Near-IR Spectral Observations of the Didymos System: Daily Evolution Before and After the DART Impact Indicates that Dimorphos Originated from Didymos

Abstract

Ejecta from Dimorphos following the DART mission impact significantly increased the brightness of the Didymos–Dimorphos system, allowing us to examine subsurface material. We report daily near-IR spectroscopic observations of the Didymos system using NASA’s IRTF that follow the evolution of the spectral signature of the ejecta cloud over 1 week, from 1 day before the impact. Overall, the spectral features remained fixed (S-type classification) while the ejecta dissipated, confirming that both Didymos and Dimorphos are constructed from the same silicate material. This novel result strongly supports binary asteroid formation models that include the breaking up of a single body due to rotational breakup of kilometer-wide bodies. At impact time +14 and +38 hr, the spectral slope decreased, but the following nights presented an increasing spectral slope that almost returned to the preimpact slope. However, the parameters of the 1 μm band remained fixed, and no “fresh”/Q-type-like spectrum was measured. We interpret this as follows. (1) The ejecta cloud is the main contributor (60%–70%) to the overall light during the ∼40 hr after impact. (2) Coarser debris (≥100 μm) dominated the ejecta cloud, decreasing the spectral slope (after radiation pressure removed the fine grains ≤10 hr after impact). (3) After approximately 1 week, the ejecta cloud dispersed enough to make the fine grains on Didymos’s surface the dominant part of the light, increasing the spectral slope to the preimpact level. (4) A negligible amount of nonweathered material was ejected from Dimorphos’s subsurface, suggesting that Dimorphos was accumulated from weathered material ejected from Didymos’s surface.

Achievement of the Planetary Defense Investigations of the Double Asteroid Redirection Test (DART) Mission

“…However, it has also been suggested that the gravitational interactions between slower ejecta and the Didymos binary system naturally can produce a secondary tail without the need for additional impact or disruption events (Ferrari et al 2024) or that the appearance of a secondary tail may be due to the projection of the ejecta cone and the viewing geometry of the observations . Spectral and color observations (Lin et al 2023;Polishook et al 2023;Ieva et al 2024) as well as polarimetry observations (Bagnulo et al 2023;Gray et al 2024) provide evidence that the ejecta excavated from Dimorphos share similar properties to Didymos as an S-type asteroid. Extended polarimetry observations show that an immediate drop in polarization was associated with DARTʼs impact event and that lower polarization persisted through all the subsequent months of observations; this suggests that smaller or brighter particles than the pre-impact surface were ejected by DART and that these particles continue to exist in the system, either in orbit or deposited on the surface (Gray et al 2024;Penttilä et al 2024).…”

“…The total mass of the binary system is constrained by the pre-impact orbit period; though, given that Didymos is significantly larger than Dimorphos, this does not provide a direct constraint on the mass of Dimorphos. Telescopic observations of the Didymos system taken after DARTʼs impact, during the following days when the signal from the Didymos system was dominated by ejecta from Dimorphos, indicate that Dimorphos, like Didymos, is an S-type asteroid (Bagnulo et al 2023;Lin et al 2023;Polishook et al 2023;Gray et al 2024;Ieva et al 2024;Lazzarin et al 2024). While these telescopic observations provide evidence that the composition of Dimorphos is the same as Didymos, the density of the objects could still be different depending on the macroporosity of the bodies.…”

Pre-impact Thermophysical Properties and the Yarkovsky Effect of NASA DART Target (65803) Didymos

“…Such a global layer would require ∼5 × 10 6 kg of material to be redeposited, which is ∼10%-40% of the estimated mass that was observed to be ejected by the DART impact (i.e., 1.3-6.4 × 10 7 kg; Roth et al 2023). Furthermore, no permanent spectral changes were observed on Didymos (Polishook et al 2023), which suggests that no system-wide resurfacing occurred at even thinner layers.…”

The Population of Small Near-Earth Objects: Composition, Source Regions, and Rotational Properties

“…An example of such a case could be the Didymos system, the target of NASAʼs Double Asteroid Redirection Test (DART) mission. Polishook et al (2023) obtained NIR spectra of Didymos before and after the impact. They found that the ejecta cloud released from Dimorphos was the main contributor to the light of the system for ∼40 hr after the impact.…”