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Saturday, August 23, 2008

Bottke: Lunar Basins May Provide Key to Solar System Evolution

The Southwest Research Institute 's William F. "Bill" Bottke provided a quite interesting response to former Apollo 17 lunar science astronaut Harrison "Jack" Schmitt questions on the Lunar Listserv Saturday afternoon.

Here are the questions and Bottke's responses.

1) What was going on with planetesimals located between the orbits of Jupiter-Neptune and those outside Neptune from the time of Jupiter's formation and the time of the putative Nice model (i.e., outer solar system reconfiguration)?

Bottke: "This is very interesting question, one that would require a fair amount of modeling work to really probe the issue. Nevertheless, we can say some things based on current work.

Our best estimates suggest the zone of planetesimals between Jupiter and Neptune was quickly depleted by planet formation processes. They probably do not make any basins on the Moon. However, a large disk of survivors outside the orbit Neptune was left relatively unaffected.

Neptune, once formed, ended up interacting with the inner edge of this leftover disk of comets, such that some comets were scattered onto planet-crossing orbits over hundreds of My. A small fraction of these objects could have struck the Moon (and produced some early basins, along with other sources). These scattering events also caused the gas giants to migrate. If the Nice model is correct, this migration, after 600 My or so, ended up being the trigger for a system wide instability, the sudden depletion of comet (and asteroid) reservoirs, and the "lunar cataclysm".

2) What is the evidence for a system-wide instability?

Bottke: "There is a lot to say here, such that it may be worthwhile for those interested to track down the original Nice model papers in Nature from 2005 (Gomes et al., Tsiganis et al., Morbidelli et al.). Much of the evidence comes from modeling work; without turning many "knobs", the Nice model is able to reproduce the following:

-- The semimajor axes and small e,i values of the gas giants. Note that models of gas accretion on the Jovian planets indicates they should be on circular, zero inclination orbits after formation.

-- The inclination distribution and approximate population size of the Trojan asteroids. The origin of the Trojans, particularly the fact that many have 10 deg <>

-- The population of irregular satellites.

-- The orbital distribution of the Kuiper belt.

-- The timing and approximately impactor mass needed to make the late basins on the Moon.

-- Possibly some interesting features of the asteroid belt.

With that said, though, this is not a solved problem by any means, and the Nice model is simply that -- a model. There is still much to be done across a range of fields to confirm what actually happened -- with the most critical constraints provided by the Moon and lunar samples."

3) Can the early basins come from a cataclysm?

Bottke: It is hard to say until more modeling work is done. Current simulations indicate that the Nice model lunar cataclysm, made up of impacting comets and then impacting asteroids, only lasts about 150-200 My, with the starting time assumed to be around 4.0 Ga. So, if some lunar basins are older, they may indeed have come from alternative sources.

4) Could we have a bi-modal or even a multi-modal distribution of basins?

Bottke: "Sure. Until all of this has been modeled and compared with constraints from the Moon and elsewhere, I do not think anything can or should be ruled out. The lunar basins still have a lot to tell us about how the solar system evolved. That is why we need to go back! :-)"

1 comment:

Joel Raupe said...

Your descretion in filtering this from the thread on Lunar-L is appreciated.

How fortunate it is that we have such a Moon as this one, a time capsule upon which may be written this history of ~4.527 Ba.

I found the comments on the thread more than interesting, but illustrative of what the models are telling our planetary scientists about drop-off of bombardment in the ongoing history of this star-system.

Senator Schmitt's cautious approach toward making final conclusions on the timing and clustering of an apparent spike in the Mascon-forming basin at 3.85 Ba shows wisdom. There is evidence, and it's strong. He's not one to hesitate about offering interesting ideas, either, such as his paper a few years ago about the possibility of a nearside megaimpact. Considering the obvious number of planet-splitting impacts our Moon has apparently endured, it's amazing we still have a Moon at all.

Of course, we wouldn't have the Moon we have had those events not happened. Amazing, isn't it?