New maps of the asteroid Psyche reveal an ancient world of metal and rock

New maps of the asteroid Psyche reveal an ancient world of metal and rock

Later this year, NASA will launch a probe the size of a tennis court to the asteroid belt, a region between Mars and Jupiter’s courts where remnants of the early solar system orbit the sun. Once inside the asteroid belt, the spacecraft will reach Psyche, a large, metal-rich asteroid believed to be the ancient core of an early planet. The probe, named after its asteroid target, will then spend nearly two years orbiting and analyzing Psyche’s surface for clues as to how early planetary bodies evolved.

Ahead of the mission, led by lead researcher Lindy Elkins-Tanton ’87, SM ’87, PhD ’02, planetary scientists at MIT and elsewhere have now given a sneak peek at what the Psyche spacecraft can see when it reaches its destination.

In a newspaper published today in Journal of Geophysical Research: Planets, the team presents the most detailed maps of the asteroid’s surface properties to date, based on observations made by a large number of ground telescopes in northern Chile. The maps reveal large metal-rich areas that sweep across the asteroid’s surface, along with a large depression that appears to have a different surface structure between the inside and its edge; this difference may reflect a crater filled with finer sand and lined with rockier material.

Overall, Psyche’s surface turned out to be surprisingly variable in its properties.

The new maps suggest the history of the asteroid. Its rocky regions may be remnants of an ancient mantle – similar in composition to the rocky outermost layer of Earth, Mars and the asteroid Vesta – or the imprint of earlier impact from space rocks. Finally, craters containing metallic material support the idea suggested by previous studies that the asteroid may have experienced early eruptions of metallic lava as its ancient core cooled.

“The surface of the psyche is very heterogeneous,” said lead author Saverio Cambioni, Crosby Distinguished Postdoc Fellow at MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “It is a developed surface, and these maps confirm that metal-rich asteroids are interesting, enigmatic worlds. There is another reason to look forward to the Psyche mission going to the asteroid.”

Cambioni’s co – authors are Katherine de Kleer, Assistant Professor of Planetary Science and Astronomy at Caltech, and Michael Shepard, Professor of Environmental, Geographical and Geological Sciences at Bloomsburg University.

Telescopic power

The surface of Psyche has been the focus of many previous mapping efforts. Scientists have observed the asteroid using various telescopes to measure light emitted from the asteroid at infrared wavelengths, which carries information about Psyche’s surface composition. However, these studies could not spatially resolve variations in composition across the surface.

Cambioni and his colleagues could instead see Psyche in finer detail, with a resolution of about 20 miles per pixel, with the combined power of the 66 radio antennas in the Atacama Large Millimeter / Submillimeter Array (ALMA) in northern Chile. Each antenna at 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 from the Alma antennas can be combined into a synthetic signal that corresponds to a telescope with a diameter of 16 kilometers (10 miles),” says de Kleer. “The larger the telescope, the higher the resolution.”

On June 19, 2019, ALMA focused its entire array on Psyche as it orbited and rotated within the asteroid belt. De Kleer collected data during this period and turned it into a map of thermal emissions over the asteroid’s surface, which the team reported in a study from 2021. The same data was used by Shepard to produce the latest high-resolution 3D shape model of Psyche, also published 2021.

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On the left this map shows surface properties of Psyche, from sandy areas (purple / low) to rocky areas (yellow / high). The map to the right shows the metal abundance on Psyche, from low (purple) to high (yellow).

To capture a match

In the new study, Cambioni ran simulations of Psyche to see which surface properties best match and explain the measured thermal emissions. In each of hundreds of simulated scenarios, he set the surface of the asteroid with different combinations of materials, such as areas with different metal deposits. He modeled the rotation of the asteroid and measured how simulated materials on the asteroid would emit thermal emissions. Cambioni then looked for the simulated emissions that best matched the actual emissions measured by ALMA. That scenario, he reasoned, would reveal the most likely map of the asteroid’s surface material.

“We ran these simulations area by area so that we could capture differences in surface properties,” says Cambioni.

The study produced detailed maps of Psyche’s surface properties, which show that the asteroid’s facade is likely to be covered by a large variety of materials. The researchers confirmed that Psyche’s surface is rich in metals overall, but the amount of metals and silicates varies over its surface. This may be another indication that the asteroid may have had a silicate-rich mantle early in its formation which has since disappeared.

They also found that when the asteroid rotates, the material at the bottom of a large depression – probably a crater – changes temperature much faster than the material along the edge. This indicates that the crater floor is covered by “ponds” of fine-grained material, such as sand on the ground, which heat up quickly, while the crater edges are composed of stony, slower to warm materials.

“Ponds of fine-grained material have been seen on small asteroids, whose gravity is low enough for shocks to shake the surface and cause finer materials to accumulate,” says Cambioni. “But the Psyche is a big body, so if fine-grained material accumulates at the bottom of the depression, this is interesting and a little mysterious.”

“These data show that Psyche’s surface is heterogeneous, with possible notable variations in composition,” said Simone Marchi, a staff researcher at the Southwest Research Institute and a co-investigator on NASA’s Psyche mission, who was not involved in the current study. “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 former. So the possible presence of compositional heterogeneities is something that the Psyche Science Team is keen to study more.”

This research was supported by the EAPS Crosby Distinguished Postodoctoral Fellowship, and in part by the Heising-Simons Foundation.

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