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Jupiter's moon Io 
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Joined: Fri Mar 13, 2009 4:18 pm
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Io!

Io's a pretty cool moon, mm-kay. At a radius of 1821 km, it's just a little larger than our own Moon, but as the innermost of Jupiter's four Galilean moons (the ones Galileo spotted way back in 1610), it's so close to the gas giant that the "tidal" force of the planet's gravity squishes and squeezes the moon by as much as 100 km. This massive mechanical force heats up the moon's innards, and makes it the most volcanically active body in our solar system. Instead of being covered by icy craters like most moons, Io, which may have as many as 400 active volcanoes, is covered with the sulfuric outpourings of its inner layers:

Image
(image by NASA (source))

The moon shoots out so much stuff--in plumes that go as high as 1000 km above its surface--that the heavy particles form significant parts of magnetically controlled dust fields around Jupiter, even forming a high radiation zone around the moon due to their ionization by Jupiter's magnetosphere. And Io itself has a very large iron core, making it an unusually dense moon; this core interacts with Jupiter's magnetosphere, even causing visible aurora activity at Jupiter's poles, and in Io's very thin atmosphere, as seen here with Io in eclipse:

Image
(image by NASA (source))

Massive volcanic craters form in the surface, like this one, Tupan Patera:

Image
(image by NASA (source))

Explosive blasts scar the surface. These separate images show the formation of a 400 km (249 mile) black scar around Pillan Patera ("patera" means "volcanic depression") between April and September 1997:

Image
(image by NASA (source))

The large red ash circle seen nearby surrounds one of the moon's most prominent volcanoes, Pele.

The volcanism came as a bit of a surprise; and although some suspected it, it wasn't definite until the approaching Voyager 1 probe caught this photo of two eruption plumes (one over the left horizon, and the bright one in the middle, rising above the "terminator"--night/day shadow) in 1979:

Image
(image by NASA (source))

The Galileo probe, which took the other photos you've seen, got this more detailed photo of two eruption plumes in the late 90's:

Image
(image by NASA (source))

The second plume is harder to see: it's a little blue spot near the terminator, with a dark reddish plume shadow to its right. That plume, called "Prometheus," was one of the two visible in the original Voyager photo, and has possibly been continuously active since at least '79!

Here's a plume in action:

Image
(image by NASA (source))

That plume was 330 km high, but we're only seeing about the upper half of it, since its origin was over the horizon from the camera's vantage point. It was taken by the probe New Horizons; New Horizons, lauched from Earth in 2006, passed by Jupiter in 2007, using it as a gravitational slingshot to accelerate its voyage to Pluto, where it should arrive in 2015, within 10,000 km, to get the first real photos of that decommissioned planet! I was surprised when I first looked into it earlier this year that we don't actually have any decent pictures of Pluto, simply because it's so tiny (~1150 km radius--much smaller than Io) and far away and nothing's been sent out there before. So 2015 should be pretty exciting. New Horizons is in fact designed to escape from the solar system entirely (you need a launch speed of about 16.5 km/s from Earth to escape the Sun's gravity), and has a decent shot at doing a fly-by of another Kuiper Belt object on its way out--Pluto being just one of thousands of large objects believed to exist out there.


Sun Nov 29, 2009 1:07 am
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Joined: Fri Mar 13, 2009 4:18 pm
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Galileo's fly-bys of Io showed odd magnetic signatures in the magnetic field, and thanks to recent research in mineral physics, these have been attributed to "ultramafic" rocks--a type of rock formed from cooling magma, and which, when melted, can carry "a substantial electric current." In fact, it is now thought that the aberration in the magnetic field around Io is due to a 50-kilometer-deep magma ocean 30-50 km beneath its surface.


Thu May 19, 2011 2:55 am
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