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Astronomers using archival data from NASA’s Hubble 
Space Telescope found evidence of water vapor in  

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the thin atmosphere of Jupiter’s moon Ganymede.

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Ganymede is the largest moon in our solar  

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system, larger than Mercury and Pluto, 
and three-quarters the size of Mars.  

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It’s an ice-covered world that may hold more 
water than all of Earth’s oceans combined,  

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but unlike Earth, Ganymede’s oceans are 
below its 100-mile-thick, icy crust.

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Ultraviolet images taken in 1998 and 2010 using 
Hubble’s Space Telescope Imaging Spectrograph  

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revealed bands of charged particles at Ganymede 
similar to Earth’s polar auroral ovals,  

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the regions that give us our Northern and 
Southern lights, but the two observations revealed  

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differences in Ganymede’s auroral bands over time. 
The researchers thought these differences were  

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due to the presence of atomic oxygen, or single oxygen atoms, in Ganymede’s atmosphere.  

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Atomic oxygen affects one of the wavelengths of 
ultraviolet light the scientists were looking at. 

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In 2018, researchers used Hubble’s 
Cosmic Origins Spectrograph to measure  

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the amount of atomic oxygen in Ganymede’s 
atmosphere. They compared the 1998, 2010,  

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and 2018 data. To their surprise, the 2018 data 
revealed hardly any atomic oxygen in Ganymede’s  

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atmosphere. There had to be another explanation 
for the differences in the aurora images. 

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Ganymede’s surface temperature varies widely 
throughout the day, and around noon, near its  

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equator, it may get warm enough for surface 
ice to release water molecules. Instead of  

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melting and forming liquid water, the icy surface 
warms and changes from a solid directly to a gas,  

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a process called sublimation. Looking back at 
the original images, the team discovered the  

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differences they observed in Ganymede’s auroral 
bands are related to where water vapor would be  

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expected in the moon’s atmosphere.

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Understanding the Jovian system and  

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unravelling its history, from its origin to the 
possible emergence of habitable environments,  

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will provide us with a better understanding 
of how gas giant planets and their satellites  

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form and evolve. Along with learning more 
about Jupiter and its mysterious moon Ganymede,  

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this exciting new discovery has brought us closer 
to understanding our own place in the universe.

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