Joshua Rhoades was standing near an abandoned farmhouse in rural Illinois on a windy night in early March, fiddling with his camera, when he noticed what he called “a faint, eerie, ethereal glow” above him. A pillar of light had illuminated the darkness, stretching from the horizon—a hint of sun, but it was nearly 8 p.m.
There was nothing alarming about the sight, though it was, by one definition, a little alien: The glow was the result of an interplanetary cloud of dust particles reflecting sunlight toward Earth. To Rhoades, a geologist with a photography hobby, the display was an item on his bucket list, best viewed around this time of year in places far from light pollution. For astronomers who study space dust, the phenomenon, known as zodiacal light, is at the center of a startling discovery.
Scientists have long known that a cloud of interplanetary dust is responsible for the glow, and that its particles come from asteroids and comets, which shed dust as they travel into the inner solar system from afar. Researchers have collected some of these particles, sweeping them from Earth’s atmosphere and excavating them from Antarctic snow, and their composition lines up with the known properties of those celestial objects. But new research suggests that some of the dust might be coming from elsewhere in the solar system. Rhoades actually captured the potential source in the photograph he took that night: Mars, a little orb trapped in the column of light.
The evidence comes from the NASA spacecraft Juno and its journey from Earth to Jupiter a few years ago. Scientists working on the mission weren’t looking for the dust that contributes to zodiacal light, or any kind of dust, but it smacked right into the spacecraft. Juno’s cameras captured images full of mysterious streaks, “like someone was shaking a dusty tablecloth out their window,” John Leif Jørgensen, a Juno scientist and professor at the Technical University of Denmark, explained in a recent statement. The streaks turned out to be pieces of Juno’s solar panels floating in space; dust particles had slammed into the panels at high speed and chipped them off. By the look of the spacecraft fragments, the bombarding particles were about the same size as the ones associated with zodiacal light.
That the empty void all around us can be a dusty place seems counterintuitive, but it’s true, and “the inner solar system is certainly dusty in comparison to the outer solar system,” Hope Ishii, a professor at the Hawaii Institute of Geophysics and Planetology, who was not involved in the new research, told me. Juno wasn’t designed to hold on to these particles or study them. But scientists could examine the flurry that occurred along Juno’s journey and use it to learn about the particles’ origin and distribution across space. Their calculations suggested that, to create the dusty landscape Juno experienced, the source of the particles must have a certain set of orbital dynamics, and the only spot that matches up is in the vicinity of Mars.
This doesn’t mean that the glow Rhoades captured was made by dust particles straight from the surface of Mars. Scientists don’t know how such particles could manage to escape the red planet’s gravity and sail away into space. The researchers have floated the idea that the dust could have originated on Mars’s two moons. (Yes, Mars has moons. Most planets in our solar system do, and, at the risk of offending Earth, theirs are more interesting than our own.) Phobos and Deimos—irregularly shaped, more like lumpy balls of cookie dough than marbles—are thought to resemble asteroids, the same kinds that scientists believe sprinkle dust through the solar system. But dust from these moons would have a difficult time breaking away too.
“It’s really difficult to envision how this dust could be coming from Mars or its moons,” Larry Nittler, a cosmic-dust expert at the Carnegie Institution for Science, who was not involved in the research, told me. But “dynamically, it seems to be right.”
Juno’s solar panels survived their stint as accidental dust detectors, and today the probe is in orbit around Jupiter, capturing stunning views of the giant planet’s swirling storms. Cosmic-dust experts must now mull over the discovery they made, and astrophotographers like Rhoades might view the glow of the zodiacal light a little differently.
The zodiacal light might be one of the most underrated cosmic sights visible from Earth. It is far less dramatic than a solar eclipse and more subtle than a planetary alignment. While the Milky Way is hard to miss on a moonless night over a rural landscape, zodiacal light can be easily mistaken for the yellow glow of a distant city.
Electric light has blotted it out for most of us. But we can still see Jupiter, one of the brightest points in the sky, even in the busiest cities. And while trying to understand our distant neighbor, scientists have come to learn more about what we see in the night sky. Pioneer 10, a NASA spacecraft on its way to Jupiter, made the connection between interplanetary dust and zodiacal light in the 1970s. Juno provided another clue, and so could future spacecraft headed in Jupiter’s direction—a quirk of modern times, when human beings have dispatched machines across the solar system to try to decipher things they can marvel at only from here.