The origin of pits on 9P/Tempel 1 and the geologic signature of outbursts in Stardust-NExT images

We consider the origin of ∼380 quasi-circular depressions (pits) seen to be distributed in a broad band across the surface of 9P/Tempel 1 and show that possibly ∼96% may be due to outburst activity. Of the rest, <4%, are probably due to a mix of cryo-volcanic collapse events and collisional impacts with asteroidal material. We estimate the mass ejected during the June 14, 2005, mini-outburst on 9P to be in the range (6–30) × 104 kg and find that the resulting pit should have a diameter in the range 10–30 m. Published locations of mini-outbursts are revised to account for changes in the nucleus shape, rotation rate, and rotation pole that have resulted from observations made during the Stardust-NExT mission. Both of these locations are found to fall in, or on the edge of, the band of pits that encircles the nucleus. We have identified features in high-resolution images near one of these locations as the possible places of origin of the mini-outbursts. These features show close packing of multiple pits in the appropriate diameter range.We consider the distribution of pit diameters and show that the largest pits follow a power–law with exponent −2.24 ± 0.09. Using the June 14, 2005, mini-outburst and the Deep Impact crater to provide a calibration, we establish empirical relationships between pit diameter, D, the total outburst energy, E, and the visual magnitude change, Δmabs, which is the visual amplitude of the outburst referenced to a standard initial brightness. We find Log10 D ∼ 0.107(±0.004)Δmabs + 1.3(±0.4) and Log10 E ∼ 0.32(±0.01)Δmabs + 10.1(±1.2) where the uncertainties represent the range of values for the coefficient rather than formal error. We apply these approximate relationships to the mega-outburst on 17P/Holmes and predict that it left a pit-like scar on the surface with a diameter in the range 160–1300 m, that the total energy released was in the range 7 × 1012–3 × 1015 J, and that between 6 × 107 and 1.3 × 1011 kg of material was ejected from the surface. While these predictions are crude they encompass, particularly near the upper end of the range, the results on kinetic energy release and mass loss found by Reach et al. (Reach, W.T., Vaubaillon, J., Lisse, C.M., Holloway, M., Rho, J. [2010]. Icarus 208, 276–292) based on IR observations of 17P.

► We demonstrate that the pits observed on the surface of comet 9P/Tempel 1 are due to explosive outbursts.
► Geologic features are identified on the surface that are probably related to outbursts.
► We develop empirical relations that allow the total energy released and the size of the pit formed in an outburst from the magnitude of the outbursts.