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Mars Atmospheric Evolution : What Can Dynamical Models Tell Us?

Mars Atmospheric Evolution : What Can Dynamical Models Tell Us?. Stephen W. Bougher Jared M. Bell (University of Michigan). Jane L. Fox (Wright State University). Martian Atmospheric Regions and Escape Processes.

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Mars Atmospheric Evolution : What Can Dynamical Models Tell Us?

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  1. Mars Atmospheric Evolution :What Can Dynamical Models Tell Us? Stephen W. Bougher Jared M. Bell (University of Michigan) Jane L. Fox (Wright State University)

  2. Martian Atmospheric Regions and Escape Processes

  3. Summary of Mars Volatile Escape Mechanisms • Thermal (Jean’s) escape : e.g. H • Non-thermal escape: • Photochemical escape : DR of O2+, N2+, CO+… forming energetic (hot) neutrals (O, N, C …). (2) Pick-up ion escape : ions produced in the corona and exosphere are dragged along by solar B-field lines to partially escape in the SW (O+, H+, C+…). (3) Ionospheric outflows: planetary ions are accelerated by the SW convection E-field and partially lost (e.g. O2+). (4) Ion sputtering : a portion of pick-up ions re-impact the neutral atmosphere with enough energy to eject neutral atmospheric particles (e.g. CO2, N2, CO, O, N, C...).

  4. Requirements for Evolution Models of Mars Volatile Escape • Model for the early solar EUV fluxes (Ayres, 1997). ~3 x EUV at ~2.5 GYA. • Model for the history of the solar wind properties (Newkirk, 1981; Wood et al., 2002). • Models for the ancient upper atmosphere neutral densities and temperatures (Zhang et al., 1993; Bougher and Fox, 1996; this work). • An assumed history of the planetary magnetic field; Mars turn-off ~3.7 GYA (Acuna et al., 1998).

  5. Interaction of Key Models : Volatile Escape

  6. MTGCM Input Parameters, Fields, and Domain • Domain : ~70-300 km; 33-levels; 5x5 ° resolution • Major Fields and Species : T, U, V, W, CO2, CO, O, N2 • Minor Species : O2, He, Ar, N(4S) • Ions (PCE) : CO2+, O2+, O+, NO+, CO+, N2+ (<180 km) • Homopause Kzz ~ 1 x 107 cm2/s (at ~125 km) • Prescribed Heating efficiencies : EUV and FUV (22%) • Fast NLTE 15-µm cooling and IR heating schemes from M. Valverde 1-D NLTE code (Spain). • Ko-co2 = 3.0 x 10-12 cm3/s at 300K (Lopez-Puertas et al., 1992). • Simplified ion-neutral chemistry (Fox and Sung, 2001) • Scaled benchmark Ti and Te based upon Viking (Fox).

  7. MGCM-MTGCM Simulation: Coupling Configuration • Separate but coupled NASA Ames MGCM (0-90 km) and NCAR/Michigan MTGCM (70-300 km) codes, linked across an interface at 1.32-microbars on 5x5 ° grid. • Fields passed upward at interface (T, U, V, Z) on 2-min time-step intervals. No downward coupling enabled. • MGCM-MTGCM captures upward propagating migrating and non-migrating tidal oscillations, as well as in-situ driven solar EUV-UV migrating tides.

  8. Current vs. Ancient Model Inputs and Parameters • Both : Ls = 270 (perihelion, S. Summer, TES dust) • Current(today): --F10.7-cm= 130 solar EUV/FUV fluxes --1.0 solar IR fluxes. • Ancient (2.5 GYA) : --F10.7-cm = 390 solar EUV/FUV fluxes (Ayres, 1997) --0.79 current solar IR fluxes (Gough, 1981).

  9. Thermal Structure Exobase Altitude : ~215 km (C) ~250 km (A)

  10. Heat Balances Solid = cond Dash = adia D.Dash = heat 3D.Dash = CO2 Dotted = adv

  11. Heat Balances

  12. Neutral Composition Solid = CO2 3D-Dash = O D-Dash = N2 Dash = CO Dotted = Ar

  13. Neutral Composition

  14. O/CO2 Ratios(Current vs. Ancient) At 135 km: O/CO2 = 1.75% (C) O/CO2 = 3.75% (A)

  15. Electron Densities Ionospheric peak : 1.94 x 105 cm-3 (C) 2.90 x 105 cm-3 (A)

  16. Current : T+(U,V) at Exobase

  17. Ancient : T+(U,V) at Exobase

  18. Summary and Conclusions • Enhanced solar EUV-UV fluxes drive a warmer (290 to 430 K) ancient Mars dayside exobase, faster global winds, and a lower thermosphere more abundant in O (1.75 to 3.75% near 135 km). • Dayside (upwelling) winds have a significant impact upon adiabatic cooling, strongly regulating dayside temperatures. Advection of O is enhanced. • A strong dayside thermostat also results from enhanced CO2 cooling, due to more abundant atomic-O. Similar to present day Venus. • Exobase rises (on average) from ~195 to 230 km. Enhanced O and CO2 densities at these heights.

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