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

Martian Atmospheric Regions and Escape Processes

Summary of Mars Volatile Escape MechanismsThermal (Jeans) escape : e.g. HNon-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...).

Requirements for Evolution Models of Mars Volatile EscapeModel 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).

Interaction of Key Models : Volatile Escape

MTGCM Input Parameters, Fields, and DomainDomain : ~70-300 km; 33-levels; 5x5 resolutionMajor Fields and Species : T, U, V, W, CO2, CO, O, N2Minor Species : O2, He, Ar, N(4S)Ions (PCE) : CO2+, O2+, O+, NO+, CO+, N2+ (

MGCM-MTGCM Simulation: Coupling ConfigurationSeparate 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.

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).

Thermal StructureExobase Altitude :~215 km (C)~250 km (A)

Heat BalancesSolid = condDash = adiaD.Dash = heat3D.Dash = CO2Dotted = adv

Heat Balances

Neutral CompositionSolid = CO23D-Dash = OD-Dash = N2Dash = CODotted = Ar

Neutral Composition

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

Electron DensitiesIonospheric peak :1.94 x 105 cm-3 (C)2.90 x 105 cm-3 (A)

Current : T+(U,V) at Exobase

Ancient : T+(U,V) at Exobase

Summary and ConclusionsEnhanced 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.