Spectral evolution of a dark asteroid surface after ten years of space weathering

When asteroid 596 Scheila collided with an object in the asteroid belt between Mars and Jupiter in December 2010, a fresh layer of material was exposed on the asteroid surface . An international research team observed the spectrum from the asteroid approximately ten years later, to see how space weathering affects the surface over a genuine timescale. Within the uncertainty of the observations, the observed spectra was consistent with that observed immediately after the 2010 impact event. This suggests that the surface colour of dark asteroids is not significantly altered by space weathering over a time period of ten years.

This study is the first example that investigates spectral changes due to space weathering on the surface of an actual asteroid in the Solar System. Models based on this observation suggests that asteroids with relatively young surfaces (less than about 1000 years) can be considered to have experienced negligible evolution through space weathering, potentially changing how we understand their properties.

The research was led by HASEGAWA Sunao, Associate Senior Researcher at ISAS JAXA, together with an international team from the National Astronomical Observatory of Japan, the University of Tokyo, Japan Spaceguard Association, Kobe University, Massachusetts Institute of Technology, European Southern Observatory, Charles University, Cote d'Azur Observatory, University of Hawaii, NASA, University of Liege, Laboratoire d'Astrophysique de Marseille, Diego Portales University, and Seoul National University.

The results of this research were published in the Astrophysical Journal of Letters, an academic journal published by the American Astronomical Society, on October 27, 2022.

We now know that the surface layer of asteroids, which are airless celestial bodies without rain, wind or an atmosphere, are not always fresh, and that the colour of the surface material can change due to the effects of space weathering. However, it has only been in the last ten years that this fact has been clearly established.

The mystery first arose in the 1970s, when the spectra of asteroids and meteorites were first being compared. No asteroids in the asteroid belt were discovered that had a spectra similar to that of the ordinary chondrites, which are the most frequently discovered meteorites on Earth (comprising roughly 90% of those discovered). Instead, the asteroid belt was populated with S-type asteroids which had a redder spectra (reflect more strongly at red wavelengths).

At that time, two theories emerged to explain the identity of the S-type asteroids. The first suggested they were stony-iron meteorites which showed a similar spectra, but were much rarer as they consisted of just 1% of the meteorites found. The second said the S-type asteroids were the commonly-found ordinary chondrites, whose spectra had changed due to space weathering. One method to tackle this problem was to undertake experiments on Earth that replicate the process of space weathering.

Both domestic and overseas laboratories conducted experiments that reproduced the effect of space weathering on ordinary chondrites. The resulting weathered spectra successfully reproduced the spectra from the S-type asteroids.

In addition, the surface of the weathered ordinary chondrite was also found to have iron nanoparticles equivalent to those found in the surface layer of lunar regolith grains, which is also subject to space weathering. Once the sample returned from the S-type asteroid Itokawa by Hayabusa was found to be an ordinary chondrite, the origin of the S-type asteroid was confirmed. The results from the laboratory experiments to reproduce the effect of space weathering therefore played an important role in settling this debate.

But while laboratory experiments are clearly an important research method in space weathering, there is one difference between the space weathering experiments performed in the laboratory and the phenomena that actually occurs on the surface of celestial bodies. That issue is "time"; the duration over which space weathering occurs. Spectra changes due to space weathering are thought to occur over a time period ranging from thousands to tens of millions of years. This means that the laboratory sample experienced space weathering rate that per unit time was over 10 orders of magnitude stronger. Of course, this is unavoidable for reproductions of space weathering, when the true time scale may be longer than human history.

However, this becomes a different story if an asteroid can be observed with a fresh surface.

Research results
In December 2010, the T-type asteroid 596 Scheila was hit by an asteroid several tens of meters in size. Ejecta from the collision crater covered the top layer of material to reveal a completely renewed surface layer. This was revealed in the slope of the asteroid spectrum at near-infrared wavelengths (0.8 - 2.5 microns) which became red after the collision, indicated a fresh surface layer. Space weathering on the surface of 596 Scheila therefore has the effect of creating a bluer asteroid spectrum. Similar results have been obtained in laboratory experiments on the effect of space weathering on primitive meteorites that originate from dark asteroids such as 596 Scheila. This trend for the spectral change due to space weathering is therefore consistent.

This year (2022) will be about ten years since the collision with 596 Scheila occurred. The asteroid's refreshed surface has therefore been exposed to space weathering for about a decade, after it was exposed in December 2010. This presents a great opportunity to observe how the spectrum changes due to space weathering in real time over a ten year period.

Our team therefore obtained spectra of 596 Scheila at near-infrared and visible wavelengths using the 3.0m NASA Infrared Telescope Facility (IRTF), and the 1.05m Murikabushi telescope at the National Astronomical Observatory of Japan's Ishigakijima Observatory, respectively. Changes since the 2010 collision were then investigated by comparing the new spectra of 596 Scheila with data from just after the collision.

At visible wavelengths, the shape of the spectrum was unchanged even from before the impact event. At the longer near-infrared wavelengths, the spectrum has not changed in the years after the collision ten years ago. In other words, we found that the spectrum of a dark asteroid like 596 Scheila does not evolve in 10 years due to the effects of space weathering.

Scientific significance of this research
In these observations, the asteroid spectrum did not change even after being exposed to the effects of space weathering for about ten years. This is within the range of predictions from past laboratory experiments. However, this is the first time that we have observed that there is no spectral change from space weathering on the surface of an actual asteroid over a timescale of ten years under conditions where the exposure time can be accurately known. As with the sample from asteroid Itokawa, these observations therefore also support the validity of the laboratory experiments (at least on timescales of a decade).

The next question was whether spectral change due to space weathering is a phenomenon that occurs linearly or logarithmically with time. From the spectra of 596 Scheila shortly after the impact, we modelled the spectral change assuming a linear and then logarithmic change from this time. Comparing this with our recent observations suggested that the change had been linear.

Furthermore, Figure 2 suggests that the spectral change due to space weathering would not be significant for about 1,000 years. This means that if a dark asteroid with a spectrum like 596 Scheila is discovered with a surface age of about 1,000 years, we can assume that this is a surface that has not undergone substantial space weathering. Through a similar method, this research also found that the spectral changes of minor celestial bodies that are redder than D-type asteroids also did not change over hundreds of years.

On September 26, 2022, NASA conducted an experiment in which the DART spacecraft crashed into the asteroid moon Dimorphos of asteroid 65803 Didymos. Observations after the impact by probes and telescopes have revealed a substantial quantity of ejecta was expelled from the surface. In other words, the surface layer of Dimorphos has been renewed in a similar manner to 596 Scheila.

The European Space Agency plans to rendezvous with Didymos in 2026 with the spacecraft Hera to observe the asteroid and its moon. Hera is equipped with a thermal camera of the type that was onboard Hayabusa2. Didymos and Dimorphos are both S-type asteroids with a spectra and albedo that are different from that of 596 Scheila, and laboratory experiments have shown that space weathering over thousands of years will not change the spectrum. Therefore, it is expected that the surface layer Dimorphos can be observed in a fresh state even after the arrival of the Hera spacecraft.

Research Report:A celestial body with a fresh surface

Related Links
Institute of Space and Astronautical Science
Asteroid and Comet Mission News, Science and Technology

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