Cosmic fireworks, invisible to our eyes, fill the night sky. We can get a glimpse of this elusive light show thanks to the Large Area Telescope (LAT) aboard NASA’s Fermi Gamma-ray Space Telescope, which observes the sky in gamma rays, the highest-energy form of light. 

This animation shows the gamma-ray sky’s frenzied activity during a year of observations from February 2022 to February 2023. The pulsing circles represent just a subset of more than 1,500 light curves – records of how sources change in brightness over time – collected by the LAT over nearly 15 years in space.

Thanks to the work of an international team of astronomers, this data is now publicly available in a continually updated interactive library. A paper about the repository was published on March 15, 2023, in The Astrophysical Journal Supplement Series.

“We were inspired to put this database together by astronomers who study galaxies and wanted to compare visible and gamma-ray light curves over long time scales,” said Daniel Kocevski, a repository co-author and an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama. “We were getting requests to process one object at a time. Now the scientific community has access to all the analyzed data for the whole catalog.”

Over 90% of the sources in the dataset are blazars, central regions of galaxies hosting active supermassive black holes that produce powerful particle jets pointed almost directly at Earth. Ground-based observatories, like the National Science Foundation’s IceCube Neutrino Observatory in Antarctica, can sometimes detect high-energy particles produced in these jets. Blazars are important sources for multimessenger astronomy, where scientists use combinations of light, particles, and space-time ripples to study the cosmos.

“In 2018, astronomers announced a candidate joint detection of gamma rays and a high-energy particle called a neutrino from a blazar for the first time, thanks to Fermi LAT and IceCube,” said co-author Michela Negro, an astrophysicist at the University of Maryland, Baltimore County, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Having the historical light curve database could lead to new multimessenger insights into past events.”

In the animation, each frame represents three days of observations. Each object's magenta circle grows as it brightens and shrinks as it dims. Some objects fluctuate throughout the entire year. The reddish-orange band running across the middle of the sky is the central plane of our Milky Way galaxy, a consistent gamma-ray producer. Lighter colors there indicate a brighter glow. The yellow circle shows the Sun’s apparent annual trajectory across the sky.

Processing the full catalog required about three months, or more than 400 computer years of processing time distributed over 1,000 nodes on a supercomputer cluster located at the SLAC National Accelerator Laboratory in Menlo Park, California.

The LAT, Fermi’s primary instrument, scans the entire sky every three hours. It detects gamma rays with energies ranging from 20 million to over 300 billion electron volts. For comparison, the energy of visible light mostly falls between 2 to 3 electron volts.

The Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership managed by Goddard. Fermi was developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.

Research reveals a new explanation for how the icy shell of Jupiter’s moon Europa rotates at a different rate than its interior. NASA’s Europa Clipper will take a closer look.

NASA scientists have strong evidence that Jupiter’s moon Europa has an internal ocean under its icy outer shell – an enormous body of salty water swirling around the moon’s rocky interior. New supercomputer modeling suggests the water may be pushing the ice shell along, possibly speeding up and slowing down the rotation of the moon’s icy shell over time.

Scientists have known that Europa’s shell is probably free-floating, rotating at a different rate than the ocean below and the rocky interior. The new modeling is the first to show that Europa’s ocean currents could be contributing to the rotation of its icy shell.

A key element of the study involved calculating drag – the horizontal force that the moon’s ocean exerts on the ice above it. The research hints at how the power of the ocean flow and its drag against the ice layer could even account for some of the geology seen on Europa’s surface. Cracks and ridges could result from the icy shell slowly stretching and collapsing over time as it is pushed and tugged by the ocean currents.

“Before this, it was known through laboratory experiments and modeling that heating and cooling of Europa’s ocean may drive currents,” said Hamish Hay, a researcher at the University of Oxford and lead author of the study published in JGR: Planets. Hay performed the research while a postdoctoral research associate at NASA’s Jet Propulsion Laboratory in Southern California. “Now our results highlight a coupling between the ocean and the rotation of the icy shell that was never previously considered.”

It might even be possible, using measurements gathered by NASA’s upcoming Europa Clipper mission, to determine with precision how fast the icy shell rotates. When scientists compare images gathered by Europa Clipper with those captured in the past by NASA’s Galileo and Voyager missions, they will be able to examine locations of ice surface features and potentially determine if the position of the moon’s icy shell has changed over time.

For decades, planetary scientists have debated whether Europa’s icy shell might be rotating faster than the deep interior. But rather than tying it to the ocean’s movement, scientists focused on an outside force: Jupiter. They theorized that as the gas giant’s gravity pulls on Europa, it also tugs on the moon’s shell and causes it to spin slightly faster.

“To me, it was completely unexpected that what happens in the ocean’s circulation could be enough to affect the icy shell. That was a huge surprise,” said co-author and Europa Clipper Project Scientist Robert Pappalardo of JPL. “And the idea that the cracks and ridges we see on Europa’s surface could be tied to the circulation of the ocean below – geologists don’t usually think, ‘Maybe it’s the ocean doing that.’”

Europa Clipper, now in its assembly, test, and launch operations phase at JPL, is set to launch in 2024. The spacecraft will begin orbiting Jupiter in 2030 and will use its suite of sophisticated instruments to gather science data as it flies by the moon about 50 times. The mission aims to determine if Europa, with its deep internal ocean, has conditions that could be suitable for life.

Like a Pot of Water

Using techniques developed to study Earth’s oceans, the paper’s authors relied on NASA supercomputers to make large-scale models of Europa’s oceans. They explored the complexities of how the water circulates, and how heating and cooling affect that movement.

Scientists believe that Europa’s internal ocean is heated from below, due to radioactive decay and tidal heating within the moon’s rocky core. Like water heating in a pot on a stove, Europa’s warm water rises to the top of the ocean.

In the simulations, the circulation initially moved vertically, but the rotation of the moon as a whole caused the flowing water to veer in a more horizontal direction – in east-west and west-east currents. The researchers, by including drag in their simulations, were able to determine that if the currents are fast enough, there could be an adequate drag on the ice above to speed up or slow down the shell’s rotation speed. The amount of interior heating – and thus, circulation patterns in the ocean – may change over time, potentially speeding up or slowing the rotation of the icy shell above.

“The work could be important in understanding how other ocean worlds’ rotation speeds may have changed over time,” Hay said. “And now that we know about the potential coupling of interior oceans with the surfaces of these bodies, we may learn more about their geological histories as well as Europa’s.”