New observations suggest the main-belt asteroid Psyche is an intriguing world, possibly one with volcanic flows of iron on its surface.
Walking on asteroid 16 Psyche might feel like walking on the rusty hull of a huge space freighter in a science fiction movie. According to a team led by Tracy Becker (Southwest Research Institute), iron and iron oxides make up parts of the asteroid’s surface.
NASA has picked the main-belt asteroid as the target for its eponymous Psyche mission, set for arrival in 2026, because the rock’s high density suggests it’s basically a lump of metals. The rock is 220 kilometers (140 miles) wide and is classified as M-type, where M stands for metallic.
“The idea is that it’s the iron-and-nickel core of a protoplanet,” says Becker, who presented her results at the virtual meeting of the Division of Planetary Sciences (DPS) on October 26th. “Subsequent collisions would have torn most of the crust and mantle away, revealing the planetary core.”
The high radar reflectivity of Psyche, as measured by the Arecibo telescope, had already suggested the presence of metals on its surface. Other observations also point to the presence of hydrated rocks – maybe leftovers from the stripped mantle. Now, Becker’s ultraviolet observations with the Hubble Space Telescope confirm surface iron and signs of oxidation. At least 10% of Psyche’s surface would need to be iron to explain the UV signal, the team reports in the December Planetary Science Journal.
The new Hubble observations seem to support the view that Psyche is a planetary core. But Lauri Siltala (University of Helsinki) is not so sure. At the same DPS meeting, he presented a new mass measurement for the asteroid. Combining that result with the latest size and shape estimates based on observations from the European Southern Observatory’s Very Large Telescope, he calculates a density of just 3.3 grams per cubic centimeter – much lower than expected for an iron-nickel body and lower even than the density of iron meteorites.
Siltala precisely measured the way in which Psyche’s gravity deflects the paths of small main-belt asteroids when they pass by. The technique has been used before to “weigh” the body, but Siltala says, “most earlier studies used a significantly lower number of ‘test asteroids’ than we did in our recent study. Therefore, I believe our results to be the most realistic.”
If the relatively low density is correct, Psyche might be a mix of metals and rocks after all, albeit more iron-rich than most asteroids. A process known as ferrovolcanism could force the core material of a differentiated and solidifying planetesimal into the overlying rocky mantle, according to a study that Brandon Johnson (Purdue University, West Lafayette, Indiana) and colleagues published in the September 16, 2019 Nature Astronomy. If the mantle is relatively thin, iron could even erupt on the surface, in which case Becker may have detected metallic lava flows.
Becker argues that a lower density doesn’t necessarily rule out the planetary core theory. “The core could be relatively porous, or mixed up with other material,” she says. The Psyche mission, to be launched in 2022, will still provide planetary scientists with a unique window on the interiors of planets, she believes. “Anyway, we need to get close to understand it fully.”