Io moon surface characteristics11/11/2023 Iapetus is also ringed by a mountainous equatorial ridge that is 8 miles (13 km) high and 12 miles (20 km) wide, giving the moon its distinctive walnut shape. The process was likely started by dust from the outer moons accumulating on the leading hemisphere, but once it began, the tendency of the dark surface to absorb heat has caused a runaway sublimation effect. Most of the dark material seems to come from within Iapetus, left behind as dark 'lag' when dust-laden ice from the moon's surface sublimates - turns from solid to vapor. However, images from Cassini reveal a more complex story. One early theory to explain the color difference was that the leading side is covered in dust generated by tiny meteorite impacts on small outer moons, which spirals towards Saturn, as found by NASA. Its leading hemisphere-the half that faces 'forwards' as it orbits Saturn-is dark brown, while its trailing hemisphere is light gray. The first became obvious when it was discovered in 1671 - it is much dimmer when seen on one side of its orbit compared to the other. Iapetus has two distinct claims to a place in any list of weird satellites. (Image credit: NASA/JPL-Caltech/Space Science Institute) Left-hand panel shows the moon's leading hemisphere and the right-hand panel shows the moon's trailing side. Another theory is that Dactyl has suffered a disrupting impact, but has pulled itself back together in its orbit, as found by NASA - which might explain its surprisingly spherical shape. One theory is that the Koronis family is younger than it appears, and Ida's heavy cratering is due to a storm of impacts triggered in the original break-up. So how can it be over a billion years old? Dactyl could be a smaller fragment of debris from the collision that ended up in orbit around Ida, but there is a problem - computer models suggest Dactyl would almost certainly be destroyed by an impact from another asteroid. The family is thought to have formed 1 or 2 billion years ago during an asteroid collision. Ida is a major member of the Koronis family of over 300 asteroids, all of which share similar orbits. ![]() Thanks to the larger asteroid's weak gravity, Dactyl is unlikely to be an object captured into orbit, but the alternative - that Ida and Dactyl formed alongside each other - raises as many questions as it answers. (Image credit: NASA / JPL / Ted Stryk)Ģ43 Ida, an asteroid designated as a minor planet, has a moon, measuring just 0.99 miles (1.6 km) across on its longest axis. Dactylĭactyl imaged by NASA's Galileo Orbiter. Callisto's surface, however, has remained essentially unchanged for more than 4.5 billion years, developing its dense landscape of overlapping craters across aeons. Jupiter's larger moons are directly in the firing line, and end up soaking up more than their fair share of impacts, but Callisto's inner neighbors - influenced by greater tidal forces - have all experienced geological processes that wiped away most of their ancient craters. Its main claim to fame is the title of most heavily cratered object in the solar system its dark surface is covered in craters down to the limit of visibility, the deepest of which have exposed fresh ice from beneath and scattered bright 'ejecta' debris across the surface.Ĭallisto owes its cratered surface to its location in the Jupiter system - the giant planet's gravity exerts a powerful influence, disrupting the orbits of passing comets and often pulling them to their doom, most spectacularly demonstrated in the 1994 impact of Comet Shoemaker-Levy 9. ![]() The outermost of Jupiter's Galilean moons, Callisto is the third-largest moon in the solar system, and is only slightly smaller than Mercury. This image of Callisto was taken from NASA's Galileo spacecraft.
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