"Missing" moonlets, a huge volcanic eruption, and new maps of possible ocean ice are among the results of a recent flyby of Jupiter's moons.
The observations were made as NASA's New Horizons spacecraft whipped around the gas giant this past spring on its way to rendezvous with the dwarf planet Pluto in 2015.
Using Jupiter's gravity like a slingshot allowed New Horizons to shave years off its travel time to distant, icy Pluto.
The approach was also the first time a probe had been that close to Jupiter since the Galileo mission ended in September 2003.
Several teams of scientists used the opportunity to capture detailed images that allowed them to study different aspects of Jupiter.
Their findings appear in this week's issue of the journal Science.
No Tiny Moons
Perhaps the most surprising find for astronomers was an absence of tiny moonlets within the planet's rings.
Prior missions had revealed four small moons orbiting inside the orbit of the large moon Io.
The smallest of these are Adrastea, at 12 miles (20 kilometers) in diameter, and Metis, at 25 miles (40 kilometers) in diameter.
Earlier instruments weren't sensitive enough to spot moons much smaller than these. But scientists presumed that moonlets existed, because where there are big objects, there are usually lots of smaller ones.
"That's sort of how these things work," said Mark Showalter, a planetary scientist at the SETI Institute and lead author of a study on the "missing" moonlets.
Looking for moonlets is important, Showalter said, because they are thought to be the source of the dust that forms Jupiter's faint rings.
Such dust can be created when meteorites hit small moons within the rings, forming puffs of smokelike particles.
But despite the probe's ability to spot objects as small as 0.6 mile (a kilometer) in diameter, New Horizons found no new moons.
"It was really a surprise," Showalter said.
The most likely explanation, he continued, is that meteorite bombardment has battered such small bodies to the point that they are too small for New Horizons to detect.
But the new pictures did produce an unexpected find: clumps of debris within the planet's main ring.
While clumps of material aren't unusual, Jupiter's clumps were found in clusters that should have been rapidly dispersed by orbital forces.
"We haven't seen anything like this in any other planetary rings," Showalter said.
Lava and Frost
Other scientists examined three of Jupiter's larger moons—Io, Europa, and Ganymede.
Io is the most volcanically active body in the solar system.
During the flyby, a team led by John Spencer of the Southwest Research Institute in Boulder, Colorado, took a close look at the moon's giant Tvashtar volcano.
The massive mound happened to be undergoing a major eruption at the time of the flyby—as seen in an image released by NASA in May.
The volcanic plume was more than 215 miles (350 kilometers) high and 680 miles (1,100 kilometers) wide.
In their new study of the volcano images, Spencer and colleagues suggest that the plume's visible particles were not spewed from inside the moon but are instead gases that condensed in the frigid vacuum of space.
The team also notes that the magma inside Tvashtar and other Ionian volcanoes appear to be hot enough to be made of an Earthlike basaltic rock.
Another team, led by Kurt Retherford of the Southwest Research Institute, tried to determine what fraction of Io's atmosphere comes from volcanic emissions.
Io, Retherford said, has a very tenuous atmosphere made up of sulfur dioxide.
The scientists predicted that some of this gas comes from volcanism but that the rest comes from the evaporation of sulfur dioxide frost beneath the daytime sun.
The New Horizons flyby allowed the team to test this theory by studying Io's atmosphere when the moon passed out of the sun into Jupiter's shadow.
When there was no longer any solar heating, they found, 97 percent of the atmosphere quickly turned back into frost.
Ice Maps
When examining Europa and Ganymede, New Horizons scientists were primarily interested in mapping the distribution of "dirty" versus "clean" ice on the moons' surfaces.
The ultimate goal is to resolve a longstanding debate over the source of the material that contaminates the ice, particularly on Europa.
Europa is widely believed to have an ocean beneath its permanently frozen surface.
Scientists hope that the "dirty" ice is salt water that once welled up from below, which would provide proof of the undersurface ocean. But it could also be ice contaminated with sulfur compounds from nearby Io.
Studying the ice might offer clues to whether the underlying ocean is suitable for life, said William Grundy, a planetary scientist at the Lowell Observatory in Flagstaff, Arizona, who led the study.
The New Horizons instruments are sophisticated enough to tell what materials might be in the dirty ice, Grundy added, but at the time of the flyby the instruments hadn't been calibrated.
Once calibration is complete, he hopes to reanalyze the data to get a better answer.
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