Space dust has been found to be concentrated in ways consistent with it coming from Mars. This could overturn theories on the origins of the phenomenon known as zodiacal light. Even though the evidence comes from a space probe, it's not any of the probes sent to the red planet. Instead, the phenomenon has been revealed through an unexpected application of the Juno spacecraft's solar panels.
Under the right conditions, just after sunset or shortly before dawn, it is possible to see a faint, almost triangular glow in the sky known as the zodiacal light. The light is a product of space dust reflecting the Sun's rays. We see it only in parts of the sky because the dust is concentrated in bands close to the plane in which the planets orbit the Sun.
Other stars have something similar, and astronomers have debated the dust's source, with the most popular explanation being it represents the remains of comets and the debris from asteroid collisions. Although this makes intuitive sense, attempts to model the size and density of the zodiacal dust cloud haven't been entirely successful. Now, Professor John Leif Jørgensen of the Technical University of Denmark has arrived at a different explanation, via a very unexpected route.
Jørgensen thinks the dust is coming from Mars, but he hasn't seen any escaping the Martian gravity field, nor can he offer an explanation of how it could achieve the 5 kilometers per second (3 miles per second) velocity required. Jørgensen's case rests on the dust's distribution through the inner solar system,
Jørgensen didn't set out to study zodiacal dust – he was just trying to keep the Juno to Jupiter mission safe. In order to confirm its orientation, Juno has cameras that take photographs every quarter of a second, allowing computers to check the right stars are in each camera. Jørgensen hoped to find some asteroids as well, with one camera programmed to report anything out of the ordinary.
Instead of the occasional rare asteroid, the camera detected thousands of brief flashes. “We thought, 'Something is really wrong.' The images looked like someone was shaking a dusty tablecloth out their window.” Jørgensen said in a statement. The Juno team even briefly worried a fuel tank might be leaking.
Instead, it turned out that dust was slamming into Juno's large solar panel array. At speeds relative to the spacecraft of 16,000 kilometers an hour (10,000 mph), even tiny particles could blast chips off the panel's rear side, which were then seen by Jørgensen's camera.
Previous spacecraft have carried dust detectors, but these were dwarfed Juno's large solar arrays. The camera records allowed Juno's controllers to measure how much dust the probe passed through each day. Juno's looping path to Jupiter needed to get a gravitational assist, providing some extra detail.
Jørgensen and co-authors report in the Journal of Geophysical Research: Planets that dust is concentrated just beyond the Earth's orbit out to a little beyond Mars. The quantities of dust observed, and the forces dispersing it, suggest around 30 kilograms (66 pounds) need to be added every second to maintain what Juno encountered. Jupiter's gravity constrains the cloud's outer edge, while the inner side is swept up by the Earth to form the bands that give us the zodiacal light.
Mars, on the other hand, doesn't seem to be performing a similar sweeping function. Instead, dust is being generated at a distance from the Sun where the red planet and its moons are the only apparent source, but there is still no explanation for how any could escape, let alone so much.