rose to the surface. It's possible that not long after its materials separated (what's called differentiation) it was hit by a series of impacts that stripped off most of its rocky upper layers, exposing the core to space. There may have been some mixing at the surface, too.
It's also possible Psyche's metal core remained molten during this process. If so, it would have cooled from the outside in, forming a crust of solid metal, which shrank as it cooled. Compression would cause it to crack, and then molten metal could have flowed onto the surface.
Aha! So these experiments done with the rocky/metal mix may predict what the flows on the surface of Psyche look like (specifically, since the metal flow was low viscosity, they think eruptions on the surface may have a low profile; that is, they wouldn't be piled up very high since the liquid flow would spread rapidly). And, as it happens, NASA is currently in the process of building a mission to Psyche (confusingly also named Psyche) that is scheduled to launch in 2022 and arrive at the asteroid in 2026. When it gets there, we may very well see evidence of flows similar to what was seen in the experiment.
This is just so terribly cool. The asteroids we've looked at up close have all been rocky, so nothing like Psyche has ever been investigated in situ. It's not at all clear what the spacecraft will see when it gets there. But if it sees flows like these, then ferrovolcanism will no longer be theoretical.