Metal Shards Shed Light on the Origin of Asteroid Kalliope

The asteroid 22 Kalliope, a metal-rich denizen of the main belt between the orbits of Mars and Jupiter, is the densest asteroid known. Now, an astronomer has found that a number of small asteroids with orbits similar to Kalliope’s are likewise made mainly of iron. The finding suggests that the planetesimal that they and Kalliope were once part of had a metal core like Earth’s.

Chrysa Avdellidou (University of Leicester, UK) announced the analysis of the Kalliope family of asteroids on April 29th at a meeting of the European Geosciences Union in Vienna, Austria; the results were also published online in the Monthly Notices of the Royal Astronomical Society.

Metal Detector

In 2022, Miroslav Brož (Charles University of Prague) and colleagues concluded that a group of objects had incorrectly been associated with an asteroid called San Marcello; their orbits instead suggest they belong to the same family as Kalliope, all broken off a parent body in ancient collision.

For Avdellidou, that correction was an opportunity to find out what Kalliope itself is made of. "We immediately started to observe them,” she says. From earlier work and from observations from the European Space Agency’s Gaia mission that ended operations in January this year, she obtained spectra for 22 of the objects.

“We used all the spectra that we have in two very big repositories of meteorites, and compared them [to the asteroids’ spectra],” she explains. “We saw that they matched with iron and with stony iron meteorites.”

Avdellidou thinks all of the metallic pieces — including Kalliope, which is 166 kilometers (103 miles) across, and the much smaller objects of its family — were struck from the core of a planetesimal during a violent collision early in the solar system’s history.

Planetesimal Origins

Planetesimals are thought to have been the first solid bodies to arise from the disk of gas and dusk that ultimately became the solar system. Through many collisions and mergers, most of that material ended up building the planets. But millions of remnants of those collisions found a stable home in the asteroid belt between Mars and Jupiter.

“The belt is our test zone,” says Avdellidou. The objects in it “formed in different locations and that's why they have different compositions.”

Astronomers already thought Kalliope came from the core of a former planetesimal, due to its high density, and the discovery of metallic family members strengthens that hypothesis.

There are not many metallic-type asteroids. Perhaps the most famous of the select group is Psyche, the 220-kilometer body to be explored by the NASA spacecraft of the same name. But Kalliope is the only one with a family to investigate.

The iron-dominated objects in the Kalliope family probably came from the core of the planetesimal, Avdellidou says, while the ones with higher proportions of stone came from layers closer to the surface.

This result puts the parent planetesimal of Kalliope in the category of differentiated objects in the solar system, just like Earth. These have dense, metal-rich cores because they were hot enough for their material to melt, allowing heavier elements such as iron to sink to the center, leaving stony material on the outside.

Planetesimals’ heat came from the decay of radioactive isotopes of aluminum and iron. These have relative short half-lives (717,000 years and 2.6 million years, respectively) and so were only abundant in planetesimals that formed early on. The later a planetesimal formed, the less internal heat it had, and the less differentiated it ended up.

In view of its iron core, says Avdellidou, Kalliope’s parent object must date from 500,000 years to 1 million years after the solar system’s protoplanetary disk formed.