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Space Topics: Asteroids and Comets

Binary Trojan Asteroid 617 Patroclus and Menoetius

Click to enlarge > Artist's rendering of the binary asteroids Patroclus (center) and Menoetius. Jupiter and its four Galilean satellites are visible in the distance, while the sun is out of sight to the left. Credit: W. M. Keck Observatory / Lynette Cook

Asteroid 617 Patroclus straddles classification schemes, challenging scientists' ideas about what asteroids are and where and how they formed.  It is a Trojan asteroid, meaning that it orbits as a companion to Jupiter, 60 degrees behind the giant in its orbit (at the gravitationally stable point known as "L5").  It is a binary asteroid, having a companion, Menoetius, nearly as large as itself.  And one recent study has suggested that unlike most "asteroids" it may not be made of rock but instead of ice, like the denizens of the Kuiper belt.

Basic Facts

Patroclus and Menoetius circle each other every 4.3 days at a distance of 680 kilometers (423 miles):

Click to enlarge > Mutual orbits of Patroclus and Menoetius This animation plays only once; to see it again, refresh your screen. Credit: Franck Marchis, UC Berkeley

Size: Patroclus: 122 kilometers (76 miles); Menoetius: 112 kilometers (70 miles)
Mass: Total system mass (Patroclus plus Menoetius) = 1.36 x 1018 kilograms
Density: 0.8 grams per cubic centimeter, less than 1/3 that of rock and less than water ice
Composition: Based on the density, probably loosely packed water ice with about 15% open space with a thin covering of dirt.

How Do Scientists Know What Patroclus and Menoetius Are Made Of?

It is because Patroclus and Menoetius orbit each other that it is possible to find out their densities.  A team of scientists led by Franck Marchis of the University of California at Berkeley observed the pair in 2004 and 2005 with the Laser Guide Star Adaptive Optics system on the Keck II telescope at Mauna Kea.  The powerful telescope was able to resolve the closely orbiting binary pair as two separate bodies.  By determining the size and shape of their mutual orbit, the team could determine the mass of the system.  The team was also able to measure the diameters of the two objects.  The diameters allowed the scientists to estimate the volumes of the two bodies, and the density could be calculated from the mass and volume.

There are uncertainties in the estimates of the volumes that cause uncertainties in the estimates of the density.  Still, the bulk density is likely to be between 0.6 and 1.0 grams per cubic centimeter, which is extremely close to the density of ice with some empty space.  It is theoretically possible to imagine that the bodies could be made largely of rock, but in order to match the density measured by Marchis' team, the bodies would have to be made of almost 50% empty space, which seems structurally unlikely.

Click to enlarge > Estimating the composition of asteroid 617 Patroclus This graph explains how scientists estimated the composition of asteroid 617 Patroclus. The Y axis of the graph represents density. Without any pore space, pure water ice has a bulk density just below 1 gram per cubic centimeter (g/cc); large icy moons, around 1.6 g/cc; rocky/dirty asteroids called carbonaceous chondrites, around 2.5 g/cc; and solid rock called anhydrous silicates, 3.3 g/cc. The asteroid Patroclus has a density lower than any of these at about 0.8 g/cc. But Patroclus, being small, is likely to have open, empty pore spaces in its interior. Allowing for porosity (x-axis on the graph), Patroclus could be made of anything from pure water ice with a porosity of roughly 15% to a Ganymede-like composition (ice plus rock) with a porosity of roughly 50%. Any denser composition is very unlikely because of the high amount of pore space required. Credit: Franck Marchis et al., Nature

Where Did Patroclus and Menoetius Come From?

If Patroclus and Menoetius truly are made of ice, they could not have formed in the rock- and metal-rich asteroid belt.  Instead, they must have formed at a greater distance from the Sun, beyond the orbit of Jupiter and Saturn, perhaps even among the Kuiper belt or Oort cloud.  In other words, Patroclus and Menoetius could have began their lives as comets.

This image, captured using the Laser Guide Star Adaptive Optics System on the 10-meter Keck II telescope on Mauna Kea, is the first to separate asteroid 617 Patroclus' binary companion, Menoetius. The two bodies are barely larger than the diffraction limit of the telescope, so the image does not resolve any details on the surfaces of the bodies. Credit: Franck Marchis et al., Nature

In a study published in 2006 in the journal Nature, Marchis and his team suggested that Patroclus and Menoetius could have arrived at their present position at the Trojan point on Jupiter's orbit around 650 million years after the solar system formed (or about 4 billion years ago), when the inner solar system may have been bombarded by a barrage of comets.  However, not all scientists are certain that the "Late Heavy Bombardment" that has been theorized based upon the lunar cratering record is a real event and not just an artifact of the record.

Regardless of whether the Late Heavy Bombardment happened, it is likely that there was a period in the history of the solar system when comets were disturbed from their distant orbit at a high rate.  Current theories for the formation of the solar system suggest that the gas giants, Jupiter, Saturn, Uranus, and Neptune, originally formed closer to the Sun and migrated outward over time.  As the gas giants moved, their gravitational influence moved outward as well, and they ended up stirring up the original population of planetesimals, ejecting some from the solar system, placing some in the Kuiper belt, and capturing some in the Trojan points of their orbits.

Resources

Study of Patroclus and Menoetius: A Double Trojan System, on Franck Marchis' website
A low density of 0.8 g cm-3 for the Trojan binary asteroid 617 Patroclus, Marchis et al. Nature paper from January 2006 (320k, PDF format)