Cambioni 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.

ALMA creates detailed maps of the asteroid Psyche in the main belt |

The Atacama Large Millimeter/submillimeter Array located on the Chajnantor Plateau in Chile, made it possible to obtain temperature images of (16) Psyche, an M-type asteroid about 220 km in size, with a resolution of 30 km/pixel – the highest ever reached 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 a function of latitude and longitude. The new maps hint at Psyche’s history: the rocky areas may be remnants of an ancient cloak or the imprint of previous space rock impacts; craters containing metallic material support the idea suggested by previous studies that Psyche may have experienced early eruptions of metallic lava as the ancient core cooled.

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

“It’s an evolved surface and these maps confirm that metal-rich asteroids are interesting, puzzling 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 past mapping efforts.

Researchers observed the asteroid using several telescopes to measure the light emitted by the asteroid at infrared wavelengths, which contain information about the composition of Psyche’s surface.

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

dr. Cambioni and colleagues were instead 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 ALMA antenna measures the light emitted by an object at millimeter wavelengths, within a range sensitive to temperature and certain electrical properties of surface materials.

“The signals from the ALMA antennas can be combined into a synthetic signal comparable 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.

Cambion et al† provide evidence that Psyche is a metal-rich asteroid whose surface is heterogeneous, exhibits both metal and silicate materials, and appears to have evolved by impact. Image credit: Elena Hartley.

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

In each of the hundreds of simulated scenarios, they deploy the asteroid’s surface with different combinations of materials, such as regions of different metal concentrations.

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

They then set out to find the simulated emissions that best matched the actual emissions measured by ALMA. That scenario would reveal the most likely map of the asteroid’s surface materials.

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

The researchers created detailed maps of Psyche’s surface features, showing that the asteroid’s facade is likely covered with a wide variety of materials.

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

They also found that, as the asteroid rotates, the material at the bottom of a major depression — likely a crater — changes temperature much faster than material along the rim. This suggests that the crater floor is covered with “ponds” of fine-grained material, such as Earth’s sand, that warms quickly, while the crater rims are composed of rockier, slower-to-warm material.

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

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

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

“One of the main goals of the Psyche mission is to study the composition of the asteroid’s surface using its gamma-ray and neutron spectrometer and a color image sensor.”

“So the possible presence of compositional heterogeneities is something the Psyche science team would like to study more.”

the teams paper was published in the Journal of Geophysical Research Planets

Saverio Cambodia 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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