Astrobiology Dr. Paul Wesselius 14 Nov 2018 Lecture 1: The Earth Its Youth, Geology and Oxygen

Contents Introduction First billion years of Earth Geology of Earth Earliest life on Earth Oxygen on Earth 2

INTRODUCTION 3

Overview of courses 4 DateTitlePerson Earth, its youth, geology and oxygenWesselius Life Poolman Tutorial 1Frantseva Habitability solar systemBarthel Tutorial 2Frantseva ExoplanetsWesselius Tutorial 3Frantseva Habitability exoplanetsBarthel Tutorial 4Frantseva Origin Solar SystemFrantseva Tutorial 5Frantseva Biosignatures, FrameworkWesselius Tutorial 6Frantseva Biosignatures, Observational prospectsBarthel Tutorial 7Frantseva Examination Essays

Literature Life in the Universe, A beginner’s guide, Lewis Dartnell, Oneworld Publications, 2009 How to find a habitable planet, James Kasting, Princeton University Press, 2010 Astrobiology, Charles Cockell, Wiley Blackwell, 2015; an excellent, complete and very thorough book. It is possible to download powerpoints from Cockell, and I have extensively used those for this lecture Constraining the Time Interval for the Origin of Life on Earth, Pearce et al.,Astrobiology 18 Issue 3: March 1, 2018 The Detectability of Earth's Biosignatures Across Time, Palle, Handbook of Exoplanets, Springer,

Websites search for extraterrestrial intelligence ; the excellent NASA site a journal by NASA From: Spaceref, a space news and reference site y/ nice article in National Geographics y/ 6

Topic of this lecture In this lecture we look at our Earth as if it is an exoplanet What can we learn from the history of our Earth, extending over 4.56 billion years, that is relevant for finding life elsewhere, within our solar system or on exoplanets? Many aspects can be covered. In this lecture I concentrate on: – The first 1 billion years of the Earth, – The geology of the Earth, – Early signs of Life, – The rise of oxygen on Earth. 7

8 Van: astr.gsu.edu/hbase/Geophys/geotime.htmlhttp://hyperphysics.phy- astr.gsu.edu/hbase/Geophys/geotime.html ‘Era’ is in units of million years ago.

FIRST BILLION YEARS OF EARTH 9

The early Earth When the Earth formed, it was a homogenous mass of molten rock. Three major heat sources were important at this time: 1) Material pummelling into the surface during the formation of the Earth, 2)Gravitational forces compressed material into the newly forming Earth, generating heat, 3)Three major elements release energy from radioactive decay in planetary materials: uranium ( 235 U and 238 U), potassium ( 40 K) and thorium ( 232 Th). 10

Differentiation 11 Within 50 million years temperatures on Earth cooled to the melting point of iron. Global differentiation was completed by about 4.3 billion years ago leading to the composition and structure we are familiar with today.

Early oceans and comets There exist two forms of hydrogen in water, ‘normal’ hydrogen (H), containing one proton and one electron, and deuterium (D), containing one neutron as well as one proton and one electron. The ratio of D/H provides information on the origin of the water on Earth The D/H ratio in comets are about two to three times higher than the ratio in Earth’s oceans  the water of the Earth’s oceans was not delivered by comets Comets probably did bring in a range of volatiles, including CH 4, CO, NH 3, N 2 12

Origin of the moon to 100 million years after the solar system coalesced the hypothetical Mars-sized planet Theia collided with the Earth and that formed the Moon. (Wikipedia)

Early oceans After Theia collided with the Earth to form the Moon oceans and (part of) the mantle vaporised  an atmosphere of rock vapour and silicate clouds During ~1000 years one meter of silicates per day rained down Once the magma ocean solidified water rained down at 1 meter/year  oceans within a few 1000 years Thus by 4.3 to 4.5 Ga ago, oceans had formed on the surface of the Earth These oceans were probably warmer than today and could have been somewhere between 50 and 80°C 14

The early atmosphere 15 Originally Earth had a thin atmosphere, composed primarily of helium and hydrogen. Because these are light gases they easily escaped into outer space. Some gases produced by volcanic eruptions would have contributed significantly to the atmosphere of the very early Earth, as they do today.

The Faint young Sun paradox 4.5 Ga ago the Sun was ~ 30% less luminous than it is today  significantly reduced solar insolation That would have led today to an icy world: the Faint young Sun paradox, even considering the higher heat flux from the interior of the planet at that early time Greenhouse gases such as CO 2 and CH 4 could have compensated; CO 2 thousand times more than today and CH times; also NH 3, a very strong greenhouse gas, could have warmed the surface of the early Earth 16

Major events 17 Some of the major events in the first billion years of Earth history

GEOLOGY OF EARTH 18

Colours of the Earth * In various stages the Earth had different colours. It started black, just after the surface had sufficiently cooled, mostly basalt; there were of order 250 minerals present New minerals via crystallization and other processes  the Earth became grey from granite (lighter than basalt!; 4 billion years ago) Then, water could remain in liquid form: blue Earth Then, ice ages arrived: white Earth Of order 550 million years ago that ended and there was a (Cambrian) explosion of life  green Earth 19 * Solar system exists since 4.56 billion years

20 Earth maybe looked mainly black in its earliest eons (Wikipedia)

21 Earth, 4 billion years ago, grey rocks and blue water. See: com/re/eart h-billion- years-ago- during-the- hadean-eon / com/re/eart h-billion- years-ago- during-the- hadean-eon /

22 Artist impression of snowball Earth, 720 million years ago; snowballhttps://news.cnrs.fr/articles/when-earth-was-a- snowball

23 "The Blue Marble" is a photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft on its way to the Moon, at a distance of 29,000 kilometers. It shows Africa, Antarctica, and the Arabian Peninsula.

24 A variety of silicate structures form when silicon binds to oxygen. These structures are the main ingredient of most rocks and minerals. There are roughly 4,500 named minerals in the world and approximately 30 to 50 new minerals are described each year.

Minerals 25 The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs.

Other minerals; rocks There are many other minerals such as carbonates (e.g. limestone; CaCO 3 ) and sulphides (e.g. fool’s gold or pyrite, FeS 2 ) These are caused by weathering or by biological processes Carbonates are very important because of the preservation of the past evidence for life All of these minerals form rocks; a rock describes a solid lump of planetary material; it can be comprised of many individual minerals An example of a rock is basalt which contains olivine, pyroxenes and feldspars 26

Glasses A rock need not contain minerals If crystals of particular minerals have not had time to form (a lava eruption), it could be a homogenous material or a glass In glass the silica and cations are evenly distributed through the material, but in a non- crystalline structure A good example of a glass is obsidian, a silica-rich volcanic glass that was used by ancient people to make cutting implements, because the glass can be fractured to produce very sharp edges 27

Basalt and obsidian 28 basaltobsidian

Rock cycle 29 A rock cycle describes the different types of rocks produced on the Earth and their fate in the Earth system.

Continental drift 30

Radioactive dating 31 Parent isotope in rockDaughter isotopeHalf-lifeType of decay Rubidium-87 ( 87 Rb)Strontium-87 ( 87 Sr)47 Gabeta Uranium-238 ( 238 U)Lead-206 ( 206 Pb)4.47 Gaalpha (8), beta (6) Potassium-40 ( 40 K)Argon-40 ( 40 Ar)1.25 Gabeta Uranium-235 ( 235 U)Lead-207 ( 207 Pb)704 Maalpha (7), beta (4) Carbon-14 ( 14 C)Nitrogen-14 ( 14 N)5730 abeta

EARLY LIFE ON EARTH 32

Early life on Earth Evidence for the earliest life is highly controversial and still debated Much of the debate surrounds the oldest fossils The emergence of life on the Earth includes the Hadean and the early Archean The Hadean eon ran from 4.56 to 4.0 Ga ago Archean eon ran from 4.0 to 2.5 Ga ago Pearce et al. define a habitability boundary (less than 4.5 Ga ago) and a biosignature boundary (less than 3.7 Ga ago): carbon isotope ratios and stromatolite fossils point to this value Life must have appeared between 3.7 and 4.5 Ga 33

34 Constraining the times of habitability and biosignature boundaries (paper of Pearce et al.)

Minerals and Life The site two-thirds-of-earths-minerals-were-created-by- life/ describes the ideas of Robert Hazen two-thirds-of-earths-minerals-were-created-by- life/ Hazen and colleagues have shown that life didn’t start in isolation—minerals likely helped it along the way As life evolved, it created a myriad of chemical niches that allowed new minerals to form The advent of oxygen, made by life, created many new minerals 35

Isotopes Many elements of low atomic weight (including those used extensively by living beings) have two or more stable isotopes Some examples are shown below with their atomic mass numbers shown as superscripts: – Hydrogen: 1 H, 2 H (sometimes written as D) (deuterium) – Carbon: 12 C, 13 C – Nitrogen: 14 N, 15 N – Oxygen: 16 O, 17 O, 18 O – Sulfur: 32 S, 33 S, 34 S, 36 S 36

Isotopic fractionization Different numbers of neutrons confer on elements different chemical properties A lighter isotope has higher molecular vibrational frequency, it forms a weaker bond and is more reactive The lighter molecules become concentrated in the products of reactions This is called isotopic fractionation This fractionation is proportional to the mass of the isotopes The principle is shown for 12 C and 13 C 37

Morphology We can look for signs of spirals, filaments and other shapes that suggest cellular structures William Schopf presented photo-montages of inferred microfossils from rocks, the oldest from the Warrawoona Group of the Pilbara Craton in West Australia, ~ 3.5 Ga old 38 Later, the microfossils were claimed to be associated with a hydrothermal vein created by the interaction of hot water (over 200°C) in the ancient rock that was emplaced more recently  no life

Criteria Criteria were proposed by which all suggested microfossils should be judged Putative ancient fossils should: 1.Be in rocks that are shown to be sedimentary and have undergone very low levels of geological alteration (metamorphism), 2.Be made of kerogen *, 3.Exist with other fossils (and not just be an isolated occurrence), 4.Be of a size at least as great as the minimum known size for viable cells, 5.Be hollow (suggesting a cellular origin). 39 * Kerogen is a solid organic matter in sedimentary rocks. Petroleum and natural gas form from kerogen.

Stromatolites Stromatolites form another type of morphological feature of life These can be observed at the macroscopic scale and are found in some locations on the present-day Earth Shark Bay stromatolites in Australia, for instance, result from the interaction between microbes and the physical and chemical environment 40 Present-day stromatolites growing in the Shark Bay, Australia. Each one is about half a meter across.

41 (a) Stromatolites in an exposed outcrop of 3.7- Gyr-old metacarbonate rocks in the Isua supracrustal belt in southwest Greenland. (b) Interpretation of (a) (c) Asymmetrical, peak- shaped stromatolite and (d) linked, dome-shaped stromatolites from the ~2-Gyr-old Wooly Dolomite carbonate platform in Western Australia. (From Pearce et al.)

42 Stromatolites in the Hoyt Limestone (Cambrian period, 500 million years ago) exposed at Lester Park, near Saratoga Springs, New York.

OXYGEN ON EARTH 43

Rise of Oxygen on Earth The most significant environmental change that has occurred throughout the history of the Earth is the rise in atmospheric oxygen About 2.4 billion years ago (the Great Oxidation Event), and again about 700 million years ago, there was a dramatic rise in oxygen in the Earth’s atmosphere leading to levels we are familiar with today It is unknown at present how this increase in oxygen came about, although there are many theories 44

Geological proxies Scientists rely on geological proxies Geological proxies are changes in the characteristics of rocks or minerals or compounds that can tell us something about the environment in which they were formed In short, a proxy is a measurement of some sort that provides information about something we cannot measure directly We look at the rock record to get proxies for oxygen abundance further back in Earth history 45

Oxygen increase Geological proxies suggest that Earth has experienced two major rises of oxygen throughout its history Before 2.4 Ga ago oxygen concentrations in the atmosphere were probably much less than times the present atmospheric level (PAL) The first rise ~2.4 Ga ago took oxygen concentrations to a few percent of present-day levels About 700 Ma ago oxygen concentrations rose to at least 10% of the present day value, with a rise or rises after that leading to its present-day value In the Paleozoic, there is even evidence for a rise in oxygen concentrations to above present day values (~30%) 46

Oxygen increase 47

Minerals at low oxygen Pyrite (FeS 2 ) is formed at an O concentration below 0.1% of the Present Atmospheric Level (PAL) Uraninite (uranium oxide) is formed at oxygen concentrations below 0.01% PAL Siderite (iron carbonate; FeCO 3 ) forms at 0.001% PAL Determine concentrations of these oxides in minerals deposited in sediments In modern rivers, pyrite, uraninite and siderite are completely oxidized during transport and so are absent in detrital * sediments 48 * Detrital sediment is material that settles to the bottom of a liquid.

Solubility of minerals Some elements are soluble in only one oxidation state; e.g. iron and manganese are much more soluble when they are in a reduced form (Fe 2+ and Mn 4+ ) compared to their more oxidised form (Fe 3+ and Mn 6+ ) Uranium and molybdenum, by contrast, are much more soluble when they are in their oxidised forms (U 6+ and Mo 6+ ), compared to their reduced form (U 4+ and Mo 4+ ) Soils older than about 2.4 Ga are significantly depleted in iron relative to those younger than 1.8 Ga; this points to a change in the oxidation state of iron between 2.4 – 1.8 Ga 49

50 Aerial view of the Marandoo open pit mine in Pilbara, Western Australia. Zircon crystals are found here. The oxygen isotope ratios point to the existence of water! Water on earth then already existed 4.4 billion years ago! Red Earth

Source of Oxygen The primary source of free oxygen was cyanobacterial oxygenic photosynthesis; oxygen gas is produced as these organisms use water as the electron donor: 6 CO H 2 O → C 6 H 12 O O 2 Photosynthesis is thought to have arisen between 3.5 and 2.7 Ga, well before the Great Oxidation Event  the oxygen produced by cyanobacteria must have been quite some time mopped up before it was capable of causing a large-scale increase in atmospheric oxygen concentrations 51

Sinks of Oxygen Two major processes to remove oxygen from the atmosphere are: 1)The reaction pathway for photosynthesis is reversible in two ways: 1) One reverse reaction is aerobic respiration whereby organics are oxidised using oxygen to produce energy, thus mopping up oxygen 2)The reaction can also be reversed when organic material from dead organisms oxidises abiotically in an oxygen- containing atmosphere 2)Oxygen reacts with reduced gases such as hydrogen and hydrogen sulphide exhaled from volcanoes and reduced elements such as ferrous iron (Fe 2+ ) in water bodies; in the process it is removed from the atmosphere 52

Snowball Earth periods One of the most dramatic climatic changes that has been linked with the increases in atmospheric oxygen concentration are Snowball Earth episodes 53 This process seems irreversible. However, the outgassing by volcanoes of e.g. CO 2 and CH 4 led – slowly – to an enhanced greenhouse effect  the ice melted again and often a very hot period followed.

Survival 54 Cryoconite holes on the Juneau Icefield, Alaska. Just one type of microbial community that can be successful on the surface of ice. Each hole is several centimetres in diameter and is filled with sediment and microorganisms.

Life and rise of oxygen 55 The rise in oxygen itself would destroy quite some forms of Life existing. Oxygen is reactive and in combination with ultraviolet radiation or other ionic species it produces reactive oxygen chemical species such as oxygen free radicals. Free radicals can oxidise membrane lipids, tear cofactors from proteins and oxidise key biological molecules such as proteins and nucleic acids.

Oxygen and animals The rise of oxygen leads to the emergence of more complex organisms, taking advantage of the possibility of aerobic respiration The second rise of oxygen is coincidental with the rise of large animals: tubular and frond-shaped organisms have been found in rocks between 585 Ma and 542 Ma ago. Sufficient oxygen (~10 %) became available to drive energy intensive multicellular structures: animal respiration It seems likely that this contributed to greater complexity in ecosystems, since aerobic respiration allowed for longer food chains and successively larger organisms. 56

Oxygen and large animals Why would organisms have tended to get larger? This has interested evolutionary biologists for decades Larger organisms would have been more capable of overwhelming prey: an evolutionary “arms race” Trilobite fossils preserve the injuries of attacks by one of the Cambrian’s top predators, Anomalocaris providing evidence for interactions between predator and prey at that time. 57 A model of Anomalocaris. The organism was about one meter long.