Here on Earth, an aurora borealis is a wondrous natural event that too few of us ever get a chance to see. Their occurrence remains unpredictable enough that a glimpse of one may remain elusive even for people who live in the United States' northern latitudes.
Imagine, though, that you could see one every few minutes. That's what happens at Jupiter's north and south poles every 27 minutes. Not only that, but each auroral event blasts out enough X-ray energy to power our entire civilization. A new study, from University College London and the Chinese Academy of Science and published in Science Advances, solves the mystery of how and why this occurs.
Surfing waves of plasma
Credit: ESA / NASA / Yao / Dunn
"We have seen Jupiter producing X-ray aurora for four decades," says the study's co-lead author William Dunn, "but we didn't know how this happened. We only knew they were produced when ions crashed into the planet's atmosphere."
To unravel the mystery behind what is happening, the researchers aligned observations made over a 26-hour period by NASA's Juno spacecraft (which orbits Jupiter) with X-ray measurements made by the European Space Agency's XMM-Newton Observatory (which orbits Earth). Having time-aligned the two sets of observations, computer modeling revealed the mechanics behind the auroral bursts.
Jupiter has a massive magnetic field — some 20,000 times stronger than Earth's — extending out around the planet. Plasma, or ionized gas whose atoms have been stripped of electrons as they collide with each other, races along these lines. Periodic vibrations in the magnetic field lines, the cause of which is still unknown but thought to involve interactions with the solar wind or magnetosphere, produce waves in the plasma.
Jupiter's moon Io releases ion particles from gigantic volcanoes, which get swept up and carried along by the plasma waves, eventually smashing into Jupiter's atmosphere. This results in the massive release of visible and invisible light, including X-rays.
Dunn says, "Now we know these ions are transported by plasma waves — an explanation that has not been proposed before, even though a similar process produces Earth's own aurora. It could, therefore, be a universal phenomenon, present across many different environments in space."
"Now we have identified this fundamental process, there is a wealth of possibilities for where it could be studied next," says co-lead author Zhonghua Yao. "Similar processes likely occur around Saturn, Uranus, Neptune, and probably exoplanets as well, with different kinds of charged particles 'surfing' the waves."
A black hole lab
Co-author Graziella Branduardi-Raymont says, "X-rays are typically produced by extremely powerful and violent phenomena such as black holes and neutron stars, so it seems strange that mere planets produce them too."
Jupiter thus represents a promising research opportunity.
"We can never visit black holes," says Branduardi-Raymont, "as they are beyond space travel, but Jupiter is on our doorstep. With the arrival of the satellite Juno into Jupiter's orbit, astronomers now have a fantastic opportunity to study an environment that produces X-rays up close."
Astronomers detected a first of its kind hot Jupiter-like planet without clouds or haze. Such planets are very rare, with only one exoplanet with a clear atmosphere previously found – that one classified as a "hot Saturn".
The "hot Jupiter" exoplanet WASP-62b is 575 light years away from Earth, coming in at about half the mass of our Jupiter. It completes a rotation around its sun in only 4.5 days (compared to 12 years for Jupiter). That closeness to the star makes the planet extremely hot.
The discovery was made by astronomers at the Center for Astrophysics | Harvard & Smithsonian. The gas giant was actually first located in 2012 using the Wide Angle Search for Planets (WASP) South survey, but the unique state of its atmosphere has only been understood now.
Munazza Alam, a graduate student from the Center for Astrophysics who led the study, was working on her thesis that involved exoplanet characterization when she zeroed in on the atmosphere of WASP-62b.
She used the Hubble Space Telescope for data and observations that were made via spectroscopy, a method of detecting chemical elements by studying electromagnetic radiation. In particular, Alam focused on how WASP-62b looked as it came in front of its host star on three occasions. Observing visible light in such instances can show the existence of sodium and potassium in the atmosphere of the planet. The scientist could see no potassium, but a complete fingerprint of sodium's presence. This led her team to conclude that the exoplanet's atmosphere lacked clouds or haze, which would have hidden the sodium's clear signature.
Munazza Alam – a graduate student at the Center for Astrophysics | Harvard & Smithsonian.
Credit: Jackie Faherty
"I'll admit that at first I wasn't too excited about this planet," Alam said in a press release. "But once I started to take a look at the data, I got excited." Seeing the sodium was "the smoking gun evidence that we are seeing a clear atmosphere," she added.
Finding such a planet is very unlikely since astronomers estimate under 7 percent of exoplanets have clear atmospheres. Studying them can help us understand why they were formed in a way that is different from most planets, according to Alam. Without clouds and haze getting in the way, it is also easier to study the chemical makeup of such a planet.
Jupiter itself has a complex and chaotic cloud structure, formed at different altitudes:
Jupiter's Colorful Cloud Bands Studied by Spacecraft
The astronomers plan to study WASP-62b further upon the launch of the next-generation James Web Space Telescope later in 2021.
Check out the new study published in The Astrophysical Journal Letters.
The Earth has a magnetosphere. So does Jupiter. But Jupiter's has a million times the volume of ours. As a result, Jupiter slams its moon, Europa, with a steady blast of high-energy radiation. This can't help but have an effect on the satellite, and new research from NASA's Jet Propulsion Laboratory has an idea what that effect is: Europa glows, in shades of green, blue, and white. The moon's night side even glows in the dark. The discovery was made by exploring the behavior of a Europa laboratory model bombarded with radiation.
The research is published in the journal Nature Astronomy.
Europa
Enhanced closeup of the "chaos terrain" that is the icy surface of Europa
Credit: NASA/JPL-Caltech/SETI Institute
Europa is believed to have an ocean of water or slush beneath its chaotically-featured water-ice surface. According to NASA, it's suspected that the moon's ice layer is 10 to 15 miles thick and floats atop an ocean 40 to 100 miles deep. Europa is just a quarter the size of Earth, but its vastness and depth may mean that it has twice as much water as all of our oceans combined.
With water considered to be a prerequisite for life, scientists' interest in Europa is obvious. NASA is sending the radiation-resistant Europa Clipper there to have a look. The spacecraft will conduct 45 flybys at different distances, ranging from 1,675 miles to 16 miles above the ice. The Europa Clipper will carry cameras, spectrometers, ice-penetrating radar, magnetometer, thermal instruments, a device for measuring gravity, and more.
NASA has previously detected what may be vapor plumes extending outward from Europa. If the Europa Clipper confirms their existence, it may be possible in the future to sample the moon's escaping vapors without needing to land or drill through the ice.
ICE-HEART
Artist's impression of Europa against a backdrop of Jupiter
Credit: NASA/JPL-Caltech
The researchers modeled Europa's response to Jupiter's radiation using a special instrument they constructed called the Ice Chamber for Europa's High-Energy Electron and Radiation Environment Testing (ICE-HEART). To blast it with radiation, they took it to the Medical Industrial Radiation Facility at the National Institute of Standards and Technology in Gaithersburg, Maryland, a high-energy electron beam facility.
Expecting that Europa's oceans would contain a mix of water and salts similar to those on Earth, they were investigating the response of various materials to radiation. They began with magnesium sulfate and sodium chloride — essentially Epsom salt and table salt — both believed to be in Europa's ice.
They weren't surprised to see some glowing caused by energetic electrons getting through the moon's ice and energizing molecules beneath it. The glow is generated when the molecules relax after exposure.
However, the variety of colored glows emitted by radiated compounds was a surprise, according to co-author Bryana Henderson. "We never imagined that we would see what we ended up seeing," Henderson said. "When we tried new ice compositions, the glow looked different. And we all just stared at it for a while and then said, 'This is new, right? This is definitely a different glow?' So we pointed a spectrometer at it, and each type of ice had a different spectrum." (Spectrometers divide light into wavelengths that can signify specific compounds.)
"Seeing the sodium chloride brine with a significantly lower level of glow was the 'aha' moment that changed the course of the research," said co-author Fred Bateman.
Both sides now
We can see our own moon because it reflects sunlight. Most spectrometer readings of Europa have thus far been derived from observations of its light-reflecting bright side.
"If Europa weren't under this radiation," said Gudipati, "it would look the way our moon looks to us — dark on the shadowed side. But because it's bombarded by the radiation from Jupiter, it glows in the dark."
This means that the moon's dark side also emits light in the form of its glow, so here come the spectrometers. Gudipati said of the research, "We were able to predict that this nightside ice glow could provide additional information on Europa's surface composition. How that composition varies could give us clues about whether Europa harbors conditions suitable for life."
He adds, "It's not often that you're in a lab and say, 'We might find this when we get there. Usually, it's the other way around — you go there and find something and try to explain it in the lab. But our prediction goes back to a simple observation, and that's what science is about."
Since 2016, NASA's Juno spacecraft has been observing Jupiter's atmosphere, magnetosphere, and gravitational field. It has already managed to take fantastic images, discovered new cyclones, and analyze the gasses that make up the planet in the time it has spent investigating it.
This week, Juno was able to add another discovery to its name with the unexpected finding of lightning in the upper atmosphere of the Solar System's largest planet.
The findings are described in the study "Small lightning flashes from shallow electrical storms on Jupiter," published in Nature. Previous missions to Jupiter, including Voyager 1, Galileo, and New Horizons all observed lightning, but without the benefits of the equipment on Juno or more recent developments in models of the Jovian atmosphere.
In this case, the lighting is notable for how high it is occurring in the atmosphere. While previous observations suggested lightning in water-based clouds deep inside the gas planet, the new data suggests lightning exists in the upper atmosphere in clouds of water and ammonia. This lightning is dubbed "shallow lightning."
According to a press release by Cornell University, the ammonia is vital in creating the lightning, as it functions as an "anti-freeze" of sorts to keep the water in the clouds from freezing. The collision of droplets of mixed ammonia and water with ice water particles creates the charge needed for lightning strikes.
This is different from any process that creates lightning on Earth.
That wasn't the only bit of strangeness the probe noticed. While Juno saw plenty of ammonia near the equator and at lower levels of the atmosphere, it was hard-pressed to find much anywhere else. To explain this, researchers developed a new model of atmospheric mixing. They suggest that the ammonia at lower levels of the atmosphere rises into storm clouds, interacts with water to cause the aforementioned lightning, and then falls back down in the form of hailstones.
The scientists gave these ammonia and water ice hailstones the name "mushballs."
This model explains many things, including why Juno couldn't detect ammonia where it expected to: the mushballs would be more challenging to detect than ammonia or water vapor. The scientists further speculated that the weight of the mushballs pulls the ammonia to lower levels of the atmosphere where it is detected in more significant amounts.
A NASA designed graphic demonstrating the weather systems theorized to create "mushballs." The liquid water and ammonia rises in the storm clouds until they reach points where the extremely low temperatures cause them to freeze. Freezing into semi-solid "mushballs" causes them to fall where they redistribute ammonia throughout the lower atmosphere.
Credit: NASA/JPL-Caltech/SwRI/CNRS
How can we possibly know all of this?
Juno relies on several pieces of equipment. The most relevant in this case is the microwave radiometer. This device uses microwaves to measure the Jovian atmosphere's composition. When microwaves hit water or ammonia particles, they begin to heat up. By hitting the planet with microwaves and then looking for changes in the particles' observed temperature, the probe can determine what chemicals are present.
The findings of these studies demonstrate that Jupiter's atmosphere is more complicated than previously thought. Given how we already knew about the storms larger than Earth, temperatures that swing between extremes in different layers of the atmosphere, and winds that blow at 100 meters per second, that is saying something.
