Astronomers first noticed the subtle first light, known as zodiacal light, centuries ago. In fact, identifying this phenomenon has been important to the practice of Islam. The fajr, or dawn prayer, is required before sunrise every day, but how long before? Unlike noon and sunset, the time of dawn is harder to pin down precisely.
The phenomenon of zodiacal light was significant enough to be named in the teachings of the Islamic prophet Muhammad: it was called "the false dawn", al-fajr al-kazib, to be distinguished from the later "true dawn", al-fajr as-sadiq. The latter time is widely accepted to be the correct one for the dawn prayer.
But what causes this light? In some ways, the cause is mundane: astronomers have long known that the glow originates from sunlight reflecting off of small dust particles in space. The long-narrow shape of the glow occurs because dust is not distributed uniformly throughout interplanetary space. Nearly all objects orbiting the Sun, from the largest planets to the smallest dust particles, cluster near a single plane, the plane of the Solar System. In our night sky, this plane appears as a line known as the ecliptic, along which the planets, Sun, and Moon travel. In turn, the constellations on the ecliptic are known as the zodiac. Voilà! Zodiacal light! Though most pronounced near the Sun at dawn, dim zodiacal light can be observed on a moonless night to follow the entire ecliptic.
However, this explanation merely raises another question. Where does the dust come from? Some debris comes from comets and asteroids, but some more evidence recently surfaced on this subject from an unlikely source.
On August 5, 2011, the NASA spacecraft Juno launched from Earth, the beginning of its five-year journey to Jupiter. Elsewhere on this blog, I've written in depth about Juno's mission and its many discoveries orbiting the Solar System's largest planet. However, the probe began gathering important data long before reaching its destination. Along its trajectory (pictured above*), Juno returned to Earth for a gravity assist to give it a boost to Jupiter. During its journey, scientists on the Juno team noticed something alarming: thousands of tiny streaks of light appeared on images of what should have been empty space!
Further analysis revealed that the streaks were tiny bits of debris knocked from Juno's solar panels, dislodged by microscopic dust particles impacting at high velocities. Unintentionally, the large solar panels of Juno became a interplanetary dust collector! Fortunately, the impacts were not large enough to cause damage detrimental to the mission, but they did reveal a surprising story.
In all, over 15,000 dust impact events were recorded, enough to measure in unprecendented detail the distribution throughout the inner Solar System. The figure above* (click to enlarge) shows the data. The top panel shows events on Juno's inbound stage from the farthest point of its first orbit (on the trajectory image, the point marked DSM or "Deep Space Manuever") to Earth flyby. The second shows impacts recorded outbound from Earth flyby through the asteroid belt, on its way to Jupiter. The horizontal axis measures distance from the Sun, while the black line indicates frequency of imapct events. For context, various significant distances and the distribution of asteroids (gray) in the asteroid belt are also included.
As a bonus, the outbound portion of the trajectory shown above was tilted from the ecliptic plane, while the inbound one lay within it (see again the trajectory image). Hence, we can clearly see that less impacts occur away from the plane of the Solar System. It is also clear that the impacts are reduced near Earth, since our planet's gravitational influence "clears its orbit" of dust. Nevertheless, it is the dust closest to us that causes most of the zodiacal light we observe.
In contrast, the vicinity of Mars appears riddled with dust, even more so than some parts of the asteroid belt! This was a great surprise, for it was expected most of the dust was from asteroids and that Mars would clear its orbit, just like Earth does. However, further data analysis confirmed that Mars must in fact be a major source of the dust that ultimately causes zodiacal light! There is much more to be discovered about this story: what mechanisms cause Mars to lose so much dust to space? Currently, the answer is not definitively known, though further space missions could provide clues. But after thousands of years of mystery, at least we now know that when we see that before-dawn glow, we have the red planet to thank.
Sources: https://www.al-islam.org/articles/al-fajr-sadiq-new-perspective-sayyid-muhammad-rizvi, https://earthsky.org/astronomy-essentials/everything-you-need-to-know-zodiacal-light-or-false-dusk/, https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2020JE006509, https://www.jpl.nasa.gov/news/serendipitous-juno-detections-shatter-ideas-about-origin-of-zodiacal-light
*The last few figures in this post are from the paper "Distribution of Interplanetary Dust Detected by the Juno Spacecraft and Its Contribution to the Zodiacal Light" by J. Jorgensen et al.
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