hi I'm dr. Johnson Hoss and welcome to earth parts so let's look at the time scale events that we've covered so far around 4.6 billion years ago earth begins to accrete from the stellar nursery cloud where the Sun form is the Sun forms from that region's mass and the Sun is most of the solar system but the earth begins to accrete with the Sun and the rest of the planets around 4.6 billion years ago by 4.5 billion years ago Earth's had begun to accrete as a large surviving planetary object gaining mass as it sweeps clear its orbit of stray debris from the interior of Earth begins to boil out primordial atmosphere that would be composed of initially solar light bottles but hydrogen and helium are easily lost his space we would have had initial atmosphere once the hydrogen plate away retained helium that eventually would lead away nitrogen we kept along with co2 methane ammonia and other light volatiles like hydrogen sulfide and things like that by 4.4 billion years ago accretion was winding down and most of the planet cores and large objects had been forming for a while and at that point one stray object hit earth and caused the formation of our moon this object called Theia was mars-sized Mars mast and it struck the earth turning both into a cloud of debris that settled back down to form earth 2.0 if you want to call it that and a debris ring that coalesced into the moon forming earth with a slightly larger metal core than it would have had otherwise and the moon which has almost no metal core and no lipolysis and is dry as a bone now what seems to happen next is interesting the oldest rocks in the world that have been recovered are grains of rains of rock from original rocks that had been weathered into small particles and carried away and deposited somewhere else so these are grains of the mineral a mineral called zircon we can look at the uranium led ratios in these crystals and determine their age and date them to about four point three seventy five billion years ago so the zircon is recovered from Jack Hills in Australia and this is the Jack Hills a part of it right here these tiny bits of rock tell us a lot because they tell us from their own geochemistry their oxygen isotopic ratios for example what the temperature of formation was when these crystals formed at the Earth's surface and it turns out these things formed about four point three seventy five billion years ago in a temperature environment not too different from what we have today meaning that in just a few dozen million years after the moon formation event the Earth's surface had cooled to the point where you could have liquid water so it appears that after the moon formation we didn't remain a seething magma inferno planet for millions and millions of years we cooled fairly quickly and the oceans condensed out and geologic processes could continue by about 4.3 billion years ago the oceans have formed they've condensed out of the atmosphere the atmosphere is cooled we would have had an atmosphere of primarily nitrogen co2 bits of methane ammonia hydrogen sulfide still at this point there aren't really any large continents early on like this there would have been island chains maybe small proto continents beginning to form but no large continents and most of the earth would have appeared from space to be a water planet as time goes on by about 4.2 billion years ago the earth is still a water planet but progressive geologic processes are converting some of the crustal basaltic rock into lighter silicates and you're starting to form continental rock and the continents are beginning to accrue so about by about 4.2 billion years ago we expect that there were small island continents around the planet moving around tectonic lee and the earth continues to out gas carbon dioxide hydrogen sulfide methane ammonia from its interior nitrogen and the atmosphere continues to enrich with these materials as well as organics organics that organic compounds that were present in the accretionary material that's also in the ocean and we will talk about that later because that's relevant to the origins of life so by 4 point 1 billion years ago the earth is cool their oceans may be small continents organics in the ocean perhaps life has already begun perhaps not we'll come back to that then something happens you would expect accretion to trail away to nothing and eventually just be your random occasional small meteor hitting the earth or something like that an asteroid striking near 30 few million years but around 4.1 to 4 billion years ago something happened to change that earth started to be bombarded again intensely it's called the late heavy bombardment it was a dramatic increase in the frequency of Comet and asteroid impacts massive impacts of whichever composition into the earth and this continued until about 3.9 maybe 3.8 billion years ago and sort of trailed away that needs an explanation so how do we know that that even happened at all how do we know that the late heavy bombardment occurred any cratering from that time on earth would have been long since gone to erosion so we're not looking at our planet for the evidence for that we're looking at the moon lunar samples returned by Apollo astronauts some of them are some of the oldest rocks that indicate the age of the moon around 4.4 billion years but there's a number of them that cluster around an age range of around 4.1 2 3.8 billion years meaning that these are rocks from impacts from from igneous processes that occurred all in this sort of narrow span of time and the crater is appear to be that old so why so long after a collision collisions should have trailed off after accretion did impact suddenly looming back up again as a major issue that's the question lane heavy bombardment is an isolated episode of late impacting unto the planet so there are many ideas as to why this happen we see evidence on the moon several of the large basins of the Moon the nectarous embryo McCrory and tala basins all seem to date from this period where they should their big impact basins they should have formed much earlier nothing that big should be forming as impacts that late after accretion so there are several hypotheses about how this works that have come and gone a leading hypothesis proposes that heavy bombardment might be the result of events occurring further out in the solar system around 4.1 billion years ago accretion was over but you don't necessarily get to assume that all the planets are in permanently stable orbits yet the big outer planets sometimes the math of how orbits suddenly change over time takes time to play out it appears that Jupiter and Saturn were slightly different orbits then than they are than they are now Jupiter and Saturn then perhaps entered what we call a 2:1 orbital resonance meaning that for every one Jupiter orbit Saturn goes around the Sun twice so every second Saturn orbit Jupiter and Saturn are on the same side of the Sun two planets that are further out the outer planets Uranus and Neptune that combin combining of gravitational influences would have produced an occasional extra tug on their orbits an extra spike in gravitational disturbance and it seems that what eventually happened was that this resonance disturbed the orbit of Neptune causing it to spiral out further from the Sun in this model Neptune actually formed closer to the Sun than Uranus making Uranus the original outermost planet and Neptune having formed closer in between the orbits of Uranus and Saturn and this is actually consistent with the geochemistry of the two planets which I won't get into much here but the composition of Neptune compared to the composition of Uranus and their temperatures indicate that they're in the wrong place that Neptune should not have formed where it is and in fact should have formed closer in so that's another piece of evidence that's added to this so anyway what seems to happen is that when Neptune moved out to a more distant orbit it disturbed what was until then a fairly sizable population of small-to-medium Kuiper belt objects icy objects like Pluto in the outer reaches of the solar system beyond the orbit of Uranus and mostly even today beyond the orbit of Neptune this simulation illustrates the migration of Neptune and shows you visually what I'm trying to describe here when Jupiter and Saturn combined gravitational influence and destabilize the orbit of Neptune it moves outward neptune in this simulation is in the blue orbit so pay attention to the blue orbit beyond that these this cloud of green material out beyond the orbit of in this the purple orbit of Uranus presently that green material is the Kuiper belt it's or at least the inner part of it let me proceed with this animation and we can see what happens you watch what happens when Neptune's orbit moves outward suddenly changes positions goes into an outer orbit and scatters all that material now when that material scatters what I mean by that is it's scattering inward and outward some of its going to go into hyperbolic orbits that will leave the solar system some of it is going to a lot of it is going to fall inward toward the Sun the migration of neptune scattered lots of commentary material from the outer solar system and it would have come raining in over the next hundreds of millions of years these are some of these are long periods slow orbits out there so it takes a long time for this to play out but over the next few hundred million years the earth would have been pelted with objects sometimes big ones and this is a major event in Earth's history the earth had settled down and was coalescing and chemically differentiating in its interior and then this happens and so this in many ways resets a lot of the processes that we're already occurring on earth what I've set up here is a series of simulations that would depict broadly the results of impactors of various sizes striking the earth during the late heavy bombardment I'm gonna start with a fairly small impactor roughly the size of Ceres the largest of the asteroids in the asteroid belt a few hundred kilometers across and what would be the effect of that striking the earth in a simulation you can see immediately it creates a gigantic impact basin on the side of the earth hit and ejecta from that impact are being tossed around the planet to land in ballistic trajectories on the other side of the planet and therefore you start creating lots more impact basins and a lot more disturbance in this particular example though the ocean survives you'll note that there are areas where the oceans are probably boiling but lots of stretches of the Earth's surface where that is relatively unaffected now let's look at what happens if you impact the earth with something substantially larger like something the size of eros or Triton a Kuiper belt object a couple of thousand kilometers across what would happen in the earth we were struck by something like that during late heavy bombardment because we may have been several times let's see in this case you can see the oceans are boiling one impact from that object is enough to turn half the earth into an impact basin and the result heats the atmosphere up heats the surface enough that the oceans are born in the way in either case the result of the late heavy bombardment would have been a sieving earth again at least the surface would have been partially molten but remember the oldest rocks in the world the zircon stone in the jack hills of australia they indicated that just after the moon formation event giant impact from the moon the earth a few tens of millions of years later was able to condense water to form oceans again after the late heavy bombardment the oceans would persist and conditions became possible for life to emerge soon thereafter but that's a story for another day you