Date: January 5, 2011

Title: Planetesimal-Driven Migration

Podcaster: Dr. David Minton and Nancy Atkinson

Organization: NASA Lunar Science Institute (NLSI) – http://lunarscience.arc.nasa.gov/

Description: The early solar system looked nothing like our planetary neighborhood today that we know and love. It was a violent place with mountain-sized and even planet-sized piles of rock and ice coming together in collisions, or sometimes, these planet in the making, called “planetesimals” could even have a gravitational effect on other bodies, causing them to move around like dancers on a dance floor, shifting the orbits of objects around the Sun. David Minton from the Southwest Research Institute in Boulder, Colorado has been working on creating models of what he calls planetesimal-driven migration and he explains how visiting the Moon could tell us even more about our solar system’s storied past.

Bio: The NLSI brings together leading lunar scientists from around the world to further NASA lunar science and exploration.

Dr. David Minton is a graduate of the Department of Planetary Sciences at the University of Arizona, and is currently a research scientist at the Southwest Research Institute in Boulder, CO studying planetary dynamics.

Nancy Atkinson is a science journalist and is the Senior Editor for Universe Today

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Transcript:
Planetesimal-Driven Migration by the NASA Lunar Science Institute
Voice: You are listening to the NASA Lunar Science Institute podcast which highlights the latest news information of the Moon, on the Moon and from the Moon. It is produced from the NASA Lunar Science Institute at the Ames Research Center in Moffett Field, California.

Nancy: This is Nancy Atkinson for the NASA Lunar Science Institute. The early solar system looked nothing like our planetary neighborhood today that we know and love. It was a violent place with mountain-sized and even planet-sized piles of rock and ice coming together in collisions, or sometimes, these planet in the making, called “planetesimals” could even have a gravitational effect on other bodies, causing them to move around like dancers on a dance floor, shifting the orbits of objects around the Sun. David Minton from the Southwest Research Institute in Boulder, Colorado has been working on creating models of what he calls planetesimal-driven migration and he explains how visiting the Moon could tell us even more about our solar system’s storied past.

I’m with David Minton who is studying the bombardment history of the Moon. Could you tell us about the work that you are doing.

Minton: Right now I’m actually looking at how you actually make terrestrial planets, like Earth, Venus Mercury and Mars. I know this doesn’t necessarily sound like its related to the Moon, but I can relate this to the Moon!

So, one really outstanding problem in terrestrial planet formation is Mars. Mars is too small. Everybody who does simulations of how you form terrestrial planets always ends up with a Mars that is 5-10 times bigger than it is in real life. It has always been a puzzle to why Mars is so small. So myself and Hal Levinson have has been looking at new some models for making Mars, and one of the consequences of these new models for making Mars small has resulted in something that could be very important for the Late Heavy Bombardment later.

So, to sort of recap, we think that terrestrial planets formed very quickly within the first 50-100 million years of the solar system’s history and the Moon was formed out of an impact between a Mars-sized object and the proto-Earth somewhere between 50-100 million years, it was sort of the last event of planet formation. Much later was the Heavy Bombardment, so several hundred million years we think that the Late Bombardment, ended 600 – 700 million years after planet formation ended. So that is sort of a separate period of solar system history.

In order to make Mars small, one idea that we had, and a problem that a lot of planet simulations have had to date is that they have been too low of a resolution to capture what we think is a very important process in planet formation which is called planetesimal driven migration. It turns out planetesimal driven migration is also thought to be responsible for the Late Heavy Bombardment; it was what drove the migration of the giant planets, and we think the giant planets formed in a more compact formation then migrated to a their present location due to interactions between a really massive disk of icy cometary objects. So what we are actually doing is trying to apply the same physics of PDM to forming planets.

This idea we have is that, imagine you have this disk out of which the planets eventually formed. It is a disk that is basically like the rings of Saturn, but instead of it being a little disk around Saturn, it is a gigantic disk around the Sun. And instead of tiny icy things as in Saturn’s case, you have these big, basically asteroidal things, big objects. Those objects collide together and build up planets. When a planet gets big enough and is sitting in a a “sea” of smaller planetesimals, like something the size of a moon embedded in a disk full of objects that are much smaller, like the size of Lutitia, the asteroid that Rosetta mission just flew by – so about a 100 km sized object.

You have basically a moon-sized embryo that is forming by grabbing up all these planetesimals and getting bigger and bigger and embedded in this big huge sea of tiny little things. Every time these little things encounter the big thing, it actually causes a little nudge in the position of the big thing. It turns out if you work out the math, if there is any sort of slight imbalance to the number of objects encountering on the sunward side versus encountering on the anti-sunward side, you can actually cause a net movement of the big body.

You take this lunar sized embryo that is encountering all these little planetesimals and it will start to move within the disk – it won’t stay in one place but will actually scoot along the disk. The way we think it might work in the case of Mars is, imagine these planetary embryos for preferentially, say, in the Earth-Venus zone.

Then you have a little lunar sized guy growing to become a Mars-sized, it would start migrating because of PDM, and it scoots away from the other guys. So it has kind of left the pack, and as it moves through the disk, it gets stranded way out, away from where all the action is going on. So you can imagine that Mars is sort of an embryo that got pulled away from where all the other embryos were growing because it was embedded in this disk and basically got migrated out. So Mars’ growth got stalled at a Mars size because it basically migrated away.

It turns out that – and this works pretty well – we’ve been doing a lot of math and the migration is pretty rapid and you could migrate through the disk before any Mars guys could form. In an early solar system where you have a Mars stranded off at the edge of the disk at 1.5 AU, which is where it is right now and all the other action going on in the Earth-Venus zone, and Earth and Venus were able to grow to the size they are now, and they are roughly the same size and mass and Mars is sort of stranded on its own.

But during this migration, the migrating Mars can actually pick up planetesimals in its resonances. It is not at all obvious why that is, but the same thing is thought to have happened in the outer solar system which is what gave Pluto its orbit. We think Pluto was actually picked up in the 3:2 resonance with Neptune when Neptune migrated out, and that’s why Pluto and the other “Plutinos” livng in these resonances with Neptune. That means Pluto goes around the Sun three times for every 2 times Neptune does. The Plutinos are other Kuiper Belt objects where Pluto is. There is also Two-tinos [which are caught in a 1:2 resonance with Neptune – and which are found towards the outer edge of the Kuiper belt], so you have these lines of resonances where it is almost like a snowplow, as Neptune migrated out it picked up all these little icy things and Pluto is one of them.

This also could have happened to Mars, and as Mars migrated through the disk it would have also picked up little objects. I’ve decided to calls these Marstinis, to keep in the Plutino, Two-tino: Marstinis. I don’t know if that will stick or not.

But the interesting thing about the Marstinis is that, for instance, one of the problems is the resonance location for Kuiper Belt objects. It turns out where the 3:2 resonance with Mars is actually in a very unstable zone. There is actually a resonance there with Saturn that only existed in the time of the Late Heavy Bombardment, so before that, Saturn we think was in a different position, so this particular resonance was in a different position. So it was only after the giant planets migrated to their current location that this resonance location became unstable. So we think that these Marstinis would have been stable and in that interim period between the end of planet formation and the Late Heavy Bombardment, all of a sudden this region became unstable when the planets shifted positions and everything got to its current location, that whole region became unstable. We have reasons to think that the objects that hit the Moon during the LHB were sort of like asteroids but not exactly like the asteroids we have now. So, there is some chemical arguments you can make, also you can make some arguments from the impact probabilities that may not have been enough mass in the asteroid belt to supply all the asteroids and impacts we see on the Moon.

At the same time, the size distribution of craters on the Moon makes it look like the same size distribution of asteroids in the main asteroid belt. We think that size distribution is sort of the relative numbers of big objects and small objects have a shape, and you see that shape in the craters when you count up all the craters on the Moon. So we think the late heavy bombardment, at least the craters we see are dominated by asteroids but they have a slightly different chemical signature than the asteroids we currently have in the asteroid belt. So, what sets that size distribution has to do with the location – what they are made of, the rocky things that are in the 2 AU area where the asteroid belt is are going to have some more size distribution.

So this idea of Mars migrating, pushing all these little objects into resonance with Mars and that being responsible for the LHB kind of works because these seems to be a little unusual because they come from planet forming regions, closer to the sun than the asteroid belt is now. These things were pushed out from the planet forming regions out to the asteroid belt. They are in a region that was unstable of the LHB, so they would have been stable then, and all of a sudden the planets migrated and this whole region became unstable and so they all go flinging into the inner solar system and end up hitting the Earth.

Nancy:And as promised, Minton then explained how the Moon is important for continuing to understand the early days of the solar system.

Minton: The Moon is great because the Moon has basically witnessed all the events of solar system history. It is an airless, relatively large body, it has had very little geology on it in billions of years. It had some volcanism, 3-4 billion years ago, but it wasn’t very extensive volcanism. So it has basically recorded all these events in solar system history. WE wouldn’t have even known about the LHB and picked up rocks there and saw all the impacts that cluster around 3.9 billion years ago, the time the LHB happened. And all the trace minerals we find in all the rocks tell you about where things came from. Of all the places in the solar system, the Moon makes the most sense as far as trying to understand broad strokes of solar system history because it is like a big collection bucket collecting all this crud in the solar system, telling us all about everything that has happened.

There are all these outstanding issues as far as how long the LHB lasted, when it started, were comets ever important in the bombardment history of the Moon or was it all asteroids? These are all things that we really need to go to the Moon to find out and there is almost nowhere else you can go to do it. So the Moon is our next door neighbor, it is right there. It only takes a couple of days to send a spacecraft there. So it really is one of the best places to go to understand all the solar system history.

Voice: To find out more about this topic, visit our website at www.lunarscience.nasa.gov. Any opinions expressed are the individuals and do not necessarily reflect the opinion of NASA or the NASA Lunar Science Institute. This podcast is produced for educational purposes only. On behalf of the NASA Lunar Science Institute, thanks for listening.

End of podcast:

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