Thermal Atmospheric Escape: From Jupiters to Earths

Transcription

1 Thermal Atmospheric Escape: From Jupiters to Earths Ruth Murray-Clay University of California, Santa Barbara Image Credit: Mark Garlick

2 Super-Earths vs. Mini Neptunes primordial hydrogen envelope accreted from the protoplanetary nebula vs. atmospheric composition set by outgassing and loss

3 Two classes of escape mechanisms: Each can be thermal or non-thermal kinetic loss to space of individual atoms hydrodynamic bulk outflow of a collisional fluid exobase Jeans escape non-thermal processes, often mediated by B-fields limits of thermal escape hydrodynamic escape Roche lobe overflow ram pressure stripping

4 Thermal escape is driven by ionizing photons Usually EUV; Occasionally X-ray Before: After: XUV photon e - collisions distribute energy from ejected electron H p + heating small fraction of irradiation energy but large cross-section

5 Escape of trace species is often limited by diffusion e.g. surface water stability Kasting et al NASA/Ames/JPL-Caltech

6 A Tale of Two Thermospheres Both heated primarily by ev EUV photons Current Earth/Venus Current Hot Jupiters EUV absorption conduction CO2 15 micron emission EUV absorption collisional, fluid outflow

7 Thermospheric Energy Balance Venus HD b log heating/cooling rates (erg/s/cm 3 ) s photoionization heating c.i. c.c. Ly cooling c.h. r.r. cooling by PdV work f.f r/r p Fox & Bougher 1991 Murray-Clay et al. 2009

8 Earth exobase: minimal escape conduction of deposited heat lower Altitude mesopause is where the atmosphere is dense enough to radiate the absorbed energy away Temperature ~1000K

9 Hot Jupiters: Observations with unclear interpretation #1-100 km/s 100 km/s Lammer et al. 2003, Baraffe et al suggested photoevaporation Vidal-Madjar et al Additional detections: Lyman-alpha Ehrenreich et al. 2008, Lecavelier des Etangs et al CII, OI, SiIV, MgII Vidal-Madjar et al. 2004, Linsky et al Haswell et al X-ray Poppenhaeger et al. 2013

10 Hot Jupiter exobase hydrodynamic wind mean free path = scale height Roche lobe radius 4.5 R p Sonic point 2-5 R p T wind 10,000 K H, H +, He Photoionization base (τuv = 1) 1.1 R p 1 bar surface of planet H 2 R p ~ cm T eff 1300K Murray-Clay et al. 2009, Yelle 2004, Garcia Munoz 2007

11 What generates a Parker wind? > 0: bad! fluid, isothermal accelerates the gas outward energy for PdV work in hydrostatic v outward flow comes from this assumption

12 Parker winds flow through a critical point T : sonic point: cs ~ vesc x r s = GM p /(2c 2 s) De Laval Nozzle Ṁ =4 r 2 v rs exponential dropoff Von Braun with the Saturn V rocket

13 Drop isothermal assumption still assume fluid (collisional) heating from photoionization sets lower boundary condition only photoionization heating and pdv work deposited primarily atτ~ 1 : n 0 1 σh For high XUV flux: F UV hν 0 σ ν0 n 0 n 2 +α rec P ~ nanobars, altitude set by lower atmosphere

14 Jeans escape If FUV low many scale heights to sonic point; no longer collisional & model isn t self-consistent (modified) Jeans escape fluid outflow v < cs

15 Follow the Energy Three fates for deposited energy 1. radiated away in place 2. conducted lower in the hot Jupiters orbiting T Tauri stars (in place radiation + outflow) atmosphere then radiated away Earth and Venus 3. drives an outflow, which can be energy limited : several possible structures hot Jupiters; early Earth and Venus (inflated by conduction)

16 Three fates for deposited energy 1. radiated away in place 2. conducted lower in the atmosphere then radiated away 3. drives an outflow, which can be energy limited : several possible structures Difficulty: temperature and escape rate are coupled

17 transition depends on σcoll/σabs Normalized dm/dt immediate wind velocity-modified Jeans (Chassefiere 1996, Tucker et al. 2012) Jeans v /v esc,0 th

18 No local energy loss Ruth Murray-Clay: Atmospheric Escape hot Jupiter 10 5 Parker wind T 10 4 conduction Jeans dm/dt

19 No local energy loss Earth 10 5 Parker wind T conduction Jeans dm/dt

20 No local energy loss Ruth Murray-Clay: Atmospheric Escape Earth: CO Parker wind T 10 5 Jeans conduction dm/dt

21 Radiative cooling at high and low EUV flux log heating/cooling rates (erg/s/cm 3 ) photoionization heating Ly cooling c.c. c.i. cooling by PdV work f.f. r.r. s l r/r p Murray-Clay et al c.h. Koskinen et al. 2007

22 Tales of Two MoreThermospheres Early Earth/Venus Early Hot Jupiters EUV absorption collisional, fluid outflow conduction X-ray absorption collisional, fluid outflow metal line emission

23 Terrestrial history: Observations with unclear interpretation #2 Lammer et al. 2008

24 Conduction low EUV flux regime T r Watson et al. 1981

25 Hydrodynamic hydrogen loss from terrestrial planets, limited by conduction: Concern: collisionality Note: Loss of water from Venus (Kasting & Pollack 1983) is limited by transport of water to the thermosphere. Watson et al Difficulty: Models hydrogen escape from early Earth (e.g. Kuramoto et al ) parametrize the lower boundary because low hydrogen mixing ratios are assumed.

26 Early X-ray heating Owen & Jackson 2012 Caveat: molecular cooling could operate here

27 -100 km/s 100 km/s absorption at mysteriously large velocity offsets Vidal-Madjar et al. 2003

28 Both the stellar wind ram pressure and magnetic fields can reduce and/or shape mass loss wind star hot Jupiter

29 Magnetic Structure Stellar Wind Interaction Owen & Adams 2014 Stone & Proga 2009

30 Early inflated planets can lose mass more easily: Coupling to radius evolution Lopez et al Owen & Wu 2013

31 Additional physics tidal correction dissociation, ionization chemistry: composition and coolants

32 Mass-Loss rates remain roughly energy limited at late times for hydrogen-dominated super-earths Lyα cooling :;66!,<66*1;=0*/2367!"!$!"!#!"!!!"!"!" 9 > ">9 :*!*+ ()?@*#"9%&8A B-C01!D;1=E > ">' :*!*+ ()!" 8!" #!" $!" %!" &!" ' Energy-limited ()*+,-.*/ # 367

33 Summary: Follow the Energy 1. radiated away in place 2. conducted lower in the hot Jupiters orbiting T Tauri stars (in place radiation + outflow) atmosphere then radiated away Earth and Venus 3. drives an outflow, which can be energy limited : several possible structures hot Jupiters; early Earth and Venus (inflated by conduction) Note that energy-limited escape is not good: at early times (high XUV flux), any time molecular coolants are present (often for low XUV flux), if you don t know if your planet is inflated by conduction