Many people (including me) have never really seen the zodiacal light, because you need to be in a dark-sky area and have a decent view of the horizon. It appears at the spot where the Sun sets, not long after it does so. As you can see in the main image above, it’s sort of a triangular glow that leans over at an angle. Although it is diffuse, it’s actually quite bright. If you compressed all of its light into a small area, it would be the second-brightest object in the sky, outshone only by the full Moon.
It’s long been known that the glow is caused by sunlight reflected off of some sort of dust that orbits in the same plane as the planets. That’s why it’s seen around the ecliptic, or the path the Sun seems to trace through the sky. Spring is the best time to see the zodiacal light after sunset (unless you enjoy getting up before dawn — count me out — in which case you can check it out in autumn.)
The angle of the ecliptic at sunset is the steepest in springtime, making the zodiacal light easiest to see:
Let’s compare the angle of the ecliptic during astronomical twilight (90-ish minutes after sunset) on March 15 and July 15 here in the Northern Hemisphere, picking Omaha, Nebraska just because it’s central. You can see how much steeper that angle is in March than in July:
That might seem counterintuitive; shouldn’t the ecliptic be steeper in July, when the Sun gets highest into the sky? Well, it is steeper in July — but at midday, when the Sun is actually out. By sunset, because of the rotation of the Earth, its orientation changes.
So anyway, what’s Juno got to do with all of this?
Up to now, the dust that causes the zodiacal light has been conjectured to be debris from comets or asteroids or to be stuff left over from planet formation or … something. It’s just that no one has had a terrific explanation for exactly what it is or how it got there, so we took some reasonable guesses.
Juno, on its way to Jupiter from 2011-2016, was doing a lot of star tracking, because when there’s no “up” or “down”, you have to look at something to figure out what your orientation is. But on certain parts of its journey, it kept spotting fast-moving objects. They couldn’t have been asteroids or anything like that, because they were zooming by too quickly.
It turned out that they were little pieces of debris being chipped off the rear of the large solar panels, and that debris was being created by collisions with dust particles. Most of those are tiny, but you don’t want to be in their way as they approach you at 10,000 miles per hour. Other spacecraft surely have been hit by these dust particles, but the large solar panels combined with the star tracking inadvertently made Juno the largest dust detector ever used. Not exactly what the Juno people had in mind, I’m sure.
On its way to Jupiter, Juno spent some time going outbound and some time coming inbound. It spent some time being on and some time being off the ecliptic plane. Below is an animation of its path. (None of the inner planets were harmed during the creation of this simulation.)
NASA was able to track where in the Solar System each dust particle had struck Juno and also got information about the trajectories of those particles. There were some periods of no collisions and some periods of many. Reconstructing all the data into a 3-D view, the dust distribution looked like this:
Centered on the orbit of Mars, having nice circular orbits themselves. The authors concluded that the dust particles couldn’t have orbits like this unless they had originated in the vicinity of Mars.
Then they were able to simulate what the zodiacal light ought to look like from Earth if the dust that causes it is distributed like this.
“The distribution of dust that we measure better be consistent with the variation of zodiacal light that has been observed,” [author Jack] 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 only thing that’s still not clear, though, is how they originate. Mars has a lot of dust storms, all right, but it would be pretty hard for that dust to escape Mars’ gravitational pull. Maybe it comes from a long history of meteor impacts with Mars? Maybe Mars’ moons, Phobos and Deimos, are responsible somehow?
Welp, just another element of our Solar System that we need to learn more about. As if astronomers didn’t already have plenty to do.
It’s a bit sad that more of us, with all of our light pollution these days, can’t experience the zodiacal light without going to a great deal of trouble. It was once very common to witness it, as this 1902 article from Popular Astronomy nonchalantly illustrates:
If you do have dark skies, first of all, I envy you greatly, but second, do get out and see this phenomenon if you can. (And send pictures!)
Just as it is getting warmer and the days are getting longer, and as you get back outside and watch the sunsets again, Nature welcomes you back with this extra gift. It doesn’t last long, but nothing gold can stay…..