The geology and evolution of the Near-Earth binary asteroid system (65803) Didymos

Abstract

Images collected during NASA's Double Asteroid Redirection Test (DART) mission provide the first resolved views of the Didymos binary asteroid system. These images reveal that the primary asteroid, Didymos, is flattened and has plausible undulations along its equatorial perimeter. At high elevations, its surface is rough and contains large boulders and craters; at low elevations its surface is smooth and possesses fewer large boulders and craters. Didymos' moon, Dimorphos, possesses an intimate mixture of boulders, several asteroid-wide lineaments, and a handful of craters. The surfaces of both asteroids include boulders that are large relative to their host body, suggesting that both asteroids are rubble piles. Based on these observations, our models indicate that Didymos has a surface cohesion ≤ 1 Pa and an interior cohesion of ∼10 Pa, while Dimorphos has a surface cohesion of <0.9 Pa. Crater size-frequency analyzes indicate the surface age of Didymos is 40-130 times older than Dimorphos, with likely absolute ages of $~$ 12.5 Myr and <0.3 Myr, respectively. Solar radiation could have increased Didymos' spin rate leading to internal deformation and surface mass shedding, which likely created Dimorphos.

Fig. 1. Aspect ratios of asteroids visited by spacecraft and observed by radar.

The intermediate to major axis ratio, $b/a$, and minor to major axis ratio, $c/a$, shows that Didymos possesses a smaller $c/a$ relative to other top-shaped asteroids. Dimorphos has a large $b/a$ relative to other secondaries. The four secondaries included are the satellites of the S-type binaries 66391 Moshup (formerly 1999 KW₄) and 2000 DP₁₀₇ as well as the two satellites of the C-type triple 2001 SN₂₆₃ ^–. The black line defines $b/a=c/a$.

Fig. 2. Geological properties, craters and shape of Didymos.

Images (a–c), geological units (c), shape (d, e), elevation (d), and crater candidates (e, f) of Didymos. a DRACO image (dart_0401929893_44497) highlights the triangular shaped ridge (yellow arrow and dashed lines), protruding boulders, boulders with north-south tracks (upper white arrow), and an area with evidence for a plausible mass movement (lower white arrow). The orange arrow indicates the location of the bright spot. b LUKE image (liciacube_luke_l2_1664234227_01003) indicates plausible corners or longitudinal N-S ridges. The solid white arrows point to 160-m and 85-m boulders. Units shown on mosaic (c; dart_0401929893_44497 and dart_0401929913_07357) may be the result of elevation changes, overlain here on the Didymos GDTM (d). Crater candidates are shown on the GDTM in (e) and (f). Consistent with (b), GDTM (f) shows some non-circular perimeter attributes when viewed from -Z pole. GDTM does not include limb data, and thus lacks the triangular outline seen in the images (a–c; see Methods).

Fig. 3. Illuminated and backlit outlines of Dimorphos and Didymos.

Composite images of Dimorphos (DRACO image collected on Sept 26, at 23:14:09 UTC; a) and Didymos (LUKE images collected on Sept 26, at 23:17:27; b). The images are composites of two stretches to show the full outline of the bodies. Regions of Dimorphos illuminated by Didymos shine appear light gray in (a). Arrows in (b) point to the dark limb of Didymos back-lit by DART ejecta. In (b), the +Z (N) direction is tilted by a few 10 s of degrees into the page. Dimorphos and DART ejecta can be seen in (b) at roughly 8:30 clock position from the center of Didymos.

Fig. 4. Geological features on Dimorphos.

Arrows indicate a boulder with fractures (a), a crater on boulders with evident spall (b), randomly orientated debris aprons (white arrows; c) rocks on boulders (orange arrows; c) multiple lineaments on a single boulder (magenta arrow; c), a possible camp-fire structure (white arrows; d) and a linear fabric (dashed lines; d) with aligned boulders (orange arrows; d) and surface lineaments (e, f). All images were collected by DRACO moments before the DART spacecraft collided with Dimorphos. The global view (e) shows the location of the DART impact (star), an asteroid-wide trough (upper set of arrows) and several asteroid-wide fractures. The trough (white arrow) and one of the fractures (orange arrows) are evident in oblique-views (f) of Dimorphos’ global digital terrain model.

Fig. 5. Spatial distribution of boulders on Dimorphos.

Total boulder distribution as a function of latitude (a) and longitude (b) on Dimorphos, for different boulder sizes. Errors bars show 1-σ uncertainty.

Fig. 6. Global slopes relative to gravity on Dimorphos.

All slopes include the gravitational effects of Didymos and Dimorphos’s assumed synchronous rotation. a Locations of boulders on Dimorphos (ellipses). The magenta ellipses indicate boulders located on other boulders. b A global slope map of Dimorphos with arrows showing the downslope direction. Regions that show no roughness were not part of the sunlit terrain seen by DART. c The global distribution of slopes weighted by surface area. The slope tail ($>$10° in (c) is dominated by the surface boulders; the rest of the distribution reflects the overall oblate ellipsoidal shape of the asteroid. d The distribution of boulders on other boulders, which exist on slopes $<$35°.

Fig. 7. Crater shapes and surface ages of Dimorphos, and surface ages of Didymos.

Geometric crater depth to diameter ratio (a) measured on Dimorphos, and crater size frequency distribution on Didymos (b) and Dimorphos (c). Errors bars show 1-σ uncertainty. Dotted red-line shows (a) $d/D$ results for Dimorphos from DTMs using the latest GDTM. Didymos’ GDTM was of insufficient quality to measure reliable $d/D$.

Fig. 8. Slope stability assessement of regolith on Didymos.

Didymos slope (a) and factor of safety (FS) for surface cohesion, $C=$0 Pa (b, c), 0.5 Pa (d, e) and 1 Pa (f, g), assuming a 10 m-thick unconsolidated layer with friction angle $\varphi$ = ${35}^{\circ }$. Arrows in d show plausible boulder track and landslide sources.

Fig. 9. Failure and reaccumulation of Didymos.

a Failure-mode diagram of Didymos for a density of 2800 kg/m³. These curves mark the minimum cohesion required to prevent structural failure in Didymos across different spin periods. The analysis considers three potential failure modes common to rubble-pile objects. Each curve is derived based on the corresponding failure criterion. Interior deformation that affects the entire asteroid occurs if C is below the red curve, which for Didymos requires an interior bulk $C\lesssim$ 10 Pa near its current spin rate of 2.26 h (vertical green line). Landsliding and mass-shedding (orange) can be initiated when the surface $C<$ 1 Pa. Mass shedding (blue curves) is derived by placing a 10 m-radius boulder on the surface that slides over a given distance under the local gravity. For 1 Pa surface cohesion and the current spin rate of Didymos, a boulder would need to roll $\gtrsim$100 m before it lifted off the surface by the Coriolis force. b A soft-sphere discrete element simulation of a Didymos-shaped rubble pile shows that the mass movement and redeposition of shed material onto the equatorial region could help to form a triangular ridge on Didymos.