Habitability and High Energy Emissions: Limitations and

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1 Habitability and High Energy Emissions: Limitations and IIII AstrobiON Timescales Gustavo F. Porto de Mello Observatório do Valongo Universidade Federal do Rio Janeiro, Brazil Ignasi Ribas (CSIC/Spain) Ximena C. Abrevaya (CONICET/Argentina) Diego L. de Oliveira (IAG/Univ. São Paulo/Brazil) José Dias do Nascimento Jr. (UFRN/Brazil and Harvard-CfA) Aline Vidotto (Univ. St. Andrews/UK) Riano E. Giribaldi - PhD (OV/UFRJ/Brazil and ESO/Garching) Maria L. Ubaldo-Melo - PhD (OV/UFRJ/Brazil) Leticia Dutra-Ferreira (CAp/UERJ/Brazil) Ellen C. de Almeida - undergrad (OV/UFRJ/Brazil)

2 Created 2017 President: Dr. Eduardo Janot- Pacheco (USP) ~50 people and counting Congregate efforts and provide common ground for discussion and progress Future multi-institute MSc and PhD course in Astrobiology

3 You are not alone!

4 How massive is your star? How old is your star? Where do you reside in the Galaxy? Habitable Zone as a Unifying Concept How is your tectonics going for you? Do you have enough radiogenic heating? What is your Si/Fe ratio? What is your crust/mantle/core ratio? How good are you at surviving catastrophes? How massive is your planet? Can it hold its own? Can you keep your magnetic field? geodynamical extension How close are you to the edge of the ZH? How long do you have? What is your water inventory? Do you need geodynamics to stay habitable? How long do you have?

5 Magnetic activity in sun-like stars differential rotation + convection + plasma = dynamo effect differential rotation: warps field lines + convective envelope: cyclonic turbulence = generation & amplification of magnetic fields poloidal toroidal Non thermal magnetic energy is dissipated in the upper atmosphere

6 Magnetic activity in sun-like stars The magnetically-generated energy is released in the form of flares, high-energy radiation and mass ejection (solar wind) MODULATIONS: Hours: stellar flares Days: rotational period (active regions coming on/off view) Years: activity cycle (Sun s 11 yr sunspot cycle) Billions of years: rotational spin-down (over nuclear timescales) weak magnetic network strongly magnetized regions solar minimum solar maximum

7 Magnetic activity in sun-like stars The magnetically-generated energy is released in the form of flares, high-energy radiation and mass ejection (solar wind) Winds carry away momentum and spin down the star weak magnetic network MODULATIONS: Hours: stellar flares Days: rotational period (active regions coming on/off view) Years: activity cycle (Sun s 11 yr sunspot cycle) solar minimum Billions of years: rotational spin-down (over nuclear timescales)

8 Planetary evolution (even Earth s) driven by stellar emissions Star as the overwhelmingly dominant source of energy Non-thermal energy dissipated as: Flares + Particle fluxes (WINDS) and Far ultraviolet + X-ray (PHOTONS) Stellar radiation critically affects composition, thermal properties and existence of atmospheres Solar Spectrum Models deviate from observations for λ < 1700 Å : strongly non-thermal regime (magnetic)

9 The Sun in Time Sample of nearby stars (high fluxes) in narrow mass and [Fe/H] range (same convective properties) PLUS good age estimates not easy! Güdel et al (1997) Young Sun: 10x faster rotation MUCH enhanced magnetic activity solar twin Porto de Mello & da Silva 1997 exponential-like behavior EK Dra 100 Myr π 1 UMa 300 Myr Kappa Ceti 700 Myr Sun 4.56 Gyr

10 EUV X-rays FUV Evolution in Time UV

11 The Sun in Time Ribas et al. (2005) Ribas et al. (2010) X-rays EUV Very Young Sun had stronger emissions: >1000 X-rays 10 to 100 Far/Extreme UV 5 to 10 UV

12 Stellar Winds: Particle Fluxes Low-mass stars have hot coronae: lose mass in open flux magnetic tubes Mass-loss correlates well with X-ray luminosity Zero Age Sun >1000 today s particle flux Possible solution to the Faint Young Sun Paradox? Zero Age Sun slightly more massive? More luminous! Wood et al do Nascimento Jr. et al. 2016

13 Short-term Variability (Flares) Flares Relative variations 2 to 10 in X-rays (over a few hours) 1.2 to 1.5 in FUV-UV several times in particle flux L X (Sun) ~ erg.s -1 Strongest solar flares ~10 32 erg ~one/decade Low mass stars may have weakened winds as scaled with L X (mean wind dependent on open flux tubes) Flare rates also seem to scale with L X (Audard et al. 2000)

14 The Young Sun: a summary X-ray, EUV: x present values Visible: 70% present values FUV, UV: 5-60x present values Flares: more frequent and energetic (>10 per day) Solar wind: 1000x present values (?)

15 Radiative Effects on Planets Earth: Present Flux XUV Young Sun Flux XUV High altitude thermalization tiny fraction... why bother? = F total = F total Energy deposited in low density upper atmosphere Non-linear behavior: sheer power is not all there is the process matters Photons carry a lot of punch Particle winds: ionization, ion pick-up and sputtering Planetary magnetic fields to the rescue

16 Water loss: Venus & Mars Mars: small without (presently) magnetic field Intense erosion of atmosphere Surface water ~3.8 Gyr ago: greenhouse by CO2 and CH4 Tsurface > 273 K Large impacts, core solidified, volcanoes stopped replenishing atmosphere erosion wins over Loss of global Martian ocean ~10m deep in 3.5 Gyr H escapes, O incorporated to ground Kulikov et al 2007 Lammer et al 2003 Venus: loss of 1% to 100% of a full terrestrial ocean in < 1 Gyr surface oxidized down to a few meters below surface Three habitable planets in the Solar System ~4 Gyr ago?

17 Many Faces of Habitability 1 habitable planet Lammer et al. (2009, ARA&A) Today 3 habitable planets? 4 Gyr ago There is a lot more to this simplistic picture: A planet inside the HZ may not be habitable! A planet outside the habitable zone may be habitable! Other possible factors: Planetary mass? Chemical composition (mantle/core/radiogenic heating)? Atmosphere? Plate tectonics? Magnetic dipole? Parent star s irradiance?

18 M dwarfs and their perils Saturated levels L X /L bol ~ 10-3 much longer Very low mass red dwarfs: keep highly energetic emission phases for up to ~7 Gyr in contrast to solartype stars only ~100 Myr Very fast spin-down, but VERY EFFICIENT dynamo If emissions scale similarly to solar-mass G stars: K stars (0.7 < M < 0.9 M sun ): XUV 3-4 than G stars at same age M stars (< 0.6 M sun ): XUV than G stars at same age

19 M- dwarfs and their perils Segura et al. (2005) active inactive

20 M- dwarfs and their perils For rocky planets in the habitable zone it remains to be seen if atmospheres are stable Especially relevant for M stars (Scalo et al. 2007) Exosphere temperature X H = 1.5 blow off Calculations show that only very CO 2 -rich atmospheres (>1.5 bar) can survive and keep the water (IR cooling + avoid condensation on the dark side) Very high loss rates for O, N, C atoms Kulikov et al. (2006, P&SS) Rocky planets in M star HZs may not evolve into habitable worlds!

21 Case Study: Proxima Centauri_b THE STAR: MASS 0.12 solar LUMINOSITY 1.6 x 10-3 solar AGE ~4.8 Gyr (Bazot et al 2016, Porto de Mello et al 2008) AN EARTHLIKE PLANET: 1.27 Earth mass receives 65% of present Earth s flux Much closer HZ Much larger exposure to XUV emissions + winds Much extended phase of highly energetic magnetic activity Anglada-Escudé et al 2016

22 Proxima Centauri_b: wind and XUV Extended phase of settling to stable H-burning Ribas et et al 2016 Proxima Centauri: much higher luminosity than present for ~1 Gyr: Proxima_b was actually outside the HZ high rates of volatile loss are expected Up to 10 higher X-ray/EUV Up to 100 higher particle flux integrated over Proxima_b and Earth s lifetimes Also much harder X-ray spectrum Initial water inventory? Larger than Earth s? Volatile losses 1 to 20 EO H Total volatile loss depends critically on: stellar wind evolution magnetic field of planet

23 distance (AU) Proxima Centauri b: Tidal Lock? 1,2 1,1 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 Distance for tidal lock at ~3 Gyr CHZ outside TLR for all of the star's life M = 0.66 M = 0.72 metallicity effect outer edge of CHZ inner edge of CHZ tidal lock radius at 3 Gyr 0,6 0,7 0,8 0,9 1,0 stellar mass Runaway phase S eff ~ 1.1 S Earth non-synchronous S eff ~ 1.5 S Earth synchronous Kopparapu et al Thick CO 2 -rich atmospheres (>1-1.5 bar) can survive and keep the water (IR cooling + circulation: avoid atmosphere condensation on the dark side) Synchronous planets: strong convective updraft, perennial cloud deck in the substellar point, increases albedo & hardens atmosphere against water loss even so, Proxima b may have spent ~200 Myr in the runaway phase (against Earth s ~few Myr)

24 Case study: Kappa Ceti HD V = 4.84 G3V-G5V P rot = 9.2 days HR 996 distance 9.2 pc 30 light-years away Ca II K Lyra & Porto de Mello (2005) 100 Myr 500 Myr 630 Myr

25 Kappa Ceti An analogue of the Sun when life arose on Earth Ribas et al 2010 Age 0.5 to 1.0 Gyr Mass 1.03 to 1.05 M Sun 30 light-years away Lyra & Porto de Mello 2005 Lorenzo-Oliveira, Porto de Mello et al a,b

26 Kappa Ceti 30 light-years away Age 0.5 to 1.0 Gyr Mass 1.03 to 1.05 M Sun An analogue of the Sun when life arose on Earth EUV and far UV flux: < 1000 Å 10 UV flux: 1500 Å 2x 1700 Å the same Å 20-30% less Ribas et al 2010 Lyra & Porto de Mello 2005 Wind: 50 to 100 present Sun: Erosion of Mars atmosphere

27 Kappa Ceti: particle flux do Nascimento Jr et al (2016) Wood et al (2014) Winds are 50 to 100 stronger than present solar mass loss! X-ray-scaled values are higher than present Sun Young Earth: magnestopheric sizes 30% to 48% present sizes Complex field topology polar cap develops, enhanced particle loss in exosphere: relevant for the evolution of Mars volatile inventory Surface Averaged Field: 20x to 60x times stronger than solar Spectropolarimetric NARVAL observations Least Squares Deconvolution of the Stokes V parameters Zeeman signature Surface magnetic field reconstruction

28 Young Sun Prebiotic and Archean Earth s atmosphere: CO 2 -rich and CH 4 -rich (reducing) Prebiotic photochemistry in the far UV? Model Earth atmospheres Kasting et al. (1993) mildly reducing Pavlov et al. (2001) very reducing Efficient photodissociation rates mostly in the far UV

29 Young Sun Model Earth atmosphere Prebiotic photochemistry in the far UV? Prebiotic and Archean Earth s atmosphere: guessed Sun Kappa Ceti CO 2 -rich and CH 4 -rich Kasting et al. (1993) mildly reducing early atmosphere 10 or more higher photodissociation rates at high altitude

30 Kappa Ceti Prebiotic photochemistry in the far UV RNA/DNA synthesis and the far UV Powner et al (Nature) UV Prebiotic nitrogenous base synthesis Pyrimidine ribonucleotide assembly bypassing ribose by means of UV flux

31 Panspermia: The Biosun Project Brazilian Synchroton LNLS Abrevaya et al. submitted Assuming very short transit time of 10 5 years: Fluence ~10 11 Joule/m 2 Pressure ~10-4 Pa (low earth orbit) Survivorship Radioresistant microbes in halite hypersaline crystals (brines) halites (both Earth/Mars) Panspermia model = Nakhla martian meteorite Halite: survival up to 10,000 J.m -2 (D. Radiodurans) Non-halite: survival up to 2,000 J.m -2 H. volcanii Halophilic Archaea

32 Highlights It is soooo nice under the sun... Stellar energetic emissions of magnetic origin span a wide range of behavior in spectrum, total power and time scales across low mass stars Little known about the magnetic evolution of the Sun until recently Sun much more active in the past: much stronger high-energy emissions (up to many orders of magnitude) and particle winds All low-mass stars go through a similar high activity phase Major loss effects on atmospheres & volatile inventories of planets, but also including the drive of possible biochemistry Severe threats to habitability complete loss of atmospheres and/or oceans, particularly for red dwarfs, which are the most abundant hosts of Earthlike rocky planets