Near-infrared light curve of Comet 9P/Tempel 1 during Deep Impact

On UT 2005 July 4 we observed Comet 9P/Tempel 1 during its encounter with the Deep Impact flyby spacecraft and impactor. Using the SpeX near-infrared spectrograph mounted on NASA's Infrared Telescope Facility, we obtained 0.8-to-2.5 μm flux-calibrated spectral light curves of the comet for 12 min before and 14 min after impact. Our cadence was just 1.1 s. The light curve shows constant flux before the impact and an overall brightening trend after the impact, but not at a constant rate. Within a 0.8-arcsec-radius circular aperture, the comet rapidly-brightened by 0.63 mag at 1.2 μm in the first minute. Thereafter, brightening was more modest, averaging about 0.091 mag/min at 1.2 μm, although apparently not quite constant. In addition we see a bluing in the spectrum over the post-impact period of about 0.07 mag in J-H and 0.35 mag in J-K. The majority of this bluing happened in the first minute, and the dust only marginally blued after that, in stark contrast to the continued brightening. The photometric behavior in the light curve is due to a combination of crater formation effects, expansion of the ejecta cloud, and evolution of liberated dust grains. The bluing is likely due to an icy component on those grains, and the icy grains would have had to have a devolatilization timescale longer than 14 min (unless they were shielded by the optical depth of the cloud). The bluing could also have been caused by the decrease in the “typical” size of the dust grains after impact. Ejecta dominated by submicron grains, as inferred from other observations, would have stronger scattering at shorter wavelengths than the much larger grains observed before impact.