Why Dimorphos Has a Surprisingly Fresh Surface

Why Dimorphos Has a Surprisingly Fresh Surface

Dimorphos, the small moonlet of the asteroid Didymos, has an unexpectedly young and fresh surface, which has intrigued scientists since the DART mission impact in 2022. Researchers discovered that Dimorphos is a rubble pile, likely formed from material shed by Didymos, with its boulders showing signs of thermal fatigue. These findings provide insights into asteroid geology, the effectiveness of kinetic impactors for planetary defense, and the history of our Solar System.

Summary

• Dimorphos’ surface age is estimated at 300,000 years, much younger than Didymos’ 12.5 million years.
• Both Didymos and Dimorphos are rubble pile asteroids, consisting of loosely held together boulders and gravel.
• The DART mission significantly altered Dimorphos’ orbit, showcasing the impact potential of kinetic impactors.
• Thermal fatigue plays a crucial role in breaking up surface boulders on Dimorphos.
• Researchers observed a lower bearing capacity on Didymos compared to Earth’s dry sand.
• ESA’s Hera mission will further study Dimorphos in 2026 to understand the long-term effects of the DART impact.

In five new Nature Communications papers, the team behind @NASA's successful #DARTMission sheds new light on the structure and origins of the asteroid system encountered in 2022. https://t.co/G7x5tQyriq@NASASolarSystem @AsteroidWatch pic.twitter.com/i82oxbrXxw

— Johns Hopkins APL (@JHUAPL) July 30, 2024

Main Article

The asteroid Dimorphos, a small moonlet orbiting the larger asteroid Didymos, has captured the attention of scientists worldwide. Following NASA’s Double Asteroid Redirection Test (DART) mission in September 2022, researchers have explored into the geology and formation of these celestial bodies. Surprisingly, they found that Dimorphos has a much fresher surface compared to Didymos.

The Age of Dimorphos and Didymos

One of the most striking discoveries about Dimorphos is its surface age. While Didymos has a surface age of approximately 12.5 million years, Dimorphos’ surface is estimated to be only 300,000 years old. This significant age difference suggests that Dimorphos’ surface has been resurfaced relatively recently in geological terms. The younger surface of Dimorphos has led scientists to investigate the processes that could contribute to this rapid resurfacing.

Rubble Pile Composition

Both Didymos and Dimorphos are classified as rubble pile asteroids. This means they are not solid bodies but rather loose aggregates of rocks, boulders, gravel, and dust held together by their own gravity. Andy Rivkin, DART investigation team co-lead at the Johns Hopkins Applied Physics Lab (APL), described Dimorphos as “a pile of gravel and boulders (and some sand/dust) held together by its own gravity, and really not anything else.” This lack of cohesion between the different pieces makes rubble pile asteroids particularly interesting and challenging to study.

Impact of the DART Mission

The DART mission aimed to test the kinetic impactor technique as a method for planetary defense. By intentionally crashing into Dimorphos, the mission successfully altered the moonlet’s orbit, decreasing its orbital period by about 34 minutes. The significant change in Dimorphos’ orbit can be attributed to its rubble pile composition. A collection of loosely bound boulders is easier to shift than a solid object, highlighting the potential effectiveness of kinetic impactors in diverting hazardous asteroids.

Geology and Surface Characteristics

The images and data collected by DART provided a close-up view of the Didymos/Dimorphos system. Olivier Barnouin, Ronald-Louis Ballouz, and their team at APL used this information to determine the surface characteristics and ages of both asteroids. They found that the weak surface characteristics of these bodies contributed to the effectiveness of the DART impact. Dimorphos, covered with boulders of varying sizes, contrasts with the smoother, though still rocky, surface of Didymos at lower elevations.

Formation of Dimorphos

Researchers believe that Dimorphos likely formed from material shed by Didymos. The spin-up of Didymos, leading to a large mass shedding event, could have resulted in the formation of Dimorphos. This process was confirmed in a study by Maurizio Pajola and his team from the National Institute for Astrophysics (INAF) in Rome. They concluded that both Didymos and Dimorphos are primarily composed of boulders formed through the catastrophic disruption of their progenitors.

Thermal Fatigue and Boulder Fracturing

One of the critical processes affecting Dimorphos’ surface is thermal fatigue. Alice Lucchetti and colleagues from INAF discovered that the size and distribution of boulders on Dimorphos are consistent with thermal fatigue. This phenomenon involves the gradual weakening and cracking of materials due to heat, causing boulders to break up more rapidly than previously thought. Thermal fatigue significantly alters the physical characteristics of asteroids, contributing to their rapid resurfacing.

“The presence of boulder fields affected by thermal fracturing on near-Earth asteroid surfaces may contribute to an enhancement in the ejected mass and momentum from kinetic impactors when deflecting asteroids,” noted the authors of the study.

Bearing Capacity of Didymos

Understanding the surface’s ability to support applied loads, or bearing capacity, is crucial for predicting how an asteroid’s surface will respond to impacts. Jeanne Bigot and Pauline Lombardo from ISAE-SUPAERO in Toulouse, France, led a study that estimated Didymos’ bearing capacity. They found it to be only 0.1% that of dry sand on Earth. This low bearing capacity is an essential parameter for planetary defense strategies and future missions targeting asteroid displacement.

Comparative Analysis of Rubble Pile Asteroids

Colas Robin and co-authors conducted a comparative analysis of surface boulders on Dimorphos and other rubble pile asteroids, such as Itokawa, Ryugu, and Bennu. They found striking similarities in the boulders across these asteroids, suggesting they formed and evolved in a similar fashion. The data gathered from these comparisons provide valuable insights for future planetary defense missions and the interpretation of impactor missions.

Future Missions and Studies

The DART mission has paved the way for further research and exploration of the Didymos/Dimorphos system. ESA’s Hera mission, set to launch in 2024, will arrive at Didymos and Dimorphos in December 2026. Hera will conduct a detailed study of Dimorphos, examining the long-term effects of the DART impact and providing more insights into the moonlet’s geology and evolution.

Conclusion

Dimorphos’ surprisingly fresh surface and its rubble pile composition have provided scientists with valuable information about the formation and evolution of asteroids. The DART mission’s successful alteration of Dimorphos’ orbit demonstrates the potential of kinetic impactors for planetary defense. As researchers continue to study the Didymos/Dimorphos system, our understanding of these celestial bodies and their role in the history of our Solar System will continue to grow. The findings from the DART mission and future missions like Hera will inform strategies for protecting Earth from potential asteroid threats.

Tables

References

1. Barnouin, O., Ballouz, R.-L., et al. (2024). The geology and evolution of the Near-Earth binary asteroid system (65803) Didymos. Nature Communications. https://doi.org/10.1038/s41467-024-50146-x
2. Pajola, M., et al. (2024). Evidence for multi-fragmentation and mass shedding of boulders on rubble-pile binary asteroid system (65803) Didymos. Nature Communications. https://doi.org/10.1038/s41467-024-50148-9
3. Lucchetti, A., et al. (2024). Fast boulder fracturing by thermal fatigue detected on stony asteroids. Nature Communications. https://doi.org/10.1038/s41467-024-50145-y
4. Bigot, J., Lombardo, P., et al. (2024). The bearing capacity of asteroid (65803) Didymos estimated from boulder tracks. Nature Communications. https://doi.org/10.1038/s41467-024-50149-8
5. Robin, C., et al. (2024). Mechanical properties of rubble pile asteroids (Dimorphos, Itokawa, Ryugu, and Bennu) through surface boulder morphological analysis. Nature Communications. https://doi.org/10.1038/s41467-024-50147-w
6. Barnouin, O., Ballouz, R.-L., et al. (2024). The geology and evolution of the Near-Earth binary asteroid system (65803) Didymos. Nature Communications. https://doi.org/10.1038/s41467-024-50146-x
7. Pajola, M., et al. (2024). Evidence for multi-fragmentation and mass shedding of boulders on rubble-pile binary asteroid system (65803) Didymos. Nature Communications. https://doi.org/10.1038/s41467-024-50148-9
8. Lucchetti, A., et al. (2024). Fast boulder fracturing by thermal fatigue detected on stony asteroids. Nature Communications. https://doi.org/10.1038/s41467-024-50145-y
9. Bigot, J., Lombardo, P., et al. (2024). The bearing capacity of asteroid (65803) Didymos estimated from boulder tracks. Nature Communications. https://doi.org/10.1038/s41467-024-50149-8
10. Robin, C., et al. (2024). Mechanical properties of rubble pile asteroids (Dimorphos, Itokawa, Ryugu, and Bennu) through surface boulder morphological analysis. Nature Communications. https://doi.org/10.1038/s41467-024-50147-w
11. NASA. (2023). NASA’s DART Mission Sheds New Light on Target Binary Asteroid System. NASA Science
12. Newswise. (2023). DART Forward: Five Papers Shed New Light on Asteroids from World’s First Planetary Defense Test. Newswise

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#Dimorphos, #AsteroidGeology, #DARTMission, #RubblePile, #ThermalFatigue, #PlanetaryDefense, #HeraMission, #Didymos, #AsteroidResearch, #SpaceExploration