Shyama Narendranath Space Astronomy Group ISRO Satellite Centre Planetary exospheres Shyama Narendranath Space Astronomy Group ISRO Satellite Centre I the human wander lone in wonder amid this grand universe of unbounded space and time You the great keeper of universe Exist in its infinite wonders Lone in silence In the grand home of your own being! Rabindranath Tagore 2nd Venus Workshop 28-29 October 2014

2nd Venus Workshop 28-29 October 2014 Planetary exospheres Exosphere: outermost part of a planet Mean free path(l) of a molecule > pressure scale (H) height (l/H is called Knudsen number) Collisions are rare, particles follow ballistic trajectories Primarily light species (H, He, O) Connects the planet to outer space Lower boundary is called the exobase, upper boundary not defined Atmospheric constituents escape from here Hundreds of kms above the surface for Venus, Earth and Mars, starts from the surface for Moon and Mercury Planet Rc Vesc Eesc (H) Esc (O) Earth 500 10.8 0.61 9.69 Venus 200 10.2 0.54 8.64 Mars 250 4.8 0.12 1.91 2nd Venus Workshop 28-29 October 2014

2nd Venus Workshop 28-29 October 2014 Escape Processes Thermal (Jean’s escape) Lighter gases such as H Non-thermal Collisions between exospheric constituents and energetic species (ions and electrons) Dissociative re-combination, disscoiation by uv photons and electrons, charge exchange, plasma induced sputtering Important mechanism of escape for heavier species such as O, N and C (thermal escape rate is small) 2nd Venus Workshop 28-29 October 2014

2nd Venus Workshop 28-29 October 2014 Modeling exospheres Kinetic theory and individual particle dynamics Hydrodynamic models Collisionless atmosphere Kn > > 1 Kn=1 Kn <<1 Escape flux ∞ Escape energy/thermal energy Maxwellian distribution Vertical distribution based on mass Fluid like atmosphere Turbulentmixing Composition homogenous 2nd Venus Workshop 28-29 October 2014

2nd Venus Workshop 28-29 October 2014 Measurements EUV spectroscopy Emission lines from atmospheric constituents H Lyman α at 121.6 nm H Lyman β at 102.6 nm He at 58.4 nm O at 130.4 nm Insitu mass spectrometry ENA imaging X-ray spectrosocopy Emission lines from excited solar wind ions as a result of collisions with exospheric neutrals in the 100 eV – 2 keV energy range 2nd Venus Workshop 28-29 October 2014

2nd Venus Workshop 28-29 October 2014 Venus exosphere H corona of Venus: Mariner 5 Anderson, 1976 Day-night side asymmetry in temp- H bulge Low exospheric temperatures (300-330 K) even on the day side Non-thermal escape processes may be dominant 2nd Venus Workshop 28-29 October 2014

2nd Venus Workshop 28-29 October 2014 D/H Venus D/H = 150 x SMOW (upper atmosphere average) Measurements Lyman-α of H and D NIR lines of H2O and HDO Mesosphere- 250 times SMOW HDO/H2O in Venus mesosphere from SOIR on Venus Express, Fedorova et al,2008 2nd Venus Workshop 28-29 October 2014

X-ray spectroscopy of exosphere Objective is to study the contribution of solar wind charge exchange to the atmospheric loss process Charge exchange at Mars Courtesy: Herbert Gunell, BISA Work in progress 2nd Venus Workshop 28-29 October 2014

2nd Venus Workshop 28-29 October 2014 Summary Atmospheric escape Relative importance of thermal and the many non-thermal processes to be understood Exospheric constituents, their temperature and density profiles required Transition region at exobase and diffusion from troposphere to be understood A suite- EUV spectrograph/Lyman α photometer, High resolution X-ray spectrometer, Mass spectrometer can measurements to understand loss processes in the exospheres 2nd Venus Workshop 28-29 October 2014

2nd Venus Workshop 28-29 October 2014