The asteroid 16 Psyche, which NASA intends to visit with a spacecraft in
2026, may be less heavy metal and more hard rock than scientists have
surmised, according to a new study by researchers from Brown and Purdue
universities.
Psyche, which orbits the sun in the asteroid belt between Mars and Jupiter,
is the largest of the M-type asteroids, which are composed chiefly of iron
and nickel as opposed to the silicate rocks that make up most other
asteroids. But when viewed from Earth, Psyche sends mixed signals about its
composition.
The light it reflects tells scientists that the surface is indeed mostly
metal. That has led to conjecture that Psyche may be the exposed iron core
of a primordial planetary body—one whose rocky crust and mantle were blasted
away by an ancient collision. However, measurements of Psyche's mass and
density tell a different story. The way its gravity tugs on neighboring
bodies suggests that Psyche is far less dense than a giant hunk of iron
should be. So if Psyche is indeed all metal, it would have to be highly
porous—a bit like a giant ball of steel wool with nearly equal parts void
space and solid metal.
"What we wanted to do with this study was see whether it was possible for an
iron body the size of Psyche to maintain that near-50% porosity," said Fiona
Nichols-Fleming, a Ph.D. student at Brown and study's lead author. "We found
that it's very unlikely."
For the study, published in Journal of Geophysical Research: 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 known thermal properties of metallic iron, to
estimate how the porosity of a large iron body would evolve over time.
The model shows that to remain highly porous, Psyche's internal temperature
would have to cool below 800 Kelvin very shortly after its formation. At
temperatures above that, iron would have been so malleable that Psyche's own
gravity would have collapsed most of the pore space within its bulk. Based
on what is known about conditions in the early solar system, the researchers
say, it's extremely unlikely that a body of Psyche's size—about 140 miles in
diameter—could have cooled so quickly.
In addition, any event that may have added porosity to Psyche after its
formation—a massive impact, for example—would likely have also heated Psyche
back up above 800 K. So any newly introduced porosity would have been
unlikely to last.
Taken together, the results suggest that Psyche probably isn't a porous,
all-iron body, the researchers conclude. More likely, it's harboring a
hidden rocky component that drives its density down. But if Psyche does have
a rocky component, why does its surface look so metallic when viewed from
Earth? There are few possible explanations, the researchers say.
One of those possibilities is ferrovolcanism—iron-spewing volcanoes. It's
possible, the researchers say, that Psyche is actually a differentiated body
with a rocky mantle and an iron core. But widespread ferrovolcanic activity
may have brought large amounts of Psyche's core up to the surface, putting
an iron coating atop its rocky mantle. Prior research by Johnson and Evans
has shown that ferrovolcanism is possible on a body like Psyche.
Whatever the case, scientists will soon get a much clearer picture of this
mysterious asteroid. Later this year, NASA plans to launch a spacecraft that
will rendezvous with Psyche after a four-year journey to the asteroid belt.
"The mission is exciting because Psyche is such a bizarre and mysterious
thing," Nichols-Fleming said. "So anything the mission finds will be really
important new data points for the solar system."
Reference:
Fiona Nichols‐Fleming et al, Porosity Evolution in Metallic Asteroids:
Implications for the Origin and Thermal History of Asteroid 16 Psyche,
Journal of Geophysical Research: Planets (2022).
DOI: 10.1029/2021JE007063
Tags:
Space & Astrophysics