NOVA; To the Moon; Footage of a roundtable conversation among lunar scientists, part 1 of 2

How this so at least the inner solar system got put together it makes physical sense is beginning to make chemical sense actually I'm curious great you study the rocks does it make any petrological sense to the rocks get in the system with us I've been getting the opinion that it doesn't all make consistent sense yet and there's a lot of problems that have got to be worked out I don't know which way they're gonna go to to work them out what kind of problems are you seeing well particularly geochemical problems the isotopic problems just don't understand how these things are well they're as messy it's so complicated that way right when I see many different kinds of isotopes being studied it's like you you you end up looking at so many different possibilities that you can't see the forest for the trees in a set everybody seems to worry about different isotopic signatures in different parts of the moon but why aren't those explained by the the necessary fact that you you you're going to have several big impactors coming in after the really big impact I mean the moon's going to get lots of 150 and 200 kilometer plan of Thessimos coming into it and why doesn't that give you the option of

explaining a lot of isotopic variety isn't too many options you can't have too many well too many choices that you can't decide which is the correct sort of option right now it seems like there's too many inconsistencies one line of evidence says one way and another says another we haven't reconciled them all yet isn't the problem that you know we're looking back through four and a half billion years of solar system history we sit on our planet today we have rocks picked up from the moon today we're trying to figure out what happened during that early formation time and we're inevitably forced to go to very simple ideas you know I've made the joke occasionally where does an egg come from consider a spherical chicken the problem is we got feathers that we're finding and they don't fit on a spherical chicken we've got three feathers we've been to the moon a few times we brought back a few hundred clout you know kilograms of rocks it would be we work those rocks over and over again I mean in a sense we don't truly understand the moon and we can't from the samples we have just you're saying there's too few samples or too few sites well if we could really explore the moon the way geologists have explored the earth for

centuries now I mean we'd have a much better picture how it evolved and how it fit it and I bet it does fit it the real problem is if you did the same thing for the earth you went to a total of nine sites and they had some random rocks as such as we have delivered as meteorites you'd have an incredibly imperfect picture of what the present day earth looked like and we're to some extent deluding ourselves into thinking we understand what the moon and other planetary bodies look like on the basis of this phenomenally tiny sample that in many cases we really don't know the provenance of we don't know where it came from in detail I sort of encouraged I I'm not an experimentalist but I I think the sophistication of the kind of experiments that we heard about today is is a is mind-boggling at those incredible temperatures and pressures of things that are being measured and the sophistication of the computer simulations that can be done now are are phenomenal as well I mean it a sense we fool ourselves in thinking that we can figure it all out but it they just compared a few years ago it's incredible it's remarkable and the thing that likes it that everybody's kind of accepting an internally consistent view

of what's going on in the early solar system so we're all I mean the efforts are kind of more coordinated than they were ten years ago even of course the planet formation models now you're coming out with a stage of giant impacts no matter what you start with it seems inevitable that you go from the system of tens of roughly maris eyes bodies to the final planets that even multiple impacts could likely have been involved in the late and final stages that's making everybody think about the consequences of these impacts for the origin of life and the origin of continents and also gives you more free parameters and it makes things more complicated if you don't have just one giant impact if you had a post impact if you had a pre-big impact what you're seeing I think developing is that just as planet formation appears to be a natural consequence of star formation we're finding the formation of the moon is a natural consequence of the formation of the earth that has implications for the temperature of the early earth when did for example liquid water becomes

stable it has implications for the origin of life why we're even here having this discussion today and it's but it is a self-consistent picture that's beginning to evolve that in a sense everything is an accident of star formation including themselves don't you think there's too much or don't you think there's still a tendency for people to sort of glum onto one end of the mass spectrum or the other at this meeting I mean I I keep feeling like I hear some people start talking about while we can add you know veneers of all this what I picture they're thinking of like sand grains you know a big fall out of small stuff and then somebody else will come along well we can have a few middle-sized impacts or or somebody else is talking in terms of explaining everything in terms of one big impact and you can't get you can't get one big impact without having lots of little impacts and conversely you can't have a lot of little ones without getting a big one so people have to get a more

integrated way of realizing that along with all the small regular rising small impacts are a few big impacts and is that message really getting through yet it's it's an interesting question I'd be interested in what what everybody else thinks about this but my perception is we have a tendency to try a one-size-fits-all approach to science because it's simple it's something that's tractable and in practice nature is far more complicated and far more clever than we are there is a simplifying thing here that there is this size distribution and if you get one big impact you get a handful of palace size vesticize impacts and then the gazillion little tiny impacts the relation show that things can grow at that large end to a rather sizeable body so you could have some large impacts and a lot of small debris but that's sort of a missing zone where you wouldn't necessarily have the intermediate size because there isn't much mass in there because things have grown and accumulated in those but here we're talking about the kinds of things that did happen or the kind of things that

could happen we're in a historical science what we really want to know is what specific things did happen and we're into a stage where we don't really have the evidence for the earth or the moon right at the start we're looking at a moon that we have things preserved from for four billion years but not for 4.6 and similar for the earth there was a big conceptual jump for our field though to to even get into what you're calling the kinds of things that could happen because at at the beginning say in 75 there was this very strong reaction you can't have big catastrophes we shouldn't even be doing theoretical work on that incredibly skeptical yeah yeah yeah you were one of the guys no but but I like his theory better now right and but you don't I mean what you're doing now in terms of modeling lots of impacts on the growth of the moon out of things orbiting around the earth I mean you don't you don't face people saying no no this is a you know this is a crazy thing to do this isn't uniformitarian this is no actually I get the opposite question and that is you

mean no one until now is actually formed the moon after the giant impact theory this is the leading theory and people haven't done this yet the question is why did it take so long oh I think a couple of people came up with the idea of the impact but not the accretion of them but I think that's fairly simple to understand I mean back at the time 20 years ago when Bill first came up with this theory think about what computers were I mean you know the Macintosh was still 10 years away as an example it wasn't until 1990 we could reliably simulate the interiors of planets at very high pressures and temperatures it's in case of Robin's work is fundamentally the advances of computational power the advance of technology that has allowed us to now go after some problems and I think we're intractable before that time either because they took far too long with existing computers or the memory simply wasn't big enough it's also a question of context we didn't know a lot about the whole rest of the solar system and I made 70 teeth explored a lot of it at least to zero order and it all makes sense in that that there should have been a lot of big impacts on all the satellites of the giant planets and

huge objects colliding together to make the giant planets it just goes on and on the very fact that asteroids are now discovered to have little satellites around them is tying into this because it looks like debris from those collisions can go into orbit and leave satellites around satellites or around asteroids do you think the same thing is true of our ability to study the rocks I mean you just said the computer technology's better and so we can make better miles but can we do more with the rocks now than we do to be careful about what you mean by rocks there's the kind of rocks Graham studies which are the physical samples of the moon or the earth that have been subjected to incredibly complex processes and the key I think is to understand to the best of your ability what those processes are and then find isotopic elemental physical indicators that have somehow been insensitive to all those complicated processes that have gone on so you can see back through solar system history to 4.5 billion years ago that's an astonishing thing when you think about it seeing back through 4.5

billion years of solar system history that's what we're really doing we're being time-travelers if I could just interrupt you guys are doing fabulous could just urge you though where do you want to be 20 years from now I mean where do you what do you want to do what needs to happen to move you along the next step what do you want to have okay let's nail Robin with this one show hey Robin what do you think we're gonna be 20 years from now with these calculations about how you really put a moon together after a giant impact where I'd like to be is to have a consistent model so we can reasonably well explain our solar system with our giant planets so that we can have some hope at looking at the systems of extra solar planets where we can only detect the large planets and we can say is it reasonable there's an earth-like planet with a moon in those systems theoretically of course that's the holy grail being to to model being

able to model our solar system and extrapolate that's where we want to point and much more advanced you know telescope and space and it can enter from error metric space interferometer that would actually be able to see Jupiter's and Earth's and Uranus's around their stars and it would help to know what we think are good candidates if you can make very basic dynamic arguments for the architect