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: http://www.astro.cornell.edu/academics/courses/astro2299/

Moon: many more old craters than new ones

Era of heavy bombardment

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.

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

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 H2O molecule stays liquid over a wider range of

temperatures than many other solvents. •  Interesting properties: Expands when freezing;

Slippery; Wet; High heat capacity.

HABITABLE ZONES No place like home…

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.)

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?

Stellar Habitable Zone •  Liquid water on a planetary surface requires a specific

range of temperature. •  Freezing temperature of pure water: 0o C = 273.15K •  Boiling temperature: 100o C = 373.15K

•  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.

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).

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 CO2 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.

CO2 and the Carbonate-Silicate Cycle •  Weathering of rocks removes CO2 from atmosphere:

–  Silicate rocks + CO2 = limestone + SiO2. •  Feedback cycle: regulation of CO2.

–  Warmer planet ! more weathering ! less CO2 ! cooling (less greenhouse effect).

–  Cooler planet ! less weathering ! more CO2 ! warmer.

•  Tectonics ! volcanoes ! CO2 recycling.

NOTE: This feedback loop operates on geological timescales.

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].

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].

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].

HZs and other stars:"Stellar Radiation

A star’s spectrum is essentially black-body radiation at an effective temperature T:

Planck curve

Empirical Estimates of Habitable Zones •  Stellar luminosity:

more massive stars are hotter and bigger. •  So, Earth-equivalent radiation further away from star:

L★ / D2equiv = LSun/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.

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.

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”.

Sun’s evolution !in luminosity !and diameter.!!!(Don’t wait around for another 7 billion years!)

HAZARDOUS ZONES Danger, Will Robinson!

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.

J Laskar & M Gastineau Nature 459, 817-819 (2009) doi:10.1038/nature08096

Mercury’seccentricityover5Gyr.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.

J Laskar & M Gastineau Nature 459, 817-819 (2009) doi:10.1038/nature08096

ExampleofcollisionaltrajectoryforMarsandtheEarth.

Evolu<onofthemaximumeccentricityofMercury(red),Mars(green)andtheEarth(blue),recordedover1-Myrintervals.

J Laskar & M Gastineau Nature 459, 817-819 (2009) doi:10.1038/nature08096

CollisionaltrajectoriesforMarsandVenuswiththeEarth.

Eccentricity(a)andsemi-majoraxis(b)ploGedversus<meforMercury(red),Venus(pink),theEarth(blue)andMars(green).(c)MinimumEarth-Mars(green)andEarth-Venus(pink)distancesinastronomicalunits,recordedovereach1,000-yr<meinterval.ThehorizontallinesaretheMars–Earth(green)andVenus–Earth(pink)distancesofcollision,Dmin,correspondingtothesumoftheplanets'radii.

GALACTIC HABITABLE ZONES Our home in the Milky Way

Habitable Zones Around Stars "and in the Milky Way Galaxy

Continuously Habitable Zone around main-sequence stars Galactic Habitable Zone

GALAXY-SCALE CATASTROPHES GRBs, SGRs, etc.

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 light-years to 12.8 billion light-years away. Since cosmic time and distance are both measured by the speed of light, the explosion known as GRB050904 took place 12.8 billion years ago, when the universe was a relatively youthful 900 million years old.

Ordovician: 488-444 Myr ago bracketed by a minor and a major extinction event

Gamma-ray burst energetics Gamma-Ray Bursts: Energy Release with Human Experience ------------------------------------ 1 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 ------------------------------------------- 10^33 ergs/s = Sun 10^39 ergs/s = nova 10^41 ergs/s = SN 10^45 ergs/s = galaxy 10^52 ergs/s = GRB

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

ISSN 1063-7737, Astronomy Letters, 2007, Vol. 33, No. 1, pp. 1–18. c⃝ Pleiades Publishing, Inc., 2007.Original Russian Text c⃝ D.D. Frederiks, S.V. Golenetskii, V.D. Palshin, R.L. Aptekar, V.N. Ilyinskii, F.P. Oleinik, E.P. Mazets, T.L. Cline, 2007, published in Pis’ma vAstronomicheskiı Zhurnal, 2007, Vol. 33, No. 1, pp. 3–21.

Giant Flare in SGR 1806–20and Its Compton Reflection from the Moon

D. D. Frederiks1, S. V. Golenetskii1, V. D. Palshin1, R. L. Aptekar1*,V. N. Ilyinskii1, F. P. Oleinik1, E. P. Mazets1, and T. L. Cline2

1Ioffe Physical–Technical Institute, Russian Academy of Sciences,ul. Politekhnicheskaya 26, St. Petersburg, 194021 Russia

2Goddard Space Flight Center, NASA, Greenbelt, MD 20771, USAReceived August 17, 2006

Abstract—We analyze the data obtained when the Konus–Wind gamma-ray spectrometer detected agiant flare in SGR 1806–20 on December 27, 2004. The flare is similar in appearance to the two knownflares in SGR 0526–66 and SGR 1900+14 while exceeding them significantly in intensity. The enormousX-ray and gamma-ray flux in the narrow initial pulse of the flare leads to almost instantaneous deepsaturation of the gamma-ray detectors, ruling out the possibility of directly measuring the intensity, timeprofile, and energy spectrum of the initial pulse. In this situation, the detection of an attenuated signalof inverse Compton scattering of the initial pulse emission by the Moon with the Helicon gamma-rayspectrometer onboard the Coronas-F satellite was an extremely favorable circumstance. Analysis of thissignal has yielded the most reliable temporal, energy, and spectral characteristics of the pulse. The temporaland spectral characteristics of the pulsating flare tail have been determined from Konus–Wind data. Itssoft spectra have been found to contain also a hard power-law component extending to 10 MeV. A weakafterglow of SGR 1806–20 decaying over several hours is traceable up to 1 MeV. We also consider theoverall picture of activity of SGR 1806–20 in the emission of recurrent bursts before and after the giantflare.

PACS numbers : 97.60.Jd; 98.70.Rz; 95.85.Pw; 95.30.JxDOI: 10.1134/S106377370701001X

Key words: neutron stars, flares, gamma rays, Compton scattering.

INTRODUCTION

The first two soft gamma repeaters, SGR 0526–66(Mazets et al. 1979a; Golenetskii et al. 1984) andSGR 1900+14 (Mazets et al. 1979b), were dis-covered and localized in March 1979. The thirdSGR 1806–20 was discovered in 1983 (Atteia et al.1987; Laros et al. 1987). And only in 1998 was thefourth SGR 1627–41 discovered (Woods et al. 1999).The situation with the possible fifth SGR 1801–23(Cline et al. 2000) arouses scepticism, since only twosoft bursts separated by an interval of several hourshave been detected from this source.

The emission of recurrent bursts by the gamma re-peaters is highly nonuniform in time. The gamma re-peaters are predominantly in quiescence. This phasecan last for years, being interrupted by reactivationperiods that can be very intense.

The temporal and spectral characteristics for all ofthe above gamma repeaters that have been observed

*E-mail: aptekar@mail.ioffe.ru

over several years in the Konus–Wind experimentare summarized in a unified catalog of SGR activity(Aptekar et al. 2001).

Giant flares, very rare events comparable in peakemission power in the source (∼1045–1047 erg s−1)to the luminosity of quasars, are the second, incom-parably more impressive type of SGR activity.

The giant flare of March 5, 1979, had remained aunique event for more than 19 years. On August 27,1998, a giant flare came from SGR 1900+14. All themain features of the flare in SGR 0526–66 manifestedthemselves in this flare: a narrow, very intense initialemission peak with a hard energy spectrum accom-panied by a relatively weaker, spectrally soft tail thatdecayed for several minutes while pulsating (Mazetset al. 1999a; Hurley et al. 1999; Feroci et al. 1999).The third similar, but even more intense flare thatcame from SGR 1806–20 on December 27, 2004,was observed on many spacecraft equipped with X-ray and gamma-ray detectors: INTEGRAL, MarsOdyssey, Wind, Swift, RXTE, RHESSI, and others

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