The origin of pits on 9P/Tempel 1 and the geologic signature of outbursts in Stardust-NExT images Michael J.S. Belton a,⇑ , Peter Thomas b , Brian Carcich b , Andrew Quick b , Joseph Veverka b , H. Jay Melosh c , Michael F. A’Hearn d , Jian-Yang Li d , Donald Brownlee e , Peter Schultz f , Kenneth Klaasen g , Gal Sarid h a Belton Space Exploration Initiatives, LLC, 430 S Randolph Way, Tucson, AZ 85716, USA b Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, USA c Department of Earth and Atmospheric Sciences, Purdue University, Lafayette, IN 47907, USA d Department of Astronomy, University of Maryland, College Park, MD 20742, USA e Department of Astronomy, University of Washington, Seattle, WA 98195, USA f Department of Geological Sciences, Brown University, Providence, RI 02412, USA g Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA h Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA article info Article history: Available online 29 March 2012 Keywords: Comets, Nucleus Comets, Coma Comet Tempel-1 abstract 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)  10 4 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 mis- sion. 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 appropri- ate 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 cal- ibration, we establish empirical relationships between pit diameter, D, the total outburst energy, E, and the visual magnitude change, Dm abs , which is the visual amplitude of the outburst referenced to a standard ini- tial brightness. We find Log 10 D  0.107(±0.004)Dm abs + 1.3(±0.4) and Log 10 E  0.32(±0.01)Dm abs + 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  10 12 –3  10 15 J, and that between 6  10 7 and 1.3  10 11 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. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction The recent encounter of the Stardust-NExT spacecraft with the nucleus of Comet 9P/Tempel 1 (Veverka et al., 2012) expanded the high-resolution imaging coverage already available from the Deep Impact mission (A’Hearn et al., 2005) to 70% of the surface reveal- ing a wide range of landforms in various stages of sublimational evolution (Thomas et al., 2012). The dominant features include out- crops of deep-seated layers, two new examples of smooth terrain, and a large population of quasi-circular depressions (hereafter called ‘‘pits’’) that are arranged in a broad band that encircles the nucleus. In earlier work we showed how layers and smooth terrain maybe related to cryo-volcanic activity and/or to primitive pro- cesses during the accumulation phase (Belton et al., 2007; Belton and Melosh, 2009). Here we present evidence that the origins of the majority of the pits are cometary outbursts. Hughes (1975, 1991) concluded that cometary outbursts are not rare and that they occur episodically on all comets. Within 7.5 AU 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.03.007 ⇑ Corresponding author. Fax: +1 520 795 6286. E-mail address: mbelton@dakotacom.net (M.J.S. Belton). Icarus 222 (2013) 477–486 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus