If you wanted to do some forensic study of the solar system, you could head to the main asteroid belt between March and Jupiter. This is where you can find ancient rocks dating back to the early days of the solar system. Out there, in the cold vacuum of space, far from the Sun, the 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 hold.
Of particular interest is the asteroid Psyche, and NASA is sending a mission to investigate the unusual piece of rock.
Prior to this mission, a team of researchers combined observations of Psyche from 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. They make up about 75% of known asteroids and contain large amounts of carbon. The carbon makes them dark and their albedo is low.
Silica or S-type asteroids are the second most common type. They make up about 17% of known asteroids and are mostly made of iron and magnesium silicates.
Metallic or M-type asteroids are the rarest types of asteroids and make up about 8% of known asteroids. They appear to contain more metal than 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 mi) 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 called “the gold mine asteroid” because of the richness in iron and nickel it contains. Although to be clear, no one thinks it’s rich in gold.
Visible light images of Psyche don’t tell us much. The European Southern Observatory’s VLT captured a few images of the asteroid, but they revealed no details.
The story of Psyche is a story 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 a series of collisions tore the crust and the mantle from the body.
The larger body would have been fully differentiated and measured something like 500 km (310 miles) in diameter. With the crust and mantle gone, only the iron-rich core remained.
This idea fell out of favor over time and astronomers continued to observe it. The evidence showed that it was not dense enough to be solid iron and was probably porous.
Other researchers have suggested that Psyche was somehow disrupted and then reconstituted as a mixture of metals and silicates. A study indicated that Psyche is not as rich in metals as thought and is more of a pile of rubble. In this 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 origins of Psyche is the ferro-volcanic idea. A A 2019 study presented evidence that Psyche was once a molten blob. In this scenario, the outer layers cooled and formed stress cracks, and the floating molten core erupted as iron volcanoes.
The only way to know for sure what Psyche is is to go see her. So that’s what NASA does.
The mission is called Psyche and is scheduled to launch in the fall of 2022. The spacecraft will rely on solar-electric propulsion and a gravity assist maneuver with Mars to arrive at Psyche in 2026.
It will spend 21 months studying the asteroid and follow four distinct orbital paths, each successively closer than the last.
As it gets closer to the asteroid, it will focus on different scientific goals.
A team of researchers has constructed a new map of Psyche’s surface to help prepare for the mission.
The map is in an article published in the Journal of Geophysical Research: Planets. The title is “The heterogeneous surface of the asteroid (16) Psyche,and lead author is Saverio Cambioni of MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS).
“The surface of Psyche is very heterogeneous”, Cambioni said in a press release. “It’s an evolved surface, and these maps confirm that metal-rich asteroids are interesting enigmatic worlds. This is another reason to look forward to the Psyche mission coming to the asteroid.”
In this study, the authors used the Atacama Large Millimeter/submillimeter Array (ALMA) to better watch 16 Psyche. ALMA is a radio telescope made up of 66 high-precision antennas. The separate antennas work together as a high resolution interferometer.
ALMA operates at wavelengths sensitive to temperature and certain electrical properties of materials on the surface of Psyche.
“The signals from the ALMA antennas can be combined into a synthetic signal equivalent to a telescope with a diameter of 16 kilometers (10 miles)”, said 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, i.e. the time that a material takes to reach the temperature of its environment. Higher thermal inertia means it takes longer.
The second is the dielectric constant. The dielectric constant describes the ability of a material to conduct heat, electricity or sound. A material with a low dielectric constant conducts poorly and is a good insulator and vice versa.
The researchers took ALMA’s observations of thermal inertia and dielectric constant and ran hundreds of simulations to see what combinations of materials might explain them. “We ran these simulations area by area so we could detect differences in surface properties,” Cambioni said. said.
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 very dense.
Thus, the combination of thermal inertia and dielectric constant measurements gives a good idea of which surface regions of Psyche are rich in iron and other metals.
Researchers call a curious feature about Psyche the Bravo-Golf region. This region has a systematically lower thermal inertia than the altitude regions. The Bravo-Golf region is the the Depression just to the right of the asteroid’s prime meridian in the image below.
Why does an area at low altitude have lower thermal inertia? Other studies show that the region is also bright on radar. Why is that? The researchers proposed three possibilities.
Lowlands could be rich in metals but covered with fine, porous regolith which lowers their thermal inertia compared to uplands covered with coarser regolith. Thermal inertia increases with particle size. In this scenario, finer-grained regolith would have accumulated in the lowlands.
“Pools of fine-grained material have been observed on small asteroids, whose gravity is low enough that impacts shake the surface and cause finer material to accumulate,” Cambioni said. said. “But Psyche is a large body, so if fine-grained material accumulates 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 could also make lowlands more porous.
The third hypothesis is that the lowlands have more silicate-rich materials than the highlands, giving them a lower dielectric constant than some areas of the highlands. The idea is that the Bravo-Golf Depression could have been formed by impact with a silicate-rich impactor and left behind silicate-rich tailings.
Overall, the study 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 rich in metals, although the abundance of metals and silicates varies greatly by region.
It also suggests that the asteroid could be a remnant core of a differentiated body that lost its mantle and crust a long time ago.
“In conclusion, we provide evidence that Psyche is a metal-rich asteroid whose surface is heterogeneous, shows both metallic and silicate materials, and appears to have evolved by impacts,” the authors said. conclude.
Simone Marchi is a scientist at the Southwest Research Institute and a co-investigator on NASA’s Psyche mission. Marchi did not participate in this study but commented about its importance in a press release. “These data show that the surface of Psyche is heterogeneous, with possible remarkable variations in composition. One of the main objectives of the Psyche mission is to study the composition of the asteroid’s surface using its spectrometer gamma-ray and neutron, and a color imager. So the possible presence of compositional heterogeneities is something the Psyche science team is eager to investigate further.”
It will be up to NASA’s Psyche mission to more rigorously confirm these findings.
But sending a spaceship up to Psyche to understand it in more detail isn’t limited to Psyche herself.
If Psyche is the residual core of a rocky, differentiated planetesimal, it will reveal something about our planet and the formation of differentiated bodies. Will it contain some of the same light elements that we expect in the Earth’s core? Earth’s core is not dense enough to be pure iron and nickel. Scientists believe it contains lighter elements like sulfur, silicon, oxygen, carbon and hydrogen.
The Psyche mission will also determine whether the asteroid formed under more oxidizing or more reducing conditions than Earth’s core. This 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 rich in metals. An object of its size would contain an enormous amount of iron, although this value is unlikely to be realized or accessible anytime soon.
But if knowledge is as precious as iron, then 16 Psyche could still be a gold mine.
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