Astrobiology Final
What is the evidence for global warming?
Measurements show that human activity is causing a substantial increase in the atmospheric concentration of CO2. The well-understood mechanism of the greenhouse effect suggests that this increase could lead to an increase in the global average temperature, and such an increase has indeed been observed over the past century. Climate models indicate that this temperature increase is due primarily to human contribution to global warming, rather than to natural factors.
Galactic Habitable Zone
There may be an ideal location and time within the galaxy for habitable planets suitable for advanced life. Stars too close to the center of the galaxy are subject to frequent collisions and more supernovae (exploding stars) triggering gamma ray bursts that may be harmful. Stars that are too far away from the center of the galaxy may be too metal-poor (depleted in elements heavier than helium).
Why is Venus so hot?
Venus's distance from the Sun ultimately led to a runaway greenhouse effect: Venus became too hot to develop (or keep) liquid oceans like those on Earth. Without oceans to dissolve outgassed carbon dioxide and lock it away in carbonate rocks, all of Venus's carbon dioxide remained in its atmosphere, creating its intense greenhouse effect.
Astrobiology and Oxygen
*Complex organisms on Earth require oxygen* (*aerobic respiration*), yields the most energy of any type of metabolism Where does oxygen come from? - *cyanobacteria* (in plant cells, chloroplasts) - oxygenic photosynthesis = CO2 + H2O → CH2O + O2 (aerobic respiration is opposite: CH2O + O2 → CO2 + H2O) No oxygen in atmosphere on early Earth, then sudden shift to high levels of oxidation (Great Oxidation Event)
Two Major Hominin Groups w/ Signs of Intelligence
*Genus homo* = genus that includes the species 'humans' *Genus Australopithecus* = smaller in stature and brain size -------------------------------------------------------- *Homo sapiens* = humans
Difficulties for life on Gas and Ice Giants
*No solid surface* (hydrogen and helium) *Strong winds* (could carry any potential life to depths where it couldn't survive) Different temperatures at different layers But *Jupiter and Saturn have strong magnetic fields (b/c convecting liquid hydrogen)* Pros: *liquid water clouds, magnetic field* Cons: *strong vertical winds, little chemical "free energy"*
Gaia Hypothesis
Life creates and maintains conditions for its own existence by stabilizing planetary climate and other aspects of the planetary system.
Moons: *Ganymede*, *Callisto*, Io, Enceladus, Triton
*All four may have subsurface oceans that could support life.* *Io has most volcanic activity in the solar system (because of tidal flexing from Jupiter)* Europa may have a *magnetic field (indicates a conducting liquid below the icy surface*). *Young ice on the surface may indicate that this liquid is a salty ocean*. *Jets of water vapor* discovered. *Least cratered surface in our solar system (suggests plate tectonics*). *Cryovolcanism* = ice volcanoes. Periodic melting of ice. *Lenticulae* = upwellings of warm ice? (*linae* = bands pulled apart rather than compressed and then filled by new ice growth). Cycloids = caused by tidal stresses during orbit. Enceladus is *ice-covered with recent fractures and signs of fresh ice (places with less craters than other places*). *Jets* discharging internal volatiles (water vapor, ice, methane, ammonia) into space through cracks in the ice. *Hot spot* in south pole (unclear source, maybe ammonia antifreeze?). *Vapor plumes mean a higher possibility of life (b/c organic compounds occur in vapor plumes*). Titan covered in *clouds of organic haze*. Large polar *lakes of liquid methane-ethane*. Blocks of water ice on surface. *Few craters*. *Highly eccentric orbit (causes tidal friction and so, so hydrothermal energy?*). Cassini/Huygens landed on Titan. Triton has *young surface* (b/c lack of craters). *Pink* color from organic materials. Possible *frozen lakes*. Prebiotic molecules that may have impact-induced liquid water. Maybe subsurface ocean; *"weird" life in methane lakes* (but methane and ethane poor at dissolving large organic molecules so this may prevent life) OR subsurface life; has a *methane-ethane cycle that suggests subsurface ocean*. Methane must be replenished because its destroyed but it's probably a geological reason (b/c lakes too small to replenish methane).
Extrasolar planet detection methods: Astrometry, Stellar Doppler Shift, Transiting Planets, Gravitational Lensing
*Astrometric Method* = detection of exoplanets through the side-to-side motion of a star causes by gravitational tugs from the planet *Stellar Doppler Shift* = detection of exoplanets through the motion of a star toward and away from the observer caused by gravitational tugs from the planet *Transiting Planets* = when a planet passes in front of a planet during its orbit *Gravitational Lensing* = the magnification or distortion (into arcs, rings, or multiple images) of an image caused by light bending through a gravitational field, as predicted by Einstein's general theory of relativity
Evidence of water on past Mars (5)
*heavily eroded craters* (from rainfall?) *old valley networks* (river channels? ancient) *large channels* (floods? young) *old deltas* (clay minerals may indicate standing water?) *recent gully features* (meltwater?)
5 Mass Extinctions
*Ordovician* = cold climate (440 million years ago) *Devonian* = climate change? induced by first forests? ocean anoxia? (359 million years ago) *Permian* = WORST; *global warming from volcanism in Siberia in coal beds that caused ocean anoxia (251 million years ago)* *Triassic* = climate change? associated with volcanism (206 million years ago) *Cretaceous (K-T)* = climate change induced by asteroid impact and volcanism (65 million years ago) - know most about. Asteroid hit Mexico. Ejected molten rock that rained down and caused fires and acid rain. 99.9% of all animal species that have ever existed have gone extinct After K-T no land animals greater than 25kg survived.
Mars Landers & Rovers
*Viking Lander* = discovered that temperatures of Mars vary a lot per day (*sol*), and pressure varies by season (b/c amount of water vapor and CO2 depends on seasons - freezes in winter); designed specifically to look for signs of life (results ambiguous b/c peroxides [H2O2 caused by ultraviolet light hitting ice]) *Phoenix Lander* = found dater ice in trenches, ice in craters *Opportunity Rover* = found sulfate rocks, hematite ("blueberries") formed by subsurface water *Spirit Rover* = subsurface salts, silica-rich deposits and carbonate deposits (usually formed with hydrothermal fluids) *Curiosity Rover* = landed at Gale Crater. in middle of crater = a layered sedimentary mountain. site shows signs of water activity. Found that rocks contain CHNOPS (elements of life). Organic molecules in soil. High radiation levels. Ancient stream bed (water slightly salty, neutral pH). Not much methane in atmosphere (seasonal).
Tidal Force
*a difference between two gravitational forces; friction caused by tides heats planetary interiors or oceans* in elliptical orbits, tidal force is strong when close to the object it orbits but weak when far away
Animals (and vertebrates)
*animals* = have a coelom (houses internal organs) - appeared in a short period of time - earliest? = *ediacaran biota* = dickinsonia (*?*) - Cambrian explosion = modern animals appear (*causes* = 1. increased O2 in seawater = bigger animals [unlikely because O2 levels rose well before Cambrian explosion]; 2. biominerals allow predator-prey arms race [likely]; 3. diversification of homeobox genes regulating body shape [likely]) - Burgess Shale (505 million years old) = has first evidence of soft-bodied animals *vertebrates* = have a backbone - *pikaia* = possible ancestor of all vertebrates
How we look for life (including extinct life on Mars)
*microscopic fossils* (body fossils; usually unicellular, found in rocks, distinguish from minerals by looking for cell division, clonality, organic carbon, oldest uncontested microfossils = 2.6 billion years old [show cell division, integration with surrounding environment]) *trace fossils* (stromatolites made from microbial communities, build toward light, 3.48 billion years old) - IS THIS WRONG? *molecular fossils* (not all biochemicals decompose at the same rate, oldest evidence from oil in droplets of water in quartz crystals = 2.45 billion years old) *atomic fossils* (isotopic signatures that have been concentrated/depleted by life; autotrophs concentrate C12 in their tissue in preference to C13, oldest probable evidence is 3.8 billion years ago) -------------------------------------------------------- *sulfur* = isotopic evidence of metabolism (clear record back to 2.7 billion years ago, shows early evolution of respiration (ATP generated enzymes as catalysts = modern microbial biochemistry) *Evidence from North Pole = 3.5 billion years ago (sulfur isotopes) = earliest confirmed life*
Cold Origin of life
*sea ice*. pros: *concentrates organic molecules (freezes them so they don't dilute; trapped) in brine pockets*, *nucleotides are stable in cold temperatures* (e.g. freeze DNA = *preserved*, dig up mammoths) cons: *source of organic molecules?*, *what energy source is there?*
Archaen Environment
- *Earth's heat flow (mantle) hotter (more residual heat of accretion, more volcanic activity), probably smaller or faster plates (tectonics)* - *smaller continents, no life on land* - *oceans have lots of microbial life* - *atmosphere = no oxygen* - *lots of igneous rocks* (basalts, underwater = suggest more volcanic activity) & *komatiite*, which are extinct rocks, hot lava that's water-like in density - *submerged continents* possibly (*WHY?*) - *bonded iron formation* (extinct now; imply that oceans had no oxygen) - *oceans maybe 4.4 billion years old*
Early Eukaryote Fossils & Reproduction
- *acritarchs* = unicellular organic-walled fossils, oldest 3.2 billion years ago, acritarchs with complex walls and complex structures = earliest multicellular eukaryotes (*spiral seaweed*) = 1.6 billion years old - *bangiomorpha* = multicellular, first evidence of sexual reproduction *eukaryote sexuality* = forces genetic recombination by chromosome crossing, meiosis, fertilization
Possible Sources of Carbon on Early Earth
- *atmosphere* = Miller-Urey (but early atmosphere not reducing(*?*) enough) - *space* = micrometeteroites, some contain organic compounds like amino acids. comets too. panspermia - *seafloor vents* = hydrogeen-rich, hydrogen released in interactions between rock and hot water can react with CO2 to make organic molecules (Fischer-Tropsch synthesis). hydrothermal origin of life? - many early-evolved microbes are thermophiles (live @ high temperatures)
How do we know oxygen levels low on early earth?
- *paleosols* = fossil soils, iron was leached from soils. around 2.4 billion years ago but not after this date - suggests no oxygen in atmosphere - *retrial grains* = sedimentary minerals that don't dissolve in weathering (some of these would have been dissolved in oxygen rich water but weren't so no oxygen in the water) - *redbeds* = red sandstones derived from windblown dust or river-transported particles; only exist in an oxygenic atmosphere; don't exist before 2.3 billion years ago - *banded iron formation* = requires oxygen (*?*)
4 Steps Life (from simple to complex)
1. *Organic molecules formation* (carbon source) 2. *Concentration, catalysis* (cause polymerization reactions to happen, simple to complex) 3. *Energy transfer* (extracellular growth using external energy to allow them to grow and reproduce using external energy) 4. *Encapsulation* (well-developed metabolisms inside cell, protected from external environment, develop a membrane; increased rates of reaction by concentrating solutes; evolutionary advantage to self-replicating molecules); two types of pre-cells: - warm solution of amino acids cools into spheres - lipids in water form oily spheres spontaneously [first replicating molecule that carried genetic information probs RNA b/c its simpler the DNA] -------------------------------------------------------- 1. Earliest life microbial 2. Earliest microbial life was anaerobic (didn't use oxygen) 3. Earliest photosynthesis didn't produce oxygen (anoxygenic photosynthesis) THEREFORE, required the eukaryotic cell: - Primitive eukaryotes developed internal membranes around the nucleus - Got organelles by endosymbiosis: prokaryotes integrated into larger cells, originally ingested as food, parasites, or symbionts
How small planets lose their atmospheres (3)
1. *thermal escape* jeans escape = molecules collide and shoot off into space hydrodynamic escape = atoms collide and shoot off into space 2. *non-thermal escape* = solar wind particles break down molecules to atoms that escape 3. *impact erosion* = any impact blasts away all the air Mars lost atmosphere probably b/c all three
SETI (Search for Extraterrestrial Intelligence) 3 Signals
1. Local communications (e.g. our radio and tv get leaked out to space) 2. Interplanetary communications (e.g. Earth with Mars rovers) 3. Intentional signals (e.g. Drake and Sagan signal to star cluster M13) Optical SETI = light flashes (less common b/c requires more energy than noise) Extraterrestrial artifacts = stuff left behind (Lagrange points = good place to look) G (3) - astroengineering - planetary technology - interstellar spacecraft
How do the orbits of the known extrasolar planets differ from those of planets in our solar system? (3)
1. Many of the known extrasolar planets orbit their stars more closely than Mercury orbits the Sun, and almost none are located as far from their stars as are the jovian planets of our solar system. 2. Many of the planets with close-in orbits are Jupiter-like in mass, making them examples of what astronomers call hot Jupiters, because they are presumed to be jovian in nature but must be hot owing to proximity in their star. 3. Many of the planets also have large orbital eccentricities, telling us that their elliptical orbits have very stretched-out shapes.
What factors influence surface habitability?
1. within its star's habitable zone 2. large enough to retain internal heat and have plate tectonics 3. has enough of an atmosphere for liquid water to be stable on its surface
Great Oxidation Event
2.4 Ga around time of ice ages the change of Earth's atmosphere from reducing to oxidizing, brought about by oxygen-generating photosynthesis
Mars Then: 3 eras & how we know
3 Eras: *Noachian* = before 3.7 Ga (wet; impacts; clay; valley networks, lakes) *Hesperian* = 3.7 Ga-3Ga (wet → dry; sulfates, last valley networks, outflow channels) *Amazonian* = 3 Ga-present (dry; few hydrated mineral deposits, iron oxides, we know there is clay b/c spectra [clay reflects differently]) Know about Mars then b/c orbiters photograph and image Mars (Mars Global Surveyor discovered Mars has no active magnetic field, but as strong in the past)
What properties of extrasolar planets can we measure?
All detection methods allow us to determine a planet's orbital period and distance from its star. The astrometric and Doppler methods can provide masses (or minimum masses), which the transit method can provide sizes and, for some multiple-planet systems, also masses. When the transit and Doppler methods can be used together, we can determine average density. In some cases, transits (and eclipses) can provide other data, including limited data from atmospheric composition and temperature.
How do other stars differ from the Sun?
All stars are born from interstellar clouds made mostly of hydrogen and helium, and their compositions differ only in the small proportions of other chemical elements. Because of their similar compositions, the major differences among stars are a result of differing masses: more massive stars have higher surface temperatures, higher luminosities, and shorter lifetimes than less massive stars. Many stars are members of binary or multiple star systems.
Exoplanet properties: size, eccentricity, etc.
Almost all exoplanets orbit their stars closer (than mercury orbits out sun). Many exoplanets are the size of Jupiter (hot Jupiters) Very stretched out orbits (ellipses)
How long can life survive on earth?
At minimum, Earth should remain habitable for another several hundred million years. By about a billion years from now, a *moist greenhouse effect could cause Earth's oceans to evaporate away*, though natural feedback processes might prevent this from occurring so soon. In 3-4 billion years, the Sun will become bright enough that our planet will certainly be subject to a runaway greenhouse effect, ending surface habitability.
What is needed for intelligence?
Big brains (relative to body mass) Humans = 7 Dolphins = 5 and less Encephalization Quotient = >5 (for intelligence) = brain size compared to body
Tidal Locking
Biggest effect by red dwarf suns (M stars) Big gravitational attractor with smaller one nearby (smaller one's orbit shortened, orbits once every rotation; e.g. Moon to Earth, Mercury to Sun).= causes a ONE HOT AND ONE COLD SIDE (uninhabitable? no b/c thick atmosphere and it can transport heat to cold side)
What is the Hertzsprung-Russell diagram?
Classifies stars by luminosities and sizes. OBA = not suitable stars for habitability (b/c bright and short lifetime). FGK have wide habitable zones. Order (from coldest and smallest to brightest and hottest) = O B A F G K M Y-axis goes up by size X-axis decreases by luminosity (ones on left are more luminous) *Our sun is a G star* The H-R diagram plots stars according to their spectral type or surface temperature on the horizontal axis and luminosity of the vertical axis. Most stars fall along a continuous swath of diagrams, called the *main sequence*, that runs from hot, luminous stars at the upper left to cool, dim stars at the lower right. Other stars--giants and supergiants--clump in the part of the diagram where stars are luminous but have cool surface temperatures. The stellar corpses known as white dwarfs are dumb but hot, so they are found in the lower left of the diagram. Studying H-R diagrams helped astronomers realize that mass is a star's most fundamental property, and the organizational power of the diagram makes it one of astronomy's most useful tools.
Mars climate & geology today
Climate = Cold and dry. Low atmospheric pressure (water is unstable). Weather driven by seasonal changes that cause CO2 to condense at the polls in the winter. Sometimes wind creates dust storms. Geology = Mars has regions that are densely cratered and must be old, and other regions with fewer craters that are probably younger. Younger terrain is generally in low-lying plains and older terrain in southern highlands. There are giant (inactive) volcanoes like Valles Marineris.
Life's environmental requirements
Complex organisms on Earth require oxygen (aerobic respiration), yields the most energy of any type of metabolism (comes from cyanobacteria, which in plants are chloroplasts) Liquid water (solvent) - dependent on planetary temperature Habitable zone (depends on type of star) & greenhouse effect Large enough planet to retain internal heat and have plate tectonics (For surface habitability) Has a thick enough of an atmosphere to retain liquid water on its surface Organic molecules (like carbon, from space, atmosphere, or sea floor vents?) In summary = *water*, *organic compounds* (carbon), and an *energy source*
What are the potential consequences of global warming?
Continued global warming could raise the average worldwide temperature by 2 to 5 degrees Celsius during this century. Regional climate change will be greater, and we can expect increased polar melting and a rise in sea level. Additional heat should increase ocean evaporation, which may lead to more numerous and more intense storms. Many other serious effects could also occur, though precise consequences are difficult to predict.
Are planetary systems like ours common?
Current evidence suggests that most stars have planets and at lest some are Earth-sized and in their star's habitable zone. Nevertheless, we don't yet have enough data to know for certain whether planetary systems like ours are common.
How do we detect planets around other stars?
Current technology is limited in direct-detection capabilities, but we can detect planets indirectly through three major methods. We can look for a planet's gravitations effect on its star through the *astrometric method*, which looks for small sifts in stellar position. Or the *Doppler method* which looks for the back-and-forth motion of stars reveled by Doppler shifts. For the small fraction of planetary systems with orbits aligned edge-on to Earth, we can search for *transits*, in which a planet blocks a little of its star's light as it passes in front of it.
Gilese 581G
DOESNT EXIST (sun spots look like planets orbiting in front of the star) Planet (sun is Gilese 581) Earth-size
Pioneer 10 & 11 Plaques
Depict people in case Pioneer 20 or 11 intercepted by aliens
Chemical Equilibrium/Disequilibrium; reduction, oxidation
Equilibrium = sides of equation are balanced Disequilibrium = more products than reactants or vice versa (could mean higher possibility of life because *"free energy"*) *Reduction = gaining electrons* *Oxidation = remove oxygen from the atmosphere* *Redox reactions = exchange of electrons between things. E.g. aerobic respiration (produces energy)*
Convergent Evolution
Evolution toward similar characteristics in unrelated species (intelligence could be common b/c survival, e.g. dolphins and humans have signs of intelligence, some birds too, primates) Is intelligence inevitable? context = living within a group, benefit to reproduction, basic survival advantage
Why are extrasolar planets hard to detect directly? What are the two general approaches to indirect detection?
Extrasolar planets are difficult to detect because they are incredibly far away. Since stars are usually a billion times brighter than the visible light reflected by an orbiting planet, it is difficult to detect planets in photographs. The problem can be somewhat lessened by observing infrared lights, because planets emit their own infrared light and stars are typically dimmer in infrared. To directly search for an extrasolar planet means to obtain images or spectra of the object. To indirectly search for an extrasolar planet means to infer the object's existence or properties without actually seeing it. The latter can be done two ways: 1. By observing the motion of a star to detect the subtle gravitational effects of orbiting planets; or 2. Observing changes in a star's brightness that occurs when one of its planets passes in front of the star as viewed from Earth. NASA's Kepler mission uses the second method. A transit is when a planet appears to move across the face of a star. Because other star systems are so far away, we can't actually see a planet move in front of the face of a star, but it will block a portion of that star's light which allows is to detect its passage as the star appears temporarily dimmer. The other indirect approach for extrasolar planet detection revolves around the detection of gravitational effects on a star caused by orbiting planets. This approach can be split into two methods: the astrometric method and the Doppler method. The astrometric method infers a planet's existence from small changes in a star's position in the sky, while the Doppler method infers a planet's existence from star's motion toward/away from us as revealed by Doppler shifts in its spectrum.
Europa's liquid ocean (Galileo spacecraft), evidence
Fairly *low density despite rocky core*. Heat input by tidal heating from Jupiter. Young icy surface (lots of craters). Surface features show convection and liquid water upwelling. *Weak magnetic field requires electrically conductive convecting liquid (salty seawater subsurface ocean?*). *Pink surface color (may indicate magnesium sulfate or irradiated sodium chloride possibly precipitated by salty sea water?*). Water vapor plume erupting near south pole. *Maybe origin of life around hydrothermal vents (but if surface color is from magnesium sulfate so unlikely because sulfate is absent around hydrothermal vents on Earth*). Only simple life forms b/c energy source from particles and radiation breaking up surface ice and producing oxygen and hydrogen peroxide (H2O2) that might work its way down to liquid water.
ALH84001 meteorite
From Mars Found in Antarctica Only Mars meteorite thats from the subsurface of Mars; igneous rock from 4.1 billion years ago Pres. Clinton agreed that US found fossil life from Mars = 4 claims (microfossils, organic molecules, carbonate globules, magnetite crystal chains) = actually not evidence of life, but probably chemicals because they were too small to be cells
How could we detect life on extrasolar planets?
Future telescopes should allow us to obtain crude images or spectra of planets within stellar habitable zones. An image of an extrasolar planet--even if only a few pixels in size--might indicate the presence of snow or clouds, and would tell us the planet's rotation period. Spectroscopic analysis could tell us much more, and might reveal combinations of atmospheric gases, such as oxygen and methane, that would be evidence for life. SETI experiments might directly detect the presence of technologically sophisticated life.
Earth's Temperature Stable B/C
Greenhouse effect and atmosphere even though sun has gotten hotter = 4 hydrogen to 1 helium, increasing density in core of star increases rate of fusion so luminosity increases = *Faint Young Star Paradox*
Hypothesized Chain of Life
Inorganic material → synthesis of organic compounds from inorganic → polymerization → "RNA World" → membrane enclosure, DNA → protein mechanism
Potential for life on asteroids (Ceres, etc.)
Largest asteroids have enough gravity to be spherical, but are *not large enough to maintain an atmosphere* There are *hydrated minerals in many meteorites* that suggest that there was liquid water when the minerals formed Ceres is the largest asteroid *Ceres has an icy mantel that might cover liquid water underneath (water vapor emissions detected)*
Detecting Habitability
Look for *O2, liquid water (oceans, clouds, etc.), surface temperature, atmosphere, methane (de-equilibrum).* All using reflected light (spectra) Color: blue = oceans? white = clouds? Galileo spectral image of earth shows red continents because *chlorophyll (b/c vegetation)* Spectra = infrared absorption bands??? Surface temp can be determined using spectra = shape of spectra varies by temp (longer wavelengths = colder). BUT on planets with thick atmospheres spectrum wouldn't work (so, infer the amount of greenhouse gases from spectra and model surface temp)
Why was Mars warmer and wetter in the past? Why did it change?
Mar's atmosphere must once have been thicker with a much stronger greenhouse effect, though we do not yet know for certain whether this made Mars warm and wet for an extended period of time or only intermittently. Change must have occurred due to loss of atmospheric gas, which weakened the greenhouse effect (caused by the freezing of Mars' core). Some gas was probably blasted away by impacts, but more was stripped away by the solar wind as Mars cooled and lost its magnetic field and protective magnetosphere. Water was probably also lost because ultraviolet light could break apart water molecules in the atmosphere, and the lightweight hydrogen then escaped to space. Mars warmer and wetter in past because of a strong greenhouse effect (b/c Mars far from the sun and the sun was dimmer then)
Evidence suggesting that Mars must have been warm and wet, possibly with rainfall, in its distant past.
Mars most likely had a stronger greenhouse effect in the past. Calculations suggest that martian volcanoes could have outgassed enough carbon dioxide to make the atmosphere 400 times denser than it is today. There also would have been a lot of liquid water. However, the sun was much dimmer in the past, meaning that Mars would have to have either an even stronger greenhouse effect or another source of heat for liquid water to flow. Some scientists hypothesize that carbon dioxide ice clouds or methane or sulfur gases could have provided this additional heating. Another theory is that Mars wasn't constantly heated, but had periods of heat caused by large impacts of volcanic activity that may have produced wet periods. The conditions on Mars most likely changed because Mars lost a large quantity of carbon dioxide gas, which would have weakened the greenhouse effect. The gas was probably lost by large impacts or by a weakening magnetic field caused by the freezing over of Mars' core, which would have allowed solar winds to strip particles away from Mars' atmosphere, thinning it.
Mars Geology Now
No plate tectonics Rigid crust Cold temperatures (less greenhouse effect, less internal heating) → no liquid water *KEY FEATURES*: - *Valles Marineris* = canyon, long as US, 4x deeper than Grand Canyon (maybe evidence of water on Mars, could have carved the canyon) - *Olympus Mons* = mountain, 3x as tall as Everest
What Planets Habitable/Not
Not Habitable = *Mercury, Moon, Venus, and Gas/Ice Giants* (maybe early Venus) Maybe Once = *Venus* and *MARS* (Mars b/c evidence of past water on its surface [river channels, islands, *recurrent slope linae* = water-rich salty minerals that have made streaks on steep slopes facing the sun - evidence of upwelling groundwater], possibly liquid water under its present surface Maybe Now = *Europa (evidence: young icy surfaces, low density despite rocky core, convection, water-upwelling, weak magnetic field may suggest salty seawater, water vapor plume erupting near South Pole, pink surface), Callisto, Ganymede, Enceladus* (may have subsurface oceans that could support life, maybe asteroid Ceres too, young surfaces) & *Titan* (existing molecules that may have impact-induced liquid water and maybe subsurface ocean; "weird" life in methane lakes OR subsurface life; has a methane-ethane cycle that suggests subsurface ocean) & *Triton* (possible frozen lakes) & *Pluto* (maybe liquid water under fresh surface ice) & *Io* (volcanism) & *Lineae* (upwellings of warm ice)
What is a runaway greenhouse effect, and why did it occur on Venus but not on Earth? What does this tell us about the inner boundary of the Sun's habitable zone?
On Venus, nearly all the carbon dioxide is trapped in the atmosphere, while on Earth nearly all the carbon dioxide is trapped in carbonate rocks or dissolved in oceans. Venus lacks a carbon dioxide cycle like Earth because the cycle requires liquid water to dissolve carbon dioxide, so it can undergo chemical reactions to make carbonate minerals. While the Earth has lots of liquid water that can be outgassed from volcanoes, Venus' surface is far too hot for liquid water. One way Venus could have lost its water is from ultraviolet light from the Sun that could break apart water from molecules in Venus' atmosphere. Venus lacks a protective magnetic field so this hypothesis makes sense. The primary reason behind the runaway greenhouse effect on Venus is that Venus is closer to the Sun than the Earth. The greater intensity of sunlight on Venus because of its proximity to the sun has made it so perhaps oceans were never able to form (because it was too hot for liquid water), or that if oceans did form, they were eventually heated to the point that all of the water evaporates into the atmosphere (and was probably lost through thermal heating or from solar wind particles). Without these oceans to dissolve carbon dioxide, all of Venus' carbon dioxide buildup in the atmosphere, causing a runaway greenhouse effect where no of it was getting cycled out, thus heating the atmosphere. This means that Venus is not in the Sun's habitable zone, especially because if Earth was at Venus' distance, the Earth would face the same fate as Venus. The inner boundary of the Sun's habitable zone must thus lie somewhere in between Venus and the Earth.
Phanerozoic 3 Eras & Periods of Mammals, Dinosaurs, and Humans
Paleozoic, Mesozoic, Cenozoic Dinosaurs = *triassic, jurassic, cretaceous (250 million years ago - 2 million years ago)* Mammals = *tertiary (60 million years ago - 2 million years ago)* Humans = *quaternary (2 million years ago - present)*
Keppler 22b
Planet (sun is Keppler 22) Planet in habitable zone of a sun-like star (K star, less luminous than our sun).; water world? Like 600 light years away
Rare Earth Hypothesis
Plate tectonics rare (Carbonate-Silicate cycle rare) Earth lucky to have a Moon (moon-forming impacts rare) (without moon = extreme ice ages) Jupiters take the hit
Drake Equation
Probability of another intelligent civilization that is capable of communication N = R * F(planet) * Ne * F(life) * F(intelligence) * F(civilizations) * L Estimates: R = 10 (GKM stars born/year) F(planet) = 1/3 (how may stars have planets) Ne = 1/250 (habitable planets) F(life) = 1/2 (guess) F(intelligence) = 1/2 (guess) F(civilizations) = 1/10 (guess) L = 10,000 (guess; lifetime of intelligence) = 3 communicating civilizations in Milky Way
Continuously Habitable Zone
Region in which a planet could remain habitable for some period of time (our sun = 0.95-1.01 AU OR 0.85-1.7 AU. Inner edge = runaway greenhouse effect. Outer edge = "snowball" effect. When sun becomes a red giant, Earth will have a runaway greenhouse effect (4 billion years from now).
How do we search for intelligent life?
SETI. This uses radio or optical signals transmitted by distant civilizations. (has not yet succeeded)
Kepler Space Telescope
Searches for "transiting" planets (pass in front of a star, works for planets we face head on). *Infer that 1/3 of stars have planets* Habitable zone = 240-373 degrees Kelvin (more optimistic is 180 - 310) Kepler found 4 Earth's or "Super-Earths" within this range (but Kepler not very good at finding small Earth-size planets)
Mars Now
Small Frozen early in history (b/c *low magnetic field* b/c low convection in mantle) *Atmosphere mostly CO2* Southern Hemisphere = heavily bombarded (northern not) *KEY FEATURES*: - polar ice caps (water ice, in winter covered with CO2 ice) - seasons (polar caps change with seasons) - atmosphere is thin Sometimes surface is *frosty* b/c atmospheric water vapor precipitates and freezes.
What kinds of extrasolar worlds might be habitable?
Surface habitability seems possible for planets or moons similar in size and composition to Earth and located within the habitable zone, and the habitable zone may extend farther for super-Earths or water worlds with thick hydrogen atmospheres. Subsurface habitability may be even more common, since it is possible on any world with enough internal heat to keep water liquid beneath the surface. Orphan planets, which do not orbit a star, also offer intriguing possibilities for subsurface life, and possibly even for surface life they have thick enough atmospheres.
What is the Drake equation?
The Drake equation gives us a way to organize our thinking about the question of the number of civilizations in the Milky Way Galaxy. In its modified form, it says that the number of civilizations with which we could potentially communicate is N = N(hp) * f(life) * f(civ) * f(now), where N(hp) is the number of habitable planets in the galaxy, f(life) is the fraction of habitable planets that actually have life on them, f(civ) is the fraction of life-bearing planets in which a civilization capable of interstellar communications has at some time arisen, and f(now) is the fraction of all these civilizations that exist now.
How did we learn to classify stars?
The advent of spectroscopy allowed astronomers to study stars by categorizing them according to their spectra. The women astronomers of Harvard recognized the spectral sequence OBAFGKM and, later, that this sequence represents a sequence in surface temperatures.
Where are the boundaries of the Sun's habitable zone today? Will it change?
The boundary currently extends from a distance of about 0.84 AU to 1.7 AU (more conservatively, 0.95 AU to 1.4 AU). The habitable zone will change as the Sun ages. Its luminosity gradually increases, and as a result the habitable zone gradually moves outward with time.
Viking Experiments
The first experiment was called the *carbon assimilation experiment*. It mixed a sample of martian soil with carbon dioxide and carbon monoxide gas that was brought from Earth. These gases from Earth were distinguished from the martian atmosphere because they had been "tagged" with radioactive carbon-14. The results of the test showed that the carbon-14 became a part of the soil (this first suggested that life was present and was using carbon for metabolism). The experiment was then adjusted to use soil heated to 175℃ and the carbon-14, again, became part of the soil. 175℃ is considered hot enough to break chemical bonds between carbon and other atoms (and kill carbon-based organisms) so most scientists concluded that a chemical rather than a biological process was responsible for the experiment's results. The second experiment was called the *gas exchange experiment*. It mixed martian soil with a "broth" that contained organic nutrients from Earth. When the soil was exposed to the nutrients from Earth, oxygen was released into the chamber (suggesting photosynthesis, but, the process took place in the dark, so it couldn't have been photosynthesis). Oxygen was released even when the soil was exposed to water vapor. The reactions continued when the soil was heated to temperatures that would kill carbon-based organisms (175℃, like in the first experiment). Scientists concluded that the results were due to chemical and not biological processes. The third experiment was called the *labeled release experiment*. It mixed martian soil with organic nutrients from Earth which was tagged with radioactive carbon-14 and sulfur-35. It looked for changes in the level of radioactivity in the chamber has that might occur if living organisms had consumed the nutrients and released the gases. If this occurred due to living organisms, the level of radioactivity would rise at first and then level off as the nutrients were used up. This is what happened, suggesting life. When the soil was heated to 175℃ it eliminated any sign of the tagged isotopes in the chamber gas (suggesting that the life died). The fourth experiment was called the *gas chromatograph/mass spectrometer experiment*. It sought to measure the abundance of organic molecules in the martian soil. The experiment found no sign of organic molecules in the martian soil, which would have ruled out the possibility of carbon-based life in the samples studied. *Because the results of these experiments contradicted each other, they do not constitute evidence of life on Mars*. Scientists agree that different experiment designs will be needed for more definitive searches for life on Mars. *Perchlorate* found in Martian soil thought to have been brought from Earth b/c it oxides and destroys organic compounds at higher temperatures. Study done in desert in Chile (Atacama desert). Could have been reason for mixed results.
How do extrasolar planets compare with planets in our solar system?
The known extrasolar planets have a much wider range of properties than the planets in our solar system. Many orbits much closer to their stars and with more eccentric orbital paths; some Jovian planets, called hot Jupiters, are also found close to their stars. We have also observed properties indicating planetary types, such as super-Earths and water worlds, and differ from the terrestrial and Jovian planets in our solar system.
Wow Signal
a radio signal of apparently inteligent life in space. This signal was received one time and one time only. Lasted 72 seconds. From Sagittarius. (no info encoded = no signs of intelligence)
Why Mars' interior froze
The total amount of heat contained in a planet depends on the planet's volume, and the heat can only escape from the planet's surface. As heat escapes, heat flows upward from the interior to replace it, until the interior eventually cools much more. The time it takes for a planet to lose its internal heat is thus related to the surface area-to-volume ratio. Larger planets thus cool in less time than smaller ones.
Tides, tidal heating
Tidal force = difference between two gravitational forces *Friction caused by tides heats planetary interiors or oceans* Changing tides flex a planet's interior and cause tidal heating Io has tidal heating (highly elliptical orbit b/c Jupiter)
Which stars make good suns?
To make a good "sun" for a habitable world, a star should have a high enough proportion of elements besides hydrogen and helium to allow planet formation (most stars probably qualify); it should be low enough in mass that it will live long enough for life to take hold and evolve; it should be in the hydrogen-fusing state of life that allows for steady sunlight and a habitable zone; and if it is in a binary or multiple-star system, it should allow for stable orbits within the habitable zone.
Type I-III Civilizations
Type I civilization can manage the entire energy and material resources of a planet Type II civilization is capable of harnessing the energy and material resources of a star and its planetary system Type III civilization is able to marshal the energy and material resources of an entire galaxy
Is Venus in the habitable zone?
Venus clearly is not in the habitable zone today, as any planet that once had Earth-like conditions would have suffered a runaway greenhouse effect. However, early in its history, when the Sun was some 30% dimmer than it is today, Venus may have been within the Sun's habitable zone and hence could have had rain, oceans, and perhaps even life.
Is there evidence of life on Mars?
Viking experiments are inconclusive. Possibly methane gas in the atmosphere which could be the result of biological or geological activity (internal heat that means life could be on Mars). If there was ever life on Mars, it was probably in first billion years of its history (b/c small planets lose their atmospheres easier)
Are Earth-like planets rare or common?
We don't know. Some of the key questions are whether our galaxy, like a star, has a relatively narrow habitable zone; whether the role of Jupiter has played in lowering our solar system's impact rate is rare or critical to life; and whether Earth's relatively stable climate, due largely to plate tectonics and our large Moon, is likely on otherwise similar worlds. Arguments can be made on both sides of each question, and at a present we lack the data to determine which side is correct.
Secondary Transit Spectroscopy
When a planet passes in front of a star E.g. James Webb Space Telescope (may star with M star (red dwarf) which has planets with short orbit, so takes least amount of time to gather evidence about them).
Jovian planet characteristics
no solid surface, outer layers made of hydrogen and helium at much cooler temperatures, clouds driven by solar and internal heating, strong magnetic field Jupiter has liquid water but strong vertical winds that carry any life to depths would destroy them and little chemical "free energy" to do useful biological work
Fermi Paradox
the apparent contradiction between the lack of evidence and high probability estimates for the existence of extraterrestrial civilizations.
Is intelligence common?
we don't know. studies indicate a drive towards intelligence, so it is plausible to imagine intelligence appearing on any planet with life, given enough time.