ALMA Creates Detailed Maps of Main-Belt Asteroid Psyche

The Atacama Large Millimeter/submillimeter Array located on the Chajnantor Plateau in Chile allowed the acquisition of temperature images of (16) Psyche, an M-type asteroid of about 220 km in size, at a resolution of 30 km/pixel — the highest ever achieved from Earth. In new research, scientists from MIT, Caltech and Bloomsburg University analyzed these images to map the metal content of the first millimeters of the asteroid’s surface as function of longitude and latitude. The new maps hint at Psyche’s history: its rocky regions could be vestiges of an ancient mantle or the imprint of past impacts by space rocks; craters that contain metallic material support the idea proposed by previous studies that Psyche may have experienced early eruptions of metallic lava as its ancient core cooled.

“Psyche’s surface is very heterogeneous,” says lead author Saverio Cambioni, the Crosby Distinguished Postdoctoral Fellow in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS).

“It’s an evolution surface, and these maps confirm that metal-rich asteroids are interesting, enigmatic worlds. It’s another reason to look forward to the Psyche mission going to the asteroid.”

The surface of Psyche has been a focus of numerous previous mapping efforts.

Researchers have observed the asteroid using various telescopes to measure light emitted from the asteroid at infrared wavelengths, which carry information about Psyche’s surface composition.

However, these studies could not spatially resolve variations in composition over the surface.

Dr. Cambioni and colleagues instead were able to see Psyche in finer detail, at a resolution of 30 km/pixel, using the combined power of the 66 radio antennas of the Atacama Large Millimeter/submillimeter Array (ALMA).

Each antenna of ALMA measures light emitted from an object at millimeter wavelengths, within a range that is sensitive to temperature and certain electrical properties of surface materials.

“The signals of the ALMA antennas can be combined into a synthetic signal that’s equivalent to a telescope with a diameter of 16 km. The larger the telescope, the higher the resolution,” said Dr. Katherine de Kleer, an astronomer at Caltech.

Campioni et al. provide evidence that Psyche is a metal-rich asteroid whose surface is heterogeneous, shows both metal and silicate materials, and appear evolved by impacts. Image credit: Elena Hartley.

The authors ran simulations of Psyche to see which surface properties might best match and explain the measured thermal emissions.

In each of hundreds of simulated scenarios, they set the asteroid’s surface with different combinations of materials, such as areas of different metal abundances.

They modeled the asteroid’s rotation and measured how simulated materials on the asteroid would give off thermal emissions.

They then looked for the simulated emissions that best matched the actual emissions measured by ALMA. That scenario would reveal the likeliest map of the asteroid’s surface materials.

“We ran these simulations area by area so we could catch differences in surface properties,” Dr. Campioni said.

The researchers produced detailed maps of Psyche’s surface properties, showing that the asteroid’s façade is likely covered in a large diversity of materials.

They confirmed that, overall, Psyche’s surface is rich in metals, but the abundance of metals and silates vary across its surface. This may be a further hint that, early in its formation, the asteroid may have had a silicate-rich mantle that has since disappeared.

They also found that, as the asteroid rotates, the material at the bottom of a large depression — likely a crater — changes temperature much faster than material along the rim. This suggests that the crater bottom is covered in ‘ponds’ of fine-grained material, like sand on Earth, which heats up quickly, whereas the crater rims are composed of rockier, slower-to-warm materials.

“Ponds of fine-grained materials have been seen on small asteroids, whose gravity is low enough for impacts to shake the surface and cause finer materials to pool,” Dr. Campioni said.

“But Psyche is a large body, so if fine-grained materials accumulated at the bottom of the depression, this is interesting and somewhat mysterious.”

“These data show that Psyche’s surface is heterogeneous, with possible remarkable variations in composition,” said Dr. Simone Marchi, a researcher at the Southwest Research Institute.

“One of the primary goals of the Psyche mission is to study the composition of the asteroid surface using its gamma rays and neutron spectrometer and a color imager.”

“So, the possible presence of compositional heterogeneties is something that the Psyche science team is eager to study more.”

The team’s paper was published in the Journal of Geophysical Research Planets.

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Saverio Campioni et al. The Heterogeneous Surface of Asteroid (16) Psyche. Journal of Geophysical Research Planets, published online May 19, 2022; doi: 10.1029/2021JE007091

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