Enlarge / NASA's PACE spacecraft last year at Goddard Space Flight Center, Maryland. (credit: NASA)

NASA's latest mission dedicated to observing Earth's oceans and atmosphere from space rocketed into orbit from Florida early Thursday on a SpaceX launch vehicle.

This mission will study phytoplankton, microscopic plants fundamental to the marine food chain, and tiny particles called aerosols that play a key role in cloud formation. These two constituents in the ocean and the atmosphere are important to scientists' understanding of climate change. The mission's acronym, PACE, stands for Plankton, Aerosol, Cloud, ocean Ecosystem.

Nestled in the nose cone of a Falcon 9 rocket, the PACE satellite took off from Cape Canaveral Space Force Station, Florida, at 1:33 am EST (06:33 UTC) Thursday after a two-day delay caused by poor weather.

Enlarge / That is actually a moon. (credit: NASA/JPL/Space Science Institute)

The once-exclusive club of Solar System objects that host oceans is getting increasingly crowded. On Wednesday, Nature released a paper providing evidence that Saturn's moon Mimas has a subsurface ocean beneath its heavily cratered crust. The evidence for this ocean comes in the form of orbital oddities that are seemingly impossible to explain by anything other than the presence of an ocean.

Solid looks

Of Saturn's seven major moons, Mimas orbits closest to the planet, taking less than a day to complete an orbit. It's also the smallest of the major moons, with a diameter of just under 400 kilometers (about 250 miles). Despite its diminutive size, Mimas hosts the second-largest crater on any moon in the Solar System. The Herschel Crater dominates the surface of the moon, giving it an appearance that evokes the Death Star.

Even outside of Herschel, the moon's surface is heavily pocked by craters, suggesting it has been static for most of the moon's history. That's in sharp contrast to moons like Europa and Enceladus, where the subsurface oceans allow the constant remodeling of their surfaces, leaving them with much sparser crater histories. So Mimas seemed like a very poor candidate for hosting an ocean.

Enlarge (credit: USGS)

With Europa and Enceladus getting most of the attention for their subsurface oceans and potential to host life, other frozen worlds have been left in the shadows—but the mysterious Jovian moon Ganymede is now making headlines.

While Ganymede hasn’t yet been observed spewing plumes of water vapor like Saturn’s moon Enceladus, Jupiter’s largest moon is most likely hiding an enormous saltwater ocean. Hubble observations suggest that the ocean—thought to sit under 150 km (95 miles) of ice—could be up to 100 km (60 miles) deep. That’s 10 times deeper than the ocean on Earth.

Ganymede is having a moment because NASA’s Juno mission observed salts and organic compounds on its surface, possibly from an ocean that lies beneath its crust of ice. While Juno’s observations can't provide decisive evidence that this moon has an ocean that makes Earth look like a kiddie pool, the Juno findings are the strongest evidence yet of salts and other chemicals making it to the exterior of Ganymede.