Last December, scientists discovered an “active” asteroid in the asteroid belt located between the orbits of Mars and Jupiter. The space rock referred to by the astronomers as 6478 Gault left two traces of dust in its active behavior, which is associated with comets, rarely seen in asteroids.
While astronomers are still puzzling over the cause of Gault’s comet-like activity, an MIT-led team reports that the asteroid was caught red-to-blue when changing color in the near infrared spectrum. It’s the first time that scientists observe a color shifting asteroid in real time. That was a very big surprise, “says Michael Marsset, postdoctoral fellow at MIT for Earth, Atmospheric and Planetary Sciences (EAPS).”We believe we saw the asteroid lose its reddish dust into space, and we see, as the asteroid underlies, fresh blue layers.
Marsset and his colleagues have also confirmed that the asteroid is rocky – a proof that the tail of the asteroid, though it looks like a comet, is caused by a completely different mechanism, since comets are not rocky, but rather like loose snowballsmade of ice and dust.
To my knowledge, this is the first time we see a rocky body giving off dust, a bit like a comet, “says Marsset.This means that a mechanism responsible for dust emission is likely to be different from comets and most other major active mechanisms-asteroids.
Marsset and his colleagues, including EAPS Research Scientist Francesca DeMeo and Professor Richard Binzel, today published their findings in the journal Astrophysical Journal Letters.
A stone with tails
The astronomers first discovered in 1988 6478 Gault and named the asteroid after the planet geologist Donald Gault. Until recently, 4 km wide space rock was considered relatively average and circled along with millions of other pieces of rock and dust in the inner area of the asteroid belt, 214 million miles from the Sun.
In January, images from various observatories, including NASA’s Hubble Space Telescope, captured two narrow, comet-like tails that followed the asteroid.
The astronomers estimate that the longer tail is half a million miles away, while the shorter tail is about a quarter as long. They concluded that the tails must consist of tens of millions of kilograms of dust, which is actively ejected from the asteroid into space. But how? The question raised interest in Gault, and studies have since revealed earlier cases of similar asteroid activity.
We know about a million bodies between Mars and Jupiter, and maybe about 20 that are active in the asteroid belt, “says Marsset.That is very rare. He and his colleagues went in search of answers to Gault’s activities in March when they secured the observation time at NASA’s Infrared Telescope Facility (IRTF) in Mauna Kea, Hawaii.
For over two nights, they observed the asteroid and divided the incident light of the asteroid into a high-precision spectrograph in various frequencies or colors, the relative intensity of which can give the scientists an idea of the composition of an object.
Based on their analysis, the team discovered that the surface of the asteroid is mainly silicate, a dry, rocky material that resembles most other asteroids and most notably resembles most comets at all.
Comets typically come from the far colder edges of the solar system. As they approach the sun, any surface ice immediately sublimes or evaporates to gas, creating the characteristic tail of the comet. Since Marsset’s team found that 6478 Gault is a dry, rocky body, it means that it is likely to create dust tails through another active mechanism.
A fresh change As the team watched the asteroid, it was to his surprise that the near-infrared rock changed color from red to blue. We have never seen such a dramatic change in such a short time, “says co-author DeMeo.
Scientists say they’ll likely see the surface dust of the asteroid exposed to the sun over millions of years and emitted into space.Below it is a fresh, less irradiated surface that appears blue at near infrared wavelengths. Interestingly, only a very thin layer needs to be removed to detect a change in the spectrum, “says DeMeo.It can be as thin as a single layer of grains only microns deep.
What could make the asteroid get paint?
The team and other groups studying 6478 Gault believe that the reason for the color shift and the comet-like activity of the asteroid is due to the same mechanism: a fast turn. The asteroid may turn fast enough to remove dust from its surface by mere centrifugal force. The researchers estimate that a rotation period of about two hours is required, which will rotate every few hours compared to the 24-hour period of the earth.
About 10 percent of the asteroids spin very fast, with a rotation time of two to three hours, and that’s probably because the sun is whirling them up, “says Marsset. This spin phenomenon is known as the YORP effect (or Yarkovsky-O “Keefe-Radzievskii paddack effect, named after the scientists who discovered it), which refers to the effect of solar radiation or photons on small, nearby bodiesasteroid.
While asteroids reflect most of this radiation back into space, some of these photons are absorbed, then released as heat and as momentum. This creates a small force that can cause the asteroid to spin faster over millions of years. Astronomers have historically observed the YORP effect on a handful of asteroids.
To confirm that a similar effect is acting on 6478 Gault, the researchers must capture its spin by light curves – measurements of the brightness of the asteroid over time. The challenge will be to see through the asteroid’s substantial dust tail, which can cover important parts of the asteroid’s light.
Marsset’s team and other groups plan to investigate the asteroid for further clues on activity the next time it becomes visible in the sky. I think [the study of the group] underpins the fact that the asteroid belt is a really dynamic place, “says DeMeo.While the asteroid fields you see in the movies are all hugely exaggerating, there is definitely a lot of passing every moment out there.