Habitability Bonnie Meinke January 27, 2009 Bonnie Meinke January 27, 2009

Introduction  Define Habitability  The Habitable Zone  Environment of early Earth  Define Habitability  The Habitable Zone  Environment of early Earth

Defining Habitability  What do we mean when we say habitable?  Earth-like animal life: specific requirements  Microbial life: broader set of conditions  What do we mean when we say habitable?  Earth-like animal life: specific requirements  Microbial life: broader set of conditions Defining Habitability

 What do we mean when we say habitable?  Earth-like animal life: specific requirements (oxygen, water, dry land, temperature range)  Microbial life: broader set of conditions (more extreme conditions ok)  What do we mean when we say habitable?  Earth-like animal life: specific requirements (oxygen, water, dry land, temperature range)  Microbial life: broader set of conditions (more extreme conditions ok) Defining Habitability

Common basic requirements for life  Water  Stable climate  Water  Stable climate Defining Habitability

What stabilizes the climate?  Size - long-term heat source  Stellar evolution - incoming solar energy  Impact rate - could result in climate change  Presence of large, natural satellite - prevents large swings in obliquity  Oceans - regulate global temperatures  Size - long-term heat source  Stellar evolution - incoming solar energy  Impact rate - could result in climate change  Presence of large, natural satellite - prevents large swings in obliquity  Oceans - regulate global temperatures Defining Habitability

Habitable Zones  Why is Earth the only (as far as we know) habitable planet in our solar system?  2 main properties:  Abundant liquid water  Environmental conditions that maintain liquid water  Why is Earth the only (as far as we know) habitable planet in our solar system?  2 main properties:  Abundant liquid water  Environmental conditions that maintain liquid water The Habitable Zone

Liquid Water  Required temperature: K  Use this as simple requirement for identifying possibly habitable planets  Where do planets in this temperature range orbit?  Required temperature: K  Use this as simple requirement for identifying possibly habitable planets  Where do planets in this temperature range orbit? The Habitable Zone

Liquid Water  Where do planets in this temperature range orbit?  Called the Habitable Zone  Let’s work it out…  Where do planets in this temperature range orbit?  Called the Habitable Zone  Let’s work it out… The Habitable Zone

How does star type affect HZ?  Different sized stars have different luminosities  T  L 1/4  Brighter stars have HZs farther out  Different sized stars have different luminosities  T  L 1/4  Brighter stars have HZs farther out The Habitable Zone

How does star type affect HZ?  Main sequence (MS) stars have different luminosities throughout their lifetimes  Continuously Habitable Zone: maintains conditions suitable for life throughout MS lifetime of star  Main sequence (MS) stars have different luminosities throughout their lifetimes  Continuously Habitable Zone: maintains conditions suitable for life throughout MS lifetime of star The Habitable Zone

Is it that simple?  Albedo, a  Atmosphere - what part of spectrum can pass through  Albedo, a  Atmosphere - what part of spectrum can pass through The Habitable Zone Moves HZ inwards Moves HZ outwards

Role of the Carbon Cycle  Kasting proposed the Carbon Dioxide Thermostat  Extends to HZ for Earth-like planets  Keeps off temperature extremes  Kasting proposed the Carbon Dioxide Thermostat  Extends to HZ for Earth-like planets  Keeps off temperature extremes  Carbon sources:  Volcanic outgassing  Decarbonation  Organic carbon  Carbon sinks:  Calcium carbonate formation  Photosynthesis The Habitable Zone

Role of the Carbon Cycle The Habitable Zone

Continuously Habitable Zone  Inner edge: 0.95 AU  Outer edge: 1.15 AU  Inner edge: 0.95 AU  Outer edge: 1.15 AU  Were other planets habitable in the past?  Will other planets be habitable in the future? The Habitable Zone

Mars: Once Habitable? Still Habitable?  Early Mars  Evidence of large amounts of flowing liquid water  Warmer temperatures:  Heat from interior would have been higher  Warm climate from greenhouse gases or CO 2 clouds  Early Mars  Evidence of large amounts of flowing liquid water  Warmer temperatures:  Heat from interior would have been higher  Warm climate from greenhouse gases or CO 2 clouds  Current Mars  Gullies may be due to underground water  Carbon cycle not as active as on Earth The Habitable Zone

Characteristics that make a habitable planet The Habitable Zone Other Heat sources to sustain liquid water Geothermal Iceland Tidal Europa Other Heat sources to sustain liquid water Geothermal Iceland Tidal Europa Size of planet Internal heat comes from Accretional heat Differentiation Radiogenic decay Allows for plate tectonics Mars cooled quickly, so no plate tectonics at present Size of planet Internal heat comes from Accretional heat Differentiation Radiogenic decay Allows for plate tectonics Mars cooled quickly, so no plate tectonics at present

Characteristics that make a habitable system  Star Type: stable luminous stars necessary  Sufficiently long lifetime for life to evolve  Large enough so planets don’t tidally lock The Habitable Zone Star system Single star: allows for stable orbit Binary system: Fewer stable orbits exist HZ calculated on individual basis Star system Single star: allows for stable orbit Binary system: Fewer stable orbits exist HZ calculated on individual basis

Characteristics that make a habitable neighborhood  Galactic Habitable Zone  Area of high metallicity (elements w/ Z>2)  Outer region of galaxy  Lower stellar density  Lower radiation levels The Habitable Zone

Early Earth Astr 3300 September 16, 2009 Astr 3300 September 16, 2009

Environment of early Earth Evidence of a habitable planet 3.8 Ga –Geological evidence near Isua, Greenland –Limestone and sandstone –We can infer presence of liquid water –Earth must have had temperatures similar to today’s Evidence of a habitable planet 3.8 Ga –Geological evidence near Isua, Greenland –Limestone and sandstone –We can infer presence of liquid water –Earth must have had temperatures similar to today’s Early Earth

Liquid water 3.8 Ga? Faint young Sun –Sun was 25-30% less luminous –Simple energy balance shows Earth’s surface temperature would have been below 273 K Other heat sources –Geological activity More internal heat from radioactive decay and primordial heat Plate tectonics release CO 2 - greenhouse traps heat Faint young Sun –Sun was 25-30% less luminous –Simple energy balance shows Earth’s surface temperature would have been below 273 K Other heat sources –Geological activity More internal heat from radioactive decay and primordial heat Plate tectonics release CO 2 - greenhouse traps heat Early Earth

Snowball Earth Global glaciations brought on by disruptions in the carbon cycle –Up to 4 occurred between 750 Ma and 580 Ma ago –Geological record shows layered deposits in tropics attributable to glacial erosion CO 2 sinks would cease, but sources would continue. 350 times current CO 2 levels would accumulate to create a severe greenhouse, causing the ice to melt w/in a few hundred years. All eukaryotes today are from the survivors of snowball earth Global glaciations brought on by disruptions in the carbon cycle –Up to 4 occurred between 750 Ma and 580 Ma ago –Geological record shows layered deposits in tropics attributable to glacial erosion CO 2 sinks would cease, but sources would continue. 350 times current CO 2 levels would accumulate to create a severe greenhouse, causing the ice to melt w/in a few hundred years. All eukaryotes today are from the survivors of snowball earth Early Earth

Early Hydrosphere How did Earth get all it’s water? Early Earth

Origin of Earth’s Water Delivered by comet impact –D/H ratios of comets are not the same as on earth –This is unlikely the delivery mechanism Solar nebula –Unlikely because relative abundance of other volatiles are higher in the solar nebula than in planetary atmospheres From un-degassed interiors of planetary embryos –Most likely scenario –Hydrated minerals could form around 1 AU Delivered by comet impact –D/H ratios of comets are not the same as on earth –This is unlikely the delivery mechanism Solar nebula –Unlikely because relative abundance of other volatiles are higher in the solar nebula than in planetary atmospheres From un-degassed interiors of planetary embryos –Most likely scenario –Hydrated minerals could form around 1 AU Early Earth

Origin of Earth’s Atmosphere Only trace amounts of oxygen for the first 1 billion years –O 2 resulted from breakdown of water vapor by UV radiation Current atmosphere is oxygen-rich, so where did it come from? PHOTOSYNTHESIS! –First developed in cyanobacteria Ga ago (Archaean era) Only trace amounts of oxygen for the first 1 billion years –O 2 resulted from breakdown of water vapor by UV radiation Current atmosphere is oxygen-rich, so where did it come from? PHOTOSYNTHESIS! –First developed in cyanobacteria Ga ago (Archaean era) Early Earth

Banded Iron Formations Geologic evidence for appearance of free oxygen are Banded Iron Formations (BIFs) BIFs provide clues as to the oxidation state of ocean and atmosphere at time of formation Usually formed in shallow seas - oxygen available here Geologic evidence for appearance of free oxygen are Banded Iron Formations (BIFs) BIFs provide clues as to the oxidation state of ocean and atmosphere at time of formation Usually formed in shallow seas - oxygen available here Early Earth

Role of hydrothermal systems Seawater flowing through hydrothermal vents dissolved iron Injected iron into deep ocean through vents Deep ocean too oxygen- poor to oxidize iron, so it cycled through system to be deposited in shallow seas. Possible iron was consumed by bacteria near vents and transported in drifts of large colonies Seawater flowing through hydrothermal vents dissolved iron Injected iron into deep ocean through vents Deep ocean too oxygen- poor to oxidize iron, so it cycled through system to be deposited in shallow seas. Possible iron was consumed by bacteria near vents and transported in drifts of large colonies Early Earth

Evidence for early life on Earth Stromatolites –Oldest known is 3.46 Ga-old –Formed from cyanobacteria and blue-green algae –Organisms for gelatinous mat and precipitate calcium carbonate, so it looks like stack of pancakes w/ alternating layers Stromatolites –Oldest known is 3.46 Ga-old –Formed from cyanobacteria and blue-green algae –Organisms for gelatinous mat and precipitate calcium carbonate, so it looks like stack of pancakes w/ alternating layers Early Earth

Carbon Isotopes Can be used as indicators of biological processes Early Earth 12 C and 13 C are stable isotopes Ratio is affected by physical processes More energy efficient to make or break 12 C bonds 12 C is preferentially incorporated into products of chemical reactions 12 C and 13 C are stable isotopes Ratio is affected by physical processes More energy efficient to make or break 12 C bonds 12 C is preferentially incorporated into products of chemical reactions

Carbon Isotopes A negative value can be used as a biomarker, indicating fractionation is due to photosynthesis Early Earth Isotopic fractionation let’s work it out Isotopic fractionation let’s work it out

Evolving Complexity Ediacaran fauna show distinctive changes in size ~670 Ma ago) Early Earth Life started small (maximum of a few mm in size) In the last 600 Ma, evolution of more larger, more complex organisms has occurred Life started small (maximum of a few mm in size) In the last 600 Ma, evolution of more larger, more complex organisms has occurred

Evolving Complexity Ediacaran fauna show distinctive changes in size ~670 Ma ago) Tubular, frond- like, radially symmetric cm-m in size Ediacaran fauna show distinctive changes in size ~670 Ma ago) Tubular, frond- like, radially symmetric cm-m in size Early Earth

Increase in size and diversity Subsequently, after 500 Ma ago, sizes increased 2 orders of magnitude Dinosaurs Larger mammals Subsequently, after 500 Ma ago, sizes increased 2 orders of magnitude Dinosaurs Larger mammals Early Earth

Major extinctions Marked periods of Earth’s biological history Reduce diversity Most recent –Possibly due to comet or asteroid impact –Die out of the dinosaurs (65 Ma ago) Demonstrates how important “environmental stability” is for a habitable planet Marked periods of Earth’s biological history Reduce diversity Most recent –Possibly due to comet or asteroid impact –Die out of the dinosaurs (65 Ma ago) Demonstrates how important “environmental stability” is for a habitable planet Early Earth

Banded-Iron Formations (BIFs) Most formed 3Ga-1.8Ga Amount of Oxygen locked in BIFs is ~20 times the volume in the modern atmosphere

Banded-Iron Formations (BIFs) Formation: –Oxygen produced by cyanobacteria combined with iron in the ocean (early ocean was acidic and iron- rich) –Oxidized iron then deposits in a layer –Process is cyclical due to oscillating availability of free oxygen –Eventually, photosynthesis caught on, the oceans because well-oxygenated, and the available iron in the Earth's oceans was precipitated out as iron oxides

Banded-Iron Formations (BIFs) Snowball Earth cycles may have been the cause of bands –During snowball periods, free oxygen not available and iron –Followed by oxidizing periods of melt Metal-rich brines may also be responsible –Carry iron from the deep ocean (near hydrothermal vents) –Deposited in shallow seas where it has access to free oxygen

Carbon Isotopes 12 C and 13 C are stable isotopes More energy efficient to make 12 C bonds 12 C is preferentially incorporated into products of chemical reactions (like photosynthesis!) Ratio of the two isotopes can be used as an indicator of biological processes

Carbon Isotopes If 12 C has preferentially been incorporated, 13 C/ 12 C will be smaller than the standard If sample < standard,  13 C is negative A negative value can be used as a biomarker, indicating fractionation is due to photosynthesis

Extreme Environments ASTR/GEOL 3300 September 18, 2009 ASTR/GEOL 3300 September 18, 2009

Overview Extreme Conditions Other Worlds Extreme Conditions Other Worlds

Extreme conditions Conditions on early earth may have been “extreme” compared to present-day Extremophiles - organisms that thrive in exteme environments –Heat/Cold –Acids/alkalines –High pressures –dessication Conditions on early earth may have been “extreme” compared to present-day Extremophiles - organisms that thrive in exteme environments –Heat/Cold –Acids/alkalines –High pressures –dessication

Temperature Majority of organisms on Earth thrive in the temperature range °C (mesophiles) Usual response to extreme temperatures: –Cold: Formation of ice crystals in the body –Hot: Structural breakdown of biological molecules (proteins and nucleic acids) Disruption of cells’ structural integrity due to increased membrane fluidity Majority of organisms on Earth thrive in the temperature range °C (mesophiles) Usual response to extreme temperatures: –Cold: Formation of ice crystals in the body –Hot: Structural breakdown of biological molecules (proteins and nucleic acids) Disruption of cells’ structural integrity due to increased membrane fluidity Extreme Conditions

Temperature Extreme Conditions

Thermophiles Extreme Conditions Thermophiles live between 50 and 80 °C –Example: Thermoplasma Archaea Lives in volcanic hot springs Hyperthermophiles live between 80 and 115 °C –Example: Sulfolobus No multicellular plants or animals can tolerate >50 °C No microbial eukarya can tolerate >60 °C

Thermophiles First true thermophile discovered in Yellowstone National Park in 1960s > 50 hyperthermophiles have been isolated to date –Many live in or near deep-sea hydrothermal systems (black smokers) First true thermophile discovered in Yellowstone National Park in 1960s > 50 hyperthermophiles have been isolated to date –Many live in or near deep-sea hydrothermal systems (black smokers) Extreme Conditions

Thermophiles: how they cope Since high temperatures change membrane fluidity, adaptation is change of membrane composition Evolution of proteins to better cope w/ high temps Since high temperatures change membrane fluidity, adaptation is change of membrane composition Evolution of proteins to better cope w/ high temps Extreme Conditions

Psychrophiles Supported in frozen environments of Earth Lowest recorded temperature for active microbial communities: -18 °C Found in all 3 domains of life Supported in frozen environments of Earth Lowest recorded temperature for active microbial communities: -18 °C Found in all 3 domains of life Extreme Conditions

Psychrophiles: how they cope Low temps mean decrease in membrane fluidity, so adaptation is adjustment of ratios of lipids in their membranes Prevent water from freezing with soluble compounds that lower freezing temp of water (e.g. thermal hysteresis proteins) Low temps mean decrease in membrane fluidity, so adaptation is adjustment of ratios of lipids in their membranes Prevent water from freezing with soluble compounds that lower freezing temp of water (e.g. thermal hysteresis proteins) Extreme Conditions

Radiation UV and ionizing radiation can do serious damage to DNA –Deinococcus radiodurans can withstand high-dose radiation because it can accurately rebuild its DNA –Also able to cope with extreme dessication, so also a xerophile - thus known as a polyextremophile UV and ionizing radiation can do serious damage to DNA –Deinococcus radiodurans can withstand high-dose radiation because it can accurately rebuild its DNA –Also able to cope with extreme dessication, so also a xerophile - thus known as a polyextremophile Extreme Conditions

pH Most biological processes occur in middle of pH scale (4-8) Acidophile - thrive at Alkaliphile - thrive at Most biological processes occur in middle of pH scale (4-8) Acidophile - thrive at Alkaliphile - thrive at Extreme Conditions

pH Acidophile - thrive at –Occur in geochemical activities Sulfur production at hot springs and deep-sea vents –Cope by pumping H+ out of cells at a high rate Alkaliphile - thrive at –Live in soils containing carbonate and soda lakes –Above pH of 8, RNA breaks down, so alkaliphiles maintain neutrality inside cells Acidophile - thrive at –Occur in geochemical activities Sulfur production at hot springs and deep-sea vents –Cope by pumping H+ out of cells at a high rate Alkaliphile - thrive at –Live in soils containing carbonate and soda lakes –Above pH of 8, RNA breaks down, so alkaliphiles maintain neutrality inside cells Extreme Conditions Acidic mudpot Acidic mudpot: located in Yellowstone NP, home of Sulfolobus acidocaldarius. Photo courtesy of National Park Service

Salinity Halophiles require high concentrations of salt to live (2-5 times that in seawater) Found in Great Salt Lake, Dead Sea, salterns Can be coincident with high alkalinity environments Survive by producing large amounts of internal solute so as to not lose water via osmosis Halophiles require high concentrations of salt to live (2-5 times that in seawater) Found in Great Salt Lake, Dead Sea, salterns Can be coincident with high alkalinity environments Survive by producing large amounts of internal solute so as to not lose water via osmosis Extreme Conditions Great Salt Lake, UT. Great Salt Lake, UT. Carotenoids seen here are biproduct of halophiles.

Dessication Some organisms survive low-water environments via anhydrobiosis, a state of suspended animation Extreme Conditions

Pressure Undersea pressures are much greater than surface pressures –Boiling point increases with pressure, so liquid water at ocean floor could be 400 ºC –Pressure compresses volume, so peizophiles have increased membrane fluidity so they don’t get “smushed” Upper atmosphere pressures are much lower than surface pressures Undersea pressures are much greater than surface pressures –Boiling point increases with pressure, so liquid water at ocean floor could be 400 ºC –Pressure compresses volume, so peizophiles have increased membrane fluidity so they don’t get “smushed” Upper atmosphere pressures are much lower than surface pressures Extreme Conditions

Oxygen Aerobic metabolism is more efficient than anaerobic, but it kills cells quicker via oxidation Many organisms with aerobic metabolisms combat oxidation with natural anti-oxidants Aerobic metabolism is more efficient than anaerobic, but it kills cells quicker via oxidation Many organisms with aerobic metabolisms combat oxidation with natural anti-oxidants Extreme Conditions

Extremes on other planets If we’ve seen life thrive in extreme circumstances on Earth, why not on other planets? Mars holds most promise What about moons in our solar system: –Europa –Titan –Enceladus If we’ve seen life thrive in extreme circumstances on Earth, why not on other planets? Mars holds most promise What about moons in our solar system: –Europa –Titan –Enceladus Other Worlds

Possible Earth analogues Hotsprings The deep sea Hypersaline environments Evaporites The atmosphere Ice, permafrost, snow Subsurface environments Hotsprings The deep sea Hypersaline environments Evaporites The atmosphere Ice, permafrost, snow Subsurface environments Other Worlds

Europa Life exists w/o photosynthesis in the deep ocean Europa has a subsurface ocean Life may exist beneath the surface Life exists w/o photosynthesis in the deep ocean Europa has a subsurface ocean Life may exist beneath the surface Other Worlds

Europa Life exists w/o photosynthesis in the deep ocean Europa has a subsurface ocean Life may exist beneath the surface –Shielded from Jupiter’s radiation –Warmer than surface temperatures Life exists w/o photosynthesis in the deep ocean Europa has a subsurface ocean Life may exist beneath the surface –Shielded from Jupiter’s radiation –Warmer than surface temperatures Other Worlds

Titan Airborne micro-organisms? Extremes to withstand: –Dessication –Radiation Airborne micro-organisms? Extremes to withstand: –Dessication –Radiation Other Worlds

Titan On Earth, spores are only things that really “live” in the atmosphere. Debate as to weather this constitutes life On Earth, spores are only things that really “live” in the atmosphere. Debate as to weather this constitutes life Other Worlds

Mars Host to several extreme environments –Deserts –Ice, permafrost, snow –Subsurface Other Worlds

Mars: deserts Driest places on earth –Hottest: Atacama –Coldest: Antarctica Bacteria, algae, fungi live on or under rocks –Endoliths –Cryptoendoliths Rocks provide shelter from –Temperature extremes –UV radiation Other Worlds

Mars: ice, permafrost, snow Microbes and algae exist in frozen environments on Earth Maybe not thriving, but microbial survivors could exist Other Worlds

Mars: subsurface environments Best chance of withstanding Martian extremes –No liquid water at surface –Low pressure –CO 2 -rich atmosphere –Only 43% solar radiation at Earth Subsurface provides –Protection –Possible liquid water –Energy source for chemolithoautotrophs Other Worlds