Thermal Atmospheric Escape: From Jupiters to Earths Ruth Murray-Clay University of California, Santa Barbara Image Credit: Mark Garlick
Super-Earths vs. Mini Neptunes primordial hydrogen envelope accreted from the protoplanetary nebula vs. atmospheric composition set by outgassing and loss
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
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
Escape of trace species is often limited by diffusion e.g. surface water stability Kasting et al. 1993 NASA/Ames/JPL-Caltech
A Tale of Two Thermospheres Both heated primarily by 15-20 ev EUV photons Current Earth/Venus Current Hot Jupiters EUV absorption conduction CO2 15 micron emission EUV absorption collisional, fluid outflow
Thermospheric Energy Balance Venus HD 209458b log heating/cooling rates (erg/s/cm 3 ) s 4 6 8 10 12 14 photoionization heating c.i. c.c. Ly cooling c.h. r.r. cooling by PdV work f.f. 1 2 3 4 r/r p Fox & Bougher 1991 Murray-Clay et al. 2009
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
Hot Jupiters: Observations with unclear interpretation #1-100 km/s 100 km/s Lammer et al. 2003, Baraffe et al. 2003 suggested photoevaporation Vidal-Madjar et al. 2003 Additional detections: Lyman-alpha Ehrenreich et al. 2008, Lecavelier des Etangs et al. 2009 CII, OI, SiIV, MgII Vidal-Madjar et al. 2004, Linsky et al. 2010 Haswell et al. 2012 X-ray Poppenhaeger et al. 2013
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 ~ 10 10 cm T eff 1300K Murray-Clay et al. 2009, Yelle 2004, Garcia Munoz 2007
What generates a Parker wind? pressure @ > 0: bad! fluid, isothermal accelerates the gas outward energy for PdV work in hydrostatic v outward flow comes from this assumption
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
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
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
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)
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
transition depends on σcoll/σabs Normalized dm/dt 10 0 10-1 10-2 10-3 10-4 10-5 immediate wind velocity-modified Jeans (Chassefiere 1996, Tucker et al. 2012) Jeans 10-6 1.0 1.5 2.0 2.5 3.0 3.5 4.0 v /v esc,0 th
No local energy loss Ruth Murray-Clay: Atmospheric Escape hot Jupiter 10 5 Parker wind T 10 4 conduction Jeans 10 3 10 2 10 4 10 6 10 8 10 10 10 12 dm/dt
No local energy loss Earth 10 5 Parker wind T 10 4 10 3 conduction Jeans 10 2 10 0 10 2 10 4 10 6 10 8 10 10 dm/dt
No local energy loss Ruth Murray-Clay: Atmospheric Escape Earth: CO2 10 6 Parker wind T 10 5 Jeans conduction 10 4 10 2 10 4 10 6 10 8 10 10 10 12 dm/dt
Radiative cooling at high and low EUV flux log heating/cooling rates (erg/s/cm 3 ) 4 6 8 10 12 photoionization heating Ly cooling c.c. c.i. cooling by PdV work f.f. r.r. s l 14 1 2 3 4 r/r p Murray-Clay et al. 2009 c.h. Koskinen et al. 2007
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
Terrestrial history: Observations with unclear interpretation #2 Lammer et al. 2008
Conduction low EUV flux regime T r Watson et al. 1981
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. 1981 Difficulty: Models hydrogen escape from early Earth (e.g. Kuramoto et al.. 2013) parametrize the lower boundary because low hydrogen mixing ratios are assumed.
Early X-ray heating Owen & Jackson 2012 Caveat: molecular cooling could operate here
-100 km/s 100 km/s absorption at mysteriously large velocity offsets Vidal-Madjar et al. 2003
Both the stellar wind ram pressure and magnetic fields can reduce and/or shape mass loss wind star hot Jupiter
Magnetic Structure Stellar Wind Interaction Owen & Adams 2014 Stone & Proga 2009
Early inflated planets can lose mass more easily: Coupling to radius evolution Lopez et al. 2012 Owen & Wu 2013
Additional physics tidal correction dissociation, ionization chemistry: composition and coolants
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 ()*+,-.*/012345 # 367
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