If you wanted to do a forensic study of the solar system, you can go to the head asteroid belt between March and Jupiter. This is where you can find ancient stones 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 refer to asteroids – and their meteorite fragments that fall to Earth – as time capsules because of the evidence they have.
The asteroid Psyche is particularly interesting, and NASA is sending a mission to investigate the unusual piece of rock.
Prior to that mission, a team of scientists combined observations of Psyche from a series of telescopes and constructed a map of the asteroid’s surface.
Astronomers divide asteroids into three categories. Carbonaceous or C-asteroids are the most common type. They make up about 75 percent of known asteroids and contain large amounts of carbon. The coal makes them dark, and they have low albedos.
Silicon or S 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 and make up about 8 percent of known asteroids. They appear to contain more metal than the other asteroid types, and scientists believe they are the source of iron meteorites that fall to the ground. 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 mi) in diameter. It is called 16 Psyche because it was the 16th smaller planet to be discovered. (Larger asteroids such as Psyche are also known as smaller planets.)
Psyche is sometimes called the “goldmine asteroid” because of the richness of iron and nickel it contains. Though to be clear, no one thinks it is rich in gold.
Visible slides of Psyche do not tell us much. The European Southern Observatory’s VLT took some pictures of the asteroid, but they did not reveal any details.
The history of the psyche is a history of uncertainty. For a long time, astronomers believed that it was the exposed iron core of a much larger body. In this hypothesis, a major collision or series of collisions peeled off the body’s crust and mantle.
The larger body would have been completely differentiated and measured approximately 500 km (310 miles) in diameter. With the crust and mantle gone, only the iron-rich core remained.
That idea fell into disfavor as time went on and astronomers continued to observe it. Evidence showed that it was not dense enough to be solid iron and probably porous.
Other researchers suggested that Psyche be disturbed in some way and then rebuilt as a mixture of metals and silicates. One study indicated that Psyche is not as rich in metal as was thought and is more of a pile of rubble. 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 Psyche’s origin is the ferro-volcanic idea. A The 2019 study presented evidence that Psyche was once a molten lump. In that scenario, the outer layers cooled and formed stress cracks, and the liquid molten core erupted like iron volcanoes.
The only way to safely find out what Psyche is is to go and look at it. So that’s what NASA is doing.
The mission is called Psyche and is scheduled to be launched sometime in the fall of 2022. The spacecraft will rely on solar-electric propulsion and a gravitational assistance maneuver with Mars to arrive in Psyche 2026.
It will spend 21 months studying the asteroid and will follow four separate orbits, each closer than the previous one.
As it moves closer to the asteroid, it will focus on various scientific objectives.
A team of researchers constructed a new map of Psyche’s surface to prepare for the mission.
The map can be found in a newspaper published in Journal of Geophysical Research: Planets. The title is “The Heterogeneous Surface of the Asteroid (16) Psyche,“and the lead author is Saverio Cambioni from MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).
“The surface of the psyche is very heterogeneous,” Cambioni sa in a press release. “It’s a developed surface, and these maps confirm that metal-rich asteroids are interesting enigmatic worlds. There’s another reason to look forward to the Psyche mission going to the asteroid.”
In this study, the authors used Atacama Large Millimeter / submillimeter Array (ALMA) to better watch 16 Psyche. ALMA is a radio telescope consisting of 66 high-precision antennas. The separate antennas work together as a high-resolution interferometer.
ALMA works at wavelengths that are sensitive to temperature and certain electrical properties of materials on Psyche’s surface.
“Alma antenna signals can be combined into a synthetic signal equivalent to a 16-kilometer (10-mile) telescope,” sa co-author Katherine de Kleer, assistant professor of planetary science and astronomy at Caltech. “The larger 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 for a material to reach the temperature in its environment. Higher thermal inertia means that it takes longer.
The other one is 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 which combinations of materials could explain them. “We ran these simulations area by area so that we could capture differences in surface properties,” Cambioni says.
Pure iron has an infinite dielectric constant. By measuring the dielectric constant on Psyche, the researchers were able to map the surface and locate regions richer in iron. Iron also has high thermal inertia because it is so dense.
How to combine thermal inertia and measurements of dielectric constant gives a good idea of which surface areas on Psyche are rich in iron and other metals.
Researchers call a strange feature about Psyche the Bravo-Golf region. That region has a systematically lower thermal inertia than the highland regions. The Bravo-Golf region is Depression just to the right of the asteroid’s zero meridian in the image below.
Why does a low area have lower thermal inertia? Other studies show that the region is also radar light. Why is it like that? The researchers came up with three possibilities.
The lowlands can be rich in metal but covered with fine, porous regolith which lowers their thermal inertia compared to the highlands covered with coarser regolith. The thermal inertia increases with the particle size. In this scenario, fine-grained regolith would have accumulated in the lowlands.
“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,” Cambioni sa. “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.”
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 cracks can also make the lowlands more porous.
The third hypothesis is that the lowlands have more silicate-rich materials than the highlands, which gives them a lower dielectric constant than certain areas in the highlands. The idea is that the Bravo-Golf depression could have been formed by a blow with a silicate-rich shock and left silica-rich residues.
Overall, the study shows that the surface of 16 Psyches is covered by a variety of materials. It also adds other evidence that the asteroid is rich in metals, although the amount of metals and silicates varies considerably in different regions.
It also suggests that the asteroid may be a remnant nucleus of a differentiated body that lost its mantle and crust long ago.
“In summary, we provide evidence that Psyche is a metal-rich asteroid whose surface is heterogeneous, shows both metal and silicate materials and appears to be developed by shocks,” the authors stop.
Simone Marchi is a staff researcher at the Southwest Research Institute and a co-investigator on NASA’s Psyche mission. Marchi was not involved in this study though commented about its significance in a press release. “These data show that the surface of the Psyche is heterogeneous, with possible remarkable variations in composition. One of the primary objectives of the Psyche mission is to study the composition of the asteroid surface using its gamma rays and neutron spectrometer and a color imager. So, the possible presence of compositional heterogeneities are something that the Psyche Science Team is keen to study more. “
It will be up to NASA’s Psyche mission to confirm 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 itself.
If Psyche is the remnant of a rocky, differentiated planetesimal, it will reveal something about our planet and how differentiated bodies are formed. Will it contain some of the same light elements that we expect to find in the Earth’s core? The earth’s core is not dense enough to be pure iron and nickel. Researchers believe that it contains lighter elements such as sulfur, silicon, oxygen, carbon and hydrogen.
The psyche mission will also determine if the asteroid was formed under conditions that are more oxidizing or more reducing than the Earth’s core. It will tell you more about the solar nebula and the protoplanetary disk.
People sometimes refer to Psyche as the gold mining asteroid because it is so metallic. An object of its size would contain a huge amount of iron, but it is unlikely that that value will be realized or reached any time soon.
But if knowledge is as valuable as iron, then 16 Psyche can still be a goldmine.
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