Saturn’s icy moon Enceladus is one of the most unique bodies in our solar system. A salty liquid ocean lies beneath its frozen crust. But the liquid ocean doesn’t stay hidden beneath the surface. Plumes of icy material erupt continuously from the south polar region.

Tomorrow, Cassini will dive closer to Enceladus’ plumes than ever before. At just 30 miles above the moon’s south polar region, the spacecraft will fly through the plume of icy spray. Cassini will train its camera and instruments to this region and hopefully learn more about what’s going on beneath the moon’s surface.

“The global nature of Enceladus’ ocean and the inference that hydrothermal systems might exist at the ocean’s base strengthen the case that this small moon of Saturn may have environments similar to those at the bottom of our own ocean,” said Jonathan Lunine, an interdisciplinary scientist on the Cassini mission. “It is therefore very tempting to imagine that life could exist in such a habitable realm, a billion miles from our home.”

In March, scientists announced data from Cassini revealed evidence of present-day hydrothermal activity. Two papers described this evidence. One paper focused on tiny grains of rock detected by the spacecraft. Four years of analysis, including computer simulations and lab experiments, show the tiny rocks likely formed when hot water from the moon’s rock interior comes into contact with cooler water near the surface. Scientists believe the temperatures required for this interaction would need to be at least 194 degrees Fahrenheit.

“It’s very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on — and beneath — the ocean floor of an icy moon,” said the paper’s lead author Sean Hsu.

Tomorrow’s flyby (probably) won’t detect life

Tomorrow’s close encounter with Enceladus’ geysers was never intended to look specifically for life. The main goal is to better understand what kind of ocean environment lies beneath the moon’s frigid surface.

Cassini scientists will be looking closely for more evidence of hydrothermal activity and how much of it is going on. According to NASA, finding molecular hydrogen will be key.

Cassini may also tell us the exact structure of the plumes. Are they individual jets, or curtain eruptions? Or both? Figuring out the structure will also give scientists clues for how the material is escaping to the surface.

Tomorrow’s flyby won’t be the last. Cassini will visit Enceladus one more time on December 19th. It will be much further away, though. At 3,106 miles away, Cassini will look at how much heat is coming from the moon’s interior.