Astronomers at MIT and elsewhere have mapped the composition of asteroid Psyche, revealing a surface of metal, sand and rock. Credit: screenshot from NASA
The varied surface of asteroid Psyche suggests a dynamic history, which can include mineral eruptions, asteroid shaking effects, and a missing rocky mantle.
Later this yearAnd the[{” attribute=””>NASA is set to launch a probe the size of a tennis court to the asteroid belt, a region between the orbits of Mars and Jupiter where remnants of the early solar system orbit the sun. Once within the asteroid belt, the spacecraft will zero in on Psyche, a large, metal-rich asteroid that is thought to be the ancient core of an early planet. The probe, named after its asteroid target, will then spend close to two years orbiting and analyzing Psyche’s surface for clues to how early planetary bodies evolved.
Ahead of the mission, which is led by principal investigator Lindy Elkins-Tanton ’87, SM ’87, PhD ’02, planetary scientists at MIT and elsewhere have now provided a sneak peek of what the Psyche spacecraft might see when it reaches its destination.
In a paper published on June 15, 2022, in the Journal of Geophysical Research: Planets, the planetary science team presents the most detailed maps of the asteroid’s surface properties to date, based on observations taken by a large array of ground telescopes in northern Chile. The maps reveal vast metal-rich regions sweeping across the asteroid’s surface, along with a large depression that appears to have a different surface texture between the interior and its rim; this difference could reflect a crater filled with finer sand and rimmed with rockier materials.
This illustration, updated in April 2022, depicts NASA’s Psyche spacecraft. Set to launch in August 2022, the Psyche mission will explore a metal-rich asteroid of the same name that lies in the main asteroid belt between Mars and Jupiter. The spacecraft will arrive in early 2026 and orbit the asteroid – also shown in this illustration – for nearly two years to investigate its composition. Credit: NASA/JPL-Caltech/ASU
Overall, Psyche’s surface was found to be surprisingly varied in its properties.
The new maps hint at the asteroid’s history. Its rocky regions could be vestiges of an ancient mantle — similar in composition to the rocky outermost layer of Earth, Mars, and the asteroid Vesta — or the imprint of past impacts by space rocks. Finally, craters that contain metallic material support the idea proposed by previous studies that the asteroid 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 evolved 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.”
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.
Telescope Power
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.
Cambioni and his colleagues instead were able to see Psyche in finer detail, at a resolution of about 20 miles per pixel, using the combined power of the 66 radio antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile. 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 kilometers (10 miles),” de Kleer says. “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 converted it into a map of thermal emissions across the asteroid’s surface, which the team reported in a 2021 study. Those same data were used by Shepard to produce the most recent high-resolution 3D shape model of Psyche, also published in 2021.
On the left, this map shows surface characteristics on Psyche, from sandy (purple/low) to rocky (yellow/high) areas. The map on the right shows the abundance of minerals on Psyche, from low (purple) to high (yellow).
to catch up with the match
In the new study, Cambioni ran Psyche simulations to find out which surface properties might best match and explain the measured heat emissions. In each of the hundreds of simulated scenarios, he mapped the asteroid’s surface with different combinations of materials, such as regions with different mineral abundances. He modeled the asteroid’s rotation and measured how simulated materials on the asteroid would emit heat. Campione then looked for simulated emissions that best matched the actual emissions measured by ALMA. This scenario, he believes, will reveal the most likely map of the asteroid’s surface material.
“We ran these simulations region by region until we could identify differences in surface properties,” Campione says.
The study produced detailed maps of Psyche’s surface properties, showing that the asteroid’s interface is likely covered with a large variety of materials. The researchers emphasized that Psyche’s surface in general is rich in minerals, but the abundance of minerals and silicates varies depending on its surface. This may be an additional hint that the asteroid, early in its formation, may have had a silicate-rich mantle that has since disappeared.
They also found that as the asteroid orbited, the material at the bottom of a large depression – likely a crater – changed temperature much faster than material along the edge. This indicates that the bottom of the crater is covered with “pools” of fine-grained material, like sand on the ground, that heat up rapidly, while the crater edges are made up of slower, warmer rocky material.
“Ponds of fine-grained material have been seen on small asteroids, their gravity is low enough for surface-shaking shocks to cause finer material to aggregate,” Campione says. “But Psyche is a big body, so if fine-grained material accumulates at the bottom of the depression, that’s rather interesting and mysterious.”
“These data show that Psyche’s surface is heterogeneous, with marked differences in composition,” says Simone Marchi, a scientist at the Southwest Research Institute and a NASA Psyche mission co-investigator, who was not involved in the current study. “One of the primary goals of Psyche’s mission is to study the composition of the asteroid’s surface using gamma rays, a neutron spectrometer, and a color imager. So, the potential presence of compositional genes is something the psychology team is eager to study further.”
Reference: “The Heterogeneous Surface of Asteroid (16) Psyche” by Saverio Campione, Catherine de Clare and Michael Shepherd, May 19, 2022, Available here. Journal of Geophysical Research: Planets.
DOI: 10.1029/2021JE007091
This research was supported by an EAPS Crosby Distinguished Postdoctoral Fellowship, and in part by the Heising-Simons Foundation.
