 |
| Pluto & Charon |
It was no surprise that Charon rotates in the same period of time as it revolves so that it always presents the same hemisphere to Pluto. All the inner satellites and all the major satellites in the solar system have synchronous rotation and revolution because they are tidally coupled to their planets. A planet's gravity creates a slight tidal bulge in its moons and pulls on that bulge so that the moons cannot turn it away from the planet. One side of the satellite always faces the planet and the other side always faces away while the planet rotates rapidly, so that the moon rises and sets for all parts of the planet. But, Pluto furnished a surprise. Pluto and Charon are so close to twins in size and so close together that Charon's gravity induces a bulge in Pluto. The bulge is great enough that pluto is tidally coupled to Charon just as Charon is tidally coupled to Pluto. Thus, Pluto always shows the same face to Charon just as Charon always shows the same face to Pluto. It is the only example of mutual tidal coupling in the solar system. The result is that for an astronaut standing in Pluto, Charon is either always visible or never visible.
The shadows we see on Charon reveal an uneven, cratered landscape. Like Pluto, Charon is light gray, although somewhat darker and more even in color than Pluto, as was know from measurements made from Earth using the Pluto-Charon eclipses. The very slightly reddish brown hue of Pluto is missing from Charon-or at least from Charon's Pluto-facing side, that is the only side we get to see from the surface of Pluto. Missing too from Charon is the methane frost which partially covers Pluto. With Charon's smaller mass and therefore weaker gravity, whatever methane ice there was at the surface has evaporated. Perhaps this in part explains why Charon is less reflective. The escaping methane has exposed frozen water to view.
On Earth, we are used to rising and setting of the Sun, Moon, and stars as our planet turns. On Pluto, the Sun rises and sets, if somewhat slowly, but Charon stays fixed in the sky. It never rises or sets, thanks to tidal coupling. As Charon revolves once around Pluto in 6.4 days, Pluto spins once around on its axis in that same period of time. The result is that Charon hangs almost stationary in the sky while the Sun and Stars glide slowly past in the background. Because Charon is so large in the sky, stars are frequently blocked from view. These stellar occultations are the only eclipses visible during the 120-year gap between seasons of solar and lunar ecplipses.
From the vantage point of Earth, Pluto and Charon pass in front of and behind one another very rarely. The Earth experiences solar and lunar eclipses at least four times and sometimes as many as seven times a year. Because of Pluto's axial tilt and Charon's position over Pluto's equator, the pair goes for almost 120 years without their shadows ever falling upon one another. Then, in a period roughly six years long, Charon's orbit is nearly edge on to Earth and every 6.39-day orbit Charon makes carries it across the face of Pluto and then around behind Pluto. The result is an eclipse frenzy. Serendipitously, that eclipse season began in 1985, soon after Charon was discovered.
During an eclipse of the Sun on Pluto, Charon would look like a giant dark hole in the sky, marked only by the absence of stars. It would be dark but not black because it would be illuminated be reflected light from Pluto. The corona-the outer atmosphere of the Sun, which makes solar eclipses seen from Earth so beautiful-would be visible only just after the Sun vanished and just before it reappeared. At mid-eclipse, the disk of Charon covers the entire orbit of the Earth. The corona is far too faint at that distance from the Sun to peer around the edges of Charon.
The shadows we see on Charon reveal an uneven, cratered landscape. Like Pluto, Charon is light gray, although somewhat darker and more even in color than Pluto, as was know from measurements made from Earth using the Pluto-Charon eclipses. The very slightly reddish brown hue of Pluto is missing from Charon-or at least from Charon's Pluto-facing side, that is the only side we get to see from the surface of Pluto. Missing too from Charon is the methane frost which partially covers Pluto. With Charon's smaller mass and therefore weaker gravity, whatever methane ice there was at the surface has evaporated. Perhaps this in part explains why Charon is less reflective. The escaping methane has exposed frozen water to view.
On Earth, we are used to rising and setting of the Sun, Moon, and stars as our planet turns. On Pluto, the Sun rises and sets, if somewhat slowly, but Charon stays fixed in the sky. It never rises or sets, thanks to tidal coupling. As Charon revolves once around Pluto in 6.4 days, Pluto spins once around on its axis in that same period of time. The result is that Charon hangs almost stationary in the sky while the Sun and Stars glide slowly past in the background. Because Charon is so large in the sky, stars are frequently blocked from view. These stellar occultations are the only eclipses visible during the 120-year gap between seasons of solar and lunar ecplipses.
From the vantage point of Earth, Pluto and Charon pass in front of and behind one another very rarely. The Earth experiences solar and lunar eclipses at least four times and sometimes as many as seven times a year. Because of Pluto's axial tilt and Charon's position over Pluto's equator, the pair goes for almost 120 years without their shadows ever falling upon one another. Then, in a period roughly six years long, Charon's orbit is nearly edge on to Earth and every 6.39-day orbit Charon makes carries it across the face of Pluto and then around behind Pluto. The result is an eclipse frenzy. Serendipitously, that eclipse season began in 1985, soon after Charon was discovered.
During an eclipse of the Sun on Pluto, Charon would look like a giant dark hole in the sky, marked only by the absence of stars. It would be dark but not black because it would be illuminated be reflected light from Pluto. The corona-the outer atmosphere of the Sun, which makes solar eclipses seen from Earth so beautiful-would be visible only just after the Sun vanished and just before it reappeared. At mid-eclipse, the disk of Charon covers the entire orbit of the Earth. The corona is far too faint at that distance from the Sun to peer around the edges of Charon.
