Study Explains Atypical Nature of Solar System Family of Asteroids

Hello reader!

It follows an article published day (08/20) in the english website of the Agência FAPESP noting that a study led by Brazilian’s Astronomer “Valerio Carruba” explains atypical nature of Solar System Family of Asteroids.

Duda Falcão


Study Explains Atypical Nature of
Solar System Family of Asteroids

José Tadeu Arantes
August 20, 2014

(Image: NASA)
Results from a study led by Brazilian
researchers were discussed at a
conference in Finland and will be
published in an article in
The Astrophysical Journal.
Agência FAPESP – The atypical nature of the asteroid family known as Euphrosyne, one of several situated between Mars and Jupiter and which for years has intrigued astronomers, has just been explained by the team led by Valerio Carruba, a professor at the State University of São Paulo (Unesp), Guaratinguetá campus. The explanation, which was presented at the conference Asteroids, Comets, Meteors 2014, held in Helsinki, Finland, is the topic of a soon-to-be-published article in The Astrophysical Journal (ApJ).

The work was part of the FAPESP-funded project entitled “Secular families.”

One particular feature of this family, which is composed of more than 2,500 objects, is that it has few large or medium-sized asteroids with diameters ranging between 8 and 12 kilometers (km). The notable exception is its principal asteroid Euphrosyne, which gives the group its name and which contains more than 99% of the family’s mass. The other objects are all very small.

“This causes the line that describes the size distribution to be quite steep,” Carruba told the Agência FAPESP. “The slope of this curve is indicated by a parameter called alpha. An alpha value on the order of 3.8 characterizes asteroid families of the same type. The alpha value for Euphrosyne, however, is much larger: on the order of 4.4.”

More than half a million asteroids are part of the so-called “main belt” situated between the orbits of Mars and Jupiter. This number is constantly increasing because of new discoveries.

Some asteroids are grouped by family, each of which supposedly originated from a parent body that fragmented after collisions with other bodies. The longstanding notion that a single body gave rise to the entire main belt has been dismissed, if for no other reason than because the chemical compositions of the various families are quite different.

“We currently recognize two types of formation of families. One is when the parent body is completely broken up. The other is when the parent body is just ‘crater-ized’ [in other words, when craters form on its surface]. The Euphrosyne family would be included in this second group. It’s very likely that all the small objects that it consists of have originated from the matter extracted from a crater found on the surface of the main asteroid,” Carruba said.

Even so, the fact that the family has very few large or medium-sized bodies was considered strange. “This is because families usually tend to lose the small objects more easily, as they break off from the group during its dynamic evolution,” the researcher explained. “So a family with so many small objects, few medium-sized bodies and a single large object would really constitute quite an original situation.”

One explanation for this was proposed by other groups of astronomers some time ago. “It was thought that the matter that formed the family had been extracted from Euphrosyne by a tangential impact. As a result, the larger objects would have formed very close to it and later fallen back into the main body,” Carruba said.

The problem with such an explanation is that this type of impact, if it existed, would have constituted an extremely rare event. In light of this, Carruba and his colleagues decided to look for an alternate explanation. “What got our immediate attention was the fact that Euphrosyne is the only family of asteroids that is crossed in the middle by the nu6 resonance,” the researcher noted.

A mean-motion resonance occurs when two bodies orbiting a third body have their orbital periods related by a ratio of two small integers.

“A classic example of resonance is that found in the Kirkwood gaps in the asteroid belt. When an asteroid’s period of revolution [the time it takes to make one complete trip around the Sun] is equal to two times the period of revolution of Jupiter, the disturbances of the latter planet on the asteroid repeat periodically and could cause increases in the eccentricity of the asteroid’s orbit, leading to instabilities,” Carruba explained.

Computer Simulation

The fact that the Euphrosyne family is crossed by the nu6 resonance strongly influences the movement of its objects.

The epicenter of the orbit of a planet, comet or asteroid is the point at which the body’s trajectory is closest to the Sun. The epicenter is not fixed, however. It changes position periodically because of gravitational disturbance caused by other planets. This periodic movement is called epicenter precession.

“Nu6 resonance occurs when the frequency of the epicenter precession of the asteroids is equal or very close to the epicenter precession of Saturn,” Carruba said.

According to the researcher, the nu6 resonance is one of the most powerful resonances in the Solar System. “Many objects that interact with this resonance are quickly lost because it increases the eccentricity of their orbits, causing them to collide with the planets or with the Sun,” he said.

Because the nu6 resonance crosses the Euphrosyne family practically in the middle, the central region is what suffers the most from its influence, and this region is precisely where the larger objects are found.

“We performed a computer simulation of the dynamic evolution of the Euphrosyne family, starting from a fictitious family with the alpha characteristic of a group of the same type. Then, we calculated the variation of this parameter on a scale of one billion years,” the researcher said.

“We determined that the alpha value that informs the size distribution of the objects of the family increases over time. At an interval of 500 million years, it reached the current value measured in the Euphrosyne family,” he said.

This means that a tangential impact, which is extremely rare, does not need to be assumed in order to explain the atypical distribution of the size of Euphrosyne’s objects. “They may have formed naturally as a function of the local dynamic, achieving the configuration observed,” Carruba noted.

Source: English WebSite of the Agência FAPESP