Please Register or Log in to view the hidden image! From Space.com When Galileo first spied four points of light that appeared to be circling Jupiter, he really tightened the thumbscrews on those who believed that all bodies in the universe revolved around the Earth. Even today, almost 400 years later, this quartet is still upsetting expectations. Astronomer's current thinking is that the Galilean moons, unlike Earth's moon or the planets in the solar system, formed by a slow building-up process, one that began in the final stages of Jupiter's growth. Our own Moon almost certainly was created when a Mars-sized object crashed into a young Earth, creating a ring of debris that gathered into a satellite. Planets, on the other hand, are thought to form fairly rapidly in the early years after a star is born, typically when small rocks collide, joining together to form larger and larger objects. But for the moons Galileo discovered -- Io, Europa, Ganymede, and Callisto -- it seems that a third process was at work, according to planetary scientist David Stevenson of Caltech. "This is pointing toward a diversity of processes, a diversity of stories, about how satellites form," Stevenson told SPACE.com. Four-and-a-half billion years ago, according to the scenario he describes in the Oct. 5 issue of the journal Science, Jupiter was almost completely formed and had drained its surroundings of a lot of the gas and dust that was the developing solar system's raw material. Gradually, over the next million years, the giant planet managed to accumulate a relatively flat disk of this grit around it. This material then coalesced into the Galilean moons over a period of about 10,000 years. "That [million years] is a long time in the normal way of thinking about accumulation," Davidson explained. Such a slow gathering of material and short assembly would be like building a house "if someone was delivering you just one brick a week," he said. Jupiter and its moons can be sliced through with a single imaginary plane in space, which is what you would expect if they had formed out of a single disk of material. But the main evidence for this idea came from measurements of Callisto's gravity taken in 1997 by the Galileo spacecraft, which has been studying Jupiter and its moons since 1995. The Galileo readings suggested that the rock and ice that make up Callisto were partially mixed throughout its insides. If the moon had formed faster, this ice would have melted and the rock would have sunk to the satellite's core. The other moons have clearer divisions, probably as a result of being closer to Jupiter, thus forming faster and absorbing heat from the giant planet, Davidson said. "But it's very likely that they also formed on this timescale." Despite their different origins, the Jovian moons and the solar system's planets share some common ground. Both lie in a plane as they orbit and the members of each group spin counter-clockwise as seen from above. So until recent years, scientists were tempted to think both could have formed the same way. Still, this theory is not fully hatched, Davidson said. Planetary scientists have taken a renewed interest in it over the last year, and one of them might yet come up with a better idea. "There are lots of details that haven't been figured out," he said.
wet1 ... Thanks for yet another beautiful picture .. and reinforcing my belief that the more we learn, the more we should realize how little we know.
