Comparative Dynamics and Aeronomy of the Atmospheres of Earth and Mars Jeffrey M. Forbes Department of Aerospace Engineering Sciences University of Colorado, Boulder, Colorado ftp://odo.colorado.edu/pub/NicoletLecture/

Some general impressions that I hope you draw from this talk Surface topography has an important influence on the thermospheres of both Mars and Earth. The atmospheres of Mars and Earth are vertically coupled systems that require a “whole atmosphere” perspective. The Earth’s mesosphere/lower thermosphere represents a realistic laboratory for the study of Mars’ aerobraking regime. Comparative study of Earth and Mars is a synergistic activity. ftp://odo.colorado.edu/pub/NicoletLecture/

~1~ Planetary & Atmospheric Characteristics

12,756 km 6,794 km g 0 = 9.8 ms -2 P 0 ~ 1000 mb g 0 = 3.73 ms -2 P 0 ~ 6 mb ≈ 2  /24h = 2  day -1 ≈ 2  /23.5h = 2  sol -1 Comparative Planetary Characteristics Smaller radius amplifies effects of mean winds on wave propagation, among other dynamical effects. However, Earth and Mars pressures & densities ~ equal near 120 km Allows for some significant similarities in the atmospheric dynamics of the two planets

Earth and Mars: Temperature Structure & Major Atmosphere and Ionosphere Constituents Altitude (km) O 2, N 2 O O 2 + NO + O2+O2+ O CO 2 Earth Mars cooling to space O+O+ O+O+ Temperature

Earth and Mars Major Energy & Momentum Drivers Temperature Altitude (km) EUV & magnetospheric inputs Mars Earth wave forcing wave forcing radiative forcing radiative forcing Aerobraking Regime The MLT of Earth is primarily governed by the same processes as the aerobraking regime of Mars (waves, CO 2 cooling, EUV, diffusion)

~2~ Plasma and Magnetic Environments

bow shock magnetopause IMF Solar wind Earth’s Atmosphere & Ionosphere are Protected from the Solar Wind and IMF by the Magnetosphere  V 2 ~ B 2 /2  0

Solar Wind and IMF Interactions with Earth’s Magnetosphere

crustal magnetism Analog to Earth During one of its Magnetic Field Reversals ?

crustal magnetism ionopause bow shock IMF O +, O 2 + scavenging H + scavenging Solar wind Nature of ionopause currents? Nature of ionosphere currents? Ionosphere shielding by magnetic ‘umbrellas’? (Mitchell et al., 2002) [Michel, 1971; Luhmann & Kozyra, 1991 & others] Magnetic reconnection?  V 2 ~ n p kT p

Vertical Transport (large-scale dynamics and diffusion) photo- dissociation H 2 O + h  H + OH H 2 + O IMF photo- ionization  H +, O +, etc. H2OH2O Lower Atmosphere A Scenario for the Loss of Water on Mars H +, O +, O 2 +, etc. scavenging McElroy et al. (1977) & others

Courtesy NASA/Nagoya University Solar Wind and IMF Interactions with Mars’ Ionosphere

~3~ Thermospheric Storms

Thermosphere Density Storm Response at 200 km Forbes et al.,1996

The Mars Thermosphere Analog of the Magnetic Storm: The Dust Storm

Dust Storm During Re-start of Phase I MGS Aerobraking Dynamic Pressure at Periapsis (Nm -2 ) Normalized to an Altitude of 121 km Johnston et al., 1998 Credit: Malin Space Science Systems /NASA dust storm Frost cap Height above Limb (km) Clancy et al., Mars Dust Storm MOC Normalized Brightness Region of enhanced radiative absorption & heating 35°S

~4~ Surface-Thermosphere Tidal Coupling

(Forbes et al., 2004; see also Wilson, 2002;Withers et al., 2003) Density Perturbations in Mars’ Atmosphere at 130 km During Phase I and Phase II of Aerobraking Note: ~Sun-Synchronous Orbits What is the True Origin of These Oscillations? The first and obvious explanation was that these are planetary-fixed features, i.e., stationary planetary waves.

A spectrum of thermal tides is generated via topographic/land-sea modulation of periodic solar radiation absorption: zonal wave number s = n ± m ‘sum’ & ‘difference’ waves hidden physics (height,latitude) Mars: Conrath, 1976; Zurek, 1976 Earth: Hendon & Woodberry, 1993; Tokioka & Yagai, 1987; et al. solar radiation  = 2  /24 (rotating planet) IR = longitude to first order

Annual-mean height-Integrated (0-15 km) diurnal heating rates (K day -1 ) from NCEP/NCAR Reanalysis Project Dominant zonal wavenumber representing low-latitude topography & land-sea contrast on Earth is s = 4 cos(  t + ) x cos4 diurnal harmonic of solar radiation n = 1 dominant topographic wavenumber m = 4 hidden physics m = 1 (short vertical wavelength) s = +5 s = -3 Example: Diurnal (24-hour or n = 1) tides excited by latent heating due to tropical convection (Earth) = cos(  t - 3 ) + cos(  t + 5 ) eastward propagating westward propagating

In terms of local time t LT = t + /  becomes From Sun-synchronous orbit (t LT = constant), a tide with frequency n (day -1 or sol -1 ), and zonal wavenumber s generated by wave-m topography appears as a wave-m longitude variation How Does the Wave Appear from Sun- Synchronous Orbit? = ±m

Example: Temperatures from TIMED/SABER 15 Jul - 20 Sep 2002 yaw cycle good longitude & local time coverage Diurnal ( n = 1), s = -3 Kelvin Wave Space-Time Decomposition Predominant waves n = 1, s = 1 n = 1, s = -3 Note: |s - n| = 4

SABER Temperature Residuals, LST = 1300, 110 km Raw temperature residuals (from the mean) exhibit the wave-4 pattern anticipated for a dominant eastward- propagating s = -3 diurnal tide.

Westward migrating solar radiation modulated by m = 2 topographic influences cos(  t + ) cos2 diurnal (24h) westward s=3 (short vertical wavelength) diurnal eastward propagating s=1 Mars solar radiation n = 1, s = 1 topography m = 2 Near-resonant oscillation (amplified response anticipated) ---> cos(  t + 3 ) + cos(  t - ) Note: |s-n| = 2

S = 4 S = 2 Topographic Effects Penetrate to the Thermospheres of Both Mars and Earth in the Form of a Spectrum of Thermal Tides A comprehensive understanding requires knowledge of the sources, and nature of wave-wave and wave-mean flow interactions. The zonal mean zonal winds generated by these waves are very significant (recent unpublished calculations).

~5~ The Solar Semidiurnal Tide in the Dusty Mars Atmosphere

Tide-Mean Flow Interactions in Mars’ Atmosphere Dissipating tides and gravity waves deposit net momentum and heat into the atmosphere, thus modifying the mean temperature and wind structure. The mean temperature and wind structure in turn modify the propagation of the waves. GCM studies show that thermal tides modify the circulation of Mars middle atmosphere (i.e., km) in important ways. “Deep forcing” of the Sun-synchronous (“migrating”) semidiurnal (12-hour) tide occurs via radiative absorption by O 3 on Earth and by dust on Mars. What are the consequences of the semidiurnal tide on Mars’ thermosphere during dusty conditions?

What if …………… You had a model that interactively solved for the zonal mean flow and the dissipating semidiurnal tide (Miyahara & Forbes, 1991); you forced the solar semidiurnal tide with realistic heating rates for a globally dusty atmosphere (Zurek, 1986); the semidiurnal tide was so large that it had the potential to undergo convective instability and break; you adopted a “saturation hypothesis”, i.e., that sufficient turbulence is generated by the breaking wave to cease exponential growth and maintain a marginally stable wave; and you furthermore implemented a “cascade hypothesis”, i.e., that the non-breaking wave could cascade to higher wavenumber waves that did break (Lindzen, 1981; Lindzen and Forbes, 1983). What would be the consequences? 3.25 x 10 4 m 2 s -1 ≤1

Perturbation Temperatures and Eddy Diffusion Coefficients due to the Solar Semidiurnal Tide, Ls = 270, Dust  ~ ~3 x 10 4 m 2 s -1 ~2 x 10 3 m 2 s -1 (~ 100 km/6 days) Potential for significant vertical transport -- relevant to H 2 O loss? ~60% density perturbations at 122 km in aerobraking regime ~80 K ‘whole atmosphere response’

Semidiurnal Temperature Perturbation

Eddy diffusion Coefficient due to Breaking Semidiurnal Tide

~250 ms -1 ~150 ms -1 Zonal Mean Eastward and Vertical Winds Driven by the Solar Semidiurnal Tide, Ls = 270, dust  ~ 2.3 Significant modifications to thermosphere circulation Potential for significant vertical transport -- relevant to H 2 O loss? (cms -1 )

~6~ Outstanding Questions

ftp://odo.colorado.edu/pub/NicoletLecture/ What are the global temperature, density and wind structures in Mars’ middle and upper atmosphere? What are the sources and sinks of small-scale waves in the atmospheres of Earth and Mars, and how do they affect the mean states of these atmospheres? What are the fundamental physics and broader consequences of wave-wave, wave-turbulence, and wave-mean flow interactions at all scales in the atmospheres of Earth and Mars? What are the implications of the above for establishing realistic and reliable aerobraking and aerocapture models? ~ Neutral Atmospheres ~

ftp://odo.colorado.edu/pub/NicoletLecture/ How does the solar wind and IMF interact with the ionosphere, neutral atmosphere and partially magnetized environment of Mars? What processes are primarily responsible for the vertical transport of constituents in Mars’ atmosphere? What are the implications for the past, present and future evolutions of the atmospheres of Mars and Earth? ~ Solar Wind Interactions at Mars ~

ftp://odo.colorado.edu/pub/NicoletLecture/ ~ CONCLUSION ~ There are compelling scientific and practical reasons for atmospheric dynamics, aeronomy and space physics missions to Mars!

Auxiliary Slides

Number Density (cm -3 ) Altitude (km) CO O 2 NO N2N2 O 2 Ar N2N2 minor constituents He O 2 H Thermosphere Composition - Earth and Mars CO 2 O O N2N2 Mars Earth Note~ equal densities despite P o (Earth) ~ 1000 mb P o (Mars) ~ 6 mb OO2N2OO2N2 CO 2

O2+O2+ O+O+ CO 2 + Altitude (km) Number Density (cm -3 ) Mars Nominal Daytime Ionospheres - Earth and Mars O + h  O + CO 2 + h  CO 2 + O + h  O O2+O2+ O+O+ N 2 +, N +, H +, He + Altitude (km) Number Density (cm -3 ) Earth NO + O 2 + h  O 2 + CO O  O CO O + + N 2  NO + + N