PHYSICS 20E - LIFE IN THE UNIVERSE (midterm 2)

Mars' Long Term Climate (or The Death of Mars)

(1) Mars has a small size, which means its interior cooled quickly! (2) With little atmosphere, H2O is broken apart by UV radiation, and (3) Mars' low surface gravity means that H2 will escape! Images from the MAVEN spacecraft show C, O, and H atoms high above the Martian surface. C and O escape due to solar wind stripping, and H is being lost to thermal escape. Mars is hydrogen depleted!

Many Small Moons

As of 2019, we know of more than 200 moons around the four giant planets! •Lots of small moons!•Most are "potato shaped" •Small moons have irregular orbits (some in opposite direction as planet spin - retrograde) •Moons with retrograde orbits are probably captured small bodies

a good criterion for calling a planet "habitable."

Atmospheres and oceans will protect organic molecules from UV radiation - this is a good criterion for calling a planet "habitable."

Cyanobacteria

Blue-green algae started adding O2 to the atmosphere ~2.7 billion years ago!

Proteins

Chains of amino acid bases. •Often described as the workhorses of cells - wide variety of functions •Enzymes: serve as catalysts in biochemical reactions inside cells, including copying DNA! (More on this later.) •Catalysts: substances that facilitate or accelerate chemical reactions that would otherwise happen more slowly •Enzymes are left unchanged after they catalyze reactions - they can be reused!

Energy Sources

Chemical energy is available if reactants are brought into contact - atmospheric / ocean + mixing is important! More possible energy sources: heat of formation, radioactive decay, heat of differentiation, tidal heating!

Oxidation Reactions

Chemical reactions that remove oxygen from the atmosphere

Uranus and Neptune

Colder atmospheres than Jupiter and Saturn — liquid water clouds would be deep in the atmosphere near strong winds. Their outer cores, made of water, methane, and ammonia, may be liquid! Deep interior oceans could be a place for life. High pressures, strange mix of liquids, and no obvious way of extracting energy make life seem unlikely. But, we can't rule it out.

Methane on Mars

Curiosity rover detected measurable and variable methane levels in Gale Crater! Methane could be produced by geology or chemoautotrophs!

Seasons and Dust Storms

Cycling of CO2 between solid and gas phases causes intense winds, which spawn dust storms and dustdevils! The dust suspended in the atmosphere also gives the sky a yellow-brown color. The color of Martian soil (and Martian dust in the atmosphere) is due to oxidation!

As long as accretion can occur, other worlds will contain elements essential for life.

Different elements and molecules will condense out of the protoplanetary disk at different temperatures (distances), and get incorporated into planets!

Lipids

Energy storage ("fats") and cell membranes •Store energy: e.g. "fats"•Repel or attract water •Play a handful of roles in cell structure, but focus for this class is membrane formation •Lipid membranes can form spontaneously in water and probably did so on early Earth •Organic molecules would be trapped inside space formed by lipid membranes, facilitating reactions in closed spaces

Adenosine Triphosphate (ATP)

Every living cell uses the same molecule to store and release energy! •ATP and ADP (adenosine diphosphate) are the "energy currency" of the cell •Energy is stored in a single bond, and released when a phosphate group is released (hydrolysis), which transforms ATP to ADP •All life on Earth uses ADP/ATP for storing and releasing energy! Evidence for a shared ancestor!

Viking and MRO orbiter photos

Evidence for liquid water flows! Crater counts suggest water has not flowed for 3 Gyr.

Impacts and Energy

Example: Meteor Crater in Arizona •Geologists have identified > 150 impact craters on Earth •Plate tectonics - more impacts than this have occurred •Meteor crater: a 1 km wide crater formed from a 50 m wide impactor; debris likely covered hundreds of square km •Typical velocities: few x 104 m/s! •These impacts carry at least as much energy as nuclear bombs!

Mutation Rate Changes

External factors can lead to increased mutation rate •E.g. UV radiation leads to mutations that can cause skin cancer. •Changes in ozone layer can lead to changes in UV radiation reaching Earth's surface. •High energy particles can also cause mutations. •Earth's magnetic field strength varies with time (and reverses!), leading to changes in protection from charged particles.

anaerobic chemoautotrophs

First microorganisms were anaerobic chemoautotrophs: carbon from CO2, energy from chemicals E.g. sulfide based "chemosynthesis" reactions:Purple sulfur bacteria in a stagnant pond 12H2S+6CO2→C6H12O6+6H2O+12S

Recent water flows!

Imagery from the Mars Reconnaissance Orbiter shows dark streaks on the wall of a crater. The streaks change from season to season, and appear to grow during the warmest times of year. This suggests they are caused by flowing (salty) water!

Potential Liquids for Life

Important consideration:freezing and boiling points as a function of atmospheric pressure

Sugars in Space

In 2012, astronomers detected the simple carbohydrate glycolaldehyde in the nebulae surrounding newly forming stars!

Perseverance

Jezero Crater- ancient river delta and lake system could have supported microbial life. Perseverance carried the first aircraft to explore another world (Ingenuity)! Mission goals: climate characterization, searching for preserved biosignatures and organics, preparation for human exploration.

Impactor Kinetic Energy

KE = 1/2 mv^2 m = mass = 5 x 10^8 kg v = velocity = 2 x 10^4 m/s KE = 10^17 J (about a 20-megaton nuclear bomb!)

Nucleic Acids

Made of nucleobases attached to a sugar-phosphate helical structure •DNA: Deoxyribonucleic acid•RNA: Ribonucleic acid •DNA and RNA contain genetic information that allow cells to function according to precise, heritable instructions •More details on these next lecture!

Flooding, Impacts, and Volcanic Activity

Mars Express images show channels likely carved by a flood, which possibly emerged from underground! Flood channels downhill from a volcano suggest eruptions caused melting of subsurface ice!

Schematic of a bacteria cell:

Much simpler than plant and animal cells! Lack membrane-bound organelles. Genetic material not located in a nucleus! (This characteristic used to define different categories of life: eukaryotes and prokaryotes. Now genetics and biochemistry are used to organize life forms.)

Jupiter and Saturn

No solid surfaces, but altitudes in atmosphere that are warm enough for water droplets could exist Problem: strong vertical winds would either rip microbes apart or send them to extreme temperature regions!

Carbohydrates

Organic molecules made up of carbon, hydrogen and oxygen

The Viking Experiments

Searching for microbes in Martian soil These contradictory and inconclusive results can be explained by the presence of perchlorate in Martian soil — chemistry, rather than biology!

Seasons Review

Seasons are caused by the relative angle between a planet's orbit and its rotation axis.

Evidence of life in Martian meteorites?

Similarities to bacteria fossils here on Earth, but non-biological processes could be at work. ALH 84001 (discovered in Antarctica in 1984) showed microscopic evidence of life in the form of magnetite crystals, as well as possible bacterial microfossils. However, nonbiological processes could be responsible, and possible contamination by Earth-based life?

Surface Liquids in the Solar System

Strength of sunlight ⍺ 1/d2 Other important effects: greenhouse effect, internal heating, albedo (reflectivity), stellar type and age, atmospheric pressure, UV radiation...

Schematic of an animal cell:

Structures inside the cell with different functions are called organelles.

Schematic of a plant cell:

Structures inside the cell with different functions are called organelles. Chloroplasts are organelles found in plant cells but not in animal cells!

How much water is there?

The Phoenix lander added to evidence suggesting water ice is widespread in the Martian arctic. The total amount of ice on Mars now, if melted, would cover the planet with a 10-meter deep ocean! There may have once been enough water on Mars to cover its entire surface to a depth of 130 meters! Some scientists hypothesize that the low-elevation northern plains once held a deep ocean! Bottom line: Mars once had a thicker atmosphere and warmer climate, which allowed surface water to exist!

Amino Acids

The building blocks of proteins - contain an amino group and a carboxyl group. •70 amino acids known •All life on Earth builds proteins from the same 20 amino acids •Evidence for common ancestor! •Amino acids also have chirality("handedness") Amino acids have handedness(chirality), where the same molecule comes in two forms that are mirror images of each other. •Non-biological amino acids have a mix of right- and left-handed versions •Life on Earth uses only left-handed amino acids to build proteins (more evidence for common ancestor) •Some meteorite evidence that left-handed versions outnumber right-handed amino acids in space •Note: life on Earth uses mainly right-handed sugars.

Metabolism

The many chemical reactions that occur in living organisms and are involved in providing energy or nutrients to cells •Reactions include: breaking down (catabolism) and building up (anabolism)molecules! •Reactions all involve breaking bonds and forming new ones. Both require energy! •A reaction where energy is released is called exothermic •A reaction where energy is absorbed is called endothermic •We compare bond energy in products and reactants to decide whether a reaction is exothermic or endothermic.

Minerals and Sedimentary Rocks

The presence of opal is evidence for hot springs or similar environments! Hematite spheres "blueberries" likely formed from sedimentary rock!

Martian Geographic Features

The southern hemisphere is 1-3 km higher than northern hemisphere, and covered in impact craters and tectonic features (older). Leading hypothesis: impacts in the south caused a magma ocean that flowed to the north and cooled.

Axial Tilt Variations

The tilt of Mars' axis may vary between 0° and 60° on timescales of 105 —106 years.During periods of high tilt (> 40°), Mars may have zones of liquid water!

Basic Metabolic Needs

These are necessary for any manufacturing process! •Raw materials: For living cells, these are the molecules that provide the cell with carbon and other basic elements of life. •A source of energy: The metabolic processes that break down old molecules and manufacture new ones require energy. (Recall: making and breaking atomic and molecular bonds involves energy absorption and release!)

Seasons and Polar Caps

This image, from Mars Global Surveyor, shows the residual south polar cap during summer. A layer of frozen CO2 around 8 meters thick overlies a thicker cap of water ice. In winter the whole area in the image is covered in CO2 ice.

The Evolution of Eukarya

Two important evolutionary changes: 1) Early eukarya probably adapted to have membrane foldings that compartmentalized cell functions 2) Endosymbiosis: cells living in symbiosis with invading bacteria, which evolved to become mitochondria and chloroplasts

Martian Geological Activity

Valles Marineris: a canyon as long as the US is wide! (> 2500 miles long, 120 miles wide, 4 miles deep) Evidence for past geological activity (surface stretching)!

Metabolic Classification

We classify life into four main categories based on its metabolic sources! •The basic metabolic needs of living things are carbon compounds and energy sources. •We use the sources of carbon and energy to classify living organisms into metabolic groups! •Two types of carbon sourcing:heterotrophs and autotrophs •Two types of energy sourcing (independent of carbon sourcing): phototrophs and chemotrophs •This leads to four possible categories!

Martian Volcanism

Weaker gravity + no plate tectonics (stationary mantle plumes) lead to huge volcanoes! Crater counts suggest lava flows from as recently as ~10 Myr ago.

Great Oxidation Event:

atmospheric levels rise to 0.002% and continue to climb.

Anaerobic

does not require oxygen for metabolism

Carbohydrates (Cellular structure)

e.g. cellulose makes up fibers of cotton and linenStrong bonds thanks to carbon's electron configuration. Life on other worlds would probably need molecules to play these same roles (remember ideas about Si-based life from a few lectures ago)!

Nucleus

hold chromosomes and DNA

Banded iron formations

iron oxides precipitated out of ocean water!

Seasons on Mars - Relevant orbital info:

length of day = 24.66 hours axial tilt = 25.2 degrees eccentricity = 0.093 orbital period = 1.881 years (For comparison, Earth's eccentricity is 0.017.)

The Viking Experiments - Gas chromatograph / mass spectrometer experiment:

measured abundance of organic molecules in Martian soil no sign of organic molecules!

Cells

microscopic units in which the living matter inside is separated from the outside world by a membrane •Some organisms are single-celled, and some have trillions of cells! •Similarities between cells from different organisms suggest that all life on Earth shares a common ancestor!

The Viking Experiments -Labeled release experiment

mixed Martian soil with organic nutrients tagged with radioactive isotopes, to see if radioactivity would change radioactivity behaved as if nutrients were getting used, and heating appeared to stop this process

The Viking Experiments - Gas exchange experiment:

mixing Martian soil with "broth" of organic nutrients, to see if oxygen would be released oxygen was released, but even in the dark, even when H2O with no nutrients was introduced, and even at high temperatures

cell membrane

protects cell from the environment

Nucleoid

region where DNA is located

Aerobic

requires oxygen for metabolism

mitochondrion

site of metabolism

chloroplast

site of photosynthesis

ribosomes

site of protein synthesis

Carbohydrates (Source of energy)

sugars and starches known as "carbs" (e.g. glucose)

Rusting

surface minerals reacting with oxygen, turning a reddish color

The Viking Experiments - Carbon assimilation experiment:

testing whether carbon was being incorporated into soil carbon was incorporated into soil, but even at temperatures that would kill organisms

Dwarf Planets

•"Ice geology" also possible•Differentiated - outer layers are ice-rich (ingredients for liquid) •Down side: isolation means lack of tidal heating. •Ceres: growing patches of ice+ cryovolcanic activity! •Pluto: young localized surface features (< 100 Myr) and evidence for subsurface ocean!

Supernovae and Gamma Ray Bursts

•1 supernova expected per century in the Milky Way (out of > 100 billion stars) •Statistical calculations: Earth is occasionally within a few tens of lightyears of a supernova •Supernovae produce cosmic rays (high energy particles)! •Particularly strong supernovae (gamma ray bursts; GRBs) produce gamma rays (high energy photons) •GRBs within a few thousand lightyears could destroy half the ozone layer (expected ~once a billion years; Ordovician mass extinction ~450 My ago)

Extreme Volcanism - End-Permian Extinction

•252 Myr ago•90% of all species killed, including some insect species (this is unique!) •Siberian Traps: hills with basalt rock in stair-like structures formed from a single set of huge volcanic eruptions •Extinction reasons: lots of ash spewed into atmosphere, leading to acid rain, extreme winter, and then enhanced greenhouse effect (Tavg as high as 104°F)

The Cambrian Explosion

•542 - 502 million years ago •Huge diversification of multicellular organisms and distinct plant and animal species •All biological phyla (next level of classification below kingdom - e.g. animal kingdom) arose in 40 Myr! •"Hard" parts: bones, claws, teeth, exoskeletons - predators arise •Eyes first evolved! (first seen in trilobite fossils ~545 Myr ago)

Implications for Evolution of Life Elsewhere

•98% of our DNA is shared with chimpanzees, yet we evolved to have complex technology while chimpanzees did not. •Humans and chimpanzees split from our common ancestor at the same time - is intelligence an inevitable consequence of evolution? •Dramatic changes in last ~40,000 years are not the result of natural selection, but cultural evolution - changes in transmission of information. •Intelligent and advanced extraterrestrial civilizations likely have complex technology, which would allow them to alter the course of evolution (like humans have done; technological evolution).

ATP is easily and completely recycled

•ADP is transformed into ATP inside cells' mitochondria! •ATP can turn back into ADP and release energy when it interacts with water (hydrolysis) •This powers all cell functions! •Each ATP is recycled about 2000-3000 times per day!

Takeaways from our solar system

•All bodies have the raw chemical elements required for life. •Possibilities are much more limited when we consider the environmental requirements for life we've found by studying Earth. •Need for liquid water (or just a liquid medium) seems to rule out life on many bodies in the solar system — e.g. Mercury, the Moon, and small bodies. •Harsh atmospheric conditions, and a lack of obvious energy sources in their liquid cores make life seem unlikely for jovian planets. •Best prospects: atmosphere of Venus, Mars, and jovian planet moons!

Primates

•Ancestral primates arose ~60 Myr ago •Tree dwellers: limber arms, dextrous hands, depth perception, extended care for young •Humans diverged from apes a few million years ago

Building Blocks of Life

•Carbon, hydrogen, oxygen, and nitrogen make up 96% of mass of living organisms! •These are among the most abundant elements in the universe, making this requirement easy to satisfy. •Remember: stars make essentially all chemical elements beyond H, He - they are everywhere.

The K-T Boundary Killed the Dinosaurs!

•Dinosaur fossils and fossils of many other species only appear below the iridium layer! •Geological record: •99% of all living plants and animals died in K-T event •75% of all existing plant and animal species went extinct •This is an example of a mass extinction!

The Emergence of Humankind

•Earliest modern human fossils appear 200,000 years ago •We shared the planet with other hominid species: Neandertals, Homo floresiensis, and perhaps others. •Up to 4% of human DNA might come from Neandertals

Large Moons - The best candidates for solar system habitability

•Internal "ice geology" with much less required internal heat than "rock geology" •Tidal heating contributes to internal heat! •Tidal heating can lead to sub-surface oceans! (Europa, Ganymede, Enceladus all have evidence for sub-surface H2O.) •Also: surface lakes of methane and ethane (Titan)

Implications for Life Elsewhere

•It took a long time for oxygen to build up on Earth! •No complex plants and/or animals for first 4 Gyr! •If oxygen buildup is slow or slower on other worlds, life there could be mainly microscopic. •Alternative: Earth was unlucky? •No way of knowing whether Earth is typical, but assuming it is continues to be our best guess!

anaerobic photoautotrophs

•Later life forms used sunlight: anaerobic photoautotrophs •Making use of "waste" water from anaerobic chemoautotrophs •New "waste" product: O2•Photosynthesis:• This created an "oxygen crisis", prompting evolution!

Mercury

•Magnetic field, but no atmosphere •Very slow rotation (58.6 days) — one Mercury "day" is about two months long! •Orbital period: 87.9 days •Daytime temperatures: 425 °C •Nighttime temperatures: -150 °C •Only frozen water in shielded craters.

The moon

•No atmosphere or geological activity •Most volatiles (chemical compounds that are easily vaporized) are gone due to formation (giant impact). •No liquid water.•Ice exists at bottom of craters near polar regions that are shielded from sunlight. •Deposited from comets and impacts, but probably not enough for life.

Mars Today

•No current geological activity, but certainly volcanism in the past •Extremely thin atmosphere (0.6% of Earth's pressure at sea level), made of 96% CO2, 2% N2, 2% argon •Extremely cold — average surface temperature -55°C, range 27°C(81°F) to -143°C

Mars

•Once had flowing water and a thicker, warmer atmosphere! •~3 Gyr ago: climate change causes deep freeze •Since then: atmosphere thinned (loss of magnetic field?) — water boils off the surface! •Enough internal heat that liquid water may exist underground!

The Colonization of Land

•Plants and large fungi ~ 475 Myr ago •Forests, plant-eating amphibians, insects by ~360 Myr ago •Organic remains of first plants not easily decomposed — compressed to form coal beds •Development of the ozone (O3) layer was important!

Population Growth - More on exponential processes

•Populations of living organisms tend to grow exponentially •"Doubling time:" the timescale over which a population size doubles (think, opposite of half-life) •Example: start with single bacterium that divides every minute - how many bacteria in 5 minutes? •Total = starting number x 2# of doubling times•1 x 2 x 2 x 2 x 2 x 2 = 1 x 25 = 32•Exponential growth eventually stops because of lack of sufficient resources - carrying capacity

The K-T Boundary 1978: Luis and Walter Alvarez

•Sediments at the Cretaceous - Tertiary boundary is unusually rich in iridium! •Iridium is rare on Earth but common in meteorites. •First noticed in Italy, and later around the world (global event)! •Alvarez team conclusion: 10-15 km asteroid or comet impactor distributed the worldwide iridium layer!

Who survived, and who repopulated first?

•Small, burrowing mammals that stored enough food to last through global winter survived the extinction event. •Ferns were likely the first plant life to re-grow. •Mammals went on to "rule Earth," like the dinosaurs once did.

Early oxygen sinks

•Surface minerals (iron rich rocks) •Dissolved iron in oceans •Volcanic gases •Free oxygen does not last long without life!

Energy Sources - Sunlight intensity falls off as distance squared!

•The same total amount of sunlight goes through any spherical area encircling the sun. •Sunlight per area decreases with (1/d)2(where d is distance from the sun). •Example: if you are twice as far away, you receive four times less sunlight per area!

Venus

•Thermal equilibrium with Earth-like atmosphere should yield T ~ 95 °F •Real surface temperature: 880 °F — hot enough to melt lead, way too hot for H2O! •Thick atmosphere: pressure 90x greater than Earth's at sea level — same as 1 km underwater! •"Runaway greenhouse": CO2 makes up 96% of Venus' atmosphere! •Venus may have previously had oceans — CO2 sink + possible habitability! •Extreme past volcanism — past geological activity! •Cratering suggests surface is less than 1 Gyr old (recent geological activity) •Only real prospect for life now: microbes in the atmosphere — H2O can exist ~50 km up — no robust detections so far.

Small Bodies

•Too small to have any remaining heat of formation. •Many are far enough from the sun to be perpetually frozen. •Impact melting would re-freeze quickly. •Early in solar system's history, heat of formation would keep liquid water around for ~several Myr.

Advantages of Water

1) It remains liquid over a wide range of temperatures, particularly toward high temperatures. no known methanol reservoirs in the solar system Colder liquids mean slower chemical reactions! 2) Water is less dense as a solid - ice floats and insulates water below! This is especially important during e.g. ice ages! (Imagine if ice sank!) 3) H2O is a polar molecule: one side net positive charge, one side net negative. This keeps living cell membranes from dissolving in water, and also allows hydrogen bonds to form! These are important for biochemistry, and are also why ice floats! Advantage #3 could get turned around. For example, cell membranes made of molecules with different charge-separation properties may dissolve in water!

Seasons on Mars - Two differences relative to Earth:

1) seasons are almost twice as long, and 2) orbital eccentricity means that the southern hemisphere has more extreme seasons than the northern hemisphere

Environmental Requirements for Habitability

1.A source of molecules from which to build living cells! Easily satisfied by most worlds! 2.A source of energyto fuel metabolism! More limiting, but lots of worlds have the right incident sunlight and/or chemical energy. 3.A liquid medium — most likely liquid water — for transporting the molecules of life! Most stringent! A world can only be habitable if it has a liquid medium, probably water but possibly one of our other options.

The Mass Extinction Why did almost all living organisms die?

1.Asteroid slams into present-day Mexico with the force of 108 hydrogen bombs 2.Tsunami sloshes more than 1000 km inland 3.Hot debris ignites wildfires all over the world. (The entire sky may have been bright enough to roast most land life.) 4.Dust and smoke hang in atmosphere for weeks to months, causing temperatures to drop. A global, extremely harsh winter follows, stopping photosynthesis for up to a year! 5.Global winter may have been followed by a period of unusual warmth (extra CO2 released from carbonate rich rocks at impact site. 6.Impact probably caused chemical reactions in atmosphere that produced harmful compounds, which then dissolved in the oceans and/or led to acid rain.

Evidence for an Impact

1.High abundances of other metals: osmium, gold, platinum 2.Shocked quartz grains: experienced high P and T 3.Spherules: spherical rock "droplets" formed by molten rock cooling / solidifying in air 4.Soot apparently produced by widespread forest fires

The Cambrian Explosion (Four contributing factors)

1.Rapid rise in oxygen 2.Evolution of genetic complexity 3.Climate change - snowball Earth episode close in time to Cambrian explosion (evolutionary pressure) 4.Absence of efficient predators

Metabolic reactions are greatly enhanced by:

1.The environment of the cell 2.Enzymes, which can act as catalysts! 3.Adenosine triphosphate:Earth's universal energy storage molecule (more soon!)

~1877 - 1907: Schiaparelli & LowellCanals on Mars?

1877: Italian astronomer Giovanni Schiaparelli identifies network of 79 canali - linear features he believed to be canals. (Note that this followed the 1869 opening of the Suez Canal.) 1894: Percival Lowell builds Lowell Observatory in Flagstaff, AZ. Mapped close to 200 "canal" features. Imagined that Martians used them to transport water from the polar ice caps to agricultural areas and cities.