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The evidence gets stronger for water on one of Jupiter’s moons

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Description: At first glance, Jupiter looks like it has a mild case of the measles. Five spots - one colored white, one blue, and three black are scattered across the upper half of the planet. Closer inspection by NASA's Hubble Space Telescope reveals that these spots are actually a rare alignment of three of Jupiter's largest moons - Io, Ganymede, and Callisto - across the planet's face. In this image, the telltale signatures of this alignment are the shadows [the three black circles] cast by the moons. Io's shadow is located just above center and to the left; Ganymede's on the planet's left edge; and Callisto's near the right edge. Only two of the moons, however, are visible in this image. Io is the white circle in the center of the image, and Ganymede is the blue circle at upper right. Callisto is out of the image and to the right. On Earth, we witness a solar eclipse when our Moon's shadow sweeps across our planet's face as it passes in front of our Sun. Jupiter, however, has four moons roughly the same size as Earth's Moon. The shadows of three of them occasionally sweep simultaneously across Jupiter. The image was taken March 28, 2004, with Hubble's Near Infrared Camera and Multi-Object Spectrometer. Seeing three shadows on Jupiter happens only about once or twice a decade. Why is this triple eclipse so unique? Io, Ganymede, and Callisto orbit Jupiter at different rates. Their shadows likewise cross Jupiter's face at different rates. For example, the outermost moon Callisto orbits the slowest of the three satellites. Callisto's shadow moves across the planet once for every 20 shadow crossings of Io. Add the crossing rate of Ganymede's shadow and the possibility of a triple eclipse becomes even more rare. Viewing the triple shadows in 2004 was even more special, because two of the moons were crossing Jupiter's face at the same time as the three shadows. Jupiter appears in pastel colors in this photo because the observation was taken in near-infrared light. Astronomers combined images taken in three near-infrared wavelengths to make this color image. The photo shows sunlight reflected from Jupiter's clouds. In the near infrared, methane gas in Jupiter's atmosphere limits the penetration of sunlight, which causes clouds to appear in different colors depending on their altitude. Studying clouds in near-infrared light is very useful for scientists studying the layers of clouds that make up Jupiter's atmosphere. Yellow colors indicate high clouds; red colors lower clouds; and blue colors even lower clouds in Jupiter's atmosphere. The green color near the poles comes from a thin haze very high in the atmosphere. Ganymede's blue color comes from the absorption of water ice on its surface at longer wavelengths. Io's white color is from light reflected off bright sulfur compounds on the satellite's surface. In viewing this rare alignment, astronomers also tested a new imaging technique. To increase the sharpness of the near-infrared camera images, astronomers speeded up Hubble's tracking system so that Jupiter traveled through the telescope's field of view much faster than normal. This technique allowed scientists to take rapid-fire snapshots of the planet and its moons. They then combined the images into one single picture to show more details of the planet and its moons. Image # SPD-SLRSY-3083 Date: March 28, 2004

Using new modeling techniques to analyze data gathered in 1997 by the NASA Galileo spacecraft, astronomers have discovered surprising new details about one of Jupiter’s moons.

A paper published in Nature Astronomy offers the clearest evidence to date that there are “plumes” — eruptions of water vapor — venting from the surface of on an icy moon called Europa. Two UCLA scientists are co-authors of the study: Margaret Kivelson, a space physicist and planetary scientist who is a professor emerita, and researcher Krishan Khurana.

Space scientists have long been interested in Europa because there is evidence to suggest that a vast ocean — like the ones on Earth — lies beneath its icy outer shell. The new research provides further support for the possibility of such an ocean, as well as evidence that there are the types of energy sources in the moon’s interior that would be required if life were to develop on the moon.

In the 1990s, Kivelson led the team that designed and built the device on Galileo that recorded magnetic field signatures. Because magnetic fields are generated by flowing currents, measuring the magnetic fields provides information about where they come from, which in turn suggests information about the characteristics of the moon itself.

Galileo collected data from as close as 124 miles above Europa’s surface, but at the time, mission scientists didn’t think to look for plumes. Over the next few years, though, new data from a separate NASA mission — the Cassini spacecraft’s study of Saturn and its moons — led Kivelson and her colleagues to suspect that there was more to be learned by analyzing the measurements Galileo had made near Europa.

In early 2017, Kivelson, Khurana and Xianzhe Jia — a University of Michigan space physicist who earned his doctorate at UCLA — attended a NASA presentation about the agency’s planned Europa Clipper mission, which is scheduled for a launch in the early 2020s. The researchers examined the information gathered during the 1997 flyby and, indeed, the high-resolution magnetometer data showed something strange.

Drawing on what scientists had learned from the exploration of plumes on Saturn’s moon Enceladus by NASA’s Cassini mission, Jia focused on an anomaly that had never been explained, and then applied it as strong evidence of plumes.

In addition to Kivelson’s magnetometer, Galileo also carried a powerful plasma wave spectrometer to measure plasma waves caused by charged particles in gases around Europa’s atmosphere. For the new study, scientists layered data from the magnetometer and spectrometer into 3D modeling software, and then incorporated models based on data from remote observations of Europa by NASA’s Hubble Space Telescope between 2014 and 2017.

“The data were there, but we needed sophisticated modeling to make sense of the observation,” Jia said.

The new findings bolster the potential for discovery of more plumes by the Europa Clipper mission, which is intended to explore the moon’s habitability. Jia, Kivelson and Khurana are members of the research teams for two of the instruments aboard Europa Clipper that will measure the moon’s magnetic and plasma environments.

“If plumes exist, and we can directly sample what’s coming from the interior of Europa, then we can more easily get at whether Europa has the ingredients for life,” said Robert Pappalardo, the project scientist for Europa Clipper at NASA’s Jet Propulsion Laboratory in Pasadena, California. “That’s what the mission is after. That’s the big picture.”

Kivelson, who in 2017 received the Gerard P. Kuiper Prize for outstanding contributions to planetary science from the American Astronomical Society, said the upcoming mission should reveal a wealth of new insights about the moon.

“Although Galileo discovered an ocean at Europa, we know little about its properties,” she said. “The Europa Clipper mission will tell us lots about the ocean, such as its depth, the thickness of the ice layer that hides it, and its electrical properties.”

IMAGE SOURCE: NASA

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