When NASA’s OSIRIS-REx spacecraft returned from its mission to asteroid Bennu in 2023, it brought back more than just ancient space rocks, it delivered answers to puzzles that have baffled astronomers for years. Among the most intriguing questions was why asteroids that should look identical through telescopes appear strikingly different colours from Earth.
The mystery cantered on two remarkably similar asteroids – Bennu and Ryugu which were both visited by sample return missions. These ancient rocks are made of the same dark, carbon rich materials and formed around the same time during the birth of our Solar System 4.5 billion years ago. Logic suggests they should reflect light identically, yet when studied by spacecraft, Ryugu appears faintly red while Bennu looks distinctly blue.
OSIRIS-REx in Launch Configuration (Credit : NASA Kennedy from United StatesNASA/Glenn Benson)
Michelle Thompson, a Purdue University planetary scientist and expert on space weathering, has been studying samples from both asteroids to crack this mystery. Her research, part of three newly published papers based on analysis of Bennu samples by worldwide experts, shows that these asteroids are undergoing the same weathering processes but at different stages of a repeating cycle.
Space weathering is the constant bombardment that rocky bodies endure from solar radiation, cosmic rays, and micrometeorite impacts. This constant assault gradually changes how asteroid surfaces reflect light, essentially giving them what we might call a ‘tan’ that alters their apparent colour when viewed from Earth.
The breakthrough came when Thompson and her colleagues compared the exposure ages of surface particles from both asteroids. They discovered that grains collected from Ryugu’s surface have been exposed to space for only a few thousand years, while surface grains from Bennu have been exposed for tens of thousands of years.
This is a coloured view of the C-type asteroid 162173 Ryugu, seen by the ONC-T camera on board of Hayabusa2 (Credit : ISAS/JAXA)
This age difference explains everything. Rather than representing two different weathering processes, the asteroids show two snapshots of the same cycle. Rubble pile asteroids regularly refresh their surfaces through impacts and gravitational shifts, exposing fresh material that then begins weathering again. Bennu’s bluer appearance indicates more advanced weathering, while Ryugu’s reddish tint represents an earlier stage.
With 1.45 million known asteroids in our Solar System, it’s neither economically nor physically feasible to visit even a fraction of them. Being able to extrapolate and understand the nature of various asteroids by analysing them from Earth is key to understanding these ancient remnants.
This research enables scientists to correlate what telescopes see with actual sample analysis. Such calibration is crucial for future asteroid exploration, whether for scientific study or potential mining operations. Knowing how surface weathering affects an asteroid’s appearance helps mission planners select targets with greater confidence.
But the Bennu samples reveal even more remarkable secrets about our Solar System’s origins. Earlier research discovered salts in the samples, including phosphates critical to life on Earth and essential for metabolism and DNA. Scientists found evidence of an environment well suited to kick start precursor compounds for the chemistry of life.
“Looking at the organic molecules from Bennu, we are getting an understanding of what kinds of molecules could have seeded life on early Earth. We won’t find life itself, but we’re looking at the building blocks that could have eventually evolved into life.” – Michelle Thompson from Purdue University
These materials remain pristine because asteroids are essentially time capsules, unchanged since the Solar System’s formation. Unlike Earth, where billions of years of geological and biological processes have mixed and transformed the original ingredients, asteroids preserve the raw materials from which planets formed.
The colour mystery’s solution demonstrates how sample return missions revolutionise our understanding of space. By bringing pieces of distant worlds back to Earth’s sophisticated laboratories, scientists can decode secrets that remain hidden when observing from afar, opening new windows into both our Solar System’s history and the potential origins of life itself.
Source : Planetary scientist decodes clues in Bennu’s surface composition to make sense of far-flung asteroids