Rare red Neptunian asteroids could shed light on solar system’s early evolution

The presence of redder asteroids suggests the "existence of a transition zone between more neutral colored and redder objects".
Mrigakshi Dixit
An artist's concept of a craggy piece of solar system debris that belongs to a class of bodies called trans-Neptunian objects.
An artist's concept of a craggy piece of solar system debris that belongs to a class of bodies called trans-Neptunian objects.

NASA, ESA, and G. Bacon (STScI). Science Credit: NASA, ESA, and C. Fuentes

Although planets and moons receive a lot of attention in space exploration, there is a lot to learn about our solar system from its many smaller bodies, such as comets and asteroids. 

An international team of scientists has now closely examined Neptune's Trojan asteroids, which could shed light on the origins of the solar system. Close inspection with larger ground-based telescopes revealed that Neptune's Trojan asteroids display a shade of red.

Reddish color asteroids

These trojan asteroids are those that share the orbit of the ice giant, Neptune. Neptune's Trojan asteroids were discovered in 2001, and scientists have only been able to describe a few of them. The asteroids are difficult to spot and study as they are located about 2.8 billion miles (4.5 billion km) from the sun.

For this study, the team tracked 18 Neptunian Trojans for over two years. The newly observed asteroids measure about 31 to 62 miles (50 to 100 km) wide. They were able to examine the color of asteroids using the data collected from telescopes. The team also compared the data with other asteroids in this group looked at in previous studies. 

The results revealed that several were significantly redder than most asteroids in this group. The redness indicates that some of these asteroids are high in volatile ices like ammonia and methanol.

The astronomers peeked into the trojan asteroids using multiple telescopes based on the Earth, including 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. 

There could be two populations of asteroids 

According to the team, this implies the presence of two populations (one redder and another neutral-colored) of asteroids in the region. The redder ones could have formed at a much greater distance from the sun.

The presence of redder asteroids in this group suggests the “existence of a transition zone between more neutral colored and redder objects".

The redder Neptunian asteroids could have formed far away from this transition boundary, and later eventually drawn and captured into Neptune's orbit. Neptune has migrated from the inner solar system to its current location, which is approximately 4.5 billion kilometers from the Sun.

“Because we have a larger sample of Neptunian Trojans with measured colors, we can now start to see major differences between asteroid groups. Our observations also show that the Neptunian Trojans are also different in color compared to asteroid groups even further from the Sun. A possible explanation may be that the processing of the surfaces of asteroids by the Sun’s heat may have different effects for asteroids at varying solar distances,” said Dr Bryce Bolin of the NASA Goddard Space Flight Centre, and lead author of this study, in a statement

These pristine space rocks are extremely valuable in space because they act as time capsules —preserving the secrets of our solar system's early years.

The study is published in the journal Monthly Notices of the Royal Astronomical Society.

Study abstract:

Neptunian Trojans (NTs), trans-Neptunian objects in 1:1 mean-motion resonance with Neptune, are generally thought to have been captured from the original trans-Neptunian protoplanetary disc into co-orbital resonance with the ice giant during its outward migration. It is possible, therefore, that the colour distribution of NTs is a constraint on the location of any colour transition zones that may have been present in the disc. In support of this possible test, we obtained g, r, and i-band observations of 18 NTs, more than doubling the sample of NTs with known visible colours to 31 objects. Out of the combined sample, we found ≈4 objects with g –i colours of >1.2 mags placing them in the very red (VR) category as typically defined. We find, without taking observational selection effects into account, that the NT g – i colour distribution is statistically distinct from other trans-Neptunian dynamical classes. The optical colours of Jovian Trojans and NTs are shown to be less similar than previously claimed with additional VR NTs. The presence of VR objects among the NTs may suggest that the location of the red to VR colour transition zone in the protoplanetary disc was interior to 30 – 35 au.

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