A team of scientists with NASA’s Juno mission have made a surprising discovery regarding a phenomenon that you may have seen yourself sometimes – the zodiacal light. That faint glow in the sky, visible just before dawn or just after dusk, is caused by dust that is orbiting the Sun in the zodiacal cloud.
It has long been thought that the dust originates from asteroids and comets as they enter the inner Solar System, but now Juno, a spacecraft that is currently orbiting Jupiter, has found a different answer. The new evidence suggests that this cosmic dust actually comes from Mars.
The new findings were published in a final peer-reviewed paper in the Journal of Geophysical Research: Planets on March 9, 2021. They had previously been published, before peer review, on Nov. 11, 2020.
The zodiacal light is visible as a faint column of light extending upward from the horizon. Tiny dust particles reflect light from the Sun toward Earth.
Since Juno is much further away, at Jupiter, how did it find clues that this dust came from Mars?
The answer comes from the fact that some of the dust particles slammed into Juno while it was traveling from Earth to Jupiter. It was a serendipitous discovery, since Juno wasn’t really meant to study such dust.
“I never thought we’d be looking for interplanetary dust,” said John Leif Jørgensen, a professor at the Technical University of Denmark.
The dust was detected by Juno’s four star tracker cameras, used to determine the orientation of the spacecraft by using known star patterns. Kind of like using star patterns or constellations to navigate on the ocean.
The images sent back showed something surprising – thousands of odd streaks that would appear and then disappear.
“We were looking at the images and saying, ‘What could this be?’” Jørgensen said.
What were they? One of the most concerning possibilities was that Juno’s fuel tank was leaking.
“We thought, ‘Something is really wrong,’” Jørgensen said. “The images looked like someone was shaking a dusty tablecloth out their window.”
If so, that would, of course, be very bad news for the mission.
It turned out, however, that these were not tiny droplets of fuel, but rather dust grains. They were impacting the spacecraft at 10,000 miles (16,000 kilometers) per hour, hard enough to chip off submillimeter pieces from Juno’s surface.
“Even though we’re talking about objects with only a tiny bit of mass, they pack a mean punch,” said Jack Connerney, Juno’s magnetometer investigation lead.
But where were these dust grains coming from? Juno’s large solar panels. As well as powering the spacecraft, they turned out to be efficient dust collectors, too.
“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,” Connerney said.
Most of the dust was seen between Earth and the asteroid belt. Data from Juno indicated that the dust cloud ended near Earth, due to Earth’s gravity. This, therefore, is the dust that creates the zodiacal light.
“That’s the dust we see as zodiacal light,” Jørgensen noted.
Before Juno, scientists had been unable to adequately measure the dust cloud and how extensive it was, because most other dust collectors are too small and limited to how much dust they can gather. Juno’s solar panels have 1,000 times more collection area, even though they weren’t actually designed to be dust collectors.
The outer edge of the dust cloud, it turned out, ended just past Mars, at about 2 astronomical units (AU) from the Sun. Just like Earth acts as a barrier for the inner edge, Jupiter acts as a barrier for the outer edge, blocking dust from traveling any further into the outer Solar System.
The dust particles are in a nearly circular orbit around the Sun, sort of like another asteroid belt, but composed only of very fine dust. Since Mars is the only large object in this region of the dust cloud, the scientists say this points to Mars as the most likely source of the dust.
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 zodiacal light that has been observed,” Connerney said. The researchers 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 the researchers plugged in the orbital elements of Mars, the distribution accurately predicted the tell-tale signature of the variation of 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,” Connerney said, “and where they originate.”
The findings present a compelling case that we can thank Mars for the subtle but beautiful zodiacal light that we see in our night sky. What’s still not known, however, is how the dust escaped Mars to begin with. Mars has huge dust storms that sometimes envelope the entire planet, one of which ended the Opportunity rover‘s mission, so maybe some dust is able to get high enough in the atmosphere to break out of Mars’s gravitational pull.
Another benefit of this research is that knowing how the dust affected Juno will help engineers design spacecraft that can withstand the tiny but forceful impacts even better than Juno did.
Juno arrived at Jupiter on July 4, 2016, and has revolutionized our understanding of the largest planet in our Solar System. Now, it seems, it has also provided important new clues about how dust is distributed throughout the Solar System, far beyond Jupiter itself.
The spacecraft was launched on Aug. 5, 2011 on a United Launch Alliance Atlas V-551 rocket from the Space Launch Complex 41 at Cape Canaveral Air Force Station in Florida.