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

g0 = 9.8 ms-2

P0 ~ 1000 mb

g0 = 3.73 ms-2

P0 ~ 6 mb

≈ 2/24h = 2day-1

≈ 2/23.5h = 2sol-

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

Alt

itu

de

(km

)

O2, N2

O

O2, N2

O2+

NO+O2

+

O

CO2

CO2

EarthMarscooling

to space

O+O+

Temperature

Earth and MarsMajor Energy & Momentum Drivers

Temperature

Alt

itu

de

(km

)

EUV & magnetospheric

inputsMars

Earth

waveforcing

waveforcing

radiativeforcing

radiativeforcing

Aerobraking

Regime

The MLT of Earth is primarily governed by the same processes as the aerobraking regime of

Mars (waves, CO2 cooling, EUV, diffusion)

~2~Plasma and Magnetic

Environments

bow shock

magnetopause

IMF

IMF

Solar wind

Solar wind

Earth’s Atmosphere & Ionosphere are Protected from the Solar Wind and IMF by the Magnetosphere

V2 ~ B2/20

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Solar Wind and IMF Interactionswith Earth’s Magnetosphere

crustal magnetism

Analog to Earth During one of its Magnetic Field Reversals ?

crustal magnetism

ionopause

bow shock

IMF

IMF

O+, O2+ scavenging

H+ scavenging

Solar wind

Solar wind

Nature ofionopause currents?

Nature ofionosphere currents?

Ionosphere shielding by magnetic ‘umbrellas’?

(Mitchell et al. , 2002)

[Michel, 1971; Luhmann & Kozyra, 1991 & others]

Magnetic reconnection?

V2 ~ npkTp

Vertical Transport(large-scale dynamics and diffusion)

photo-dissociation

H2O + h H + OH H2 + O

IMF

photo-ionization H+, O+, etc.

H2O Lower Atmosphere

A Scenario for the Loss of Water on Mars

H+, O+, O2+, etc.

scavenging

McElroy et al. (1977) & others

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Courtesy NASA/Nagoya University

Solar Wind and IMF Interactionswith 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 ofPhase I MGS Aerobraking

Dy

na

mic

Pre

ss

ure

at

P

eri

ap

sis

(N

m-2)

No

rma

lize

d t

o a

n A

ltit

ud

e o

f 1

21

km

Johnston et al., 1998 Credit: Malin Space Science Systems /NASA

dust storm

Frost cap

11-26-97

Height above Limb (km)Clancy et al., 2003

2001 Mars Dust Storm

MO

C N

orm

aliz

ed B

rig

htn

ess

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 wavenumber

s = n ± m‘sum’ & ‘difference’

waves

hidden physics

(height,latitude)

Mars: Conrath, 1976; Zurek, 1976Earth: Hendon & Woodberry, 1993; Tokioka & Yagai, 1987; et al.

solarradiation

= 2/24(rotatingplanet)

Snn=0

N

∑ cos(nt+ nλ −αn)

IR

λ = longitude

Smm=0,1,2,...

M

∑ cos(mλ −βm)

n=0

N

∑ Tn,s cos[nt+ (n±m)λm=0

M

∑ −φn,m]

tofirst

order

Annual-mean height-Integrated (0-15 km) diurnal heating rates (K day-1) from

NCEP/NCAR Reanalysis ProjectDominant zonal wavenumber representing low-latitude topography & land-sea

contrast on Earth is s = 4

cos(t + λ) x cos4λ

diurnal harmonic of solar

radiationn = 1

dominant topographicwavenumber

m = 4

hiddenphysics

m = 1

(short vertical wavelength)

s = +5s = -3

Example: Diurnal (24-hour or n = 1) tides excited by latent heating due to tropical convection (Earth)

= cos(t - 3λ) + cos(t + 5λ)

eastwardpropagating

westwardpropagating

In terms of local time tLT = t + λ/ Tn,s cos nt+ sλ −φn,s⎡⎣ ⎤⎦

Tn,s cos ntLT + (s−n)λ −φn,s⎡⎣ ⎤⎦becomes

From Sun-synchronous orbit (tLT = 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

s=−k

s=+k

∑ An cos(nt+ sλn=1

N

∑ −φ)Space-Time Decomposition

Predominant wavesn = 1, s = 1n = 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 + λ) cos2λ

diurnal (24h) westward s=3(short vertical wavelength)

diurnal eastward propagating s=1

Mars

solar radiation

n = 1, s = 1

topographym = 2

Near-resonant oscillation(amplified response

anticipated)

---> cos(t + 3λ) + cos(t - λ)

Note:|s-n| = 2

S = 4S = 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., 25-95 km) in important ways.

• “Deep forcing” of the Sun-synchronous (“migrating”) semidiurnal (12-hour) tide occurs via radiative absorption by O3 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 thenon-breaking wave could cascade tohigher wavenumber waves that did break(Lindzen, 1981; Lindzen and Forbes, 1983).

What would be the consequences?

D =Dmax

′T ⋅2 λz

Γ

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

3

3.25 x 104 m2s-1 ≤1

Perturbation Temperatures and Eddy Diffusion Coefficientsdue to the Solar Semidiurnal Tide, Ls = 270, Dust ~ 2.3

100

~3 x 104 m2s-1

~2 x 103 m2s-1

(~ 100 km/6 days)

Potential for significant vertical transport -- relevant to H2O loss?

~60% density perturbationsat 122 km in aerobraking regime

~80 K

‘whole atmosphereresponse’

QuickTime™ and aBMP decompressor

are needed to see this picture.

Semidiurnal Temperature Perturbation

QuickTime™ and aBMP decompressor

are needed to see this picture.

Eddy diffusion Coefficient

due to Breaking Semidiurnal Tide

~250 ms-1

~150 ms-1

Zonal Mean Eastward and Vertical Winds Driven bythe Solar Semidiurnal Tide, Ls = 270, dust ~ 2.3

Significant modifications to thermosphere circulation

Potential for significant vertical transport -- relevant to H2O loss?

(cms-1)

~6~OutstandingQuestions

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!

AuxiliarySlides

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260

220

180

140

100

Number Density (cm-3)

Alt

itu

de

(km

)

102 106 107 108 109 1010 1011 1012103 104 105

CO O2

NO

N2

O2

Ar

N2

min

or

con

stit

uen

ts

HeO2

H

Thermosphere Composition - Earth and Mars

CO2

OO

N2

MarsEarth

Note~ equal densities despite Po (Earth) ~ 1000 mbPo (Mars) ~ 6 mb

OO2

N2

CO2

101 102 103 104 105

400

300

200

100106

O2+

O+ CO2+

Alt

itu

de

(km

)

Number Density (cm-3)

Mars

Nominal Daytime Ionospheres - Earth and Mars

O + h O+

CO2 + h CO2+ O + h O+

102 103 104 105 106

400

300

200

100107

O2+

O+

N2+, N+, H+, He+

Alt

itu

de

(km

)

Number Density (cm-3)

Earth

NO+

O2 + h O2+

CO2+ + O O2

+ + CO

O+ + N2 NO+ + N