My Google background (supplied by "NASA Image Library") has a picture of Jupiter, along with one of its moons (Ganymede? Io?) in the foreground.

What are these two features, that look like a red dot and blue cloud?

enter image description here

enter image description here

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    $\begingroup$ I assume the answer is "aliens", btw $\endgroup$
    – Richard
    Commented Oct 27, 2021 at 11:55
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    $\begingroup$ Neither image looks to be of Ganymede. It looks like the moon is Io. $\endgroup$
    – notovny
    Commented Oct 27, 2021 at 13:08
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    $\begingroup$ Yes, it is Io solarsystem.nasa.gov/moons/jupiter-moons/io/overview Jupiter's moon Io is the most volcanically active world in the Solar System, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high. 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 farther from Jupiter—Europa and Ganymede $\endgroup$
    – Robotex
    Commented Oct 27, 2021 at 13:23
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    $\begingroup$ FYI This image was taken in 2007 by the New Horizons spacecraft as it was zipping by the Jovian system en route to Pluto. $\endgroup$ Commented Oct 27, 2021 at 16:27

1 Answer 1


The image is of Jupiter's moon Io
The image is of course a composite, massively contrast-enhanced, and color adjusted to be both clearer and more visually appealing.

The red dot is a volcanic eruption (actually a fresh lava lake)

The blue plume is another volcano's plume, seen in sunlight against space.
Possibly the same volcano, but the angles don't quite seem to match up, and Io has many volcanoes.

Here is a very similar photo on the https://www.jpl.nasa.gov/images/active-volcanic-plumes-on-io website
enter image description here

Yes, those two plumes are the same color, the apparent difference would be due to differing levels of post-processing.
The actual image, at human eye sensitivity, would be a very dim, vaguely yellow-orange-ish almost featureless sphere. And the plume would be nonobvious to the point of near invisibility.
Remember that the sunlight there is only 50W/m2, compared to 1360W/m2 at Earth. (1/27th as bright as daylight here)

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    $\begingroup$ Funny, I expected the red dot to be due to a hot pixel in the sensor :D $\endgroup$
    – Ruslan
    Commented Oct 27, 2021 at 21:02
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    $\begingroup$ @Ruslan not likely. That image is a composite of 30+ smaller images, and has had the bejeesus post-processed out of it. Anything as mundane as a camera artifact would have been redone or massaged out of existence. Besides, compare the red spot with the light on the mountains just past the terminator. That spot is 4+ pixels wider than the smallest resolved feature. $\endgroup$ Commented Oct 27, 2021 at 21:39
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    $\begingroup$ Should be noted that 50W/m2 isn’t that dim. At about four stops dimmer than a cloudless summer day, it’s about equivalent to the illumination on an overcast day on Earth. $\endgroup$
    – JohannesD
    Commented Oct 27, 2021 at 22:41
  • $\begingroup$ @JohannesD: What kind of overcast? Like over Dublin in the summer of 1988? $\endgroup$ Commented Oct 29, 2021 at 18:56

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