General Circulation of the Martian Atmosphere:Dynamics and Dust

Melissa J. Strausberg

25 August 2005

Overview

• Mars basics

• Dust on Mars

• My (future) work

Observations

• Orbiter observations (will show later)– Thermal Emission Spectrometer (TES): 1997-2004;

temperature profiles, temperature and opacity maps– Mars Orbiter Camera (MOC): 1997-2004; full-color

daily global maps, high-resolution targeted imagery

• Lander observations– Ground truth provided by Viking (late 70s) and

Pathfinder (1997)

• Upcoming data from Mars Reconnaissance Orbiter (MRO)– MARCI: Mars Color Imager for “daily weather report”– MCS: Mars Climate Sounder for temperature, humidity

and dust

Models

• Both mesoscale models and GCMs used for Mars

• Mars modeling efforts based on modified Earth models– UCLA GCM, MM5, Skyhi, WRF– Earth-specific phenomena removed, Mars-specific

phenomena added– Level of complexity rivals Earth models– Many of same problems: boundary layer, eddy

resolution

• Dust is most complicated/important cycle for Mars vs. water for Earth

Mars facts

• Located at 1.5 AU

– 591 W/m2 vs. 1373 W/m2

• 95% CO2, 2.7% N2, 1.6% Ar

• Opacity 0.1-10 due to atmospheric dust

• Surface pressure 6 mb

• Near-surface temperature 145-245 K

Mars-Earth comparison

Mars Earth

Gravity (m/s2) 3.72 9.81

Scale height (km) 10 7.8

Adiabatic lapse rate (K/km)

4.5 9.8

Seasonal Cycle

• Obliquity 25° (14.9°-35.5° over 1.2 Myr)

• Eccentricity 0.093 vs. 0.017 for Earth

• Aphelion corresponds with southern winter, while perihelion corresponds with northern winter

Seasonal cycle of pressure observed by Viking landers Taken from Leovy (1979).

Impact of Seasonal Cycle

• 25% of atmosphere freezes out onto seasonal ice caps in winter hemisphere

• Weaker circulation during southern winter than northern winter

• Temperature asymmetry between same season in each hemisphere

• All of the above impact seasonal cycling of dust

Bonus! Mars water cycle

• Most water permanently tied up in polar ice caps (north AND south)

Left: temperature contours. Taken from Leovy (1969).Center: water phase diagramRight: THEMIS image of water ice “flower clouds” over Arsia Mons.

Mean Meridional Circulation

• At equinoxes: circulation patterns similar

• At solstices– Earth: stronger winter

cell extends into tropics, weak summer cell

– Mars: one cross-equatorial cell extends into mid-latitudes

Solsticial general circulations of Earth and Mars, showing the structures of the MMC and strength of meridional winds. Taken from Leovy (1969).

Comparison of Circulations

If Earth and Mars have almost the same obliquity, why aren’t their circulations more similar?

• From simulations by Walker & Schneider (2005), solsticial cell asymmetry on Earth corresponds to an obliquity of ~6° in a model with no ocean.

• Oceans moderate the seasonal cycle on Earth. Mars has no oceans and the atmosphere has little heat capacity.

Wind patterns

• Thermal wind equation applicable to zonal wind calculation

• Few in situ observations of surface winds, but generally mild

• Intense mid-latitude jet in winter hemisphere

Southern winter wind and temperature fields predicted by a GCM in 1976. Taken from Leovy (1979).

Simulations: Northern Winter

Simulations: Southern Winter

Simulations: Equinox

Dust

• Dust present in large quantities on surface and in atmosphere– Constant haze driven by surface convective lifting (dust

devils)– “Dust storms” in excess of haze driven by wind stress

lifting• Dust storms from local to global scales occur

frequently– Polar cap-edge baroclinic zone generates local and

regional dust storms every spring– Planet-encircling storms occur in early southern spring but

only in some years• Dust is a radiatively active mineral aerosol

Pretty Pictures I (Dust Devils)

Dust devils seen during a 12 minute sequence in southern spring by MER Spirit in the vicinity of Gusev Crater.

Pretty Pictures II (Local Dust Storms)

Local dust storms in early southern spring 2001, near the polar cap edge.

Pretty Pictures III (Regional Dust Storms)

Hellas Basin in southern spring: small-scale dust activity (left) and later evolution into regional dust activity.

Pretty Picture IV (Global Dust Storms)

Mars before and during the 2001 global dust storm.

Top: MGS image of Tharsis side.

Bottom: HST image of Hellas side.

Effect of dust on atmosphere

TES temperature profile during the evolution of the 2001 global dust storm. Note atmospheric warming and expansion of the mean meridional circulation.

Life of a global dust storm

TES temperature and dust opacity data during the evolution of the 2001 global dust storm.

Teleconnections project

• Dust storms are generally unpredictable, exhibit strong interannual variability

• Goal: understand what circumstances lead to global dust storm

• Importance of Hellas Basin and Syria/Solis/Daedalia– 2001 global dust storm began with two regional dust lifting

centers: first, Hellas basin; second, Syria/Solis/Daedalia

– 1977 observations too sparse to capture entire origin, but observed initial lifting over Syria/Solis/Daedalia

• Are teleconnections responsible for the globalization of dust storms?

Effect of dust on atmosphere

NASA Ames modeling results for northern winter solstice with variable dust loads: 0.3 (left),1.0(middle),5.0(right). Dust loading warms the atmosphere and increases/expands the circulation. Taken from Haberle et al (1993).

Surface Dust Reservoir

Darker surface after a global dust storm, indicating the presence of a finite reservoir of surface dust. Taken from Szwast et al (2005).

GDS circulation project

• Positive feedback associated with increased wind stress lifting in a dusty atmosphere (more dust → stronger circulation → stronger surface winds → more dust)

• Possible limiting factors:– Eventual cooling effect as dust pall fills higher into

atmosphere– Instabilities in circulation– Limited dust available on the surface

• What governs maximum amplitude of atmospheric circulation?

• What governs storm switch-off?

WRF

• Mesoscale model has been “stitched up” at poles to make a global model

• Earth-specific constants and phenomena removed

Result: global any-planet model• Modular nature of model lends itself for use on

other planets– Current work on Mars and Titan

• Easy use of nesting allows detailed simulation over complicated/important areas– Dust source regions on Mars