The zodiacal light is sunlight reflected by interplanetary dust in the inner Solar System. Variations in the zodiacal light with ecliptic latitude reveal discrete bands of dust orbiting near the ecliptic plane. NASA’s Juno spacecraft, in transit from Earth to Jupiter, recorded a sufficient number of impacts with these dust particles to characterize their distribution in space for the first time.
The Juno spacecraft was launched in August 2011, bound for Jupiter orbit insertion on July 4, 2016, on a trajectory that carried it out to the asteroid belt where a deep space maneuver sent it back toward a rendezvous with Earth en route to Jupiter.
The orbiter was not instrumented with a dedicated dust detection instrument, but it did carry a complement of four star cameras to provide accurate attitude determination for the magnetic sensors. This system is known as the Advanced Stellar Compass, and it employs four Camera Head Units that are collocated with the magnetic sensors.
One of these cameras was configured to search for asteroids, recording observations of luminous objects that could be tracked across multiple images in transit across the star background.
This search for non-stellar objects turned up a number of luminous objects that were identified as impact products excavated from the body of the Juno spacecraft upon impact with an interplanetary dust particle traveling at 5 to 15 km/s relative velocity.
“We were looking at the images and saying, ‘What could this be?’” said Professor John Leif Jørgensen, a researcher at the Technical University of Denmark.
“We thought, ‘Something is really wrong.’ The images looked like someone was shaking a dusty tablecloth out their window.”
“Even though we’re talking about objects with only a tiny bit of mass, they pack a mean punch,” said Juno magnetometer investigation lead and deputy principal investigator Dr. Jack Connerney, a researcher at NASA’s Goddard Space Flight Center.
“That’s the dust we see as the zodiacal light,” Professor Jørgensen noted.
The scientists determined that the dust cloud ends at Earth because Earth’s gravity sucks up all the dust that gets near it.
As for the outer edge, around 2 AU (astronomical units) from the Sun, it ends just beyond Mars.
At that point, the influence of Jupiter’s gravity acts as a barrier, preventing dust particles from crossing from the inner Solar System into deep space.
This same phenomenon, known as orbital resonance, also works the other way, where it blocks dust originating in deep space from passing into the inner Solar System.
“The profound influence of the gravity barrier indicates that the dust particles are in a nearly circular orbit around the Sun,” Professor Jørgensen said.
“And the only object we know of in almost circular orbit around 2 AU is Mars, so the natural thought is that Mars is a source of this dust.”
“The distribution of dust that we measure better be consistent with the variation of the zodiacal light that has been observed,” Dr. Connerney said.
The authors also developed a computer model to predict the light reflected by the dust cloud, dispersed by gravitational interaction with Jupiter that scatters the dust into a thicker disk.
The scattering depends only on two quantities: the dust inclination to the ecliptic and its orbital eccentricity.
When they plugged in the orbital elements of Mars, the distribution accurately predicted the tell-tale signature of the variation of the zodiacal light near the ecliptic.
“That is, in my view, a confirmation that we know exactly how these particles are orbiting in our Solar System and where they originate,” Dr. Connerney said.
“While there is good evidence now that Mars, the dustiest planet we know of, is the source of the zodiacal light, we cannot yet explain how the dust could have escaped the grip of Martian gravity. We hope other scientists will help us,” the researchers said.
Their paper was published in the Journal of Geophysical Research: Planets.
J.L. Jorgensen et al. 2021. Distribution of Interplanetary Dust Detected by the Juno Spacecraft and Its Contribution to the Zodiacal Light. Journal of Geophysical Research: Planets 126 (3); doi: 10.1029/2020JE006509
This article is based on text provided by the National Aeronautics and Space Administration.
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