Universe Today
Despite their overall similarities, asteroids are usually pretty distinct from one another. Vesta has a very different spectroscopic profile than Psyche, for example. So it might come as no surprise that another of the main asteroids - Pallas - is in a class all its own except for the 300 or so members of its "family" with similar orbital profiles and spectroscopic lines. A new paper from researchers who were then Visiting Astronomers at NASA's Infrared Telescope Facility (IRTF) in Haiwi'i took a look at members of that family in the infrared for the first time and compared them to a particular Near-Earth object that might have a similar make-up.
Pallas has been studied for decades, given its spot as the third biggest asteroid in the asteroid belt, behind Ceres and Vesta. Its 513km diameter isn't the most unique thing about it, though—it's the fact that it's blue.
"Blue," in this case, is relative. Like stars, a "blue" asteroid wouldn't necessarily appear blue to the naked eye—it just has spikes at specific spectroscopic frequencies associated with that color of light. Pallas's spectroscopic profile places it firmly in the B-type category, a special version of a C-type carbonaceous asteroid with a blue tint—and it only makes up about 4% of the total number of asteroids known to science.
Fraser describes what makes the asteroid belt the asteroid belt.
While Pallas has been a well-known example of this rare type of asteroid for some time, there are plenty surrounding it that have not been studied as much. Pallas has a "family" that shares a similar spectrographic profile to its parent, but they have never been studied in detail.
Asteroid families are created when the parent body, in this case, Pallas itself, is hit by another large object, and fragments of that parent body are ejected into space, but only fast enough to fall into orbits slightly away from the parent. Typically, they keep attributes like orbital eccentricity, speed, and, importantly for this study, spectroscopic profile. A study in 1994 first noted that Pallas had a family, mainly consisting of asteroids smaller than 20 km in diameter. What they lack in overall size, though, they make up in sheer numbers - there are an estimated 300 asteroids in Pallas' family.
The study authors looked at 23 Pallas family members using the near-infrared (NIR) camera on the IRTF and found spectrographic profiles similar primarily to Pallas itself. However, those profiles also don't quite fit into standard categorizations of meteors, which are commonly used to study the compositions of asteroids without having actually to visit them. The closest fit, according to the study, are CY and CI meteorites, which are known for having undergone both water-based and heat-based transformations at some point in their lifetime. However, some family members more closely mimic CM and CK chondrite meteors, which are mainly impacted by just water.
Video describing Pallas, the third biggest asteroid in the asteroid belt.
Credit - Factoids Media YouTube Channel
The study lends credence to the idea that Pallas and its large family of asteroids are made up of hydrated materials—in other words, they contain water. That would explain the "blue" spectra, even if they are not visible to the naked eye. But the authors didn't stop there—they wanted to see if there happened to be a rogue family member much closer to home.
Pallas and its family have a very high orbital inclination of around 35°, making it difficult for spacecraft to reach them so far out of the ecliptic's plan. However, a near-Earth asteroid known as (3200) Phaethon has long been thought to be a member of the Pallas family that got knocked off its course and subsequently much closer to Earth.
Phaethon, whose orbit took it to within about 10 million km of Earth back in 2017, shares much of the spectral profile as the Pallas family. It is notably blue and, according to the study, shares similar spectral profiles to about 10% of the Pallas family asteroids in its size category. That makes a powerful argument for including Phaethon as a rogue member of the family, who was either knocked out of their vicinity by a gravitation tweak or by an impact. But, importantly, it means that we could potentially study the composition of this family of asteroids by sending a mission to it.
Such a mission is already in the works, but it will be a while before it launches. Until then, there are plenty more asteroids, even in the Pallas family, to study with as many spectral wavelengths as we can. As we begin to move out into the solar system, such work will become even more critical.
Learn More:
P. Chavan et al - Near-infrared spectroscopic characterization of the Pallas family
UT - New Images of the "Golf Ball" Asteroid Pallas
UT - Now's the Time to See Asteroid Pallas at its Best
UT - What is the Asteroid Belt?