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New Light on Charon

While we’re on the subject of Pluto — and we will be off and on as the New Horizons launch approaches — it’s intriguing to see how much we have already learned about the Pluto/Charon pair from Earth-based telescopes. We’ve just found out, for instance, that Charon’s radius is 606 kilometers, with a fudge factor of plus or minus 8 kilometers. That’s a pretty remarkable measurement for an object this distant, but it proceeded through a reliable and time-honored astronomical method: stellar occultation.

If you know when and where to look, an occultation can provide reams of information. What’s happening is that the nearer object, in this case Charon, passes in front of a distant star; observations of that event give us not just accurate size estimates but useful data on the object’s density and possible atmosphere. For by combining the occultation data with measurements from the Hubble Space Telescope, the team (from MIT and Williams College) was able to establish a density for Charon of 1.72 g/cm³. That’s about a third of Earth’s density, and it reflects the combination of rock and ice out of which Charon is made.

The early take on these findings is that they argue argainst the theory that Pluto and Charon formed by the cooling and condensation of gas and dust from the solar nebula. Instead, a collision between two objects in the early Solar System seems to be the culprit. Here’s Nature lead author Amanda Gulbis on this issue:

“Our observations show that there is no substantial atmosphere on Charon, which is consistent with an impact formation scenario. We also find that Charon contains roughly 10% less rock by mass than Pluto. This difference suggests that either, or both, objects involved in a Charon-forming collision had concentrations of heavier materials in their cores.”

Much the same impact scenario is often used to point to the origins of the Earth/Moon system. And it will be fascinating to watch stellar occultation methods applied to more distant Kuiper Belt objects like 2003 UB313 (the ’10th planet’). All told, the Charon observations took less than a minute, and were viewable only in a narrow, 650-mile wide region of South America. That’s a tiny window, but it opens up quite a useful vista as we look for ways to study the outer edges of the Solar System.

The paper is A.A.S. Gulbis, J.L. Elliot et al., “Charon’s Radius and Atmospheric Constraints from Observations of A Stellar Occultation,” which will run in the January 5 issue of Nature.