The redness of Neptunian asteroids provides important clues about the early Solar System. These asteroids are believed to have formed in the Kuiper Belt, a region beyond Neptune that is rich in icy bodies. The red color of these asteroids is due to the presence of organic molecules, which are believed to have formed on the surface of the asteroids through a process known as irradiation. This process involves the bombardment of the asteroids by high-energy particles from the Sun and other sources, which can break down the icy surface and create complex organic molecules.
According to a new study by an international team of researchers, asteroids in orbit with the planet Neptune have been observed to exist in a broad spectrum of red color, implying the existence of two populations of asteroids in the region. The findings were published in the journal Monthly Notices of the Royal Astronomical Society: Letters.
The team of scientists from the United States, California, France, the Netherlands, Chile, and Hawaii observed 18 asteroids known as Neptunian Trojans that share Neptune’s orbit. They are between 50 and 100 kilometers in size and are located approximately 4.5 billion kilometers from the Sun. Astronomers have a difficult time studying asteroids orbiting this far away because they are faint. Before the new work, only about a dozen Neptunian Trojans had been studied, requiring the use of some of the largest telescopes on Earth.
Our observations also show that the Neptunian Trojans differ in color from asteroid groups further from the Sun. One possible explanation is that the processing of asteroids’ surfaces by the Sun’s heat has different effects for asteroids at different solar distances.
Dr. Bryce Bolin
The new data were collected over the course of two years using the WASP wide field camera on the Palomar Observatory telescope in California, the GMOS cameras on the Gemini North and South telescopes in Hawaii and Chile, and the LRIS camera on the Keck Telescope in Hawaii.
Several of the 18 observed Neptunian Trojans were much redder than most asteroids when compared to other asteroids in this group studied previously. Redder asteroids are expected to have formed much further from the Sun; one population of these is known as the Cold Classical trans-Neptunian objects discovered beyond Pluto’s orbit, at around 6 billion kilometers from the Sun. The newly observed Neptunian Trojans are also unlike asteroids located in the orbit of Jupiter, which are typically more neutral in colour.
The redness of the asteroids implies that they contain a higher proportion of more volatile ices such as ammonia and methanol. These are extremely sensitive to heat, and can rapidly transform into gas if the temperature rises, so are more stable at large distances from the Sun.
The asteroids’ proximity to Neptune implies that they are stable on timescales comparable to the age of the Solar System. They serve as a time capsule, recording the Solar System’s initial conditions.
The presence of redder asteroids among the Neptunian Trojans suggests that there is a transition zone between more neutrally colored and redder objects. The redder Neptunian asteroids may have formed beyond this transition boundary before being captured by Neptune and captured into its orbit. The Neptunian Trojans would have been captured into the same orbit as the planet Neptune as it migrated from the inner solar system to where it is now, approximately 4.5 billion kilometers from the Sun.
Dr. Bryce Bolin of NASA’s Goddard Space Flight Center, the study’s lead author, stated, “We have more than doubled the number of Neptunian Trojans studied with large telescopes in our new study. It’s exciting to discover the first indications of redder asteroids in this group.”
“We can now see significant differences between asteroid groups because we have a larger sample of Neptunian Trojans with measured colors. Our observations also show that the Neptunian Trojans differ in color from asteroid groups further from the Sun. One possible explanation is that the processing of asteroids’ surfaces by the Sun’s heat has different effects for asteroids at different solar distances.”
