Callisto from afar
Published on Apr 6, 2007 at 10:59 am.
4 Comments.
Filed under extraterrestrial moons.
Callisto is one of the four Galilean moons of Jupiter, easily seen in even modest telescopes. But, from Earth, it is generally just a dot. It wasn’t until the Voyager missions that we got our first closeup views of the moon. Since then, the Hubble telescope and the latest large ground based telescopes have been able to observe some features on it, but the clearest views will always be up close. So, naturally, as the New Horizons spacecraft passed Jupiter a little over a month ago, it took lots of images of Jupiter, Jupiter’s ring, and Jupiter’s moons. One of those moons was Callisto.
Now, these images recently released by the New Horizons team show just how good the imaging system on New Horizons really is! The image on the right was taken at a distance of 4.2 million kilometers. Since Callisto was on the opposite side of Jupiter from where New Horizons was passing, this was the closest that the spacecraft got to Callisto. Even so, these images are better than anything that we can get from here on Earth.
Callisto is gray in color, not because it is composed of rock like the Earth’s Moon, but because it is covered in dust, probably micrometeorites or dust from the Jupiter system that settled on the surface. Beneath the dust is mostly ice. Impact craters excavate that ice, leaving bright marks where things have run into Callisto. The largest of these impact features is Valhalla, which is actually visible in these images on the left side of the moon, just above the middle of the left limb. Valhalla is a large impact structure, but it isn’t very deep. Presumably, the ice below flowed back into the basin (remember that ice can flow!).
Other than the impact features, though, Callisto has very little else in the way of visible geological features. This contrasts with the other Galilean moons, all of which so features on their surfaces showing quite a bit of recent, or fairly recent, activity.
New Horizons passed a bit far from Callisto to expect much in the way of new discoveries, particularly since the moon was studied much more closely by the Galileo spacecraft some years back. However, by studying Callisto both with the optical and infrared systems, the New Horizons team can study how the icy features of Callisto change with temperature (warm on the sunlit side, and cold on the night side). This will be useful when studying Pluto, Charon, and other Kuiper Belt objects, which is really the whole point of the New Horizons mission. Remember, when New Horizons passes these bodies, it will be only a one shot deal. We might not get another chance to study them, so the more we know about what we are looking at, the better.
-Astroprof
(Images courtesy NASA, JPL, New Horizons)
terry on April 6, 2007 at 12:07 pm: 1
Okay, I’ve been reading too much about early humans - I immediately thought, “How cool, Callisto from the Afar valley!”
Astroprof on April 6, 2007 at 12:30 pm: 2
Well, I always think of Callisto from the TV show, Xena: Warrior Princess.
terry on April 6, 2007 at 7:53 pm: 3
There is a bit of resemblance isn’t there? Both wear a lot of black with flashy white bits showing here and there…
A Ler…-- Rastos de Luz on April 10, 2007 at 12:28 pm: 4
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