Water ice and dust in the innermost coma of comet 103P/Hartley 2

• Presence of water ice grains in the innermost coma of 103P/Hartley 2.
• 1 μm water ice particles, thermally and physically decoupled from the dust.
• Water ice grains are likely aggregates.

On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI) successfully encountered comet 103P/Hartley 2, when it was at a heliocentric distance of 1.06 AU. Spatially resolved near-IR spectra of comet Hartley 2 were acquired in the 1.05–4.83 μm wavelength range using the HRI-IR spectrometer. We present spectral maps of the inner ∼∼10 km of the coma collected 7 min and 23 min after closest approach. The extracted reflectance spectra include well-defined absorption bands near 1.5, 2.0, and 3.0 μm consistent in position, bandwidth, and shape with the presence of water ice grains. Using Hapke’s radiative transfer model, we characterize the type of mixing (areal vs. intimate), relative abundance, grain size, and spatial distribution of water ice and refractories. Our modeling suggests that the dust, which dominates the innermost coma of Hartley 2 and is at a temperature of 300 K, is thermally and physically decoupled from the fine-grained water ice particles, which are on the order of 1 μm in size. The strong correlation between the water ice, dust, and CO2 spatial distribution supports the concept that CO2 gas drags the water ice and dust grains from the nucleus. Once in the coma, the water ice begins subliming while the dust is in a constant outflow. The derived water ice scale-length is compatible with the lifetimes expected for 1-μm pure water ice grains at 1 AU, if velocities are near 0.5 m/s. Such velocities, about three order of magnitudes lower than the expansion velocities expected for isolated 1-μm water ice particles (Hanner, 1981 and Whipple, 1951), suggest that the observed water ice grains are likely aggregates.