Planets & LifePlanets & LifePHYS 214PHYS 214

Dr Rob ThackerDr Rob ThackerDept of Physics (308A)Dept of Physics (308A)

thacker@astro.queensu.cathacker@astro.queensu.caPlease start all class related emails Please start all class related emails

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Today’s LectureToday’s Lecture Earth HistoryEarth History

Formation of the MoonFormation of the Moon Origin of the hydrosphereOrigin of the hydrosphere Plate tectonicsPlate tectonics Snowball EarthSnowball Earth

This lecture will actually draw This lecture will actually draw together a few ideas we’ve seen in together a few ideas we’ve seen in other contextsother contexts

Formation of the MoonFormation of the Moon Possible theories –Possible theories –

Earth broke-up “fissioned” to form MoonEarth broke-up “fissioned” to form Moon Not dynamically possibleNot dynamically possible

We talked about “disks within disks” for We talked about “disks within disks” for forming planetary systems - this suggests forming planetary systems - this suggests Moon should have similar composition to Moon should have similar composition to EarthEarth

It doesn’t – so Earth and Moon are unlikely to have It doesn’t – so Earth and Moon are unlikely to have formed togetherformed together

Moon is a captured planetesimalMoon is a captured planetesimal Without an impact event capture requires a 3Without an impact event capture requires a 3rdrd body to body to

remove (angular) momentum from the system – seems remove (angular) momentum from the system – seems improbableimprobable

Lunar compositionLunar composition

Much less Fe than the Earth (Fe for Much less Fe than the Earth (Fe for Earth about 35%)Earth about 35%)

Impact event: the most Impact event: the most probable theoryprobable theory

An object close to the size of Mars impacts the An object close to the size of Mars impacts the proto-Earth around 4.5 Gyr agoproto-Earth around 4.5 Gyr ago The resulting impact ejects large amounts of material The resulting impact ejects large amounts of material

into a ring or disk that orbits around the Earthinto a ring or disk that orbits around the Earth This material quickly begins to collapse together This material quickly begins to collapse together

under mutual gravitation and forms the Moonunder mutual gravitation and forms the Moon The collapse of the ring could even be a short as a yearThe collapse of the ring could even be a short as a year

The resulting Moon is still very hot, and has a The resulting Moon is still very hot, and has a mass of 1/80mass of 1/80thth that of the Earth that of the Earth Cannot maintain an atmosphereCannot maintain an atmosphere

Explains age, composition, lack of iron; and is Explains age, composition, lack of iron; and is very plausible given conditions in early solar very plausible given conditions in early solar system.system.

Simulation of the impact Simulation of the impact eventevent

Accretion of the ring Accretion of the ring materialmaterial

Movie

Early Earth atmosphereEarly Earth atmosphere Any Any first atmospherefirst atmosphere would have been swept would have been swept

away by strong solar wind from young Sunaway by strong solar wind from young Sun Secondary atmosphereSecondary atmosphere began forming from: began forming from:

deposition of volatiles (light elements: H, C, N, O) deposition of volatiles (light elements: H, C, N, O) from ongoing from ongoing cometary & asteriod impactscometary & asteriod impacts, , continued for ~500 million yearscontinued for ~500 million years

outgassingoutgassing from from volcanic activityvolcanic activity Early atmosphere probably consisted of Early atmosphere probably consisted of

COCO22, N, N22, CO, H, CO, H22OO (which went on to form (which went on to form oceansoceans), and traces of H), and traces of H22 this was a mildly this was a mildly reducingreducing atmosphere (little atmosphere (little

oxidation)oxidation)

Evolution of the Evolution of the atmosphereatmosphere

We already mentioned in lecture 20 the impact of cyanobacteria on the early atmosphere

Photodissociation by UV under perfect circumstances can produce only 1-2% of current O2 enough for ozone (O3) to form,

blocking UV and preventing further dissociation

Once life was established and evolved to the point of cyanobacteria photosynthesis became possible O2 from photosynthesis: CO2 + H2O + sunlight =

glucose + O2

The O2 produced by cyanobacteria was being used to oxidize Fe and S either in solution or in rocks (recall the banded iron formations mentioned in lecture 20)

Little free oxygen until ~2.4 Gyr ago

Once the Fe and S were fully oxidized, O2 could remain in atmosphere. from 2.4 – 1.6 Gyr ago,

atmospheric O2 levels rose from ~0.001% to ~1%

Fairly steady rise since that point

Presence of free oxygenPresence of free oxygen

Atmosphere retentionAtmosphere retention The strength of the gravitational field of a body The strength of the gravitational field of a body

determines the ability of it to retain an atmospheredetermines the ability of it to retain an atmosphere The key feature is the escape velocity, vThe key feature is the escape velocity, vescesc, the , the

velocity required to escape the planets velocity required to escape the planets gravitational field. gravitational field.

Kinetic energy must equal the amount of potential Kinetic energy must equal the amount of potential energy lost coming from infinity (where the PE is energy lost coming from infinity (where the PE is initially zero)initially zero)

We can compare this value to the average velocity We can compare this value to the average velocity of the molecules in the atmosphere…of the molecules in the atmosphere…

RGMv

RGMmmv escesc

221 2 M=mass of

BodyR=radius

Average molecule speedsAverage molecule speeds From the kinetic theory of gases, the root mean square (From the kinetic theory of gases, the root mean square (i.e.i.e.

average) velocity of a gas molecule isaverage) velocity of a gas molecule is

The dependence on 1/√m means that lighter molecules have a The dependence on 1/√m means that lighter molecules have a higher velocity and are thus more likely to escapehigher velocity and are thus more likely to escape

To maintain an atmosphere for the lifetime of the solar system To maintain an atmosphere for the lifetime of the solar system the rms velocity of the gas should be less than 1/6the rms velocity of the gas should be less than 1/6thth of the of the escape speedescape speed Why 1/6Why 1/6thth? It comes from a statistical argument about the ? It comes from a statistical argument about the

distribution of speeds of the molecules (many will have speeds distribution of speeds of the molecules (many will have speeds higher than the rms value)higher than the rms value)

For H at 300 K the speed is about 1920 m sFor H at 300 K the speed is about 1920 m s-1-1, while the Earth’s , while the Earth’s escape velocity is about 11 300 m sescape velocity is about 11 300 m s-1-1 – so H will not stay in the – so H will not stay in the Earth’s atmosphere over long periodsEarth’s atmosphere over long periods

mkTv 3

k=Boltzmann’s constantT=gas temp in Kelvinm=molecule mass

Source of Earth’s waterSource of Earth’s water Still the subject of open research – a number Still the subject of open research – a number

of possibilities have been suggested:of possibilities have been suggested: Water bearing (“wet”) asteroids being involved in Water bearing (“wet”) asteroids being involved in

both the formation and later bombardment of the both the formation and later bombardment of the EarthEarth

Earth formed ‘dry’ and comets depositing large Earth formed ‘dry’ and comets depositing large volumes of watervolumes of water

Icy particles from the outer regions migrating Icy particles from the outer regions migrating inwards (unlikely)inwards (unlikely)

Ice planets migrating (unlikely)Ice planets migrating (unlikely) Merger with a single very wet planetesimalMerger with a single very wet planetesimal

We know that material from the inner solar We know that material from the inner solar system is largely devoid of water now, so we system is largely devoid of water now, so we suspect the source of water must have been suspect the source of water must have been the outer regionsthe outer regions

Issues to explainIssues to explain A successful theory of water delivery A successful theory of water delivery

to the Earth must explain:to the Earth must explain: The abundance of water (2.8The abundance of water (2.81010-4-4 M MEE on on

the surface the surface and in the crust, and 0.8 and in the crust, and 0.8 - 8- 81010-4-4 M MEE in the mantle, in the mantle, quite quite uncertain!uncertain!))

The ratio of deuterium (The ratio of deuterium (22H) to hydrogen H) to hydrogen ((11H) isotopes in seawater (1.5H) isotopes in seawater (1.51010-4-4))

Arguments against the Arguments against the comet hypothesiscomet hypothesis

There is no doubt that some comets have hit Earth and There is no doubt that some comets have hit Earth and supplied water – the key issue is just how many and how supplied water – the key issue is just how many and how big were they?big were they? Typical comets in today’s solar system – water mass about 10Typical comets in today’s solar system – water mass about 1099 tons tons Mass of water in Earth’s oceans: 10Mass of water in Earth’s oceans: 101818 tons tons Unlikely that 10Unlikely that 1099 comets have impacted the Earth comets have impacted the Earth There are larger icy bodies out in the Kupier belt thoughThere are larger icy bodies out in the Kupier belt though

Given we know the isotopic ratio D/H of water on Earth Given we know the isotopic ratio D/H of water on Earth (1.5(1.51010-4-4) how does this compare to that for comets?) how does this compare to that for comets? Comet 1P/Halley: 3.16Comet 1P/Halley: 3.161010-4-4

Comet Hyakatuke: 2.82Comet Hyakatuke: 2.821010-4-4

So these comets contain about twice as much deuterium as Earth So these comets contain about twice as much deuterium as Earth water water

However these comets didn’t come from the Kuiper belt – is their However these comets didn’t come from the Kuiper belt – is their D/H ratio representative of earlier impacts from those objects?D/H ratio representative of earlier impacts from those objects?

HydratesHydrates These have been mentioned in relation to a These have been mentioned in relation to a

number of issues in the coursenumber of issues in the course Useful to provide a definition:Useful to provide a definition:

Hydrate: A solid compound containing water Hydrate: A solid compound containing water molecules combined in a definite ratio as an integral molecules combined in a definite ratio as an integral part of the crystal. part of the crystal.

Hydrates don’t have to be appear to be “wet”Hydrates don’t have to be appear to be “wet” Gas hydrates (Gas hydrates (e.g.e.g. CO CO22 hydrate) can be solid hydrate) can be solid

under either very high pressures or very low under either very high pressures or very low temperaturestemperatures

Can also embed water in silicate structures – Can also embed water in silicate structures – this leads to water bearing rocksthis leads to water bearing rocks

The “wet” planetesimal The “wet” planetesimal ideaidea

In this scenario hydrated minerals are involved In this scenario hydrated minerals are involved in the formation of Earthin the formation of Earth

As differentiation occurs the water is separated As differentiation occurs the water is separated from the crystal structure and rises to the from the crystal structure and rises to the surfacesurface

In this scenario wet planetesimals must arrive In this scenario wet planetesimals must arrive later so as not to lose all the water due later so as not to lose all the water due photodissociation to oxygen and hydrogenphotodissociation to oxygen and hydrogen If all the water arrives early it will escape quickly and If all the water arrives early it will escape quickly and

then be subject to the strong UV fieldthen be subject to the strong UV field Once a dense enough atmosphere is formed it Once a dense enough atmosphere is formed it

and the temperature lowers then the water can and the temperature lowers then the water can begin to condense out of the atmosphere to form begin to condense out of the atmosphere to form oceans oceans

Simulating water Simulating water deliverydelivery

Raymond, Quinn & Lunine (2004) modelled the delivery of water to terrestrial planets in an HZ using a variety of different assumptions about the distributions of comets & water bearing minerals.

Orbital properties of the gas giants in the system were also varied and importantly only objects more than 2.5 AU distant were allowed to be water bearing.

Of the 45 terrestrial planets formed in the HZ, 35 received as much or more water than the Earth (solid dot).

Water must be retained by Water must be retained by a planeta planet

Long term habitability requires that a planet Long term habitability requires that a planet retains its waterretains its water Water can be both lost to space by a combination Water can be both lost to space by a combination

of evaporation and also photodissociationof evaporation and also photodissociation Water can also be lost into the planets interior Water can also be lost into the planets interior

during continental plate subduction – water-during continental plate subduction – water-bearing minerals are formed and then buried into bearing minerals are formed and then buried into the mantle of the Earththe mantle of the Earth

On Earth, the increasing temperature of the On Earth, the increasing temperature of the Sun will boil away the oceans in about 2.5 GyrSun will boil away the oceans in about 2.5 Gyr

It is also estimated that in 1 Gyr, ¼ of the It is also estimated that in 1 Gyr, ¼ of the Earth’s current water will have been Earth’s current water will have been sequestered into the mantle!sequestered into the mantle!

Plate tectonics & lifePlate tectonics & life We met plate tectonics briefly before in relation We met plate tectonics briefly before in relation

to the carbonate-silicate cycle (lecture 11)to the carbonate-silicate cycle (lecture 11) Calcium carbonate is subducted into the Earth’s Calcium carbonate is subducted into the Earth’s

mantle, part of the COmantle, part of the CO22 storage before being storage before being released via vulcanism released via vulcanism

The mechanism behind plate tectonics is the The mechanism behind plate tectonics is the continual flow of heat from the core of the continual flow of heat from the core of the planet to surface from two main sourcesplanet to surface from two main sources Ongoing radioactive decay of unstable isotopes Ongoing radioactive decay of unstable isotopes Continued release of heat stored in the Earth during Continued release of heat stored in the Earth during

its formationits formation For smaller bodies this heat can be released more quickly For smaller bodies this heat can be released more quickly

Rifts & subduction zonesRifts & subduction zones Energy is transported by Energy is transported by

conduction, convection and conduction, convection and advection (horizontal advection (horizontal movement of material)movement of material)

The lithosphere is the The lithosphere is the outermost - rocky - part of the outermost - rocky - part of the planetplanet

The athenosphere is the The athenosphere is the weaker hotter region weaker hotter region underlaying the lithosphereunderlaying the lithosphere Rifts form where flows are Rifts form where flows are

moving plates apart (rift moving plates apart (rift volcanoes associated)volcanoes associated)

Subduction occurs where the Subduction occurs where the flows come together (needs flows come together (needs water to lubricate it)water to lubricate it)

The chemical composition of The chemical composition of the planet must be such that the planet must be such that lighter (granitic) rocks float on lighter (granitic) rocks float on the denser (basaltic) rocksthe denser (basaltic) rocks

Rodinia (1.1 Gyr ago)Pangea (0.25 Gyr ago)

Snowball EarthSnowball Earth The geological record suggests that dramatic changes The geological record suggests that dramatic changes

occurred in the climate between 0.75 and 0.58 Gyr occurred in the climate between 0.75 and 0.58 Gyr agoago Strong evidence for glaciations – glacial till deposited on all Strong evidence for glaciations – glacial till deposited on all

continentscontinents The snowball Earth theory is that these glaciations The snowball Earth theory is that these glaciations

were global, with ice perhaps being 2 kilometers thickwere global, with ice perhaps being 2 kilometers thick Life could still survive under the icesheetLife could still survive under the icesheet

Although it is unclear how the snowball starts, we can Although it is unclear how the snowball starts, we can escape the snowball Earth state as the carbonate-escape the snowball Earth state as the carbonate-silicate cycle must stop (no more weathering of rocks)silicate cycle must stop (no more weathering of rocks) COCO22 slowly builds up in the atmosphere and begins a strong slowly builds up in the atmosphere and begins a strong

greenhouse effectgreenhouse effect Some researchers argue there may have been a series Some researchers argue there may have been a series

of snowball eventsof snowball events Each cooling followed by an excessively hot period due to the Each cooling followed by an excessively hot period due to the

greenhouse effectgreenhouse effect

Snowball Earth-Cambrian Snowball Earth-Cambrian Explosion link?Explosion link?

The coincidence of the timescale for the The coincidence of the timescale for the snowball Earth period and the Cambrian snowball Earth period and the Cambrian explosion has led researchers to speculate on explosion has led researchers to speculate on cause and effectcause and effect

Could a small region of water around the Could a small region of water around the equator have enforced rapid evolutionary equator have enforced rapid evolutionary adaptation to the environment?adaptation to the environment?

Alternatively, perhaps the glaciations served to Alternatively, perhaps the glaciations served to kill a certain form of life allowing others to kill a certain form of life allowing others to thrivethrive

Much debate!Much debate!

Summary of lecture 22Summary of lecture 22 The Moon was most likely formed by a major impact The Moon was most likely formed by a major impact

event of the Earth with a Mars-sized planetesimalevent of the Earth with a Mars-sized planetesimal Recondensation of the ejected into a satellite probably Recondensation of the ejected into a satellite probably

happened very quicklyhappened very quickly The origin of Earth’s hydrosphere is not that well The origin of Earth’s hydrosphere is not that well

understoodunderstood Believed to be a result of water bearing minerals from “wet” Believed to be a result of water bearing minerals from “wet”

planetesimals, but comets did make some contributionplanetesimals, but comets did make some contribution Plate tectonics via the carbonate-silicate cycle plays a Plate tectonics via the carbonate-silicate cycle plays a

significant role in the evolution of lifesignificant role in the evolution of life Snowball Earth events may well have had a profound Snowball Earth events may well have had a profound

influence on the development of life on Earthinfluence on the development of life on Earth

Next lectureNext lecture Ideas behind the Rare Earth HypothesisIdeas behind the Rare Earth Hypothesis