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 More on Habitable zonesMore on Habitable zones

Changes in sizeChanges in size Impact of greenhouse effectImpact of greenhouse effect Galactic habitable zonesGalactic habitable zones

Possibly(?) no lecture Friday Feb 9Possibly(?) no lecture Friday Feb 9thth, , more information about this on Fridaymore information about this on Friday

Monday 5Monday 5thth Feb, guest lecture: Dr Martin Feb, guest lecture: Dr Martin Duncan (Astronomy) on formation of the Duncan (Astronomy) on formation of the outer outer solar system solar system

Impact of albedo on the Impact of albedo on the HZHZ

Because albedo can take such a wide Because albedo can take such a wide variety of values it is interesting to variety of values it is interesting to study it’s effects on the HZstudy it’s effects on the HZ

Let’s compare the average Let’s compare the average temperature of a planet with high temperature of a planet with high cloud cover (a=0.75) with dark soil cloud cover (a=0.75) with dark soil (a=0.1)(a=0.1)

HZ for soil world(low albedo)

HZ for cloudworld(high albedo)

More reflectivity, a higher albedo,moves the habitable zone inwardbecause radiation is reflected awayfrom the planet. A lower albedo,i.e. more radiation is absorbed, movesthe habitable zone outward.

Changes in stellar Changes in stellar luminosityluminosity

4 billion years ago the Sun’s luminosity 4 billion years ago the Sun’s luminosity was 70% of its current valuewas 70% of its current value In fact all main sequence stars show this In fact all main sequence stars show this

brightening over timebrightening over time What impact does it have on the What impact does it have on the

habitable zone?habitable zone? HZ must move outward over timeHZ must move outward over time Planetary orbital radii are quite stable, Planetary orbital radii are quite stable,

so as the HZ moves over time a planet so as the HZ moves over time a planet may find itself move in, or out of the HZmay find itself move in, or out of the HZ

Continuously Habitable Continuously Habitable ZoneZone

The region in The region in which a planet which a planet may reside and may reside and maintain liquid maintain liquid water water throughout most throughout most of a star’s life, is of a star’s life, is called the called the continuous continuous habitable zone habitable zone (CHZ)(CHZ)..

CHZ marks the middle region that remains habitable as the boundaries move outward.

CHZ in our simpleradiation balancemodel

Impact of the Impact of the greenhouse effectgreenhouse effect

All our preceding estimates have All our preceding estimates have neglected the greenhouse effectneglected the greenhouse effect We have also assumed our planet to be a We have also assumed our planet to be a

perfectly radiating black body – which is far perfectly radiating black body – which is far from truefrom true

As we have discussed, it is actually As we have discussed, it is actually extremely important in assessing the extremely important in assessing the extent of the CHZextent of the CHZ

Before discussing how we model its impact Before discussing how we model its impact on the CHZ, we first review the concepts on the CHZ, we first review the concepts behind the greenhouse effect itselfbehind the greenhouse effect itself

A note about the A note about the greenhouse effect in the greenhouse effect in the

context of climate changecontext of climate change As we’ve discussed already, the As we’ve discussed already, the

greenhouse effect is necessary for life greenhouse effect is necessary for life on Earthon Earth

When it is talked about in terms of When it is talked about in terms of climate change they really mean a more climate change they really mean a more severe greenhouse effect due to severe greenhouse effect due to anthropogenic COanthropogenic CO22 emissions emissions Believe it or not, we knew the temperature Believe it or not, we knew the temperature

increase to within a factor of 2 in the early increase to within a factor of 2 in the early 1960s1960sJay Leno: “According to a survey in ths week’s Time Magazine, 85% of Americans

think global warming is happening. The other 15% work for the White House.”

Greenhouse effectGreenhouse effect

The greenhouse effect can be The greenhouse effect can be understood as arising from two factsunderstood as arising from two facts(1) The Earth’s atmosphere transmits (1) The Earth’s atmosphere transmits

visible light efficiently, but strongly visible light efficiently, but strongly absorbs infra-red lightabsorbs infra-red light

(2) The Earth’s temperature is (2) The Earth’s temperature is sufficiently low that incoming light sufficiently low that incoming light energy from the Sun will be re-radiated energy from the Sun will be re-radiated at infra-red wavelengths (black-at infra-red wavelengths (black-body-“ish”)body-“ish”)

Strong absorption inthe infrared region

Wavelength ofincoming solarradiation

Greenhouse GasesGreenhouse Gases HH22O (water vapour) and COO (water vapour) and CO22 are the two are the two

dominant greenhouse gasesdominant greenhouse gases Water vapour probably contributes more (by a factor of Water vapour probably contributes more (by a factor of

2 perhaps) to the greenhouse effect than CO2 perhaps) to the greenhouse effect than CO22

Methane (CHMethane (CH44) and ozone (O) and ozone (O33) also play a roll along ) also play a roll along with other organic moleculeswith other organic molecules

These molecules have molecular structures that These molecules have molecular structures that absorb photons at infrared wavelength and incite absorb photons at infrared wavelength and incite vibrations in the moleculevibrations in the molecule

On a per molecule basis some man made CFC’s On a per molecule basis some man made CFC’s are spectacularly more capable of absorbing are spectacularly more capable of absorbing infrared than COinfrared than CO22, but fortunately the , but fortunately the atmospheric concentrations are very low atmospheric concentrations are very low

Nitrogen, N2, and oxygen, O2, which make up the bulk of the atmosphereare not greenhouse gases.

Greenhouse Greenhouse EffectEffect

Incoming radiation is at Incoming radiation is at visible wavelengthsvisible wavelengths Some is reflected due to the Some is reflected due to the

albedo of the planetalbedo of the planet Most radiation is Most radiation is

transmitted down to transmitted down to planet’s surfaceplanet’s surface

Re-radiated at long (infra Re-radiated at long (infra red) wavelengths from the red) wavelengths from the planet’s surfaceplanet’s surface A small fraction escapes out A small fraction escapes out

directlydirectly Atmospheric greenhouse Atmospheric greenhouse

gases absorb most of the gases absorb most of the infrared radiation and will infrared radiation and will re-emit toore-emit too

Most radiation is sent back Most radiation is sent back down to the Earthdown to the Earth

The Earth’s Energy The Earth’s Energy budgetbudget

Once the greenhouse effect is established it Once the greenhouse effect is established it will reach an equilibrium where the net will reach an equilibrium where the net radiation outward equals that coming inradiation outward equals that coming in

If you work out the amount of energy If you work out the amount of energy arriving from the Earth it is about 340 W marriving from the Earth it is about 340 W m-2-2

Albedo serves to reduce the net radiation Albedo serves to reduce the net radiation through the atmosphere to about 235 W mthrough the atmosphere to about 235 W m--22

Yet the Earth’s average radiation output is Yet the Earth’s average radiation output is about about 492 W mabout about 492 W m-2-2 – where is the excess – where is the excess energy going?energy going?

Kasting’s modelsKasting’s models

In the early 1990s Kasting and co-workers In the early 1990s Kasting and co-workers examined the effect of greenhouse gases on the examined the effect of greenhouse gases on the Sun’s habitable zone using computer modelsSun’s habitable zone using computer models COCO22, H, H220, N0, N22 atmosphere atmosphere

They estimated the inner edge of the HZ on the They estimated the inner edge of the HZ on the basis of when water breaks down into oxygen basis of when water breaks down into oxygen and hydrogen due to UV radiationand hydrogen due to UV radiation Hydrogen then escapes the atmosphere and water is Hydrogen then escapes the atmosphere and water is

essentially “boiled” away from the planet essentially “boiled” away from the planet They found the inner edge of the HZ to be 0.95 They found the inner edge of the HZ to be 0.95

AUAU

What about the outer What about the outer edge?edge?

How far out can COHow far out can CO22 and H and H22O still O still maintain a habitable planet?maintain a habitable planet?

Obviously depends on atmospheric Obviously depends on atmospheric concentrations of greenhouse gasesconcentrations of greenhouse gases

Model’s showed that the existence of a Model’s showed that the existence of a ““COCO22 thermostat thermostat” could extend the HZ” could extend the HZ Atmospheric COAtmospheric CO22 rises as surface gets colder rises as surface gets colder

because weathering (a sink of CObecause weathering (a sink of CO22) stops and ) stops and volcanic activity (a source of COvolcanic activity (a source of CO22) becomes ) becomes more importantmore important

The carbonate-silicate The carbonate-silicate cyclecycle

CO2 sourceCO2 sink:Turned into bicarbonateions

CO2 sink:Turned into calcium carbonatewith one CO2 released

Outer edge of the HZOuter edge of the HZ

Kasting’s work provided two estimates for the Kasting’s work provided two estimates for the outer edge of the HZouter edge of the HZ

The first estimate: 1.37 AU was based upon The first estimate: 1.37 AU was based upon where COwhere CO22 would start to condense out of the would start to condense out of the atmosphereatmosphere Without the COWithout the CO22 to drive the greenhouse effect, to drive the greenhouse effect,

temperatures are expected to fall rapidlytemperatures are expected to fall rapidly The second estimate: 1.67 AU was based upon The second estimate: 1.67 AU was based upon

establishing the maximum greenhouse effect establishing the maximum greenhouse effect that could operate and produce an average that could operate and produce an average temperature of 273 Ktemperature of 273 K

Venus & MarsVenus & Mars Venus, at 0.72 AU, is Venus, at 0.72 AU, is

clearly well inward of the clearly well inward of the inner boundary inner boundary Runaway greenhouse there Runaway greenhouse there

has lead to a temperature has lead to a temperature 500 K higher than you 500 K higher than you would estimate!would estimate!

Mars is at 1.52 AU, which Mars is at 1.52 AU, which is within the maximum is within the maximum limit but outside the COlimit but outside the CO22 condensation limitcondensation limit Average surface Average surface

temperature very low (-55 temperature very low (-55 C)C)

Thin atmosphere left Thin atmosphere left We’ll look at Mars in more We’ll look at Mars in more

detail laterdetail later

Habitable zones around Habitable zones around other starsother stars

Mass limit (star must have lifetime > 109 years)

Planets here alwayshave one face towardthe star

Galactic Habitable ZonesGalactic Habitable Zones

Just as we can define regions of space Just as we can define regions of space in the solar system, we can extend this in the solar system, we can extend this idea to the galaxy itselfidea to the galaxy itself

The main constraints areThe main constraints are(1)(1) Have enough heavy elements to create Have enough heavy elements to create

terrestrial planets been created in a region terrestrial planets been created in a region of space? (this is largely a time constraint)of space? (this is largely a time constraint)

(2)(2) Are there few enough supernova events so Are there few enough supernova events so that life is not wiped up by a large burst of that life is not wiped up by a large burst of gamma-rays?gamma-rays?

Types of galaxiesTypes of galaxies

In general ellipticals have stars thatare very old and with few heavyelements – probability of supportinglife is lower than spirals

Have both young (metal rich) andold (metal poor stars)

Distribution of stars in Distribution of stars in the galaxythe galaxy

There is more matter in the centre of the There is more matter in the centre of the galaxy than edges (especially at early times)galaxy than edges (especially at early times)

This means there will be more supernovae & This means there will be more supernovae & powerful radiation in the centre of the galaxypowerful radiation in the centre of the galaxy life will have a very hard time evolving in this life will have a very hard time evolving in this

environmentenvironment There are also more exotic events to worry about There are also more exotic events to worry about

“gamma-ray bursters”“gamma-ray bursters” At the outer edges of the galaxy star At the outer edges of the galaxy star

formation proceeds more slowly because formation proceeds more slowly because there is less gasthere is less gas Creating heavy elements will take a very long Creating heavy elements will take a very long

time, so again at the present age of the Universe time, so again at the present age of the Universe life is unlikely to have evolved out therelife is unlikely to have evolved out there

Too slow star formationto form the heavymetals for terrestrialplanets

Too high supernova rate/gamma ray events to supportlife

Galactic Habitable Zone

Fig.3. The GHZ in the disk of the Milky Way based on the star formation rate, metallicity (blue), sufficient time for evolution (gray), and freedom from life-extinguishing supernova explosions (red).

Green line is the expected age distribution of stars that support complex life (most are expected to be between 4-8 Gyrs)

C. H. Lineweaver, Y. Fenner, B. K. Gibson (2004): The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way, Science 302, 59-62.

When and where do life-When and where do life-supporting stars form?supporting stars form?

95 % chance ofsupporting life

68 % chance ofsupporting life

Series of frames showing evolution of the Milky Way to the present day. Each frame is600 million years of evolution. Red indicates regions too full of supernova activity to supportLife, blue areas have too few heavy elements, green represents most favoured regions.

Much to be learnt in this Much to be learnt in this fieldfield

The concept of galactic habitable zones The concept of galactic habitable zones was first discussed in 1991 (Gonzalez, was first discussed in 1991 (Gonzalez, Brownlee & Ward)Brownlee & Ward) The field is still very youngThe field is still very young

There remains considerable controversy There remains considerable controversy over assumptions about supernovae over assumptions about supernovae rates and the extinction efficiency of rates and the extinction efficiency of them them We’ll improve our guesses in the future, but We’ll improve our guesses in the future, but

a true answer will be elusivea true answer will be elusive

Summary of lecture 10Summary of lecture 10 Increasing albedo moves the HZ inwardIncreasing albedo moves the HZ inward The luminosity of the Sun is growing over time The luminosity of the Sun is growing over time

and is moving the HZ outwardand is moving the HZ outward The continously HZ (CHZ) defines a region of The continously HZ (CHZ) defines a region of

space that a planet can in habit while still space that a planet can in habit while still maintaining liquid watermaintaining liquid water

The greenhouse effect strongly alters the CHZ The greenhouse effect strongly alters the CHZ allowing it to push outward, possibly beyond the allowing it to push outward, possibly beyond the orbit of Marsorbit of Mars

A galactic habitable zone analysis suggests there A galactic habitable zone analysis suggests there is a region in the galaxy that is suitable for lifeis a region in the galaxy that is suitable for life

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Formation of planets (p 245-246)Formation of planets (p 245-246)