Astronomers capture most detailed images of large, rare metal asteroid

Astronomers have used the Atacama Large Millimeter/Submillimeter Array (ALMA) to gain a more detailed picture of the composition of the asteroid Psyche. The asteroid, which orbits the Sun at a distance of between 179.5 and 329 million kilometres from Earth, is the target of a NASA mission scheduled to arrive in 2026, and the latest results suggest that its surface is both rocky and highly metallic.

Psyche was discovered in 1852 by the Italian astronomer Annibale de Gasparis. It is classed as an M-type asteroid, meaning that its spectrum resembles that of an iron meteorite, and its diameter of more than 200 km means that it is the largest of its kind ever found.

Asteroid observations

In principle, Psyche’s thermal emissions could offer further clues to its composition. By monitoring the heat radiated from its surface, scientists can infer its thermal inertia, which measures “how much the surface heats during the day and cools at night,” explains Katherine de Kleer, an assistant professor of planetary science and astronomy at the California Institute of Technology and the first author of a paper in The Planetary Science Journal describing the latest measurements. However, Psyche’s small size and relatively large distance from Earth makes it difficult to get in-depth observations of the asteroid’s surface using ground-based infrared detectors. Past efforts have produced only single-pixel images.

To overcome this problem, de Kleer and her colleagues combined data from the 66 radio telescopes that make up the ALMA facility in Chile. By observing all of Psyche’s surface multiple times per day, the team gained more insight into its thermal inertia, producing 50-pixel images.

Metallic composition inferred from thermal emissions

These new images reveal that some regions of the asteroid have surface temperatures different from the average, indicating that Psyche’s composition is not uniform. The researchers also found that Psyche has a relatively high thermal inertia compared to other asteroids, yet it radiates approximately 60% less heat than would be expected for an object with such a high inertia. The researchers hypothesize that this is because the asteroid’s surface is at least 30% metal. However, the light reflecting off Psyche’s surface is unpolarized, which would not be the case for an object with a smooth or solid metallic surface. They therefore hypothesize that metallic grains are spread throughout its surface material, causing the light to scatter.

If Psyche is mostly metal, it could mean that it is a pre-planet that had a core, mantle and crust before it suffered a major collision with another object. Alternatively, the researchers suggest that an abundance of enstatite chondrite on Psyche’s surface could indicate that the asteroid formed in a different area of the solar system, from material being accreted closer to the Sun. 

While this study is the first to make highly spatially-resolved thermal observations of an M-type asteroid, and to map its thermal emissions, the nature of millimetre observations of rocky and metallic astronomical objects makes it impossible to measure their composition more than a few millimetres below the surface.  NASA’s Psyche mission, which is due to launch in August 2022 with various payloads, will explore the asteroid’s composition further, with the goal of determining whether it is a core fragment or unmelted material. According to de Kleer, “both possibilities remain viable” for now, though she hopes “that future observations, whether from Earth or from the Psyche mission, may be able to differentiate between them”.