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 ten 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.

To call what’s happening in the oceans right now an anomaly is a bit of an understatement. Since March, average sea surface temperatures have been climbing to record highs , as shown in the dark line in the graph below.

(credit: Sean Birkel/University of Maine)

Since this record-keeping began in the early 1980s—the other squiggly lines are previous years—the global average for the world’s ocean surfaces has oscillated seasonally between 19.7° and 21° Celsius (67.5° and 69.8° Fahrenheit). Toward the end of March, the average shot above the 21° mark and stayed there for a month. (The most recent reading, for April 26, was just a hair under 21°.) This temperature spike is not just unprecedented, but extreme.

“It’s surprising to me that we’re this far off the trajectory,” says Robert Rohde, lead scientist at Berkeley Earth, a nonprofit that gathers climate data. “Usually when you have a particular warming event, we’re beating the previous record by a little bit. Right now we’re sitting well above the past records for this time of year, for a considerable period of time.”

Enlarge / The boat trip went through an area of milky sea south of the lights of an island. (credit: Steven Miller, Leon Schommer, and Naomi McKinnon )

On some moonless nights, enormous patches of the Northwest Indian Ocean and seas around Indonesia begin to glow. This event has been witnessed by hundreds of sailors, but only one research vessel has ever, by pure chance, come across this bioluminescent phenomenon, known as milky seas. Thanks to that vessel, samples showed that the source of the light was a bacteria called V. harveyi, which had colonized a microalgae called Phaocystis. But that was back in 1988 , and researchers have yet to be in the right place and the right time to catch one of these events again.

Both the bacteria and algae are common to those waters, so it’s not clear what triggers these rare events. To help understand why milky seas form, researchers have gotten much better at spotting these swaths of bioluminescence from the skies. With the help of satellites, Stephen Miller, a professor of atmospheric science , has been collecting both images and eyewitness accounts of milky seas for nearly 20 years . Thanks to improvements in the imaging capabilities over the past decades, Miller published a compilation last year of probable milky seas in the time frame of 2012 to 2021, including one occurrence south of Java, Indonesia, in summer 2019.

But these satellite observations lacked surface confirmation—that is, until the crew of the yacht Ganesha reached out to Miller with their first-hand account of what they had experienced during their trip through the seas around Java that August, which was recently published in PNAS . Their eyewitness corroboration—along with the first photographs of a milky sea—show that these satellites are indeed a powerful tool for spotting these events.