Lecture 1&2: Introduction to Astrobiology

I. Snellen, Leiden Observatory, February 7, 2007IAC2007: Extrasolar planets & Astrobiology

1 Course Prerequisites

2 What is life?

3 Organic matter in the Universe

4 Origin and development of life on Earth

5 Prerequisites for life elsewhere

1 Course Prerequisites

• Books:An Introduction to Astrobiology; I. Gilmore & M. SephtonPlanetary Sciences; I. de Pater & J. LissauerPlanetary Science; G. Cole & M. WoolfsonAstrobiology: A Multidisciplinary Approach; J. Lunine

• What is astrobiology?

Astrobiologyis the study of life in the Universe, investigating its origin, distribution,and evolution. Its fundamentalquestions are:

• How does life begin and evolve?

• Does life exist elsewhere in the universe?

• What is the future of life on Earth and beyond?

Look at http://astrobiology.arc.nasa.gov/roadmap/ for the 7 NASA astrobiology goals.

Astrobiology is multi-disciplinary in nature, connectingastronomy, biology, physics, geology, and chemistry. Nonormal person can be an expert in all of these fields, not even your lecturer→ this is reflected in this lecture course.All information about the course (eg. subject contents) is available onhttp://www.strw.leidenuniv.nl/˜snellen/iac2007/

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2 What is life?

• Historical contextBefore Louis Pasteur (1822-1895) it was generally thought that life starts spontaneously and repeatedly.Evidence: - Generation of flies and maggots from rotting meat

- Generation of Eels and fish from sea mud- Generation of Frogs and mice from moist soil- van Leeuwenhoek (1632-1723) detected micro-organisms:

interpreted as resulting from spontaneous generation

Pasteur showed that a sterilized and confined substance remains sterile forever→ Life can not be generated spontaneously.This raised a very difficult question: at some point must lifehave arisen from inanimate matter. How?

• Definition of LifeTwo key features that indicate life (organism):

• The capacity for self-replication• The capacity to undergo Darwinian evolution

Every definition has its problems, but that oflife in particular.→ A mule can’t replicate (neither can anymore my cat), but is quite alive.→ Viruses and aberrant prions are often calledreplicators, and not life, because they can only reproduce undervery specific circumstances (but don’t most species?)→ What about flames which ’grow’?, certain computer software programs which are made to mutate and evolve?,machines that can move?

A more specific definition oflife says that a living organism should be capable of:Homeostasis(regulation ofinternal environment),Organization (in composition),Metabolism (production of energy),Growth , Adaptation,Response to stimuli, reproduction.Of course, many counter examples can be given.

Darwinian evolution:The concept of natural selection and its influence on successive generationsAny population of organisms consistsof individuals all slightly different. Those individuals having a variation that gives them an advantage in reproduc-tion will pass on that variation more frequently to the next generation, and subsequently become more commonand the population evolves.

2.1 Building blocks of Life

• Carbon as the basic element of life

• Since life requires a high level of chemical complexity, it is evident that a living organism needs to be buildup from a combination of large, complex molecules, to be ableto perform the necessary functions. It seemsthat onlyCarbon can form the basis for this.

• Carbon can form bonds with many other atoms, such as hydrogen, oxygen, nitrogen, sulfur and phosphorus,and metals such as iron, magnesium, and zinc.

• Carbon can form molecules that easily dissolve in liquid water. It is this ’interaction’ with liquid water thathelps the formation of highly complex molecules and/or structures (see later).

• Carbon, and the three other most common elements utilized bylife (hydrogen, oxygen, and nitrogen) are 4of the 5 most abundant chemical elements in the universe.

• Silicon is sometimes quoted as an alternative to carbon as the basic chemical element to support life, but thevariety of silicon polymeres is far less, and its interaction with water is not favorable. It is also an order ofmagnitude less abundant than Carbon.

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• Why can’t there be (highly evolved) forms of life beyond the range of our imagination? You watch too muchStar Trek. For a more scientific approach, see the last lecture of this series on SETI.

• The role of liquid Water

Liquid water also appears to be an essential requirement forlife.

• Water is a unique medium in which molecules can dissolve and chemical reactions can take place.

• Water exists as a liquid in a temperature range -not too coldso that biochemical reactions cannot take place,and -not too warm so that organic bonds cannot form.

• Water is apolar solvent− each side of the molecule carries a different electrical charge. It can easilyestablish hydrogen bonds and dissolves other polar molecules (hydrophilic) well, in contrast to apolar (non-polar) molecules (hydrophobic). Life exploits the different behavior of different molecules in water. Somemolecules are hydrophilic on one side and hydrophobic on theother.

• Ammonia (NH3) is sometimes proposed as an alternative. It is also polar, but is liquid under much coldertemperatures, which makes it much harder for biochemical reactions. Titan, one of the moons of Saturn, isvery interesting in this respect (see guest lecture of Lebreton).

• 4 types of basic organic macromoleculesExcept for water, most of the molecules in a living organism are large organic (=based on Carbon) molecules ormacromolecules. These can be subdivided infour types.

Individual molecules are calledmonomers, and combine to formpolymeres(these are the macromolecules). Thisprocess is called polymerization.

Organic Chemistryincludes all chemical processes using two or more carbon atomsbiochemical, biological, and bioticrefer to living processes.

• lipids: These are fats and oils, and are the primary component of cell membranes. They have one hydropho-bic and one hydrophilic end. They do not dissolve well in water (as grease does). They are therefore notoften found as individual molecules, but form weakly bond aggregates. These are highly flexible and acompact way to store chemical energy.

• carbohydrates: These are molecules that have many hydroxyl groups (−OH) attached. Carbohydrates arepolar and dissolve in water. Sugars are a common example thatform ringlike structures when dissolved inwater. Carbohydrates are used to store energy, and can provide structural support.

• Amino-acids/proteins: Proteins are the most complex macromolecules, and consists of long trains of amino-acids (a monomer with groups with Nitrogen as key-atom, and carboxyl C=O−OH). The particular sequenceof amino-acids that gives a protein its function (20 different amino-acids are found in living organisms).Proteins have an enormous range of roles, eg. to provide structure (hair, nails), or as catalysts (enzymes)

• nucleotides/nucleic acids: Nucleic Acids are the largest macromolecules found, made of a collection ofnucleotides. One nucleotide monomer consists of a sugar molecule, a phosphate group, and a nitrogenousbase. DNA and RNA are famous nucleic acids and store/transport information.

Carbohydrates and proteins are formed by simple reactions that involve the loss of water. In the same way nucleicacids are formed from individual nucleotides.

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2.2 How does life function?

•Deoxyribonucleic Acid (DNA), Ribonucleic Acid (RNA), and proteins

DNA contains four different nucleotides:A, G, C, and T. Threesequential nucleotides form acodon. There are 43 codes forone of the 22 different amino acids found in organisms. Hencemultiple codons can refer to the same amino acid.

In 1953, James Watson and Francis Crick showed that DNAconsists of two long molecular strands that coil about each otherto form adouble Helix

The start of the coding of a polypeptide chain is signaled by aspecific codon (AUG). The end of a chain is signaled by UAA,UAG, or UGA.

Information transfer occurs intwo steps: DNA is the primarymolecule for information storage. DNA can be repaired andcopied by proteins (DNA replicase and helicase).1: ProteinscalledRNA polymerasescopy codon sequences from DNA tomessenger DNA(mRNA). This is calledtranscription .2: Protein/RNA complexes calledribosomesthen direct proteinsynthesis from RNA by reading the codons and inserting the cor-responding amino acids into a growing polypeptide chain. Thisis calledtranslation.

The direction of information transfer in modern organisms is DNA → RNA → protein, except retroviruses. DNAmay also store information on how to regulate certain processes, affecting timing and rate of transcription. Thereare several forms of RNA with different purposes, such as mRNA (see above), tRNA (transfer RNA), and rRNA(ribosome RNA)....

• The Cell

Many different molecules must be close to one other for an organism to operate. A cell structure is needed (in mostorganisms) to stop molecules simply drifting away.

• A cell is a small bag of molecules that is separated from the outside world.

• Cells are the basic structural unit of all present-day organisms on Earth.

• In the center are strands of DNA devoted to use and storage of information

• The DNA is surrounded by a salt water solution containing enzymes and ribosomes, called cytosol.

• The contents are surrounded by a soft membrane, consisting of lipids and proteins, restricting the movementof molecules into and out of the cell.

• An outer tough cell-wall makes the cell rigid.

• Cells can reproduce by splitting into two. First DNA is replicated and the two new DNA molecules attachthemselves to different parts of the cell membrane. After this the cell divides.

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2.3 How to study life’s origin and remains

A biological marker orbio-marker: any evidence that indicates present or past life, detectedeither in situ or re-motely. There are 5 categories:

1: Cellular remains2: Textural fabrics in sediments that record the structure and/or function of biological communities3: Biologically produced organic matter4: Minerals whose deposition has been affected by biological processes5: stable isotopic patterns that indicate biological activity6: atmospheric constituencies whose relative concentrations require a biological source (such as oxygen on

earth).This will be the only way to study possible life on extrasolarplanets.

All these biomarkers can lead to controversy...

• There are two approaches to study the origin of life.The bottom-up strategyfocuses on inanimate matter andattempts to figure out how they may combine to form a living organism. Thetop-down strategylooks at presentday biology and tries to extrapolate back to the simplest living animals.

3 Organic matter in the Universe

Carbon based molecules can be observed in several phases of its voyage or life cycle. Carbon is produced duringHelium burning in massive stars. It can be observed when1) it gets expelled in the red giant phase of massive stars,2) in the interstellar medium, in the dark, dense clouds that form stars, and3) in meteorites and comets in our ownsolar systems which are thought to be the remains or remnant planetesimals that were not assembled into planets.

How we can study this: Molecules produce infrared spectral line complexes due their vibrations and rotations.These complexes can be reproduced and studied in the laboratory (using near-vacuum chemistry). Dust particlescan also be picked up in situ in the solar system using satellites.

1: Circumstellar envelopes of carbon-rich red giant stars, containing large amounts of carbon-based molecules,which the stellar wind expels into interstellar space, mostly locked in CO, C2H2.

2: Interstellar Medium. >100 different molecules have been identified in interstellar space.

The Interstellar Medium (ISM) is composed of mainly three types of clouds: Dark clouds (AV >5 mag), translucentclouds (1 mag<AV <5 mag), and diffuse clouds (AV <1 mag). Stars are formed in the dense (104−8 H/cm3), darkclouds.

Cold gas phase chemistry can simply form CO, N2, O2, C2H2, C2H4, HCN, and simple carbon chains.

The temperatures in the dark clouds are so low (T=10-30 K) that any gas molecule hitting a dust grain will imme-diately freeze out and contribute to an ice mantle. After this chemical reactions are catalyzed by the grain surface,and reaction products are further processed by UV radiationand cosmic rays. In this way one can form CO2 andCH3OH, which are later returned to the interstellar gas.

These evaporated ices can be the precursors for large organic molecules. In particular when they are incorporatedin the warmest (T=200-400 K) and dense (>100 H2/cm3) areas of gas around recently formed stars (hot cores).The largest molecules found so far are HC11N and (C2H5)2O.

When a star is being formed, a spinning disc of dust and gas is produced (the solar nebula), from which ultimatelyplanetary systems are formed (see lecture 6) The solar nebula inherits the organic molecules produced in the hotcores, but some of it may also be newly synthesized.

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Carbonaceous dust in the ISM may show strong diversity:

3: Meteorites and Comets. In situ measurements by space probes on close fly by of comet Haley in 1986 havestrongly enhanced our knowledge about the structure and composition of comets. Meteorites fall on Earth and ifrecognized as such can be studied subsequently.

In the 1950s the modern view of what comets are began with ’thedirty snowball’ theory, with a comet composed ofmainly water ice with small rock particles. Recent observations have shown that the volatile component in cometsis dominated by H2O, followed by CO and CO2, with trace amounts of other chemical species such as CH3OH,CH4, C2H2, and more. In situ space missions indicate a mass ratio between silicates and organic materials of 2:1,but there is also dust composed of CHON particles.

Carbonaceous meteorites represent a sample of interstellar matter, although highly processed.

The Murchison meteorite: A rare carbonaceous chondrite fell near Melbourne in 1969.Eyewitnesses noticedit smelled (aromates). Quickly analyzed by laboratories prepared for moon samples. Several classes of organiccompounds were found, including amino acids, some of which used by life on Earth.

Inward of the asteroid belt the amount of organic matter declines abruptly, while, as we have seen, we need liquidwater to make organic life. That is why it is proposed that organic matter was delivered to Earth by cometsand meteorites originating from the outer solar system.Ongoing debate whether building blocks of life weresynthesized on earth, or arrived from space.

4 Origin and development of life on Earth

So some organic molecules arrived on Earth from space. How could they also form on Earth itself?

4.1 Synthesis of organic molecules on Earth

The more complex the molecule, the moreenergy is needed to make it. The main energy source is the Sun(although 4 Gyr ago it was 20-30% weaker than today). Other energy sources are (locally), lightning, radio activedecay, volcanoes (primordial heat from grav. contraction), and shock waves from meteorites on entry.

• Miller’s origin of life experiment:Simple experiments in the 1950s show that using water as an ocean andmethane (CH4) ammonia (NH3) and hydrogen as an atmosphere, lightning produces organiccompounds for life,in particular amino acids. Recent models of atmospheric evolution indicate that the early Earth did not have anmethane and ammonia rich atmosphere because these molecules are easily destroyed by sun light (more likelyCO2, nitrogen, with water).

•Rate of meteors 4 Gyr ago was much higher (see our moon). Provide Carbon, but also energy through atmosphericshock-waves , and may vaporize on impact− both processes may result in the production of organic molecules.

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Evidence that the organic matter’s origin is extraterrestrial from chirality : Some molecules can exist in right-handedand left-handed forms (isomers). This is called chirality.

• 19 of the 20 amino acids can exist in left- and right-handed forms.

• The chemical reactions that make amino acids generally create equal numbers of left- and right-handedisomers.

• Mixing left- and right-handed forms of amino-acids would hinder proteins from performing their biologicalfunctions.

• Life on Earth uses only left-handed amino acids.

• Early in its history, life must have begun with left-handed amino-acids, locking biology into this preference.

• Was this random? maybe not! Organic rich meteorites have an excess of left-handed amino acids. Thisstrengthens the idea that prebiotic material came from meteorites.

• star forming regions show ultraviolet circularly polarized light (UVCPL). It destroys one type of chiralmolecules more easily that the other. This can produce the bias towards one type of isomer.

4.2 Increasing the local concentration of reactants and making more complex molecules

For two molecules to react, they have to come into contact. The rate of chemical reactions increase with theconcentration of the reactants. The organic matter assumedto have arrived from space needs to be concentrated.possible mechanisms:• Lagoons and tidal pools provide a means to concentrate dilute solutions, with temporarily cut-off solutionsevaporating out.• Freezing a solution also causes an increase of the solution,because water freezes first.• Surfaces of minerals can trap organic matter. Clays are in particular useful since they can incorporate moleculeswithin their structure.

Once the raw materials are in place, construction of life maybegin.The process from organic molecules to complexliving structures is poorly understood.

Creating polymers and macromolecules:Important chemical reactions

• Two−OH groups from two sugar molecules can react to form a bond, with the release of one water molecule.The ends of the newly formed molecule still has two reactive−OH groups and can continue to grow.

• An −NH2 and−COOH group from two amino acids can form a bond with the release of one water molecule.The new molecule still has one of each of these groups enabling the formation of an even larger molecule.Polymerization reactions can continue in principle indefinitely.

• The high level of order and complexity of micro-molecules inliving organisms need more sophisticatedmethods (making use of catalytic properties of enzymes). However, abiotic chemistry is a good start.

4.3 Boundary layers and cell structure

At some point, the complex chemicals need to be kept together, to protect them and to stop them from wanderingabout. How could these structures have formed?

• Amphiphiles, such as lipids, are molecules with a hydrophilic head and a hydrophobic tail.

• Added to water, amphiphiles float, with their heads in the water and tails in the air.

• In this way they can create a single layer of molecules, a mono-layer.

• When the water is subsequently shaken, they form small spherical structures, tails on the inside, heads onthe outside. These are called mi-cells. The double layer equivalent is called a bi-layer vesicle.

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• All these structures are formed spontaneously.

Grouping together without cell structure:Polymers, when added to water, group together to form droplets calledcoacervates. This process is related to polarity of molecules and the formation of hydrogen bonds with water(proteins, nucleic acids, polysaccharides)

Sidney Fox experiment (1958)By heating up a dry mixture of amino acids polymerization occurred. By dissolvingthese in hot water and cooling this down, these polymers formed small spheres, with a double wall resemblinga membrane. These micro-spheres can absorb more protein material from the solution. They shrink and swellaccording to the salt concentration of the water.

Deamer (1985) showed that amphiphilic molecules are also present in the Murchison meteorite, forming boundarylayers when added to water.

4.4 The role of minerals

To reach structure and complexity, minerals could play fourimportant roles:

• protectionfrom dispersal and destruction. eg. small air pockets in volcanic rock, or microscopic splits

• supportstructure for molecules to accumulate and interact.

• selection:many crystal have faces that are mirrors of each other. Left-and right-handed amino acids canbond to different faces, allowing a natural selection to make structures of purely right-handed or left-handedmolecules.

• catalysis:e.g. Nitrogen gas can flow over a metallic surface to form ammonia, in such way becoming avaluable source of nitrogen for biological reactions.

4.5 The first biological systems

No one has yet synthesized a ’protocell’ using basic components which would have the necessary properties oflife. Another approach is to engineer existing prokaryoticcells with less and less genes, to see when the minimalrequirements of life are reached. How simple organic molecules can form a protocell is largely unknown, but thereare many hypotheses.

• ’RNA’ World Hypothesis : It proposes that RNA was the first life-form on earth, later developing a cell mem-brane around itself and becoming the first prokaryotic cell.Relatively short RNA molecules could have formedspontaneously, which catalyze their own continuing replication. RNA can store, transmit and duplicate geneticinformation (just like DNA), but it can also act as an enzyme (ribozyme). In this way it can perform the tasks ofboth DNA and proteins (enzymes).

Principle: It assumes that there existed free-floating nucleotides in the primordial soup, forming bonds regularlywith each other, which easily break up again due to the littleenergy involved. However, certain sequences of pairshave certain properties that lower the energy of the chain, causing them to stay together for longer periods. Thelonger the chain, the more it attracted matching nucleotides at a faster rate than they broke down.

These RNA molecules were now prone to natural selection. A chain that is an efficient catalyzer and replicatorwould survive more easily and could evolve into modern RNA. Further competition may have favored cooper-ation between different RNA chains, further giving selective advantage to those that could serve as ribozymes.Eventually DNA (more stable), lipids, carbohydrates were recruited, leading to the first prokaryotic cells.

Some Problems:1) large RNA molecules are very fragile, and can easily be broken down into individual nu-cleotides through hydrolysis. They also absorb strongly UVradiation causing break-down.2) Prebiotic simula-tions show that the conditions to make nucleotides are incompatible with those for making sugar. Hence they mustbe synthesized and then brought together.

Some recent additions/alternatives:’PNA world’ (Peptide Nucleic Acid) is more stable than RNA and more easilymade in prebiotic conditions. Alternatively TNA, and GNA.

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Retroviruses:Some sort of RNA life is present today, called viruses. They do not have a cell structure but consistof fragments of nucleic acid with a protein coating. In the outside world they do not carry out the functions of life,but within a cell of another organism, and take the cell’s energy and ability to make proteins and nucleic acids inorder to reproduce itself. Most interesting in this respectare theretroviruses, like HIV. They ’reverse’ the cellularprocess by transcribing RNA into the cells DNA, taking over the cellular machinery to make more viral RNA.

• ’Metabolism first’ models: These hypothesize that a cycle of chemical reactions that produce energy that canbe used by other processes, predates genetics. The best example is theIron-sulfur world theory as developed byWachterhauser, in which life occurred on mineral surfaces near deep submarine vents (a smoker: high temperaturehigh pressure environments). Once primitive metabolic cycles were established, more complex compounds wereproduced. They showed that amino acids and peptides could beformed in this way. The temperature and chemicalgradient around a smoker plays an important role in this theory. Each smoker ecosystem functions equivalently toa single cell. How it would evolve into cellular life is difficult to see.

• Other theories around are: ’Lipid world’ hypothesis (First the cell-structures are formed, which have somehereditary potential), Clay theory, bubble theory, autocatalyses theory, deep-hot biosphere model, etc. etc...

Conclusion: there is no recipe for life yet, but there are many ideas floating around. Most promising is the RNA-first world’, although it has its problems.

The top-down approach

It is also possible to extrapolate back as far as possible towards the origin of life by using information contained inthe DNA of organisms. This is possible because life does not reject what evolution has created, but builds on whathas gone before, and a so called phylogenetic tree can be constructed, eg. using genetic information from smallsub-units of ribosomal RNA. It shows that all life on Earth seems to have a common ancestor (hence that all lifeon earth is related).

Our last common ancestor seems to be similar to heat-loving chemo-synthetic organisms that populate hy-

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drothermal vents.

4.6 Alternative hypothesis: Panspermia

From the early 20th century it was proposed that life itself could survive in space in the form of spores. Somespores could arrive from a distant life-bearing planet and at some point have arrived on Earth. One of the problemsis the large doses of fatal radiation it receives during the long trip. We would still need to explain how life wouldhave started in this other planet...

An alternative hypothesis of suggests that panspermia is carried through space through meteorites. It couldhave arrived from Venus or Mars (could have had much friendlier atmospheres in the early solar system). This alsoshifts the problem. How did life start on mars or venus? Once in a while the panspermia hypothesis attracts newattention, and it should not be completely disregarded. Seethe Mars meteorite ALH84001 later in the course.

4.7 Diversity of life in extreme environments

It is suggested that the first living organisms on Earth may well have arisen in the extreme environments of hy-drothermal vents. It is interesting to explore the most extreme organisms on Earth since this may indicate the rangeof environments in which life may form elsewhere.

• Extremophiles: organisms that tolerate (or thrive in) extreme environments.

• (Hyper)thermophiles:Thrive in 80-115 Celsius environments, such as sulfur dependent archea (does noteven grow below 90c).

• psychrophiles: some fish like the very cold: Many microbes can be frozen in liquid nitrogen and survive.

• radiation extremophile: Deinococcus radiodurans can accurately rebuild damaged DNA.

• vacuum tolerant: microbes, insects, seeds

• pressure extremophiles:piezophiles/barophiles

• Salinity, pH, chemical extremes, etc

• Where do we find these extreme environments on Earth?Hot springs, high pressure in deep sea, hyper-saline insalt-flats, permafrost and snow.

5 Prerequisites for life elsewhere

We have seen in the previous section that life on Earth can exist under very extreme circumstances. However, formore complex life to develop (eg. life that uses oxygen), theconditions are more narrow than the wide range thatsupport microbial life.

A planet that can host more complex life would require an ocean and some dry land, moderately high O2 (lowCO2 to allow the formation of an ozone layer) and a seasonable stable climate (planet orbital constraints, stellarevolution, comet/asteroid impact rate, etc.). See later inthe course.

5.1 The circumstellar habitable zone

One property of Earth that is crucial for the development andsupport of life is liquid water. Acircumstellarhabitable zone is defined as the range of distances from a star for which liquid water can exist on a planet’ssurface. This means that the surface temperature needs to bebetween 273 and 373 K.

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• Effective temperature Teff : The average temperature of a planet is determined by the balance of incomingsolar radiation, Ein, and thermal emission from the planet, Eout.

Ein =((1 − A)Lstar

4πd2πR

2

p

whereLstar is the stellar luminosity,A is the planet’s albedo (the fraction of light that is reflected), d is thestar-planet distance, andRp is the radius of the planet.

Eout = σT4

eff4πR2

p

whereσ is the Stefan-Boltzmann constant. Theextrafactor 4 comes from the fact that the planet receives lightfrom one side, but radiates in all directions.Ein = Eout now results in:

T4

eff =(1 − A)Lstar

16πd2σ

Time dependent habitable zone:It won’t be a surprise that for the Sun, the habitable zone is around 1 AU. Notehowever that the solar luminosity is not constant with time.4 Gyr ago the sun was about 30% weaker than today,moving the habitable zone slightly inwards. The region in which a planet may reside and maintain liquid waterthroughout most of a star’s life, is called thecontinuous habitable zone

Habitable zone as function of stellar typeThe luminosity of a star is a strong function of its mass. Hence hot, bluestars have their habitable further out, and cool, red dwarfscloser in. The most massive stars have probably a tooshort lifetime to allow the development of life.

5.2 The greenhouse effect

The temperature at the surface of a atmosphere-bearing planet may be significantly higher than its effective tem-perature, as is the case on Earth (Teff = 255 K,Tsurface ≈ 288 K.), and more extreme on Venus (Teff=238 K.,Tsurface = 733 K.). This is due to the infamous greenhouse effect. While the radiation of the Sun is primarily atoptical wavelengths (at which the Earth atmosphere is reasonably transparent), the Earth thermal energy is radiatedaway from its surface at infrared wavelengths. Since the atmosphere absorbs most of this flux, the atmosphere isheated up from below, which is radiated back to the Earth’s surface, subsequently heating it. The IR absorption isprimarily caused by H2O and CO2. The greenhouse effect can extend the habitable zone significantly further outfrom the star.

’Mickey Mouse’ one layer model:Let’s do a very simplistic calculation to prove the principle. Let us assume aplanet with an atmosphere containing one single layer of IR absorbing matter. The planet’s surface gets heated bytwo components, 1) direct sunlight, and 2) heat radiation from the atmosphere. Overall, the planet should still emitradiation as if it has an effective temperature as calculated above (to maintain energy balance). However, since thelayer in the atmosphere is optically thick at IR wavelengths, it is this atmospheric layer that is ultimately radiatingaway the planet’s heat into space, and should therefore havea temperature equal toTeff . Simple geometry meansthat in this model the planet’s surface receives as much energy directly from the star as from its atmosphere.,increasing the temperature by21/4. See: http://eesc.columbia.edu/courses/ees/climate/lectures/ghkushnir.html

multi layer model:The one layer model can explain temperature rises in the order of 20%. The surface temperatureof Venus is about a factor 3 higher than its effective temperature. This can be explained by introducing multiplelayers. In the simple model of above, anN -layer atmosphere will produce a surface temperatureN1/4 times higherthan the effective temperature. Since an atmosphere is continuous, one layer can be seen as one vertical slab of gasthat has an optical depth ofτ=1, meaning thatT 4

surf = T 4

eff (1 + τ).Proper treatment of radiative transfer givesT 4

surf = T 4

eff (1 + 3

4τ) (see de Pater & Lissauer, p61).

Note that a more dense atmosphere is likely to increase the planet’s albedo. Genuine greenhouse models are highlycomplicated.

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Stability in the greenhouse effect and surface temperature: The CO2 content is a very important factor in the Earth’sgreenhouse effect (that is why there is so much fuss about burning fossil fuels). CO2 is being removed throughchemical weathering (in rock), and is released in the atmosphere by volcanoes. Life on earth also removes CO2

by photosynthesis. This gives a certain balance. A huge amount of Carbon is stored in the Earth. On Venus thisbalance is complete shifted to one direction, with 96.5% of Venus’ atmosphere consisting of CO2. This is alsoknown as a ’runaway greenhouse effect’.

5.3 Was the young Earth a habitable planet?

One has established that the oldest rocks on Earth are∼4 Gyr old (through radiometric dating, see tutorial). Thereis strong geological evidence that these rocks have been deposited in water (hence implying the presence of liquidwater at that time). This means that the temperature must have been like now. How is this possible, the Sun was25-30% less luminous, implying an effective temperature of−33 Celsius?

• Plate tectonicswas particularly violent on the young Earth, because of moreinternal heat, due to 1) primordialheat, 2) radio-active decay, 3) iron and nickel sink to the center to form a core. The enhanced plate tectonics keepthe greenhouse gas CO2 in the air.

• Water? Where the water comes from is a problem. An obvious possibility is from comets, but these have adifferent isotope ratio than water on Earth. Ongoing debate...

5.4 Other energy sources

This section has assumed that the central star needs to provide the energy for the development and support oflife. However, other potential energy sources could be radio-active decay and/or heat generation through tidalinteraction (This is why the moons of Jupiter, far outside the Sun’s habitable zone, are still interesting as a habitatfor life).

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