Comet Woes Lamented

Space.com posted an article on “The enduring mysteries of comets.”  The mysteries include:

• Where did earth’s oceans come from?  For a long time, scientists expected comets delivered the water, until measurements showed a discrepancy in the hydrogen/deuterium ratios.  It’s also highly improbable there was enough wet stuff in those assumed delivery vehicles.  No secular scientist knows where the water came from.  Ask how the following answer would score on a high-school science test: “Earth’s oceans are likely a mixture of water from all sorts of places, but the main-belt comets are very likely one of them.”
• Is there a comet reservoir?  The proposed Oort cloud cannot be the source of short-period comets at least.  Another high-school answer to score: “Maybe there are other reservoirs of comets yet to be discovered.”
• Why are they dirty?  Until recently, this was the expectation: “Comets were long thought to be primordial relics, pristine leftovers from the protoplanetary disk that once surrounded the newborn sun.  As such, it was supposed they might hold secrets untouched for billions of years regarding the birth of our solar system.”  Wrong.  Now that we have sampled Halley, Tempel 1 and other comets, we know they have unexpected material like carbonates, silicates and complex molecules that seem to have required high temperatures: “many of them are nearly burned-out hulks, with neither the size, mass, shape nor spin they might have had before entering the solar system.”
• How long can they bake?  How main-belt comets could have survived for billions of years is another mystery.  “Until their discovery, researchers had largely supposed no comets could have lasted that close to the sun without getting baked away after a few centuries or millennia.” 
• Where are the interstellar travelers?  Escaping comets from other stars should be coming into our solar system from all angles.  That’s a deduction from calculations that show 90 to 99% of our solar system’s comets get flung outward, never to return.  None coming in from neighboring stars’ Oort clouds have been detected.

Last month in Icarus,¹ doubt was cast on whether there really is a comet reservoir out beyond the planets.  Brasser, Duncan and Levison ran simulations of comets in a planetary disk and found that only very large chunks 20km in diameter and up would be flung outward into the hypothetical Oort cloud.  “This implies that the presence of the primordial solar nebula acts as a size-sorting mechanism,” they said, “with large bodies unaffected by the gas drag and ending up in the OC while small bodies remain trapped in the planetary region, in the models studied.”  This is a difficulty on top of the calculation in 2001 that 90% of the material would be destroyed by collisions (01/31/2001, 06/21/2002)

1. Brasser, Duncan and Levison, “Embedded star clusters and the formation of the Oort cloud II. The effect of the primordial solar nebula,” Icarus, Volume 191, Issue 2, 15 November 2007, Pages 413-433.

Other mysteries were not cited in the Space.com article, such as why any comets are left after 4.5 billion years.  The solar sparklers have a short lifetime (03/27/2003).  One might ask if a hypothetical reservoir that has never been observed is worthy of scientific respectability.  To what extent is it legitimate in science to rescue a theory from lack of data?
    It’s fun to watch materialists mutter about material matters.  The committee on comet-y objects commits more comedy than comity.