# @nasa on Instagram

- **Type:** Image
- **Original URL:** https://www.instagram.com/p/DS3GFkHktUM
- **Gondola URL:** https://gondola.cc/posts/38346476-nasa-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/ad3e973e3e.jpg
- **Posted:** 2025-12-29T20:31:26.000+00:00
- **Account Owner:** NASA (@NASA) — https://gondola.cc/NASA

## Caption

How well do you know the Galilean moons?

The Galilean moons are Jupiter’s four largest moons, named for Galileo Galilei, the Italian astronomer who observed them in 1610.

Ganymede is the largest moon in our solar system—even bigger than Mercury and Pluto. It’s also the only moon in our solar system with its own magnetic field and the auroras that come with it. Its icy surface is marked by streaky craters; most are bright white, but around 1% have dark streaks.

Io is a moon covered in volcanoes and molten lava pools. It is the most volcanically active world in our solar system, with eruptions that can reach dozens of miles high. Some volcanic bursts can even be seen through large telescopes on Earth!

Callisto is the most heavily cratered object in our solar system, with its densely dotted surface revealing a long history of impacts, likely from asteroids or comets. Due to the density of these craters, scientists believe this moon has little geologic activity to clear them.

Last, but not at all least, is Europa. This moon’s heavily-cracked surface is composed primarily of water ice, and it reflects 5.5 times the amount of sunlight that our Moon does.

Can you tell which image is which moon? Answers in the image descriptions 👇

Image descriptions:
1. An image of Io, half shaded. The moon is deep tan in color with a surface textured by volcanoes and lava pools. Credit: NASA/JPL–Caltech/SwRI/MSSS; Image processing: Emma Wälimäki © CC BY

2. An image of Europa, partially in darkness. The surface is a light, frosty beige with what looks like long scrapes along its body. Credit: NASA/JPL-Caltech/SwRI/MSSS Image processing: Björn Jónsson (CC BY 3.0)

3. A close-up image of Ganymede. The moon is varying shades of light gray similar to our own Moon, but is covered in bright, streaky craters. Credit: NASA/JPL-Caltech/SwRI/MSSS

4. An image of Callisto with shadows creeping over its right side. Its surface is a dark, desaturated brown tone, densely dotted with white impact scars. Credit: NASA/JPL

#NASA #Io #Ganymede #Europa #Callisto

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## Tags

europa, io, nasa, callisto, ganymede

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