The faint glow before dawn is sunlight scattered by interplanetary dust. Recent models suggest most of it comes from Mars, not asteroids or comets.
ESO/Y. Beletsky, CC BY 4.0
A phantom pillar of light
On a clear, moonless night far from city lights, a ghostly pyramid of light can be seen rising from the horizon. This phenomenon is the zodiacal light, a faint glow caused by sunlight reflecting off a vast cloud of dust that permeates the inner solar system. For centuries, its origin was a mystery. The Italian astronomer Giovanni Domenico Cassini provided the correct explanation in 1683, suggesting it was caused by sunlight scattering off a cloud of small particles orbiting the Sun.
The interplanetary dust cloud is immense, forming a lens-shaped structure centered on the ecliptic—the plane of Earth's orbit. The dust particles are minuscule, typically ranging from 10 to 300 micrometers in size. This cloud is not static; the Poynting-Robertson effect, a form of drag from solar radiation, causes the dust particles to slowly spiral inward toward the Sun. This means the cloud requires constant replenishment to remain visible. For many years, scientists assumed the dust was supplied by disintegrating comets and collisions between asteroids in the main belt.
An unexpected Martian source
This long-held model was challenged by serendipitous findings from NASA's Juno spacecraft during its voyage to Jupiter. Juno's mission was not study interplanetary dust, but its instruments inadvertently detected thousands of dust particle impacts. The spacecraft's massive solar arrays, with a combined area of 60 square meters, were giant dust detectors.
Scientists analyzed the distribution of these impacts as Juno traveled from Earth's orbit out toward the asteroid belt. The data revealed that the dust cloud is bounded by Earth's orbit on the inside and an outer edge just beyond Mars, at about 2 astronomical units from the Sun. Earth's gravity effectively sweeps up any dust that comes near, while Jupiter's immense gravity acts as a barrier, preventing dust from the outer solar system from entering this region. The orbital characteristics of the detected dust particles were nearly circular and closely matched the orbit of Mars. This evidence shows Mars, the dustiest planet in our solar system, as the primary source of the zodiacal light. The exact mechanism for how so much dust escapes Mars's gravity is still under investigation, but the planet's massive, globe-spanning dust storms are the leading suspect.
💡Fun Facts
The term "zodiacal" comes from the glow's appearance along the zodiac, the band of sky containing the constellations traced by the Sun's path.
In especially dark skies, a faint, oval patch of light called the Gegenschein, or "counter-glow," can be seen directly opposite the Sun. It's caused by sunlight backscattering off the same interplanetary dust cloud.
The particles making up the dust cloud are so sparse that even within the densest part of the cloud, you would likely only find a few particles per cubic kilometer.
The total mass of the entire zodiacal cloud is estimated to be equivalent to an asteroid roughly 15 kilometers in diameter.
Visible year-round from dark-sky locations. In mid-latitudes, it is best seen for about 90 minutes after sunset in spring (looking west) or 90 minutes before sunrise in autumn (looking east).
Admission
Free
Accessibility
Requires a viewing location with very dark, clear skies, far from any light pollution. This often means traveling to remote areas with natural, unpaved terrain.