  Ejecta from Dimorphos following the DART mission impact, significantly
increased the brightness of the Didymos-Dimorphos system, allowing us to
examine sub-surface 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 one week, from one day
before the impact. Overall, the spectral features remained fixed (S-type
classification) while the ejecta dissipated, confirming both Didymos and
Dimorphos are constructed from the same silicate material. This novel result
strongly supports binary asteroid formation models that include breaking up of
a single body, due to rotational breakup of km-wide bodies. At impact time +14
and +38 hours, the spectral slope decreased, but following nights presented
increasing spectral slope that almost returned to the pre-impact slope.
However, the parameters of the $1~\mu m$ band remained fixed, and no "fresh" /
Q-type-like spectrum was measured. We interpret these as follow: 1. The ejecta
cloud is the main contributor ($60-70\%$) to the overall light during the
$\sim40$ hours after impact. 2. Coarser debris ($\geq 100~\mu m$) dominated the
ejecta cloud, decreasing the spectral slope (after radiation pressure removed
the fine grains at $\leq10$ hours after impact); 3. after approximately one
week, the ejecta cloud dispersed enough to make the fine grains on Didymos
surface the dominating part of the light, increasing the spectral slope to
pre-impact level. 4. a negligible amount of non-weathered material was ejected
from Dimorphos' sub-surface, suggesting Dimorphos was accumulated from
weathered material, ejected from Didymos surface.
