The giant asteroid Psyche may not be what scientists think
Psyche, orbiting the sun in the asteroid belt between Mars and Jupiter, is the largest of the M-type asteroids, which are composed mainly of iron and nickel in contrast to the silicate rocks that make up most of the other asteroids. But when viewed from Earth, Psyche sends conflicting signals about its composition.
The light it reflects has suggested to scientists that the surface is mostly metallic. This has led to the conjecture that Psyche may be the exposed iron core of a primordial planetary body, one whose rocky crust and mantle were swept away by an ancient collision. However, Psyche's measurements of mass and density tell a different story. The way its gravity acts on nearby bodies suggests that Psyche is far less dense than a giant piece of iron should be. So if Psyche is really all metal, it should be highly porous, a bit like a giant ball of steel wool with nearly equal parts of empty space and solid metal.
“What we wanted to do with this study was to see if it was possible for a Psyche-sized iron body to maintain that porosity close to 50 percent, ”said Fiona Nichols-Fleming, a PhD student at Brown and lead author of the study. "We have found that it is very unlikely."
The study was published in JGR: Planets. Nichols-Fleming worked with Alex Evans, an assistant professor at Brown, and Purdue professors Brandon Johnson and Michael Sori. The team created a computer model, based on the known thermal properties of metallic iron, to estimate how the porosity of a large iron body would evolve over time.
The model shows that for remain highly porous, Psyche's core temperature should cool below 800 Kelvin shortly after its formation. At temperatures above those, the iron would have been so malleable that Psyche's gravity would have collapsed most of the pore space within its mass. Based on what is known about the conditions of the early solar system, the researchers say, it is extremely unlikely that a body the size of Psyche, approximately 225.3 kilometers in diameter, could have cooled so rapidly.
Furthermore, , any event that may have added porosity to Psyche after its formation, a massive impact for example, would probably also have heated Psyche over 800 K. So any newly introduced porosity would have been unlikely to last.
Taken together, The findings suggest that Psyche is likely not a porous, all-ferrous body, the researchers conclude. More likely, it is hosting a hidden rock component that lowers its density. But if Psyche has a rocky component, why does its surface look so metallic when viewed from Earth? There are few possible explanations, the researchers say.
One such possibility is rail-volcanism, volcanoes that erupt iron. It is possible, the researchers say, that Psyche is actually a differentiated body with a rocky mantle and an iron core. But widespread rail-volcanic activity may have brought large amounts of Psyche's core to the surface, placing an iron-clad covering on its rocky mantle. Previous research by Johnson and Evans has shown that rail-volcanism is possible on a body like Psyche. In any case, scientists will soon have a much clearer picture of this mysterious asteroid. By the end of the year, NASA plans to launch a spacecraft that will meet with Psyche after a four-year journey in the asteroid belt. “The mission is exciting because Psyche is such a bizarre and mysterious object,” said Nichols-Fleming. “So everything the mission finds will be really important to the solar system.”
