Deep Impact and Stardust Mission Predictions and Results

The Deep Impact mission flew close to Comet Tempel 1 on 2005 July 4 and released a probe to smash into the comet’s nucleus to reveal what was below the surface. The dirty snowball for comets outlined many possible scenarios that might result, making no definite, generally-agreed predictions. The eph made the following specific, unique predictions: (1) an impact flash because the surface (under the regolith) would be a hard asteroidal rock; (2) a dusty regolith from tidal decay of orbiting debris; (3) a small, shallow impact crater, probably just 10-20 m in diameter because of the strength-dominated surface under the regolith; (4) no new “jet” from the impact because so-called comet “jets” are really flashlight beams through the dusty coma caused by the opposition effect acting on high-albedo surface spots; (5) no lasting effect on the comet because of its rocky nucleus; (6) a planet-like or moon-like chemical composition; (7) a nucleus indistinguishable from an asteroid by any observation type. [[16]]

These predictions are pretty much a description of the mission results, with one exception. The impact crater that would have decided between a gravity-dominated vs. a strength-dominated comet nucleus was not seen because so much dust was kicked up from the regolith. The mission team concluded that the comet was probably gravity-dominated based on the behavior of the dust, which obviously is gravity-dominated. (Dust has almost no intrinsic cohesive strength.) But there is nothing compelling about that interpretation. Meanwhile, the surface colors and reflectivity indicate little or no evidence for surface snow or ice, which certainly challenges the entire basis for the dirty snowball concept. And as we noted in Figure 1, the comet nucleus was visually indistinguishable from an asteroid. Other impact craters are preserved on the surface, indicating a certain degree of cohesive strength much greater than that of dust. [[17]]

The Stardust mission flew through the coma of Comet Wild 2, collected dust particles, and returned them to Earth for analysis. They were found to contain magnesium, olivine, calcium, aluminum, and titanium, all of which have high formation temperatures in excess of 1000° C. Moreover, such minerals are usually associated with volcanic outputs from deep inside planets. The dirty snowball model, in which comets formed far from the Sun in a very cold environment and are supposed to be primitive, unprocessed bodies, is still struggling with these results. One suggestion was bipolar jets from the early Sun, reaching out to great distances and enriching the distant solar nebula with hot minerals.

In brief, the standard model indicates that asteroids were bits of a planet that never formed, and comets are left-overs from the primeval solar nebula. Both should be single bodies because the conditions for satellite formation are extremely unlikely to be realized. Asteroids should be mostly rock, whereas comets should be mostly water-ice and never heated above 100° C. By contrast, the eph indicates that asteroids and comets are initially clouds of all sizes of debris orbiting a dominant rocky nucleus. Both start with volatiles, but the volatiles in asteroids get baked away by their prolonged proximity to the Sun. While both have water, it is interstitial. And they were subjected to formation temperatures in excess of 1000° C. It is now evident that only the eph expectations match reality.

While it seems trivial to note that creative theoreticians can come up with a model patch to explain virtually any unexpected observation, we all know instinctively that painting a bull’s-eye around an arrow is not a skill deserving of much credit, in archery or in science.