Possible asteroid family of dwarf planet Ceres is identified

Researchers have found 156 potential fragments of the largest body in the solar system's main asteroid belt.

By Elton Alisson  |  Agência FAPESP – Despite evidence that Ceres, the largest body in the solar system’s main asteroid belt (located between the orbits of Mars and Jupiter), should have a family of fragments that originated from collisions over billions of years, no observations could empirically confirm this hypothesis.

Now, however, a group of researchers affiliated with the Guaratinguetá campus of São Paulo State University (UNESP) in Brazil, in collaboration with colleagues at the Southwest Research Institute in the US, have found traces of what may have been a family of the dwarf planet eons ago (i.e., a paleofamily).

The researchers identified 156 asteroids whose taxonomy, color and albedo (reflected light) suggest they may be fragments of Ceres located in a primitive region of the main asteroid belt that is characterized by a relatively low density of objects.

This discovery, resulting from a project that was supported by FAPESP, has now been described in an article published by the journal Monthly Notices of the Royal Astronomical Society.

“Failure to detect a family of asteroids from Ceres has long represented one of the main problems in asteroid dynamics,” lead author Valério Carruba told Agência FAPESP. Carruba is a professor at UNESP Guaratinguetá.

“The discovery of a possible Ceres family may contribute to a better understanding of the solar system’s history,” she added.

According to Carruba, while other bodies of the same spectral type as Ceres, such as Hygeia and Euphrosyne, have recognized asteroid families, until now no group of asteroids that could be fragments of the dwarf planet had been identified.

However, there is evidence for believing that about 10 craters larger than 300 km in diameter should have formed on Ceres due to collisions with other objects in the last 4.5 billion years.

Observational data from the Dawn probe corroborate this estimate by showing that at least two craters with diameters of about 280 km were formed in the last 2 billion years on Ceres.

Dawn was launched by NASA in 2007 to examine Ceres, which has a diameter of 900 km, and Vesta, the second-largest body in the main belt, with a diameter of 400 km.

All this evidence suggests that Ceres should have expelled a significant number of fragments and formed at least two families, yet standard techniques for identifying dynamic asteroid families have not detected a Ceres family.

“The usual method concentrates on observing objects near Ceres in the central region of the main asteroid belt,” Carruba said. “This may be because close encounters and linear secular resonances with Ceres have significantly depleted the population of objects in near proximity to this body, reducing the number of close neighbors.”

Secular resonance is a type of orbital resonance, which occurs when two orbiting bodies exert a regular periodic gravitational influence on each other. Secular (long-term) resonance occurs when the orbits of a body and another, larger body display a synchronized change in precession (i.e., the orientation of their rotational axis or orbital path). A secular resonance is linear when two such bodies synchronize a precession of the point of the orbit closest to the sun (pericenter) or of the orbit’s ascending node, and it may alter the smaller body’s eccentricity or inclination, making its orbit unstable.

Another problem, according to Carruba, is that more asteroids are concentrated in the central region of the belt, and the number of C-type objects is especially high. C-type asteroids such as Ceres are the most common; they are extremely dark, less dense, and associated with outer regions of the solar system. The central region contains two asteroids of the same spectral type as Ceres: Dora and Chloris.

“When you perform a study using astronomical spectrophotometry to analyze the spectrum of electromagnetic radiation from objects observed with a telescope, it’s hard to know whether C-type objects in the main belt are part of a possible Ceres family or belong to the families of Dora and Chloris,” Carruba explained.

In addition, the initial ejection velocities from Ceres should have been significantly larger than those observed for any other parent body in the main belt, including Vesta, its second-largest body.

Thus, the collision fragments of Ceres may have spread over a much larger area of the main asteroid belt, making members of the Ceres family significantly more distant among themselves than the typical distances between objects formed in collisions from smaller bodies – hence the difficulty of identifying the Ceres family.

Primitive region

Based on these findings, the researchers decided that instead of trying to identify members of the Ceres family close to the dwarf planet in the central region of the asteroid belt, they would investigate a pristine (primitive) region of the belt between the 5J: 2a and 7J: -3a mean-motion resonances with Jupiter.

Their hypothesis is that fragments of Ceres in the order of kilometers may have reached this region of the main asteroid belt, which was depleted during the Late Heavy Bombardment. During this astrophysical event, which is believed to have occurred between 4.3 billion and 3.8 billion years ago, an immense number of asteroids collided with other objects in the solar system, making a great many craters on the moon and other bodies.

Since then, the influx of outside material from other areas of the asteroid belt into the pristine region has been limited.

“Besides the low asteroid density, another advantage of studying the pristine region is the absence of other large C-type families with eccentricities and inclinations comparable to those of Ceres, making it easier to identify possible members of the Ceres family in this region,” Carruba said. The orbital eccentricity of an astronomical object is the amount by which its orbit around another body deviates from a perfect circle.

To confirm their hypotheses, the researchers studied the albedo and color of the objects found in the pristine region. Their analysis pointed to 156 objects in the region whose photometry and albedo were compatible with those of C-type asteroids such as Ceres, which reflects only 9% of the sunlight that falls on it.

The statistical studies performed by the researchers also indicated that the distribution of these objects’ inclinations is compatible with their having originated from Ceres.

“We don’t yet have definitive proof that a Ceres family exists, because the objects we identified are C-type candidates, and complete visible and infrared spectra haven’t been obtained yet to confirm the classification. But the circumstantial evidence is very strong,” Carruba said, adding that there are no sources of C-type objects in the pristine region of the main belt capable of explaining the concentration of this type of asteroid in the region.

The article “Footprints of a possible Ceres asteroid paleofamily” (doi: 10.1093/mnras/stw380), by Carruba et al., can be read in Monthly Notices of the Royal Astronomical Society at http://mnras.oxfordjournals.org/content/458/1/1117.