El­iz­a­beth Pear­son in­ves­ti­gates how the So­lar Sys­tem was trans­formed 3.9 bil­lion years ago

Sky at Night Magazine - - CONTENTS -

How the So­lar Sys­tem as we know it rose from the ashes of plan­e­tary mi­gra­tions and an era of as­ter­oid bom­bard­ments.

When the So­lar Sys­tem first formed 4.5 bil­lion years ago it was a vi­o­lent place. But quickly, by 4.4 bil­lion years or so ago, the plan­ets had calmed into a fa­mil­iar con­fig­u­ra­tion – sev­eral rocky in­ner worlds sur­rounded by gas giants ringed by icy ob­jects. Then, four bil­lion years ago, some­thing cat­a­strophic hap­pened. The plan­ets were thrown once more into chaos, and the face of the So­lar Sys­tem changed for­ever. This pe­riod of up­heaval re­veals star­tling truths about the evo­lu­tion of our home plan­e­tary sys­tem, and per­haps the ori­gins of life it­self.

In the So­lar Sys­tem’s first hun­dred mil­lion years or so, the be­gin­nings of plan­ets clumped to­gether from the dust of a boil­ing pro­to­plan­e­tary disc around our young star. Fre­quently the young plan­ets would col­lide and grow larger,

though some­times they would be de­stroyed en­tirely. To start, this early So­lar Sys­tem was much like it is to­day, but there were sev­eral key dif­fer­ences.

“To­day, we have gi­ant plan­ets from Jupiter at about 5 AU from the Sun to Nep­tune at about 30 AU,” says Wil­liam Bot­tke from the South­west Re­search In­sti­tute in Boul­der, Colorado; 1 AU is the dis­tance be­tween the Earth and the Sun. “Mod­el­ling work shows the ice giants Nep­tune and Uranus would never have reached their cur­rent sizes if they had to form in the cur­rent con­fig­u­ra­tion of the plan­ets. Stud­ies sug­gested in­stead that all these bod­ies formed be­tween about 5 and 20 AU. Be­yond that lies the Kuiper Belt. What’s in­ter­est­ing about the Kuiper Belt now is that a lot of ob­jects have very spe­cial or­bits, called res­o­nances. It’s very hard to get them into these res­o­nances.”

Synced surprise

A res­o­nant or­bit oc­curs when the ra­tio be­tween the or­bits of two bod­ies is two whole num­bers. Pluto and Nep­tune share a res­o­nance: for ev­ery two or­bits Pluto makes, Nep­tune com­pletes three. To ex­plain what might have caused these res­o­nances, as well as the change in the ice giants’ po­si­tions, a group of re­searchers in Nice, France, came up with what is now known as the Nice model in 2005.

“They sug­gested that you had a gi­gan­tic Kuiper Belt with maybe 10 Earth masses in it,” says Bot­tke. “This lets you form Nep­tune and Uranus on rea­son­able timescales and you can make lots of Plu­to­like ob­jects in the pri­mor­dial disc.”

Us­ing this setup, the team cre­ated a com­puter model of the early So­lar Sys­tem. It’s thought that a few hun­dred years af­ter the for­ma­tion of Jupiter, left­over gas dragged on the planet and caused it to drift deeper

into the So­lar Sys­tem. In time this caused the gi­ant plan­ets to fall into a res­o­nance and their com­bined grav­ity acted on the sur­round­ing Kuiper Belt ob­jects, pulling them in­wards. In turn, these bod­ies pulled on the or­bits of the gas giants. Though only a small ef­fect com­pared to that of the plan­ets, lit­tle by lit­tle the icy rocks be­gan to up­set the pre­car­i­ous grav­i­ta­tional bal­ance.

“They found that when the sys­tem be­comes un­sta­ble, Uranus and Nep­tune move into the pri­mor­dial disc and ac­tu­ally mi­grate across it. The gi­ant plan­ets end up with al­most iden­ti­cal or­bits to what we see to­day,” says Bot­tke.

This would have thrown the So­lar Sys­tem into dis­ar­ray. Ac­cord­ing to the model, Jupiter moved in­ward while the other gas giants moved out. In turn, the in­ner plan­ets were jos­tled and shuf­fled, pulling some of them into highly ec­cen­tric or­bits which might have flung some of our sib­lings out into the Gal­axy.

“There’s pretty com­pelling ev­i­dence that we didn’t start with four gi­ant plan­ets, but five. We had an ex­tra Nep­tune and then lost it in this process. Jupiter is so mas­sive that any­thing that en­coun­ters it has a good chance of be­ing thrown out of the So­lar Sys­tem,” says Bot­tke.

The ad­di­tion of a fifth planet to the Nice model also helps to ex­plain ob­ser­va­tions of the small bod­ies around Jupiter, as well as cer­tain as­pects of the as­ter­oid belt. Could it be that our long-lost sib­ling is cur­rently float­ing be­tween the dis­tant stars?

Mass mi­gra­tion

But plan­ets are not the only things that the Nice model pre­dicts be­ing re­lo­cated dur­ing this time. The Kuiper Belt cur­rently con­tains around the same mass as Mars, mean­ing that dur­ing this plan­e­tary reshuf­fle, 99 per cent of its mass would have been re­dis­tributed. Many Kuiper Belt bod­ies have been sent hurtling to­wards the in­ner So­lar Sys­tem. And it’s the scars left be­hind by these im­pacts that could help ex­plain a lu­nar mystery that has been around since the first moon­rock sam­ples were brought back by the Apollo mis­sions.

“Many of the Apollo sam­ples, more than you would ex­pect, had been melted in im­pacts that took place around 3.9 bil­lion years ago,” says Bar­bara Co­hen, a plan­e­tary sci­en­tist at NASA’s Mar­shall Space Flight Cen­ter. “You would think that there would be a lot of im­pact craters from when the Moon formed 4.5 bil­lion years ago, which would fall off as the im­pactors got used up, but we didn’t see any. In­stead we saw a lot at 3.9 bil­lion years ago, which was a strange and un­usual re­sult.”

Apollo sci­en­tists hy­poth­e­sised that 3.9 bil­lion years ago the in­ner So­lar Sys­tem was pelted with comet-like ob­jects at an im­pact rate 100 times larger than what’s seen to­day – an era now known as the late heavy bom­bard­ment (LHB).

How­ever, all the Apollo sam­ples came from a limited area of the Moon, close to the large Im­brium Basin. This 1,150kmwide crater is the re­sult of a huge im­pact 3.85-3.9 bil­lion years ago, which then flooded with lava. While this is one of the largest ex­am­ples of the ef­fect the LHB had on the lu­nar land­scape, there is also the

chance that all the Apollo sam­ples were sim­ply the ejecta of this one event. To cre­ate a big­ger pic­ture Co­hen had to look to­wards our only other sam­ples from the Moon – lu­nar me­te­orites.

“The me­te­orites I’ve been look­ing at are from places that the as­tro­nauts didn’t go, they have dif­fer­ent geo­chem­i­cal sig­na­tures so we think they are from far­away places,” she says. “I didn’t find any of them to be very old. We see a big pile up at 3.9 bil­lion years, with a long tail off. This tells us there was a pro­longed im­pact rate on the Moon, and then over time the im­pacts got smaller and smaller.”

The same old story

Other me­te­orites from the as­ter­oid Vesta tell much the same story, in­di­cat­ing a lack of im­pacts be­tween four and 4.5 bil­lion years ago. But re­ly­ing on me­te­orites means that re­searchers in­ves­ti­gat­ing the bom­bard­ment his­tory of the So­lar Sys­tem are con­strained, as they can only study what hap­pens to ar­rive on Earth. It’s cur­rently im­pos­si­ble to iden­tify me­te­orites from Mer­cury and Venus, and all known Mar­tian rocks are vol­canic in ori­gin. This leaves large holes in the im­pact time­line, ones that are un­likely to be filled un­til we can test

the craters di­rectly. Luck­ily that day could come rel­a­tively soon.

“I’m de­vel­op­ing an in­stru­ment that we could take to Mars to find im­pact craters and get geochronol­ogy on them,” says Co­hen. “It would have a pre­ci­sion of around 100 mil­lion years, a few per cent the age of the crater. That’s good enough to dis­tin­guish ma­jor ge­o­logic events in the planet’s his­tory.” The re­cent re­nais­sance in lu­nar mis­sions means we could soon un­der­stand the Moon’s his­tory a lit­tle bet­ter. China’s Chang’e pro­gramme aims to re­turn the first sam­ple from the Moon in over 40 years in late 2017, and then to land the first ever probe on the far side of the Moon in 2019. A full im­pact his­tory will prove use­ful in solv­ing one of the main de­bates around the LHB. “The ques­tion is whether the LHB was a unique event,” says Her­bert Frey, chief of NASA God­dard Space Flight Cen­ter’s plan­e­tary ge­ol­ogy, geo­physics and geo­chem­istry lab. “As­tronomers have al­ways been keen to know if this is an im­pact rate spike or whether it is the tail end of a bom­bard­ment that had been go­ing on for a long time and we’re just see­ing the ones that man­aged to sur­vive be­cause they came in last.” Un­der­stand­ing the pre­cise tim­ing of the bom­bard­ment is key to those con­sid­er­ing the Nice model. The length of the de­lay be­tween the So­lar Sys­tem’s for­ma­tion and the bom­bard­ment is cen­tral to work­ing out what our Uni­verse looked like be­fore the mi­gra­tion. Though there are still many mys­ter­ies sur­round­ing this era this one is cer­tain – our So­lar Sys­tem be­came a very dif­fer­ent place four bil­lion years ago.

The late heavy bom­bard­ment was a time of change and de­struc­tion, shap­ing the So­lar Sys­tem into the ar­range­ment we know it to­day

It is thought that Jupiter was the main cat­a­lyst be­hind the plan­e­tary mi­gra­tion of the So­lar Sys­tem. Even af­ter the gi­ant plan­ets formed, there was still a lot of gas and dust left be­hind that de­cayed Jupiter’s or­bit, caus­ing it to mi­grate into the in­ner So­lar Sys­tem be­fore swing­ing back out. By the end of its jour­ney the rest of the So­lar Sys­tem was com­pletely re­ordered

20 AU 40 AU Uranus Nep­tune Saturn Jupiter The Nice model shows how the icy bod­ies of the Kuiper Belt could have scat­tered in­wards, up­set­ting the grav­i­ta­tional bal­ance of the gas giants in the early So­lar Sys­tem. In this ex­am­ple, the tur­moil re­sulted in Nep­tune and Uranus swap­ping places in the plan­e­tary line-up

20 AU 40 AU

20 AU 40 AU

All Apollo sam­ples come from the re­gion of the Im­brium Basin – they may all be its ejecta

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