Astronomy Picture of the Day

Explanation: How much of Jupiter's moon Europa is made of water? No one is sure, but probably a lot. Based on the Galileo probe data acquired during its exploration of the Jovian system from 1995 to 2003, Europa possesses a deep, global ocean of liquid water beneath a layer of surface ice. The subsurface ocean plus ice layer could descend over 100 kilometres in average depth. Adopting a high-end estimate of 100 kilometres depth, if all the water on Europa were gathered into a ball, it would have a radius of over 800 kilometres. To scale, this intriguing illustration compares that hypothetical ball of all the water on Europa to the size of Europa itself (left) - and similarly to all the water on planet Earth. With a volume possibly greater than Earth's oceans, the global subsurface ocean on Europa is a tantalizing destination to search for extraterrestrial life in our Solar System. NASA's robotic Europa Clipper launched last year to investigate.