Catching up with Earth and Planetary Science Letters, vol. 642 (15 Sep):
Rider-Stokes, B.G. Stephant, A. Anand, M. et al. Evidence against water delivery by impacts with… Article 118860 .2024.118860
Bekaert, D.V. Caracausi, A. Marty, B. et al. The low primordial heavy noble gas and 244Pu-derived…
Article 118886 .2024.118886
The question of water delivery to dry planets- at the start of the Solar System- is NOT what it appears. What appears to ‘make sense’ is that, at the start, the inner Solar System planets were baked dry, then re-hydrated by impacts from comets (which are icy bodies). How many things are wrong with this mental model… aside from people assuming, of course, that their mental models are valid and accurate depictions of a cosmos that didn’t ask what you think ‘makes sense’? 1) Delivery by comets- from the outer Solar System, using highly-eccentric and typically inclined transfer orbits- is awkward and inefficient, like a bucket brigade. A long bucket brigade, of small buckets. 2) Impact by comets- in those steeply-crossing orbits- does not result in significant deposition. Because of those steep orbits, comets impact at some hypervelocity; the kinetic energy of such a dramatic crossing heats the impact site and impactor material alike. And hypervelocity, hyper-heated comet material boils off its water… it boils it back to space again, and little water gets transferred. 3) Outer-Solar-System water has a different isotopic blend, which doesn’t match Earth water. That is, the ‘Comets Brought Our Water’ mental model doesn’t jibe with observations of reality. 4) Per Rider-Stokes et al., other lines of evidence- that is, corroboration with broader investigations- don’t jibe, either. And let’s not forget 5) Per the Giotto space mission, the Deep Impact space mission, and the Rosetta space mission, we have seen that comets are not as icy as we thought. We see a coma that consists of “stuff” boiling off the comet nucleus. What we had suspected- and these multiple, broader investigations had found- is that this “stuff” is one part of the comet’s composition. The fact that we see it coming off the nucleus is because that’s what comes off the nucleus when it’s heated, not because that’s fully representative of the nucleus. In other words, there’s a selection bias. Just because warmed bread gives off a little steam does not mean bread is made of steam.
…Aaand speaking of broad evidence: heavy noble gases are unreactive, and act as tracers for the history of their parent deposits. Bekaert et al. give further data on the gas makeup of rocks from Earth’s mantle. The conversation thus circularizes: if comets brought our water, then Earth should contain lots of other ‘comet stuff’ besides water. Comets also contain krypton, xenon, etc. so 4a) Earth’s blend of noble gases doesn’t particularly match comet noble gases. Now to be sure, we don’t have lots of decimal places on what ‘comet stuff’ is (Rosetta certainly helped, but…). And we have lots of decimal places on Earth’s crust, some on Earth’s upper mantle, and few decimal places on Earth’s lower mantle and core. So thanks, Bekaert et al.; we need to know better what’s inside Earth, so we can better compare it with ‘comet stuff’, ‘asteroid stuff’, etc.