Five years on, Juno science reveals answers to zodiacal lights, Jovian auroras

Juno, NASA’s flagship mission to Jupiter, marks five years at the solar system’s largest planet today, having been in space for nearly 10 years since its launch. Juno quickly became one of NASA’s most scientifically important missions upon its arrival, with its Ultraviolet Spectrograph and Advanced Stellar Compass instruments used to reveal many unique scientific phenomena at Jupiter and throughout the solar system.

Recently, two teams published research highlighting the mystery of the solar system’s zodiacal light and the origins of Jupiter’s auroral dawn storms using the Juno’s instruments.

Juno’s changed mission

While the science Juno has returned has provided tremendous insight into Jupiter and its family of moons, its journey through the Jovian system has not exactly been as NASA hoped when the craft launched on 5 August 2011 atop a United Launch Alliance Atlas V 551 rocket from what was then Cape Canaveral Air Force Station, Florida.

After a single gravity assist with Earth on 9 October 2013 when it came within 559 km of Earth sea level, Juno’s next major event was its 4 July 2016 Eastern Daylight Time (5 July UTC) orbital insertion burn at Jupiter.

Juno’s single engine fired for a 2,102 second braking maneuver, lowering the craft’s velocity by 542 m/s to achieve a highly elliptical, polar orbit capture around the planet. This initial orbit brought Juno to within 4,200 km at perijove (point of closest approach in orbit of Jupiter) and to a distance of 8.1 million km at apojove over a 53.5 day period.

Upon second perijove in October 2016, nearly 107 days after arrival, Juno was to fire its engine again to reduce its orbit to that with a period of just 14 days — its planned science orbit. However, just days before the planned firing, teams received data from the craft showing that helium valves in the propulsion system were not opening as intended.

Credit: NASA

NASA decided not to risk the burn as troubleshooting continued. On 17 February 2017, NASA announced a decision to leave Juno in its 53.5 day capture orbit and perform the mission from there after deeming further using of the engine too risky to the science of the mission.

In June 2018, the mission was extended until July 2021. Despite exposure to Jupiter’s intense radiation fields, Juno has held up remarkably well, a testament to the engineers who designed the craft and the people who built it. In January 2021, NASA again extended the mission based on this performance through September 2025.

The second extension included plans to fly the craft by Ganymede, Europa, and Io — the first of which occurred on 7 June 2021 when Juno passed within 1,038 km of Ganymede.

A 320 km distant flyby of Europa is planned for late 2022, with two 1,500 km flybys of Io following in 2024. The encounters are designed to give mission planners for the upcoming Europa Clipper and JUICE missions from NASA and ESA better data ahead of those crafts’ launches and arrivals.

In September 2025, or when system failures mandate (whichever comes first), Juno will be purposefully deorbited into Jupiter’s atmosphere. Like Galileo at Jupiter and Cassini at Saturn before it, Juno’s fiery end will protect the planet’s potentially life harboring moons from Earth-based contamination the probe could impart to them if left to uncontrollably wonder the Jovian system — where gravity could eventually cause it to slam into one of the moons.

Jupiter’s auroras

While it’s no secret that Jupiter and other planets in our solar system feature polar auroras,  the auroral dawn storms of Jupiter are similar to a type of terrestrial auroras — and extremely powerful.

Dawn auroral storms have been observed by both ground-based and orbiting laboratories; however, due to the observations being conducted away from Jupiter, these Earth-bound telescopes could never see the night side of Jupiter when observing the aurora.

“Observing Jupiter’s aurora from Earth does not allow you to see beyond the limb, into the nightside of Jupiter’s poles,” said Dr. Bertrand Bonfond, University of Liège in Belgium. “Explorations by other spacecraft — Voyager, Galileo, Cassini — happened from relatively large distances and did not fly over the poles, so they could not see the complete picture.”

Juno’s polar orbit, however, is perfect. “That’s why the Juno data is a real game-changer, allowing us a better understanding of what is happening on the nightside, where the dawn storms are born.”

The auroral dawn storms consist of brief, intense, and broadening brightening of Jupiter’s north and south poles at Jupiter’s terminator line region — the region where day is separated into night.

Bonfond et al.’s research shows that the auroral storms are born on the nightside of the planet, and become more luminous as they rotate with Jupiter into daylight. These storms, once fully illuminated, emit hundreds of thousands of gigawatts of UV light and dump ten times more energy into Jupiter’s upper and lower atmospheres than typical auroras do.

When looking at data more closely, Bonfond et al. noticed a specific trait with these storms. “When we looked at the whole dawn storm sequence, we couldn’t help but notice that they are very similar to a type of terrestrial auroras called substorms,” said Zhonghua Yao, a member of Bonfond et al. team at the University of Liège.

Substorms are seen on Earth and are caused by disturbances in the magnetosphere that release energy into the ionosphere. The curious thing, though, is that Earth and Jupiter have extremely different magnetospheres, raising questions as to how and why substorms occur on the giant of our solar system.

Illustration of ultraviolet polar aurorae on Jupiter & Earth. While the diameter of the Jovian world is 10 times larger than that of Earth, both planets have markedly similar aurora.

On Earth, these substorms are influenced by solar activity and the magnetosphere’s interaction with solar wind. At Jupiter, the magnetosphere is dominated by charged particles escaping from Io, particles that are ionized and trapped inside the magnetosphere due to Jupiter’s intense magnetic field.

“The power that Jupiter possesses is amazing. The energy in these dawn aurorae is yet another example of how powerful this giant planet really is,” said Scott Bolton, principal investigator of Juno from the Southwest Research Institute in San Antonio.

Zodiacal lights

In addition to the science about Jupiter and its moons, Juno has performed science on our solar system in general as well.

Following its launch in 2011, Juno had a five year coast phase before arriving at Jupiter. During this time, the spacecraft recorded dust particles slamming into its instruments and hull. The dust, discovered using Juno’s Advanced Stellar Compass (ASC), a small instrument on Juno’s Magnetometer that snaps pictures of the sky surrounding the craft every quarter of a second to determine the spacecraft’s orientation and rotation. Jørgensen et al. hoped one of the cameras would spot an undiscovered object in space and programmed one to report any objects that appeared in multiple consecutive images that it could not identify.

Nothing was expected; so when the camera started sending back images of unidentifiable objects around Juno, Jørgensen et al. tried to figure out what they were. “We were looking at the images and saying, ‘What could this be?’” said Jørgensen.

After calculating the size and velocity of the material in the images, the team found that the particles were submillimeter pieces of Juno’s solar panels that were being chipped off by dust grains traveling at 16,000 km/h.

“Each piece of debris we tracked records the impact of an interplanetary dust particle, allowing us to compile a distribution of dust along Juno’s path,” said Jack Connerney, the magnetometer investigation lead and deputy principal investigator of Juno.

Looking over that data, Jørgensen and Connerney noticed that a large majority of the impacts occurred between Earth and the asteroid belt. This was an important find because scientists had not yet been able to measure the distribution of these dust particles accurately as past missions to collect and analyze dust were limited by their available dust collection areas. 

However, Juno’s solar panels had 1,000 times more surface area for dust collection than previous detectors. 

But what does this have to do with the zodiacal light, the sometimes faint streaks of light extending up from the horizon just before dawn or right after sunset, caused by the reflection of tiny dust particles in particle clouds orbiting the sun throughout our solar system?

“[The dust that Juno discovered,] That’s the dust we see as zodiacal light,” Jørgensen said.

The outer edge of the dust cloud ends around 2 AU from the Sun, a little bit beyond Mars’ orbit. And Jupiter has a lot to do with that as the massive planet’s gravity acts as a sort of shield against the particles, preventing them from escaping into deep space. But while Jupiter bounds the dust the we see sporadically as zodiacal light, it is a different planet from which the dust originates: Mars.

The zodiacal light at European Southern Observatory’s (ESO’s) La Silla Observatory in Chile in September 2009. (Credit: ESO)

Using Juno data, Jørgensen et al. developed a model to represent the dust dispersion and light reflection throughout the solar system. The model depended on two quantities: the inclination of the dust to the ecliptic and its orbital eccentricity. When Jørgensen et al. used Mars’ orbital parameters (the only object at approximately 2 AU that could explain the dust’s presence), the model correctly predicted where zodiacal lights would be.

“That is, in my view, a confirmation that we know exactly how these particles are orbiting in our solar system and where they originate,” Connerney said.

In particular the team has proposed that it is Mars’ famous/infamous dust storms that are ultimately responsible for the interplanetary dust cloud, though exactly how that dust has escaped Mars will require further study.

(Lead image: Juno performs its critical Orbit Insertion burn at Jupiter on 4 July 2016. Credit: Nathan Koga for NSF/L2)

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