Snapped by the probe's Long Range Reconnaissance Imager (LORRI) as the spacecraft flew past Jupiter in 2007, this first-ever movie of an Io plume clearly shows motion in the cloud of volcanic debris, which extends 330 km (205 miles) above the moon's surface.
Only the upper part of the plume is visible from this vantage point.
The plume's source is 130 km (80 miles) below the edge of Io's disk, on the far side of the moon.
Io's hyperactive nature is emphasized by the fact that two other volcanic plumes are also visible off the edge of Io's disk: Masubi at the 7 o'clock position, and a very faint plume, possibly from the volcano Zal, at the 10 o'clock position.
Jupiter illuminates the night side of Io, and the most prominent feature visible on the disk is the dark horseshoe shape of the volcano Loki, likely an enormous lava lake.
Boosaule Mons, which at 18 km (11 miles) is the highest mountain on Io and one of the highest mountains in the solar system, pokes above the edge of the disk on the right side.
The five images were obtained over an 8-minute span, with two minutes between frames, from 23:50 to 23:58 Universal Time on 1 March 2007.
Io was 3.8 million km (2.4 million miles) from New Horizons.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
However, concentrations of volcanic activity are significantly displaced from where they are expected to be based on models that predict how the moon's interior is heated, according to NASA and European Space Agency researchers.
Io is caught in a tug-of-war between Jupiter's massive gravity and the smaller but precisely timed pulls from two neighboring moons that orbit further from Jupiter – Europa and Ganymede.
Io orbits faster than these other moons, completing two orbits every time Europa finishes one, and four orbits for each one Ganymede makes.
This regular timing means that Io feels the strongest gravitational pull from its neighboring moons in the same orbital location, which distorts Io's orbit into an oval shape.
This in turn causes Io to flex as it moves around Jupiter. For example, as Io gets closer to Jupiter, the giant planet's powerful gravity deforms the moon toward it and then, as Io moves farther away, the gravitational pull decreases and the moon relaxes.
The flexing from gravity causes tidal heating—in the same way that you can heat up a spot on a wire coat hanger by repeatedly bending it, the flexing creates friction in Io's interior, which generates the tremendous heat that powers the moon's extreme volcanism.
The question remains regarding exactly how this tidal heating affects the moon's interior. Some propose it heats up the deep interior, but the prevailing view is that most of the heating occurs within a relatively shallow layer under the crust, called the asthenosphere.
The asthenosphere is where rock behaves like putty, slowly deforming under heat and pressure.
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