June 29, 2016
On board NASA's Juno spacecraft — set to enter Jupiter orbit on July 4 — are instruments that will help scientists answer fundamental questions not just about the solar system's largest planet but also about Earth and the universe.
"I view Jupiter as a missing link," said Barry Mauk of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. Mauk leads the investigation team for the Jupiter Energetic Particle Detector Instrument (JEDI), which was built by APL. "Jupiter is the link between the nearby space environments we study at planets like Earth, and distant astrophysical systems where magnetic fields hold sway, such as early-stage star forming regions, and hyper-energetic radiation regions like the Crab Nebula. Juno is not only going to help us better understand Jupiter, it's going to help us better understand the universe around us and our place in it."
JEDI consists of three shoebox-sized detectors, each of which views a three-dimensional, 120 degree by 12 degree "slice" of the sky; the units are positioned to provide a continuous 360-degree sampling view of the space around Juno. These detectors will provide data on particles with energies in the 30 to roughly 1,000 kiloelectron volt (keV) range that surround the planet and help create the enormous and powerful auroras surrounding Jupiter's polar regions. JEDI detects electrons and ions such as protons, helium, oxygen and sulfur that interact with Jupiter's atmosphere in creating the aurora; it also measures energetic neutral atoms emanating from Jupiter's auroral atmosphere.
"A unique aspect of JEDI is its ability to simultaneously measure incoming particles from many directions, allowing JEDI to obtain nearly instantaneous snapshots of the complete particle distributions," said Dennis Haggerty, APL's instrument scientist for the JEDI investigation. "This capability is crucial for revealing the fundamental physics of Jupiter's aurora because of the extremely rapid speed of the Juno spacecraft as it passes over Jupiter's poles. Juno will be moving at more than 30 miles a second (50 km/s) across auroral structures known to be as narrow as 50 miles (80 km) wide."
Each JEDI instrument combines a time-of-flight chamber with a series of solid-state detectors (SSDs) to determine the direction of motion, the speed and the energy of energetic particles entering JEDI. These combined measurements also allow JEDI to determine the mass composition of the incoming particles.
That new information will help scientists in their quest to learn more about where the particles come from, how they are energized, and what triggers the release of energy from these particles into brilliant auroral displays. "Jupiter has the most intense and interesting aurora in the entire solar system," said Mauk, who has studied particle systems at Earth, at other planets and throughout the solar system on NASA missions including Voyager, Galileo, the Van Allen Probes, Cassini and Magnetospheric Multiscale (MMS). "Jupiter's aurora has a power density 10 times greater than Earth's, and an overall power that is a factor of 100 greater. What we want to know is, how is this system energized?"
Along with JEDI, a second particle instrument called the Jovian Auroral Distributions Experiment (JADE) will study lower energy (5- to 50-keV ions; 0.1- to 100-keV electrons) particles involved in the same processes; JADE was built by the Southwest Research Institute (SwRI) in San Antonio, Texas. Additionally, two other instruments play a role in the fields and particles investigation: the Magnetometer Experiment (MAG), which will visualize Jupiter's magnetic field in 3-D; and the Waves instrument, which will measure radio and plasma waves in Jupiter's magnetosphere.
The particle populations — electrons and ions — around Jupiter are powered in a different way than those at Earth. The solar wind provides the energy to the particles surrounding our planet, some of which are trapped in twin donut-shaped regions called radiation belts; at Jupiter, rotational forces produce the driving energy for the system (Jupiter rotates at a much higher speed than Earth; a Jupiter day lasts about 10 hours). "From NASA's Van Allen Probes mission, we learned how high-energy particles are accelerated at Earth. Will it be different at Jupiter? We're going to find out," said Mauk.
Even before Juno reaches Jupiter, the JEDI team has been at work. In January 2016, while the spacecraft was still millions of miles from Jupiter, JEDI began formal science operations in order to perform two investigations of the interplanetary medium on the approach to the planet.
The first involved looking at what are known as upstream ions. "Jupiter is a very leaky planet," Haggerty said. "It has a unique particle identity, especially in terms of sulfur — which is not found in high numbers in the solar wind — and we've seen particles from Jupiter 'upstream' of the planet from missions including Voyager, Galileo and New Horizons." In addition to Jupiter being a somewhat unique source of energetic sulfur ions, it is also believed to produce very energetic electrons that are observed at great distances from the planet.
The JEDI team will look at these upstream ions on the approach to Jupiter and answer a question about their origin. "We're going to be looking for sulfur; if we see it, we'll know it somehow managed to escape Jupiter's influence and push upstream against the solar wind. We think we've seen indications of it already," said Haggerty.
The second question that several of the Juno teams are exploring is: How, and to what extent, does the solar wind cause changes to Jupiter's massive aurora? This investigation also needs to occur before arrival because "we're coordinating with other observatories on Earth and in space to look for sudden shocks and changes that we could see prior to hitting the space environment around Jupiter," said APL's Chris Paranicas, a JEDI scientist and Juno co-investigator.
The team is also hard at work preparing for the orbital phase of the mission, where the main questions about the gas giant will hopefully be answered. "From what JEDI and JADE and the rest of the instruments on Juno will tell us, we should be able to understand more about the particles and radiation belts that we can observe around distant, more energetic objects like the Crab Nebula," said Mauk. "That's why Jupiter is a missing link — and that's one of the reasons I love Jupiter."
NASA's Jet Propulsion Laboratory, Pasadena, California, manages the Juno mission for the principal investigator, Scott Bolton, of SwRI. Juno is part of NASA's New Frontiers Program, which is managed at NASA's Marshall Space Flight Center in Huntsville, Alabama, for NASA's Science Mission Directorate. Lockheed Martin Space Systems, Denver, built the spacecraft.
Geoff Brown, 240-228-5618, firstname.lastname@example.org
D. C. Agle, NASA's Jet Propulsion Laboratory, 818-393-9011, email@example.com
The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.