Io in eclipse
Io in eclipse As New Horizons sailed past Jupiter on February 27, 2007, it watched Io pass in to Jupiter's shadow. Without the Sun illuminating the surface, New Horizons could keep its shutter open for a very long time, capturing the glow of hot spots and aurorae on Io's surface and atmosphere. NASA / JHUAPL / SwRI

The three brightest spots are from the incandescent glow of hot lava erupting from volcanoes: Pele and Reiden (south of the equator), and a previously unknown volcano near 22°N, 233°W.  The disk is outlined by an auroral glow, produced by the interaction of Europa's thin atmosphere with the intense magnetic field of Jupiter.  A spot at 2 o'clock is a cloud of atmosphere that is somehow levitating 330 kilometers above the surface.  On the left side of the disk is a spotty region on the side of Io that faces Jupiter.  Scientists are unsure what causes these glowing spots, but they hypothesize that the spots are related to electrical currents connecting Io to Jupiter's magnetosphere.

Io in eclipse (annotated)
Io in eclipse (annotated) Image: NASA / JHUAPL / SwRI