A2299 The Search for Life in the Universe

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1 ! A2299 The Search for Life in the Universe Jim Cordes, Shami Chatterjee Recently: Impacts, extinctions, water, life.! Today: Habitable Zones.! Reading: As posted.! Assignment 3 posted. Due 3/29.! Term paper or debate topics? Due 3/22.!! Web Page:

2 Moon: many more old craters than new ones Era of heavy bombardment

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4 Current picture about water Earth accreted water throughout its history. It still does! About 30,000 tons/yr of comet/asteroid material. Initial asteroid contributions were from gas drag of asteroids. Most water came from planetary embryos formed far from the Earth (+ migration). A late veneer phase brought about 10% water from comets that have high D/H water.

5 What is life? 1. A self-organized non-equilibrium system! such that 2. its processes are governed by a stored symbolic program! and 3. it can reproduce itself, including the program.! From: Smolin, The Lives of the Cosmos, p. 156

6 Life Elsewhere: Assumptions Life is based on carbon chemistry. Are there alternatives? Perhaps. Carbon perhaps the most probable route (cosmic abundances, bonding properties) so it wins. Liquid water plays a major role in the rapid formation of life and its evolution. Necessary? Perhaps not. But water is cosmically abundant. Provides a good compromise between mobility of molecules and rate of interaction (density). The H 2 O molecule stays liquid over a wider range of temperatures than many other solvents. Interesting properties: Expands when freezing; Slippery; Wet; High heat capacity.

7 No place like home HABITABLE ZONES

8 Habitable Zones Requirements: Liquid water sustained over billions of years. Need low incidence rate of high-mass impacts. Places conditions on stability of a planet s orbit. Stability of host star s luminosity and low incidence of stellar flares. Need stable overall environment: No cosmic bad days ( local gamma-ray bursts, supernovae, etc.)

9 Habitable vs. Colonizable Habitable is used to mean life could have evolved in a liquid-water environment. Solar system: Earth, early Mars. Not so obvious: Europa, Enceladus? Can imagine environments where life could not have formed but to which it could have migrated to: Ithaca. Mars, the Moon, Europa?

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11 Stellar Habitable Zone Liquid water on a planetary surface requires a specific range of temperature. Freezing temperature of pure water: 0 o C = K Boiling temperature: 100 o C = K Given the Sun s luminosity L, one can calculate the range of distances from the Sun where the planet surface temperature would fall in the range [273, 373K]. Simple? Need to account for greenhouse effect to get a plausible range of distances in which the Earth resides.! If distance range is too small, that would suggest we are improbable. Other venues besides planets + stellar luminosity? Satellites of giant planets + tidal flexing as source of heat.

12 CHZ = Continuously Habitable Zone Defined as the range of distances from the host star where liquid water can be maintained. Defined in terms of a planetary surface that can maintain liquid water for sustained periods of time. (3 Gyr often used.) Surface temperature depends, on average, on the star s luminosity, the distance from the star, and the properties of the atmosphere. Reflecting clouds: reduces temperature on surface. Greenhouse effect: increases surface temperature. But surface conditions are not always average: Impacts, volcanoes, runaway glaciations (snowball Earth).

13 Rough Constraints on the CHZ Empirical: Earth is in the CHZ. Mars: water is frozen in the soil, thin atmosphere. Venus: runaway green house effect, most CO 2 is in the atmosphere. So HZ is between 0.72 and 1.5 AU. Calculations based on solar luminosity and atmospheric conditions (simple to complex). Implies smaller range but ~ 0.5 AU wide.

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16 CO 2 and the Carbonate-Silicate Cycle Weathering of rocks removes CO 2 from atmosphere: Silicate rocks + CO 2 = limestone + SiO 2. Feedback cycle: regulation of CO 2. Warmer planet! more weathering! less CO 2! cooling (less greenhouse effect). Cooler planet! less weathering! more CO 2! warmer. Tectonics! volcanoes! CO 2 recycling. NOTE: This feedback loop operates on geological timescales.

17 Earth s Habitable Zone We know that the HZ is smaller than the distance between orbits of Venus and Mars [0.72, 1.5AU]. Venus: runaway greenhouse effect; oceans boil off. Mars: water is frozen now but there have been clear episodes of liquid water (probably very short). Another problem: Mars has a low mass. If we ignore the atmosphere: Average surface temperature = 280K HZ = [0.56,1.05AU].

18 Earth s Habitable Zone We know that the HZ is smaller than the distance between orbits of Venus and Mars [0.72, 1.5AU]. If we ignore the atmosphere: HZ = [0.56,1.05AU]. If we account for reflection of 31% of the Sun s radiation off cloud tops: Average surface temperature = 255K. HZ = [0.47,0.87AU]. (We shouldn t be here!) If we take into account the greenhouse effect): Average surface temperature = 288K. HZ = [0.6,1.11AU].

19 Earth s Habitable Zone We know that the HZ is smaller than the distance between orbits of Venus and Mars [0.72, 1.5AU]. If we ignore the atmosphere: HZ = [0.56,1.05AU]. If we account for reflection of 31% of the Sun s radiation off cloud tops: HZ = [0.47,0.87AU]. If we take into account the greenhouse effect): HZ = [0.6,1.11AU]. Feedback cycles (carbon-silicate cycle) yield Kasting et al s HZ = [0.95, 1.37AU].

20 HZs and other stars:" Stellar Radiation A star s spectrum is essentially black-body radiation at an effective temperature T: Planck curve

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23 Empirical Estimates of Habitable Zones Stellar luminosity: more massive stars are hotter and bigger. So, Earth-equivalent radiation further away from star: L / D 2 equiv = L Sun /D2 Earth HZ limits depend on many factors! Atmospheric composition and greenhouse effect. Clouds and reflection of incoming radiation. Feedback like the Carbonate-Silicate cycle. Very crudely, 170% to 25% of current solar flux at 1AU?! So ~0.75 AU to 2 AU for G-type star.

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25 Other habitable zones in the solar system? Europa: icy crust, tectonic-like features, liquid ocean below. Enceladus: Polar jets provide evidence for liquid underground lake? ocean? Future lecture. Titan: high-pressure atmosphere, methane lakes. Future lecture.

26 Continuously Habitable Zones The early Sun was about 30% less luminous than at present; consequences? CHZs around other stars? Which stars are most likely to have CHZs? Also: Galactic habitable zones.

27 Sun s evolution! in luminosity! and diameter.!!! (Don t wait around for another 7 billion years!)

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29 Danger, Will Robinson! HAZARDOUS ZONES

30 Habitable and Hazardous Habitable zones - Ingredients for life: Liquid water, Organics. Stability, Free energy, Time (Gyr). Hazards: changes in environment, catastrophes. Geophysical: Volcanism, methane clathrates. Solar system: Solar flares, impacts, orbital instabilities. Astrophysical: Supernovae, gamma-ray bursts, magnetars.

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32 Mercury s eccentricity over 5 Gyr. J Laskar & M Gastineau Nature 459, (2009) doi: /nature08096 Evolution of the maximum eccentricity of Mercury (computed over 1-Myr intervals) over 5 Gyr. (a) Pure Newtonian model without the contribution of the Moon, for 201 solutions with initial conditions that differ by only 3.8 cm in the semi-major axis of Mercury. (b) Full Solar System model with relativistic and lunar contributions, for 2,501 solutions with initial conditions that differ by only 0.38 mm in the semi-major axis of Mercury.

33 Example of collisional trajectory for Mars and the Earth. Evolu<on of the maximum eccentricity of Mercury (red), Mars (green) and the Earth (blue), recorded over 1-Myr intervals. J Laskar & M Gastineau Nature 459, (2009) doi: /nature08096

34 Collisional trajectories for Mars and Venus with the Earth. Eccentricity (a) and semi-major axis (b) ploged versus <me for Mercury (red), Venus (pink), the Earth (blue) and Mars (green). (c) Minimum Earth-Mars (green) and Earth-Venus (pink) distances in astronomical units, recorded over each 1,000-yr <me interval. The horizontal lines are the Mars Earth (green) and Venus Earth (pink) distances of collision, Dmin, corresponding to the sum of the planets' radii. J Laskar & M Gastineau Nature 459, (2009) doi: /nature08096

35 Our home in the Milky Way GALACTIC HABITABLE ZONES

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37 Habitable Zones Around Stars " and in the Milky Way Galaxy Continuously Habitable Zone around main-sequence stars Galactic Habitable Zone

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39 GRBs, SGRs, etc. GALAXY-SCALE CATASTROPHES

40 Most Distant Star Burst Detected By Larry O'Hanlon, Discovery News March 8, 2006 The discovery of the most distance and ancient stellar explosion has now been confirmed and pushed back another 100 million lightyears to 12.8 billion light-years away. Since cosmic time and distance are both measured by the speed of light, the explosion known as GRB took place 12.8 billion years ago, when the universe was a relatively youthful 900 million years old.

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42 Ordovician: Myr ago bracketed by a minor and a major extinction event

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44 Gamma-Ray Bursts: Gamma-ray burst energetics Energy Release with Human Experience erg = energy to make a mosquito jump 10^3 ergs = ball drop 10^10 ergs = hit by truck 10^15 ergs = smart bomb 10^20 ergs = H bomb 10^26 ergs = killer asteroid 10^40 ergs = Death Star Energy Release with Astronomical Experience ^33 ergs/s = Sun 10^39 ergs/s = nova 10^41 ergs/s = SN 10^45 ergs/s = galaxy 10^52 ergs/s = GRB

45 Soft gamma repeater burst of 2004 December" SGRs = neutron stars with magnetic fields strong enough to crack their crusts

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48 Giant Flare in SGR and Its Compton Reflection from the Moon D. D. Frederiks 1, S. V. Golenetskii 1, V. D. Palshin 1, R. L. Aptekar 1*, V. N. Ilyinskii 1, F. P. Oleinik 1, E. P. Mazets 1,and T.L.Cline 2 1 IoffePhysical Technical Institute, Russian Academy of Sciences, ul. Politekhnicheskaya 26, St. Petersburg, Russia 2 Goddard Space Flight Center, NASA, Greenbelt, MD 20771, USA Received August 17, 2006 Abstract We analyze the data obtained when the Konus Wind gamma-ray spectrometer detected a giant flare in SGR on December 27, The flare is similar in appearance to the two known flares in SGR and SGR while exceeding them significantly in intensity. The enormous X-ray and gamma-ray flux in the narrow initial pulse of the flare leads to almost instantaneous deep saturation of the gamma-ray detectors, ruling out the possibility of directly measuring the intensity, time profile, and energy spectrum of the initial pulse. In this situation, the detection of an attenuated signal of inverse Compton scattering of the initial pulse emission by the Moon with the Helicon gamma-ray spectrometer onboard the Coronas-F satellite was an extremely favorable circumstance. Analysis of this signal has yielded the most reliable temporal, energy, and spectral characteristics of the pulse. The temporal and spectral characteristics of the pulsating flare tail have been determined from Konus Wind data. Its soft spectra have been found to contain also a hard power-law component extending to 10 MeV. A weak afterglow of SGR decaying over several hours is traceable up to 1 MeV. We also consider the overall picture of activity of SGR in the emission of recurrent bursts before and after the giant flare. PACS numbers : Jd; Rz; Pw; Jx