Astrobiology Dr. Paul Wesselius 9 January 2019 Lecture 7: Biosignatures, Framework 1

Inhoud Introduction Physics and Life Biosignature Assessment System and exoplanet properties Biosignatures 2

INTRODUCTION 3

Literature Astrobiology Strategy 2015, NASA From Matter to Life, Information and Causality, Edited by Walker, Davies and Ellis, Cambridge Un. Press, 2017; very difficult book C.H. Lineweaver, Annual review of Earth and Planetary Sciences, 2012, 40, ; Astrobiology, 2016, 16, 7 Exoplanet Biosignatures, Parenteau et al., Astrobiology 18, Number 6, juni 2018, chapter 4, The Ladder of Life Detection, Neveu et al., Astrobiology 18, Number 11, 2018,

PHYSICS AND LIFE 5

Emergence Emergence concerns an entity having properties its parts do not have due to interaction among the parts Weak emergence: emergent property can be simulated with a computer Strong emergence: qualities produced in an entity are irreducible to to the system’s constituent parts Example: properties of hydrogen and oxygen do not lead to the properties of water; no simulation of the system can exist Other examples – Hurricanes, Crystals, Friction, Temperature, Convection, Classical mechanics, Statistical mechanics 6 Mostly from Wikipedia

Emergency levels Laws of physics are an emergent system Chemistry is emergent property of laws of physics Biology is emergent property of laws of chemistry Psychology is emergent property of neurobiological laws Economy may be an emergent property of psychology 7

Origin of Life Given the right geology and chemistry, early RNA may form RNA chains lead to natural selection Replication with moderate fidelity is necessary Acquire and process resources Complicated molecules lead to biological organization of life: proteins, etc, interact with other molecules to achieve higher biological functions Eventually creating an organism Emergence of mind is separate phenomenon 8

How to detect life? Physical and chemical signatures are probably inadequate A clear emergent property of life should be detected What would that be? Trying to detect Life elsewhere based on Life on Earth may be mostly futile Research is urgently needed to determine the ways to detect life 9

Hard problem of Life Walker and Davies (book From Matter to Life) define a ‘hard’ problem of Life Physical laws are fixed Initial state of universe must be so special that the present state of the world can be derived The information content of genome, not the chemical nature of DNA, determines how one cell develops in e.g. a human baby The hard problem of life is identification of the actual physical mechanism that permits information to get control over matter 10

Constructor theory Formulate all laws of physics as statements about what transformations are possible, which are impossible and why Constructor theory of information leads to constructor theory of life Input substrates ---  output substrates via a constructor E.g. input chemicals are transformed via a catalyst (the constructor) into output chemicals When no-design laws exist the constructor must implement a recipe Replicator and self-reproducer, e.g. DNA is copied with error correction, and then the information in the DNA (recipe) creates a new ‘self’ 11

Physics ‘Laws’ Two very basic properties of physics are conservation of energy and increase of entropy (≈disorder) Entropy is intimately linked to information: the higher the information content the lower the entropy A living being contains much information and thus decreases the entropy Its surroundings must compensate for that (compare with a refrigerator) If an energy flow is large, e.g. from the Sun to the Earth, Life helps to increase the entropy Could that lead to a detectable biosignature? 12

BIOSIGNATURE ASSESSMENT 13

Bayes Statistics We only know Life on Earth We are unable to define what Life is Several atmospheric and surface properties are caused by this Life on Earth When one would find such properties on an exoplanet could it be concluded that there is Life as well? How to determine the probability for Life on an exoplanet? Use Bayesian statistics (Bayes, mathematician and clergyman, 1702 – 1761) many present authors tell I personally doubt this because the prior is unknown 14

Smallpox and Bayes Patient goes to family doctor with clear signs of smallpox Naive conclusion (maximum likelihood): patient has smallpox But: smallpox has been eradicated on Earth In Bayes’ formula: P(smallpox| symptoms) = P ( symptoms | smallpox) x P (smallpox) The probability of having smallpox given (|) the symptoms is the product of the likelihood and the probability to have smallpox And that last probability is virtually zero 15

Applicability to Life The socalled ‘prior’, in our example the probability to have smallpox, should be reasonably known In the Bayes formula for the probability of finding Life on an exoplanet the prior is the probability that Life exists on an exoplanet This prior is (almost) completely unknown We try to find exoplanets that resemble Earth in as many aspects as possible and assume that the probability of Life being able to exist on such an exoplanet is of order one Even then finding Life remains difficult: many biosignatures can also occur via abiotic processes 16

17 Astrobiology 18, page 711

Explanation figure ‘Context’: The general properties of an exoplanetary system and its parent star ‘Data’: observations performed from the Earth or with satellites concerning the atmosphere and/or the surface of an exoplanet ‘Exo-Earth System’ models: through model calculations determine whether the ‘Data’ point to Life or no Life ‘Prior knowledge’: Probability beforehand that Life originates and continues to exist on a certain habitable exoplanet ‘Posterior probability’: Probability that Life exists on a certain exoplanet, based on the ‘Data’ and ‘Context’ 18

19 Astrobiology 18, page 715. Four aspects to determine whether potential biosignatures can be best explained by Life. 1.: General habitability concerning parent star and exoplanet 2.: Habitability concerning specific properties of a certain exoplanet 3.: Measure the best biosignatures 4.: Check whether these biosignatures can also be caused by abiotic processes

In formula P(life|D,C) = P(D|C,life) x P(life|C) P(D|C,life) x P(life|C) + P(D|C,no life) x P(no life|C) P(life|D,C) is the probability for the presence of Life on an exoplanet when biosignature observations ‘D’ are available and the habitability of the exoplanet is ‘C’ The numerator is most important. The last term is P(life|C), the probability for Life on an exoplanet, given its properties and those of its parent star When that probability is very small do not spend time on observations of that exoplanet The denominator has a value of order one 20

SYSTEM AND EXOPLANET PROPERTIES 21

Properties parent star Size and mass of an exoplanet are determined with respect to the size and mass of its parent star The properties of parent stars are derived from their spectral type, and – until early 2018 – for very few objects using their distance The distance to and precise spectral properties of the parent star should be well known Using stellar models makes it then possible to determine accurately the size, mass and spectrum of the parent star 22

Gaia On 25 April 2018 Gaia had its data release 2 The five-parameter astrometric solution - positions on the sky (α, δ), parallaxes, and proper motions - for more than 1.3 billion (10 9 ) sources, with a limiting magnitude of G = 21 - were presented The G band is 400 nm wide and has its maximum around 600 nm This means that all relevant parent stars have known distances  better star sizes and masses (using the stellar spectrum and the accurate distance) 23

Our Sun We know that our Sun, a G2V star, was able to sustain Life for 4 billion years The Sun has presently a surface temperature of about 5,800 K (G2); the ‘V’ represents a main sequence (‘normal’) star, the typical star for this temperature class The Sun is located in the outer part of the Milky Way Galaxy and was formed from material that had been processed inside a supernova The Sun is not, as is often said, a small star; there are so many dwarf stars that the Sun falls in the top 5 percent of stars in its neighborhood 24

Other stars Because of the billions of years that Life on Earth has existed we exclude stars that live shorter than a billion years as candidate Life stars, i.e. stars hotter than ~ 8,000 K Cooler stars live even much longer than the Sun and there are much more of them, but they experience several problems for the occurrence of Life; this is an active field of investigation Ultraviolet spectrum is very important: DNA is destroyed by it Stellar magnetic field evolution drives stellar activity 25

Problems with cool stars Cool stars may be very active in their earliest few 100 million years extinguishing Life Exoplanets in the habitable zone will be so close to a cool star that rotation and orbital period are the same Then, the planet is very hot on one side and very cool on the other Maybe strong winds will allow some parts to be habitable? Energy output of cool stars decreases strongly in first few billion years  habitable zone moves ‘quickly’ outwards 26

Necessary exoplanet properties Widespread liquid water Energy for metabolism – star light (but also from chemical reactions in darkness) Renewable supply of essential chemical elements – water, carbon, air with oxygen and ozone A reasonably stable climate In principle water, carbon and energy suffice Are there other bodies in the solar system that have these conditions? 27

Habitability and water Presence of water on surface of a rocky planet having 0.1 – 1.5 Earth radii, 0.1 – 5 Earth masses Effective surface temperature of an exoplanet can be calculated when the central star’s brightness, temperature and distance to the exoplanet are known The real temperature will be higher because in a sufficiently dense atmosphere a greenhouse effect will always be present At the right temperature ( o C) and a sufficiently dense atmosphere  liquid water  exoplanet in Habitable Zone (HZ) 28

Bottlenecks Look at the Life problem in a macroscopic way Lineweaver defines three kinds of possible bottlenecks that may hinder the emergence of Life An ‘emergence bottleneck’: many conditions should be fulfilled before Life can emerge A ‘self-destruction bottleneck’: a technological civilization may destruct itself very soon A ‘Gaian bottleneck’: new idea: Life has to develop rather fast in order to be able to keep the exoplanet habitable 29

30 Fraction of wet rocky exoplanets in the HZ Time since origin exoplanet in billions of years

Role of biology Predicting habitability using just physics and chemistry is too simple Life makes a very strong imprint on surface and atmosphere of an exoplanet Several biologists think that the emergence of Life is almost inevitable because of the properties of our universe when an exoplanet is in the abiotic habitable zone (AHZ) The necessary simple organic monomers to create more complicated polymers can be easily made on the exoplanet itself or be brought there by meteorites 31

32 Life normally disappears after ~ 1 Gyr. Only Life itself can extend this to ~ 6 Gyr. (Then the sun-like star will become too hot.)

33 A rocky planet can become much too hot (Venus) or much too cold (Mars). The physical properties on an average planet seem unable to maintain a stable temperature. Life can stabilize the temperature at the points A, B, C, D. But Life should then develop sufficiently fast.

Earth and habitability Microbes can live in the atmosphere, on the ground or in water Multicellular organisms, and also several kinds of microbes, cannot survive on many places of our Earth (‘deserts’) Apart from the ‘normal’ water-less deserts and polar regions there also exist regions with a severe lack of nitrogen and iron The global characteristics of an inhabited planet may be only subtly influenced by Life E.g. the free abundant oxygen molecules now on Earth were absent in the first three billion years of Life on Earth 34

35 There are 4 kinds of ‘deserts’ on Earth: -Dry (sand colour), -Cold (white), -Lack of nitrogen (dark blue), -Lack of iron (light green)

BIOSIGNATURES 36

Ladder Astrobiology 18, nr 11 describes the Ladder of Life Detection Indigenous life should be considered as the hypothesis of last resort in interpreting life- detection measurements Small set of decision rules are proposed to determine whether abiotic interpretation is disproved The Ladder does not directly point to specific biosignatures or life-detection measurements 37

Biosignature Something that indicates biological activity Should be caused by Life processes But: Life is very difficult to define It makes no sense to define Life before there exists a theory how Life is created Qualitative criteria should be replaced by quantitative criteria Bayes statistics may answer the question how probable it is that someplace Life exists The next lecture, by Barthel, will give much more information on this topic of Biosignatures 38

39 How to find Life? (a)Find hopane, a lipid, in stone (b) chert contains microfossiles (c) a stromatolite with mats of micro organisms (d) a leave of a plant that reflects light in a special way (e) Listen to signals from intelligent beings elsewhere From: NASA Astrobiology document Observing (a) and (b) from far away is very difficult. (c) and (d) might be measurable.

40 Summary of biosignatures Left: gaseous products of biological processes Middle: surface biosignatures Right: temporary variations, e.g. changes in CO 2 concentration

41 Astrobiology 18, page 646. The reflection spectrum that a 15 meter telescope in space could obtain of an ‘Earth’ at 30 lightyears distance, that revolves around a star like our Sun. Grey lines show the noise in the observations.

42 Reflection spectrum of Earth in the Proterozoïcum (2.5 billion to 541 million years ago). There is only 0.1 % oxygen in the Earth’s atmosphere.

43 Astrobiology 18, page 640. By several abiotic processes oxygen can be formed in a planet’s atmosphere. Left the Earth is shown. The encircled molecules may show that indeed an abiotic process is at work. At the bottom biological ‘forbidden’ molecules are shown.

Laboratory simulations Article in ACS Earth and Space Chemistry of 26 November 2018 by Chao He et al Haze formation was simulated in the laboratory in a range of exoplanet atmospheres Organic gas products and molecular oxygen are photochemically generated, an abiotic source for these potential biosignatures Even the copresence of molecular oxygen and organics can be abiotically caused 44

Time interval Origin of Life See Astrobiology, 2018 Mar;18(3): , Pearce et al. Habitability boundary: earliest time at which Earth became habitable, possibly between 3.9 and 4.5 billion years ago Biosignature boundary: earliest time with clear evidence for Life, earliest reliable signs ~ 3.7 billion years ago  Life has appeared on Earth in a time interval of 200 to 800 million years 45