Trojan asteroids reveal giant planet skirmish

The main camp of the Trojan asteroids, known as the "Greek camp", is located at Jupiter's Lagrangian point L4 (think "L4 forwards"), which is a special point in the orbit where the gravitational forces of the Sun perfectly balanced. Among the troop population – located in L5 and called "Trojan camp" – is a strange pair of twin Trooclans called Patroclus and Menoetius. The asteroids made of pebbles that make up this binary pair are 130 kilometers wide and are remnants of the very first moments of the formation of the solar system.

"Current observations of the Kuiper Belt show that such binaries were very prevalent in ancient times," said co-author William Bottke of SwRI's Space Studies Department. "Only a few now exist in the orbit of Neptune, the question is how to interpret the survivors."

Many existing models that attempt to explain the evolution of the solar system suggest that global migration occurred about 700 million years after the formation of the Sun. This is partly because the migration of planets is often linked to a period known as late heavy bombardment (LHB) – a cataclysmic era beginning about 3.9 billion years ago. During the LHB, a large number of asteroids have tirelessly bombed the inner planets and the Moon, leaving behind many craters that we can still see today.

However, although LHB is often associated with global migration, the authors of the new study claim that the two events occurred at distinct epochs separated by more than half a billion years.

This animation shows how the Trojan horses Patroclus and Menoetius revolve around the Sun behind Jupiter. New research suggests that, since the fragile binary pair is still intact, it must have entered the Jupiter Trojan camp much sooner than expected.

Durda / Marchi / SwRI

According to the study, if the planets did not migrate before 700 million years after the formation of the Sun, there is very little chance that the fragile Patroclus-Menoetius Trojan could survive. Instead, researchers say (and support new models and simulations) that Patroclus and Menoetius were already linked together and in place around Jupiter long before LHB occurred. Since the binary pair was probably captured by Jupiter during its clash with the other giants, the researchers argue that the very existence of the Trojan horse suggests that the planets migrated very early, about 100 million years later.

If it turns out to be true, this new model has dramatic implications for the LHB.

Many large craters observed on the Moon, mercury and Mars are generally associated with the impacts of asteroids from the outer solar system during LHB. But if the planets migrated hundreds of millions of years before the onset of LHB, then where do the impacting asteroids come from? One of the possibilities is that the impactors causing the crater that hit the inner planets do not come from the external solar system. Instead, they could have come from the rocky remains of newly formed terrestrial planets.

While there is still a lot of mystery regarding Trojan asteroids to be solved, an ambitious NASA mission will help lift the veil. The NASA mission, scheduled for 2021, will soon embark on a twelve-year journey that will lead to six Trojan asteroids, as well as a main belt asteroid. Rightly, Lucy's latest target, which she should reach in March 2033, is none other than the binary pair of Patroclus and Menoetius.

So, keep an eye on, because over the next 15 years we are ready to learn a lot about the hidden troops that make up Jupiter's Trojans. And they probably have other secrets to share.