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An Examination of the Deep Impact Collision site on comet Tempel 1:
Placing Further Constraints on Cometary Surface Properties

By James Richardson and Jay Melosh

In this work, we have conducted a detailed study of the Deep Impact collision site on comet 9P Tempel 1 as imaged by the Stardust-NExT spacecraft in February of 2011. This investigation reveals a tentatively identified, barely resolvable, shallow impact crater located at the previously identified impact site, 49 +/- 12 m in diameter; surrounded by an area of slightly brightened material marking a potential ejecta blanket 85-120 m in diameter. Assuming that this crater identification is correct, we have used this diameter to place further constraints on the surface material struck by the impactor to yield crater scaling-law constants of mu = 0.45 +/- 0.05 (exponential constant), K1  = 0.15 +/- 0.05 (linear proportionality constant), and Y-bar = 1-10 kPa (effective target strength), using the expressions and nomenclature of (Ann. Rev. Earth & Plan. Sci., 1993). These material constants are roughly analogous to the material properties of a lightly-packed, ‘dry’ mountain snow. Additional modeling yields refined estimates for the total ejected mass produced by the DI event of 1.2 x 10^6 kg (5.4 x 10^5 - 2.6 x 10^6 kg), with 60% +/- 20% of that mass ejected at greater than the comet's escape velocity of 1.4 m/s, which produced a relatively thin ejecta blanket with a thickness at the final crater rim of 2.7 mm (0.5ñ13 mm). These values are consistent with previous analyses of the Deep Impact event's ejecta behavior as seen from the DI flyby spacecraft, as well as the observations conducted by other Earth-orbiting and Earth-based telescopes.

  composite view of the impacted region of comet Tempel 1
  View larger image

Figure 1: (Left) A composite view of the impacted region of comet Tempel 1 in July 2005, constructed from images taken by the Deep Impact (DI) Impactor Targeting System (ITS). The field of view is ~1 km on a side, with a central resolution of ~2-10 m/pixel and a solar elevation angle of 42 deg. (image provided by A. Delamere and D. Stern). (Right) A contrast enhanced view of the impacted region of comet Tempel 1 in February 2011, taken by the Stardust-NExT NAVCAM just before closest approach (~12 m/pixel resolution), with a solar elevation angle of 23 deg. In the lower panels, the green circle indicates the DI impact site contained within a ~100 m error radius. Arrows are used to highlight selected albedo features, and are color coded according to the visible change that has occurred to that feature between the two images (due to either morphological, lighting, instrument, or processing effects). White arrows indicate dark regions/shadows that have grown darker; Yellow arrows indicate light/bright regions that have faded or disappeared; Cyan arrows indicate light/bright regions that have remained or grown brighter; and Purple arrows indicate dark regions/shadows that have faded or disappeared. Note that the last (purple indicated) change only occurs only at or near the impact site, perhaps due to effects of the impact itself.

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“When we see comets up in the sky they're really spectacular. But unless you get close to a comet, you can't really figure out what's going on.”

-Joe Veverka