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The Deep Impact Crater on 9P/Tempel-1 from Stardust-NExT

By P.H. Schultz, B. Hermalyn, & J. Veverka

Comparison between clear images of the pre-impact surface (ITS camera on the Deep Impact probe) and the ejection sequence over 15 minutes (Deep Impact flyby) allows inferring the size of the DI crater and its fate. Four key observations constrain the DI crater.

  1. A 50 m-diameter pit (recognized in multiple images and in stereo) occurs at the projected impact site.
  2. A surface features (small hill) identified in pre-impact images from the DI probe is missing near the impact site and indicates excavation well beyond the 50m-diameter pit.
  3. An uprange ejecta ray observed from DI correlates with position of the missing small hill and evolves in a manner consistent with the crater excavating the feature.
  4. Downrange, a small gap of the ejecta curtain downrange correlates with disruption of a surface feature downrange from the impact point.
  Comparison of common features identified in Stardust-NExT image at left (frame n30035) and the composite ITS image at right.
  Comparison of common features identified in Stardust-NExT image at left (frame n30035) and the composite ITS image at right. In order to enable comparisons, the vertical scale is preserved in deference to the horizontal scale. This is necessary due to extreme foreshortening created during approach by the DI probe (right). Small cross (black) indicates the best-determined location for the DI impact. The small mound adjacent to the impact site in the pre-impact image (right) appears highly degraded in the Stardust-NExT image (left). Small bright patches (likely ice) occur prior to the impact (noted by A, B, C, D at right) have disappeared in the Stardust-NExT image (left).
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As a result, we propose two scenarios. First the DI impact produced a large (but shallow) 180m-diameter excavation crater surrounding small pit, corresponding to the strength-controlled penetration pit. A low-relief rim is barely discerned in the Stardust-NExT images. Similar “nested” craters occur on the lunar maria where the outer rim represents excavation of the lunar regolith and inner crater formed in the competent basalt below.

Second, the DI crater produced a large crater (consistent with the observed features missing in Stardust-NExT images) but collapsed either soon after formation or after one revolution around the sun. In the first interpretation the outer limit of excavation is consistent with a porous surface layer. In the second interpretation, the final crater does not preserve the initial appearance but provides insight into the fate of impacts on the surface. Regardless, both conclusions are consistent
with estimates for the observed amount of amount of ejecta observed during the DI
collision by telescopic observations.

 

 

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