Later this year, NASA is set to launch a probe the size of a tennis court into the asteroid belt, the region between the orbits of Mars and Jupiter where the remnants of the early solar system orbit the sun. Once in the asteroid belt, the spacecraft will focus on Psyche, a large, mineral-rich asteroid believed to be the ancient core of an early planet. The probe, named after its asteroid target, will spend nearly two years circling the surface of Psyche and analyzing it in search of clues to how early planetary bodies evolved.
Before the mission, led by principal investigator Lindy Elkins-Tanton ’87, SM ’87, PhD ’02, planetary scientists at MIT and now elsewhere have provided a sneak peak of what the Psyche spacecraft might see when it reaches its destination.
In a paper appearing today in Journal of Geophysical Research: PlanetsIn this article, the team provides the most detailed maps of the asteroid’s surface characteristics to date, based on observations taken by a large array of ground-based telescopes in northern Chile. The maps reveal vast mineral-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 edge; This difference could reflect a crater filled with fine sand and surrounded by rocky material.
In general, Psyche’s surface is found to be surprisingly diverse in its properties.
New maps indicate the history of the asteroid. Its rocky regions could be the remnants of an ancient mantle — similar in composition to Earth’s rocky outer layer, Mars, and the asteroid Vesta — or the imprint of past impacts from space rocks. Finally, craters containing mineral matter support the idea suggested by previous studies that the asteroid may have experienced early eruptions of mineral lava as its ancient core cooled.
“Psyche’s surface is very heterogeneous,” says lead author Saverio Cambioni, Crosby’s Distinguished Postdoctoral Fellow in the Department of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT. “It’s a sophisticated surface, and these maps confirm that metal-rich asteroids are interesting and mysterious worlds. It’s another reason to look forward to the Psyche asteroid-going mission.”
Campione’s co-authors are Catherine de Clare, assistant professor of planetary science and astronomy at Caltech, and Michael Shepherd, professor of environmental, geographic and geological sciences at Bloomsburg University.
The Psyche surface has been the focus of many previous mapping efforts. The researchers note that the asteroid uses various telescopes to measure the light emitted by the asteroid at infrared wavelengths, which carries information about Psyche’s surface composition. However, these studies were not able to spatially identify differences in composition at the surface.
Instead, Campione and colleagues were able to see Psyche in finer detail, at a resolution of about 20 miles per pixel, using the combined energy of the 66 radio antennas of the Large Millimeter/Sub-Atacama Array (ALMA) in northern Chile. Each ALMA antenna measures light emitted by an object at millimeter wavelengths, within a range sensitive to temperature and certain electrical properties of surface materials.
“The signals of ALMA antennas can be combined into an artificial signal equivalent to a telescope with a diameter of 16 kilometers (10 miles),” says de Claire. “The larger the telescope, the higher the resolution.”
On June 19, 2019, ALMA focused its entire collection on Psyche as it orbits and rotates within the asteroid belt. De Kleer collected data during this period and turned it into a map of heat emissions across the asteroid’s surface, which the team reported in the 2021 study. This same data was used by Shepard to produce Psyche’s latest high-resolution 3D model, 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 a further 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.”
This research was supported by an EAPS Crosby Distinguished Postdoctoral Fellowship, and in part by the Heising-Simons Foundation.