A delicate binary dance

A delicate binary dance

A delicate binary dance







				

Sometimes the small things in life are the most important. 

In the case of the life of the Solar System, those little things are asteroids, comets and objects of the Kuiper belt, the remains of the debris that did not conglomerate with the planets. The giant planets (Jupiter, Saturn, Uranus and Neptune) have dragged these smaller objects during the 4,500 million years of life of the Solar System, often expelling them completely. Astronomers can deduce the behavior of giant planets in the past by studying the small bodies of the Solar System and how they are distributed. In other words, the smallest members of the Solar System are key to revealing their dynamic past. 

With several hundred thousand known asteroids and a few thousand known objects from the Kuiper Belt, astronomers are just beginning to understand the complex history of the Solar System. However, the early days of our Solar System remain largely a mystery (and in the early days, I mean the first hundreds of millions of years). They have been proposed several ideas to explain how the planets were formed and how they arrived in their current orbits. Each theory predicts variations in the observable characteristics of the Solar System, such as the way in which small bodies are distributed. All in general agree that the giant planets suffered some degree of migration, but none explains satisfactorily what we observe. The astrobito of today throws another key in the history. The topic? A peculiar binary asteroid called (617) Patroclus-Menoetius.

Figure 1. Artistic interpretation of the Patroclus-Menoetius binary Jupiter Trojan system. Objects of ~ 110 km are separated by only ~ 670 km. Credit: W.M Keck Observatory / Lynette Cook.

A strange duo

(617) Patroclus-Menoetius, which we will call P-M from now on, is a system of two Trojans of Jupiter of ~ 110 kilometers in size (objects that orbit 60 degrees ahead or behind Jupiter with the same Semi-major axis than the same). Because they are almost equal in size, Patroclus and Menoetius co-orbit around their mutual center of mass. Such binaries were probably formed when the planetesimal disk was still collapsing and the pairs could easily be formed, which made them some of the oldest solar system relics. They are powerful tools for the study of dynamic encounters throughout the history of the Solar System, since they can be easily interrupted (for example, separated or forced to collide) by a larger mass, such as Jupiter. The fact that P-M has survived for 4.5 billion years, and so close to Jupiter, imposes strong limitations on the conditions at the beginning of the history of the Solar System.

The survival of P-M is not what makes it rare, but it is also a relatively compact binary with an average separation of only ~ 670 km. It is believed that the Jupiter Trojans formed much farther than Jupiter on a planetesimal disk of 20 to 30 astronomical units (AU) from the Sun, scattered inward during a period of dynamic instability, and subsequently captured by the Trojan regions of Jupiter. . The very existence of PM means that it survived: 1) collisions within the disk at 20-30 AU that would have brought down one or both members of the binary and 2) transport from its initial location to 20-30 AU to its current location at 5.2 UA

Simulating binaries in the solar system

The authors of today's article performed simulations of N-bodies of the Solar System to determine the probability of P-M of fulfilling both conditions, given several different versions of the early Solar System. The authors first tested the likelihood of condition 2 mentioned in the previous paragraph, examining the survivability of transported binaries from 20-30 AU to the Trojan regions of Jupiter to 5.2 AU. They found that survival depends largely on the separation of the binary. Most of the narrow binaries (components separated by less than 1500 km) with masses similar to P-M survived in the simulations, while most of the wide binaries did not. Fortunately, the P-M separation of ~ 670 km falls into the "narrow" category.

However, P-M must have survived both previous conditions. Next, the authors tested the survivability against early disk collision grinding, when impacts were much more common due to the higher density of objects. Using an initial disk mass of 20 land masses (derived from previous simulations), they find that the number of objects over 100 km that survive over the lifetime of the disk (400-700 million years) is an order of magnitude smaller than that predicted by models such as what they used to prove condition 2 above. In addition, the probability of survival of binaries similar to P-M is approximately 0.2% if the disk's useful life is over 400 million years because even relatively small and non-catastrophic impacts can disrupt the binary system. Increasing the initial mass of the disk and, therefore, the total number of objects, only makes the problem worse, since the disk will be sprayed faster. The only option left is that the life of the disk was much shorter than previously thought.

Figure 2. The dependence of the binary survival rate on the useful life of the planetesimal disk. Only the lives of ≲100Myr result in a significant fraction of binaries that survive the collision processes on the disk. Credit: Figure 2 in the article.

Nesvourný et al. found that the survival of P-M is practically impossible unless the life of the planetesimal disk is less than 100 million years (Figure 2). That is, the migration of the giant planets and the subsequent dynamic instability caused by the dissipation of the planetesimal disk must have occurred within the first 100 million years of the history of the Solar System. This is a problem for the current explanations of Late Heavy Bombardment (LHB, for its acronym in English), a period of great increase in the impact rate recorded on the surfaces of terrestrial bodies (for example, the Moon) that occurred 400-700 million years after the formation of the Solar System . Previously, the LHB was linked to the dynamic instability caused by the migration of the giant planets, but today's article suggests that migration must have occurred much earlier than previously thought.

So, what caused the LHB? The truth is that we still do not know. The restrictions provided by the P-M system can only tell us that the LHB was not caused by planetesimals scattered inward during the migration of the giant planets. However, there is one thing we do know: Trojan asteroids are some of the most important objects to study if we want to understand the evolution of the Solar System. Fortunately, we will soon have the opportunity to study closely several Jupiter Trojans with the launch of the mission Lucy from NASA in 2021. Fortunately, Lucy's final goal will be none other than (617) Patroclus-Menoetius in 2033 and I, for example, can not wait to see what she discovers!



				



				

							

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