If you want to do a forensic study of the solar system, you might go to the most important asteroid belt between Mars and Jupiter† That’s where you can find ancient rocks from the early days of the solar system. Out there in the cold vacuum of space, far from the sun, asteroids are largely untouched by space weathering.
Space scientists sometimes call asteroids — and their meteorite fragments that fall to Earth — time capsules because of the evidence they contain.
The asteroid Psyche is especially interesting and NASA sends a mission to investigate the unusual piece of rock.
Prior to that mission, a team of researchers combined observations of Psyche with an array of telescopes and constructed a map of the asteroid’s surface.
Astronomers divide asteroids into three categories. Carbonaceous or C-type asteroids are the most common type. They make up about 75 percent of known asteroids and contain large amounts of carbon. The carbon makes them dark and they have low albedos.
Silica or S-type asteroids are the second most common type. They make up about 17 percent of known asteroids and are mostly made of iron and magnesium silicates.
Metal- or M-type asteroids are the rarest types of asteroids, making up about 8 percent of known asteroids. They appear to contain more metal than the other types of asteroids, and scientists believe they are the source of iron meteorites that fall to earth† M-type meteorites were one of the earliest sources of iron in human history.
Psyche (16 Psyche) is an M-type asteroid. It is also called a dwarf planet because it is about 220 kilometers (140 miles) in diameter. It is called 16 Psyche because it was the 16th minor planet discovered. (Larger asteroids like Psyche are also known as minor planets.)
Psyche is sometimes referred to as the “Gold Mine Asteroid” because of the richness of iron and nickel it contains. Just to be clear, no one thinks it’s rich in gold.
Visible light images of Psyche don’t say much. The European Southern Observatory’s VLT has taken some photos of the asteroid, but they haven’t revealed any details.
The history of Psyche is a history of uncertainty. For a long time, astronomers thought it was the exposed iron core of a much larger body. In this hypothesis, a powerful collision or series of collisions has taken away the crust and mantle of the body.
The larger body would have been fully differentiated and measured about 500 km (310 mi) in diameter. With the crust and mantle gone, only the iron-rich core remained.
That idea fell out of favor as time went on, and astronomers continued to observe it. Evidence showed it was not dense enough to be solid iron and is likely porous.
Other researchers suggested that Psyche was somehow disrupted and then reaccreted as a mix of metals and silicates. One study indicated: that Psyche is not as metal rich as thought and is more of a mess. In that scenario, collisions with more common C-type asteroids deposited a layer of carbon and other materials on Psyche’s surface.
The most exotic idea behind the origin of Psyche is the ferro-volcanic idea. A 2019 study presented evidence that Psyche was once a molten blob. In that scenario, the outer layers cooled and formed stress cracks, and the floating molten core erupted like iron volcanoes.
The only way to be sure what Psyche is is to look at it. So that’s what NASA does.
The mission is called Psyche and is expected to launch sometime in the fall of 2022. The spacecraft will rely on solar electric propulsion and a gravity-assisted maneuver with Mars to arrive at Psyche in 2026.
It will spend 21 months studying the asteroid and will follow four separate orbital paths, each successively closer than the last.
As it gets closer to the asteroid, it will focus on different scientific targets.
A team of researchers has created a new map of Psyche’s surface to prepare for the mission.
The map was published in a newspaper in the Journal of Geophysical Research: Planets† The title is “The heterogeneous surface of asteroid (16) Psyche,and the lead author is Saverio Cambioni of MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).
“Psyche’s surface is very heterogeneous,” says Cambioni. said in a press release. “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 heading to the asteroid.”
In this study, the authors used the Atacama Large Millimeter/submillimeter Array (ALMA) to get a closer look at 16 Psyche. ALMA is a radio telescope consisting of 66 high-precision antennas. The individual antennas work together as a high-resolution interferometer.
ALMA operates at wavelengths that are sensitive to the temperature and some electrical properties of materials on Psyche’s surface.
“The signals from the ALMA antennas can be combined into a synthetic signal equivalent to a telescope 16 kilometers (10 miles) in diameter.” said co-author Katherine de Kleer, an assistant professor of planetary science and astronomy at Caltech. “The bigger the telescope, the higher the resolution.”
The new map is based on two types of measurements. One is thermal inertia, which is how long it takes a material to reach the temperature of its environment. Higher thermal inertia means it lasts longer.
The second is the dielectric constant† The dielectric constant describes how well a material conducts heat, electricity or sound. A material with a low dielectric constant conducts poorly and is a good insulator and vice versa.
The researchers took the ALMA observations of thermal inertia and dielectric constant and ran hundreds of simulations to see what combinations of materials they could explain. “We ran these simulations area by area so we could detect differences in surface properties,” Cambioni say†
Pure iron has an infinite dielectric constant. By measuring the dielectric constant on Psyche, the researchers were able to map the surface and locate areas richer in iron. Iron also has a high thermal inertia because it is so dense.
Combining thermal inertia and dielectric constant measurements gives a good idea of which surface areas on Psyche are rich in iron and other metals.
The researchers call a curious feature on Psyche the Bravo-Gulf region. That region has a systematically lower thermal inertia than the highland regions. The Bravo-Golf region is the depression just to the right of the asteroid’s prime meridian in the image below.
Why does a low-lying area have a lower thermal inertia? Other studies show that the region is also radar clear. Why is that? The researchers came up with three options.
The lowlands may be metal-rich, but covered with fine, porous regolith which lowers their thermal inertia compared to the highlands covered with coarser regolith. Thermal inertia increases with particle size. In this scenario, fine-grained regolith would have accumulated in the lowlands.
“Ponds of fine-grained materials have been seen on small asteroids whose gravity is low enough to cause impacts to shake the surface and coalesce finer materials,” Cambioni says. said† “But Psyche is a large body, so if fine-grained material builds up at the bottom of the depression, it’s interesting and somewhat mysterious.”
The second hypothesis is that the surface material covering the lowlands is more porous than the highlands. The thermal inertia decreases as the porosity of the rock increases. Impact fractures can also make the lowlands more porous.
The third hypothesis is that the lowlands contain more silicate-rich materials than the highlands, giving them a lower dielectric constant than some parts of the highlands. The idea is that the Bravo-Wave Depression may have been formed by an impact with a silicate-rich impactor, leaving behind silicate-rich residue.
Overall, the research shows that the surface of 16 Psyche is covered with a wide variety of materials. It also adds to other evidence showing that the asteroid is metal-rich, although the abundance of metals and silicates varies considerably in different regions.
It also suggests that the asteroid may be a remnant of a differentiated body that lost its mantle and crust long ago.
“Finally, we provide evidence that Psyche is a metal-rich asteroid whose surface is heterogeneous, exhibiting both metal and silicate materials, and appears to have evolved by impacts,” the authors said. to conclude†
Simone Marchi is a staff scientist at the Southwest Research Institute and a co-investigator on NASA’s Psyche mission. Marchi was not involved in this research, but commented about its meaning in a press release. “These data show that Psyche’s surface is heterogeneous, with possible notable variations in composition. 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 imager. So, the possible presence of compositional heterogeneities is something the Psyche Science Team would like to study more.”
It’s up to NASA’s Psyche mission to corroborate these findings more rigorously.
But sending a spacecraft all the way to Psyche to understand it in more detail is about more than just Psyche herself.
If Psyche is the remnant core of a rocky differentiated planetesimal, it will reveal something about our planet and how differentiated bodies form. Will it contain some of the same light elements we expect in the Earth’s core? The Earth’s core is not dense enough to be pure iron and nickel. Scientists think it contains lighter elements such as sulfur, silicon, oxygen, carbon and hydrogen.
The Psyche mission will also determine whether the asteroid formed under conditions that are more oxidizing or more reducing than Earth’s core. That will tell us more about the solar nebula and the protoplanetary disk.
People sometimes refer to Psyche as the Gold Mine Asteroid because it is so metal rich. An object of its size would contain an enormous amount of iron, although that value is unlikely to be realized or accessible anytime soon.
But if knowledge is as valuable as iron, then maybe Psyche is still a gold mine.
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