09:15 〜 09:37
[PPS02-02] Exploring Didymos and Dimorphos with the DART and LICIAcube missions.
★Invited Papers
キーワード：Asteroids, DART, Didymos, Dimorphos
NASA’s Double Asteroid Redirection Test (DART), which impacted on Sept 26, 2022 the moon Dimophos of asteroid Didymos, is the first successful full-scale demonstration of the kinetic impactor technology to deflect an asteroid. Shared live, millions of viewers watched as the DART spacecraft streamed images to Earth up to a second before impact. A suite of telescopic assets revealed an early plume minutes after impact, and a more than 30-minute orbit period change for Dimorphos two weeks later. A few days later, data downlinked by LICIACube provided by Italian Space Agency (ASI) showed spectacular images of ejecta escaping Dimorphos, following DART’s impact. An international effort with over 100 different institutions that represent 28 different nations continues to explore both the outcome of the DART impact and the properties of the Didymos system. Here, we present current assessments of the shape and surfaces of Didymos and Dimorphos, and explore the possible processes responsible for their geological formation and evolution.
Shapes: Didymos measures approximately (843±5) x (838±15) x (622±5) m but as the shape construction proceeds, additional changes are likely. The shape resembles a flattened ellipsoid, with some broad concavities, and an equatorial bulge, that ends in places with a narrow ridge. The LICIAcube images show that Didymos’ equatorial region is not circular when viewed from the pole, but possesses some angularity. In contrast, Dimorphos appears to be very uniform in shape, measuring (177 ± 2) by (174 ± 4) by (116 ± 2) m, matching closely an oblate spheroid.
Geology: Didymos possesses two geological regions. The first region lacks large boulders (>25 m) and crater candidates at mid-to-low latitudes. This region possesses features that are interpreted to be surface mass movements [6]. The second region is more hummocky relative to the first and is located at mid-to-high latitudes; it possesses a greater number of both large boulders and concavities.
Current analyses shows Dimorphos has no distinct geological units, although future efforts may reveal some. The surface is dominated by cobbles and boulders ranging from 10s of cm to ~16m. No regolith is truly visible. Some of the boulders are arranged in campfire-like structures. Others show evidence for rocks on boulders, but never on slopes exceeding 30°. Despite the limitations of the viewing geometry, lineaments have been identified across the asteroid; some of these are associated with smaller cracks seen on rocks. Evidence for any type of surface motion of Dimorphos boulders has as yet to be reported.
Craters and surface ages: Dimorphos reveals 14 candidate-craters, based on a careful topographic assessment, ranging from a few 1 to 10s of meters in diameter. Didymos, has 9 candidate-craters ranging in diameter from 10s of m to the largest one at 205m. The crater counts suggest that Didymos’ age is ~30x older than the surface of Dimorphos for identical surface strengths. The main-belt age of Didymos ranges between 15-500Myr depending on the surface strength assumed.
Implications: The surface observations indicate that Didymos and Dimorphos are likely rubble piles. The nominal ~30% porosity of Didymos, the none-circular circumference of the asteroid, and the evidence for surface mass movements typically require cohesive interior aggregates, and weaker surface material.
The flattened shape of Didymos may indicate an early episode of re-adjustment due to interior and surface failure that is observed in numerical models of rubble pile that fail due to spin-up. The age of Dimorphos’ surface may constrain the approximate time when Didymos underwent re-adjustment, if Dimorphos formed during that failure event.
The oblate shape of Dimorphos remains difficult to explain using existing dynamical models of binary asteroid formation. The DART images and moderate to high momentum transfer values indicate it is a fairly weak unconsolidated object.
Shapes: Didymos measures approximately (843±5) x (838±15) x (622±5) m but as the shape construction proceeds, additional changes are likely. The shape resembles a flattened ellipsoid, with some broad concavities, and an equatorial bulge, that ends in places with a narrow ridge. The LICIAcube images show that Didymos’ equatorial region is not circular when viewed from the pole, but possesses some angularity. In contrast, Dimorphos appears to be very uniform in shape, measuring (177 ± 2) by (174 ± 4) by (116 ± 2) m, matching closely an oblate spheroid.
Geology: Didymos possesses two geological regions. The first region lacks large boulders (>25 m) and crater candidates at mid-to-low latitudes. This region possesses features that are interpreted to be surface mass movements [6]. The second region is more hummocky relative to the first and is located at mid-to-high latitudes; it possesses a greater number of both large boulders and concavities.
Current analyses shows Dimorphos has no distinct geological units, although future efforts may reveal some. The surface is dominated by cobbles and boulders ranging from 10s of cm to ~16m. No regolith is truly visible. Some of the boulders are arranged in campfire-like structures. Others show evidence for rocks on boulders, but never on slopes exceeding 30°. Despite the limitations of the viewing geometry, lineaments have been identified across the asteroid; some of these are associated with smaller cracks seen on rocks. Evidence for any type of surface motion of Dimorphos boulders has as yet to be reported.
Craters and surface ages: Dimorphos reveals 14 candidate-craters, based on a careful topographic assessment, ranging from a few 1 to 10s of meters in diameter. Didymos, has 9 candidate-craters ranging in diameter from 10s of m to the largest one at 205m. The crater counts suggest that Didymos’ age is ~30x older than the surface of Dimorphos for identical surface strengths. The main-belt age of Didymos ranges between 15-500Myr depending on the surface strength assumed.
Implications: The surface observations indicate that Didymos and Dimorphos are likely rubble piles. The nominal ~30% porosity of Didymos, the none-circular circumference of the asteroid, and the evidence for surface mass movements typically require cohesive interior aggregates, and weaker surface material.
The flattened shape of Didymos may indicate an early episode of re-adjustment due to interior and surface failure that is observed in numerical models of rubble pile that fail due to spin-up. The age of Dimorphos’ surface may constrain the approximate time when Didymos underwent re-adjustment, if Dimorphos formed during that failure event.
The oblate shape of Dimorphos remains difficult to explain using existing dynamical models of binary asteroid formation. The DART images and moderate to high momentum transfer values indicate it is a fairly weak unconsolidated object.