Size Of Pluto's Moon Charon Determined With Unprecedented Accuracy
Being in the right place at the right time gave a group of Massachusetts research astronomers a unique opportunity to study Pluto's largest moon Charon. The resulting measurements, to unprecedented accuracy, of Charon's size and possible atmosphere provide insight into the way this distant world may have formed.
Astronomers from the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts and Williams College in Williamstown, Massachusetts observed the light from a star as it disappeared behind Charon and reappeared on the other side – an event known as a stellar occultation. Occultations provide important information about the size of remote bodies, as well as the makeup of their atmospheres (if they have any).
Jim Elliot, a professor at MIT, stated "We have been waiting many years for this opportunity. Watching the star vanish as it was blocked by Charon was spectacular."
Although the star disappeared behind Charon for less than a minute, data from the observations provided considerable information about this tiny moon. The MIT-Williams collaboration determined Charon's radius to be 606 ± 8 km. The size was combined with mass measurements from Hubble Space Telescope data to establish a density for Charon of 1.72 g/cm3. This density, roughly 1/3 that of the Earth, reflects Charon's rocky-icy composition.
What makes this achievement so remarkable is that the observation could only be made from a narrow, 650-mile wide region in South America. The MIT-Williams observers were located at four telescopes in Chile and one telescope in Brazil for the event.
The largest telescope employed by the consortium was the 8-meter Gemini South Telescope on Cerro Pachón, Chile. The observations utilized the Acquisition Camera, a guider instrument that is typically used for telescope pointing and target selection, as a high-speed photometer. Portable camera systems constructed by the MIT-Williams group were mounted on the other telescopes: the 6.5-meter Clay and 2.5-meter du Pont at Las Campanas Observatory in La Serena, Chile, the 0.8-meter at the Observatório Cerro Armazones in Anofagasta, Chile, and the 0.6-meter at Observatório Pico dos Dias, Itajubá, Brazil. Observations were successful at all stations except Brazil, which was clouded out.
Jay Pasachoff, Professor of Astronomy at Williams College and a collaborator in the effort, praised the team doing the work. "It's astounding that our group could be in the right place at the right time to line up a tiny body three billion miles away," he said. "The successful observations are quite a reward for all of the people who helped predict the event, constructed and integrated the equipment, and traveled to the telescopes."
Observations taken at a high rate, 10 frames per second, from the 6.5-meter Clay telescope detected subtle optical effects caused when the starlight passed the edge of Charon's disk. By analyzing these effects, known as diffraction fringes, the MIT-Williams team concluded that any atmosphere on Charon is less than one millionth the density of Earth's atmosphere. Their analysis provided very strict limits on the amounts of various gases that could be present.
The results of the observations argue against the theory that Pluto and Charon were formed by the cooling and condensing of the gas and dust known as the solar nebula. Instead, astronomers think that Charon was formed in a collision between two objects early in the formation of the solar system.
"Our observations show that there is no substantial atmosphere on Charon, which is consistent with an impact formation scenario," said Amanda Gulbis. "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." A collisional formation like this has a parallel in theories for the formation of the Earth-Moon system.
The so-called "10th planet" (2003 UB313), recently discovered by scientists from Caltech, is a prime candidate for stellar occultation observations. Although this object is approximately twice as far away from the Earth as Charon, it is thought to be twice as large. 2003 UB313 thus covers the same angular extent in the sky as Charon.
"We are eager to use the occultation technique to probe for atmospheres around large Kuiper belt objects," remarked Jim Elliot, who has been observing stellar occultations by bodies in the solar system for more than three decades.
For More Information:
http://www.williams.edu/admin/news/releases.php?id=1002
http://www.williams.edu/admin/news/releases.php?id=1106
Astronomers from the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts and Williams College in Williamstown, Massachusetts observed the light from a star as it disappeared behind Charon and reappeared on the other side – an event known as a stellar occultation. Occultations provide important information about the size of remote bodies, as well as the makeup of their atmospheres (if they have any).
Jim Elliot, a professor at MIT, stated "We have been waiting many years for this opportunity. Watching the star vanish as it was blocked by Charon was spectacular."
Although the star disappeared behind Charon for less than a minute, data from the observations provided considerable information about this tiny moon. The MIT-Williams collaboration determined Charon's radius to be 606 ± 8 km. The size was combined with mass measurements from Hubble Space Telescope data to establish a density for Charon of 1.72 g/cm3. This density, roughly 1/3 that of the Earth, reflects Charon's rocky-icy composition.
What makes this achievement so remarkable is that the observation could only be made from a narrow, 650-mile wide region in South America. The MIT-Williams observers were located at four telescopes in Chile and one telescope in Brazil for the event.
The largest telescope employed by the consortium was the 8-meter Gemini South Telescope on Cerro Pachón, Chile. The observations utilized the Acquisition Camera, a guider instrument that is typically used for telescope pointing and target selection, as a high-speed photometer. Portable camera systems constructed by the MIT-Williams group were mounted on the other telescopes: the 6.5-meter Clay and 2.5-meter du Pont at Las Campanas Observatory in La Serena, Chile, the 0.8-meter at the Observatório Cerro Armazones in Anofagasta, Chile, and the 0.6-meter at Observatório Pico dos Dias, Itajubá, Brazil. Observations were successful at all stations except Brazil, which was clouded out.
Jay Pasachoff, Professor of Astronomy at Williams College and a collaborator in the effort, praised the team doing the work. "It's astounding that our group could be in the right place at the right time to line up a tiny body three billion miles away," he said. "The successful observations are quite a reward for all of the people who helped predict the event, constructed and integrated the equipment, and traveled to the telescopes."
Observations taken at a high rate, 10 frames per second, from the 6.5-meter Clay telescope detected subtle optical effects caused when the starlight passed the edge of Charon's disk. By analyzing these effects, known as diffraction fringes, the MIT-Williams team concluded that any atmosphere on Charon is less than one millionth the density of Earth's atmosphere. Their analysis provided very strict limits on the amounts of various gases that could be present.
The results of the observations argue against the theory that Pluto and Charon were formed by the cooling and condensing of the gas and dust known as the solar nebula. Instead, astronomers think that Charon was formed in a collision between two objects early in the formation of the solar system.
"Our observations show that there is no substantial atmosphere on Charon, which is consistent with an impact formation scenario," said Amanda Gulbis. "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." A collisional formation like this has a parallel in theories for the formation of the Earth-Moon system.
The so-called "10th planet" (2003 UB313), recently discovered by scientists from Caltech, is a prime candidate for stellar occultation observations. Although this object is approximately twice as far away from the Earth as Charon, it is thought to be twice as large. 2003 UB313 thus covers the same angular extent in the sky as Charon.
"We are eager to use the occultation technique to probe for atmospheres around large Kuiper belt objects," remarked Jim Elliot, who has been observing stellar occultations by bodies in the solar system for more than three decades.
For More Information:
http://www.williams.edu/admin/news/releases.php?id=1002
http://www.williams.edu/admin/news/releases.php?id=1106
Funding Member
