The Surface of Dimorphos is Surprisingly New

When NASA’s DART mission intentionally slammed into Dimorphos in September 2022, the orbit of the moonlet was altered. Researchers have studied the photos and data taken by DART before its impact, learning more about the geology of the Didymos/Dimorphos system. They have now estimated the surface age of both the asteroid and its moon. The asteroid Didymos has a surface age of 12.5 million years, while the moon Dimorphos is only 300,000 years old.

Additionally, the DART researchers concluded both Didymos and Dimorphos are rubble piles, with Dimorphos likely inheriting its boulders from Didymos.

“It’s a pile of gravel and boulders (and some sand/dust) held together by its own gravity, and really not anything else,” said Andy Rivkin, DART investigation team co-lead at the Johns Hopkins Applied Physics Lab (APL), on Bluesky. “There’s really no cohesion between different pieces of gravel or rocks on Dimorphos.”

That makeup explains why DART’s impact made such a such a surprising change in Dimorphos’ orbital period, decreasing it by about 34 minutes. A collection of boulders is easier to shift than a solid object.

Several DART researchers published five papers in Nature Communications, looking at the geology and geophysics of Didymos and Dimorphos as seen by DART.

“These findings give us new insights into the ways that asteroids can change over time,” said Thomas Statler, lead scientist for Solar System Small Bodies at NASA Headquarters in Washington, in a NASA press release. “This is important not just for understanding the near-Earth objects that are the focus of planetary defense, but also for our ability to read the history of our Solar System from these remnants of planet formation. This is just part of the wealth of new knowledge we’ve gained from DART.”

In “The geology and evolution of the Near-Earth binary asteroid system (65803) Didymos,” Olivier Barnouin, Ronald-Louis Ballouz, also of APL, and their team were able to determine the disparate ages of Didymos and Dimorphos. They also found that both objects have weak surface characteristics, which very likely contributed to DART’s significant impact on the moonlet’s orbit.

“The images and data that DART collected at the Didymos system provided a unique opportunity for a close-up geological look at a near-Earth asteroid binary system,” said Barnouin, in a press release from APL. “From these images alone, we were able to infer a great deal of information on geophysical properties of both Didymos and Dimorphos, and expand our understanding of the formation of these two asteroids. We also better understand why DART was so effective in moving Dimorphos.”

Images captured by DART and its cubesat companion the LICIACube – contributed by the Italian Space Agency (ASI)  — showed Dimorphos’ topography covered with boulders of varying sizes, while the larger asteroid Didymos was smoother at lower elevations, though rocky at higher elevations. It also had more craters than Dimorphos. The authors inferred that Dimorphos likely spun off from Didymos in a large mass shedding event.

This was confirmed in another paper, “Evidence for multi-fragmentation and mass shedding of boulders on rubble-pile binary asteroid system (65803) Didymos.” Maurizio Pajola, of the National Institute for Astrophysics (INAF) in Rome, and team show how both Didymos and Dimorphos are mainly comprised of a collection of boulders. This team concluded that the formation of Dimorphos likely came as Didymos shed material, creating a new asteroid moonlet.

“The size-frequency distribution of boulders larger than 5 meters on Dimorphos and larger than 22.8 meters on Didymos confirms that both asteroids are piles of fragments produced in the catastrophic disruption of their progenitors,” the team wrote. “This finding supports the hypothesis that some asteroid binary systems form through the spin up and mass shedding of a fraction of the primary asteroid.”

In another paper, “Fast boulder fracturing by thermal fatigue detected on stony asteroids” Alice Lucchetti, also of INAF, and colleagues found that the size and distribution of boulders on Dimorphos is consistant with thermal fatigue, which is the gradual weakening and cracking of a material caused by heat. This could rapidly break up boulders on the surface of Dimorphos, generating surface lines and altering the physical characteristics of this type of asteroid more quickly than previously thought. The DART mission was likely the first observation of such a phenomenon on this type of asteroid.

Thermal fatigue could also have a bearing on what happens if this type of asteroid would need to be deflected for planetary defense.

“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,” the authors wrote.

Another paper, “The bearing capacity of asteroid (65803) Didymos estimated from boulder tracks” led by students Jeanne Bigot and Pauline Lombardo of ISAE-SUPAERO in Toulouse, France show that the bearing capacity — the surface’s ability to support applied loads of asteroid Didymos’ surface is only 0.1% that of dry sand on Earth. NASA said that this is considered an important parameter for understanding and predicting the response of a surface, including for the purposes of displacing an asteroid.

Finally, “Mechanical properties of rubble pile asteroids through surface boulder morphological analysis” by Colas Robin, also of ISAE-SUPAERO, and co-authors analyzed the surface boulders on Dimorphos, comparing them with those on other rubble pile asteroids, including Itokawa, Ryugu and Bennu. The researchers found “stiking similarities” the boulders on all four asteroids, suggesting they all formed and evolved in a similar fashion, and were also changed by impacts. This data, too, informs future planetary defense missions or attempts at impactor missions.

“Planetary defense efforts rely on estimates of the mechanical properties of asteroids, which are difficult to constrain accurately from Earth,” the team wrote. “The mechanical properties of asteroid material are also important in the interpretation of the DART impact.”  

All the DART researchers team will continue to observe and study DART’s impact. Additionally, another spacecraft will launch in 2024 to study Dimorphos even closer. ESA’s Hera mission should arrive at Didymos and Dimorphos in December 2026. Hera will undertake a detailed study of Dimorphos to understand more deeply how the impact affected it.

• The Geology and Evolution of the Near-Earth Binary Asteroid System (65803) Didymos
• Evidence for Multi-Fragmentation and Mass Shedding of Boulders on Rubble-Pile Binary Asteroid System (65803) Didymos
• Fast Boulder Fracturing by Thermal Fatigue Detected on Stony Asteroids
• The Bearing Capacity of Asteroid (65803) Didymos Estimated from Boulder Tracks
• Mechanical Properties of Rubble Pile asteroids (Dimorphos, Itokawa, Ryugu, and Bennu) Through Surface Boulder Morphological Analysis