Astronomers at MIT and elsewhere have mapped the composition of the asteroid Psyche, revealing a surface of metal, sand and rock. Credit: Screenshot courtesy of NASA
The varied surface of the asteroid Psyche suggests a dynamic story, which could include metal eruptions, asteroid shaking impacts, and a lost rocky mantle.
Later this year, NASA will launch a probe the size of a tennis court into the asteroid belt, a region between the orbits of Mars and Jupiter where the remnants of the first solar system orbit the sun. Once inside the asteroid belt, the spacecraft will focus on Psyche, a large, metal-rich asteroid believed to be the ancient core of an early planet. The spacecraft, named after its asteroid target, will spend nearly two years orbiting and analyzing the surface of Psyche to find clues as to how the first planetary bodies evolved.
Prior to the mission, which is led by principal investigator Lindy Elkins-Tanton ’87, SM ’87, PhD ’02, planetary scientists from MIT and elsewhere have now provided an appetizer of what the Psyche spacecraft might see. when it reaches its destination. .
In an article 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 made by a large variety of ground-based telescopes in northern Chile. . The maps reveal vast regions rich in metals that extend across the asteroid’s surface, along with a large depression that appears to have a different surface texture between the interior and its edge; this difference could reflect a crater full of finer sand and bordered with more rocky materials.
This illustration, updated in April 2022, depicts NASA’s Psyche spacecraft. Launched in August 2022, the Psyche mission will explore a metal-rich asteroid of the same name located in the main asteroid belt between Mars and Jupiter. The spacecraft will arrive in early 2026 and will orbit the asteroid – also shown in this illustration – for almost two years to investigate its composition. Credit: NASA / JPL-Caltech / ASU
In general, the surface of Psyche was found to be surprisingly varied in its properties.
The new maps hint at the asteroid’s history. Its rocky regions could be remnants of an ancient mantle — a composition similar to the outermost rock layer of Earth, Mars, and the asteroid Vesta — or the imprint of past space rock impacts. Finally, craters containing metallic material support the idea proposed by previous studies that the asteroid may have experienced early eruptions of metal lava when its old core cooled.
“The surface of the psyche is very heterogeneous,” says lead author Saverio Cambioni, Crosby’s distinguished postdoctoral researcher 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 and enigmatic worlds. It’s another reason to expect the Psyche mission to go to the asteroid.”
Cambioni’s co-authors are Katherine de Kleer, an assistant professor of planetary science and astronomy at Caltech, and Michael Shepard, a professor of environmental, geographic, and geological sciences at Bloomsburg University.
The surface of Psyche has been the focus of numerous previous mapping efforts. Researchers have observed the asteroid using various telescopes to measure the light emitted by the asteroid at infrared wavelengths, which carry information about the composition of the surface of Psyche. However, these studies could not spatially resolve variations in surface composition.
Cambioni and his colleagues, on the other hand, were able to see Psyche in more detail, at a resolution of about 20 miles per pixel, using the combined power of the 66 radio antennas in the Atacama Large Millimeter / submillimeter Array ( ALMA) in northern Chile. Each ALMA antenna measures the light emitted by an object at wavelengths millimeters, within a range that is sensitive to temperature and certain electrical properties of surface materials.
“The signals from ALMA antennas can be combined into a synthetic signal that is equivalent to a telescope with a diameter of 16 kilometers (10 miles),” says de Kleer. “The bigger the telescope, the higher the resolution.”
On June 19, 2019, ALMA focused its entire array on Psyche as it orbited and spun within the asteroid belt. De Kleer collected data during this period and turned it into a map of thermal emissions on the asteroid’s surface, which the team reported in a 2021 study. Shepard used the same data to produce the asteroid model. Psyche’s latest high-resolution 3D shape, also released in 2021.
On the left, this map shows the surface properties in Psyche, from sandy areas (violet / low) to rocky areas (yellow / high). The map on the right shows the abundance of metals in Psyche, from low (purple) to high (yellow).
To catch a match
In the new study, Cambioni performed Psyche simulations to see which surface properties could best match and explain the measured thermal emissions. In each of the hundreds of simulated scenarios, he set the surface of the asteroid with different combinations of materials, such as areas of different abundance of metals. He modeled the rotation of the asteroid and measured how simulated asteroid materials emitted thermal emissions. Cambioni then looked for the simulated emissions that best matched the actual emissions measured by ALMA. This scenario, he reasoned, 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,” says Cambioni.
The study produced detailed maps of Psyche’s surface properties, showing that the asteroid’s façade is probably covered with a wide variety of materials. The researchers confirmed that, in general, the surface of Psyche is rich in metals, but the abundance of metals and silicates varies across its surface. This may be another clue that, at the beginning of 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, probably a crater, changes temperature much faster than the material along the edge. This suggests that the bottom of the crater is covered with “ponds” of fine-grained material, such as sand on Earth, which heats up rapidly, while the edges of the crater are made of rockier materials and slower to heat. if.
“Ponds of fine-grained materials have been seen in small asteroids, the severity of which is low enough for the impacts to shake the surface and cause finer materials to clump together,” says Cambioni. “But Psyche is a big body, so if fine-grained materials accumulate at the bottom of the depression, that’s interesting and a little mysterious.”
“These data show that the surface of Psyche is heterogeneous, with possible notable variations in composition,” said Simone Marchi, a staff scientist at the Southwest Research Institute and a co-researcher with NASA’s Psyche mission, who did not participate in the study. current. “One of the main objectives of the Psyche mission is to study the composition of the asteroid’s surface using its gamma rays and neutron spectrometer and a color image. Thus, the possible presence of compositional heterogeneities is a something the psyche science team wants to study more. “
Reference: “The Heterogeneous Surface of Asteroid (16) Psyche” by Saverio Cambioni, Katherine de Kleer and Michael Shepard, May 19, 2022, Journal of Geophysical Research: Planets.DOI: 10.1029 / 2021JE007091
This research was supported by the distinguished EAPS Crosby Postdoctoral Fellowship and, in part, the Heising-Simons Foundation.