Our best view yet of Pallas, the largest asteroid not yet visited by a spacecraft, reveals an extraordinarily violent history with numerous impacts, most likely due to its unusual orbit, a new study finds.
In 1802, Pallas became the second asteroid ever discovered. Named after Pallas Athena, the Greek goddess of wisdom, Pallas is the third most massive asteroid ever discovered, comprising an estimated 7% of the mass in the solar system's asteroid belt. This asteroid has an average diameter of about 318 miles (513 kilometers), which is about 15% of the diameter of the moon.
Much remains unknown about this large asteroid. To shed light on Pallas' many mysteries, in a new study, scientists used the Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) imager on the Very Large Telescope in the Atacama Desert of northern Chile to analyze the asteroid's shape and surface in unprecedented detail.
Based on 11 images they captured of Pallas' surface, the researchers discovered that the asteroid is pockmarked with numerous craters ranging from about 18.5 to 75 miles (30 to 120 km) wide. Their computer simulations also suggest that Pallas has about twice as many craters as the largest known asteroid, the dwarf planet Ceres, and three times as many as the second-largest (and brightest) known asteroid, Vesta.
"Pallas is heavily cratered," study co-author Miroslav Broz, an astronomer at Charles University in the Czech Republic, told Space.com. "Its surface might resemble a golf ball."
Two giant craters on Pallas — one near its south pole, the other near its equator — hint that the asteroid once experienced giant sideways impacts with projectiles about 37 to 65 miles (60 to 90 km) in diameter, the researchers said. They added that the impact that created the crater near the equator may have formed the family of several hundred small asteroids surrounding Pallas, which are less than 12 miles (20 km) wide.
"We performed numerical simulations to determine the most probable age of the family, which is 1.7 billion years, and this should correspond to the surface age of Pallas, or at least a substantial part of it," Broz said.
Computer simulations of past collisions in the asteroid belt, conducted as part of this study, suggest that the objects hitting Pallas were also traveling at unusually high speeds, averaging about 25,725 mph (41,400 km/h), compared with the average speeds of about 12,975 (20,880 km/h) for impacts generating craters the asteroid belt. These high speeds were likely caused by the way that Pallas travels in an unusually tilted and elongated orbit, according to the study. Since fast impacts are more likely to generate craters than slower ones, Pallas' strange orbit likely also helps to explain why the asteroid is so cratered compared with Ceres and Vesta, they added.
Using their images along with previous estimates of the asteroid's mass, the researchers developed a 3D model of Pallas and found that the object is denser than Ceres but less dense than Vesta. With this information, the research team suggests that Pallas possesses a greater proportion of rock to ice than Ceres.
Pallas' density, combined with how much the asteroid reflects a specific wavelength of infrared light, additionally suggests that the asteroid is most similar in composition to a kind of meteorite known as a CM chondrite, according to the study.
Chondrites are meteorites made up of tiny round pellets known as chondrules, which form when molten mineral droplets quickly cool in space; CM chondrites are the most common form of carbonaceous (that is, carbon-rich) chondrite. CM chondrites are known to possess a rich mix of complex organic molecules. Based on what previous work has found about CM chondrites, the researchers suggested that the interior of Pallas never got hot enough to separate into a dense, silicon-heavy rocky core and a water-rich mantle, so its innards are likely a dirty mix of rock and ice.
Still, Pallas' large size suggests that it may have retained heat long enough for saltwater to separate out and rise upward. The resulting salt deposits in Pallas' crust may help to explain a bright patch seen on the asteroid's surface, the scientists noted.
Future research will focus on other asteroids observed by VLT/SPHERE, such as Iris, Hygiea, Psyche, Daphne and Interamnia, Broz said.
The scientists detailed their findings in a study published Feb. 10 in the journal Nature Astronomy.
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