Here are the reasons for Europa:
"It takes longer to get there [than to get to Mars], it's more expensive, and a bigger deal to plan a mission. But if I had a choice, I'd go for Europa," said Lynn Rothschild, an astrobiologist with the NASA Ames Research Center in Moffett Field, California.Of course, there is one very compelling reason NOT to go to Europa:
Rothschild studies the origins of life on Earth and other planets. She's intrigued by Europa because it appears to contain likely key ingredients for life—water, an energy source, organic compounds, and billions of years of development.
Taken together, these ingredients are sufficient to support life, scientists say. To answer the question of whether life actually exists on Europa, however, requires further exploration with orbiters and landers like those currently exploring Mars.
