Study finds comet dust painted Mercury black
The mystery of Mercury’s excessively dark surface may be solved.
A team of researchers at Brown University, who have published their findings in Nature Geoscience, say the planet’s inky appearance may stem from cometary dust that “painted” the planet black over billions of years.
Scientists have long wondered why Mercury was so much darker than Earth’s moon, reflecting just onethird the light that the moon ref lects.
The two bodies are often compared, said lead author Megan Bruck Syal, a postdoctoral researcher at the Lawrence Livermore National Laboratory. They are about the same size, and because neither one has much of an atmosphere, both are subject to a constant bombardment from bits of space dust and other micrometeorites.
Many airless bodies get their dark color from iron-bearing minerals on their surface, but observations by NASA’s Messenger spacecraft have shown that the surface of Mercury is less than 2% iron.
Scientists had to look for another darkening agent. “One thing that hadn’t been fully considered before was carbon,” Bruck Syal said. “Carbon is really abundant in comets and could be delivered by cometary dust.”
Full-size comets could not be responsible for depositing enough carbon on Mercury’s surface because an impact would shoot material back into space.
Impacts from cometary dust, however, were a different story.
“The little dust particles less than a millimeter in size are also very carbon rich, and they come in at lower speeds,” Bruck Syal said. “We did a lot of calculations about different impact angles and velocities and found you could retain most of the material on the planet.”
The density of cometary dust increases as you get closer to the sun, and after further calculations, the researchers determined that Mercury, the closest planet to the sun, is probably struck by 50 times as many bits of cometary dust as the moon.
“It’s pretty constant,” Bruck Syal said. “Unlike asteroid or comet impacts that are kind of hard to predict, this is more of a steady state.”
To test whether the impact of carbon dust has a darkening effect, researchers turned to the NASA Ames Vertical Gun Range in Mountain View. They used a 14-foot cannon fueled by hydrogen gas to shoot projectiles into targets at speeds of more than 3.5 miles per second, imitating celestial impacts.
By creating target materials similar to what they expect to find on the surface of Mercury, the scientists were able to demonstrate that cometary dust impacts would indeed produce dark materials on the planet.
Bruck Syal said the hypothesis still needs to be tested further.
“I know the Messenger mission is doing a low-altitude campaign right now trying to get more carbon abundance data,” she said. “If we could have some outside verification of how much carbon there is … that would be the ultimate test.”
MERCURY reflects just one-third the light that the moon reflects. Researchers say carbon-rich cometary dust darkened the planet’s surface over billions of years.