  The most dramatic phases of terrestrial planet formation are thought to be
oligarchic and chaotic growth, on timescales of up to 100-200 Myr, when violent
impacts occur between large planetesimals of sizes up to proto-planets. Such
events are marked by the production of large amounts of debris as has been
observed in some exceptionally bright and young debris disks (termed extreme
debris disks). Here we report five years of Spitzer measurements of such
systems around two young solar-type stars: ID8 and P1121. The short-term
(weekly to monthly) and long-term (yearly) disk variability is consistent with
the aftermaths of large impacts involving large asteroid-size bodies. We
demonstrate that an impact-produced clump of optically thick dust, under the
influence of the dynamical and viewing geometry effects, can produce short-term
modulation in the disk light curves. The long-term disk flux variation is
related to the collisional evolution within the impact-produced fragments once
released into a circumstellar orbit. The time-variable behavior observed in the
P1121 system is consistent with a hypervelocity impact prior to 2012 that
produced vapor condensates as the dominant impact product. Two distinct
short-term modulations in the ID8 system argue for two violent impacts at
different times and locations. Its long-term variation is consistent with the
collisional evolution of two different populations of impact-produced debris
dominated by either vapor condensates or escaping boulders. The bright,
variable emission from the dust produced in large impacts from extreme debris
disks provides a unique opportunity to study violent events during the era of
terrestrial planet formation.
