Workshop on Dust and Grains in Low Gravity and Space Environment
02-04 April 2012
ESA/ESTEC, Noordwijk, The Netherlands
The Role of Dust
in the
Martian Climate
A. Chicarro, L. Colangeli, J. Vago, O. Witasse
ESA Solar System Missions Division (SRE-SM)
Research and Scientific Support Department (RSSD)
ESA-ESTEC
Noordwijk (The Netherlands)
Outline
1.
2.
3.
4.
5.
Climate on Mars
Martian atmosphere
The role of dust on Mars
Knowledge on Martian dust
Missing information
First ever view of the Martian
surface, Viking-1 landing site, Chryse
Planitia, on 20 July 1976.
Climate on Mars
• Inactive volcanism and tectonics (but active wind erosion)
• Meteorologically active planet (diurnal, seasonal, annual, secular variations)
• The two polar caps are covered by thick layers of Carbon Dioxide and Water
Ice plus dust => extension varies with seasons (condensation/sublimation
processes)
• Climate conditions depend on the planet’ s orbit and rotation axis inclination
• Northern summer is longer than southern one (orbital eccentricity), but
thermal forcing is stronger in the South (global dust storms originate there)
Mars topography from MGS-MOLA
Martian atmosphere – parameters
• Thin, dry, cold atmosphere dominated by carbon dioxide (CO2)
• Conditions depend on season, time of day, location and altitude
Atmospheric Parameters
Composition (volume fraction)
95.32% CO2, 2.70% N2, 1.60% Ar40, 0.13% O2,
0.07% CO, 0.03% H2O, 0.013% NO, 5.3 ppm
Ar36+38, 2.5 ppm Ne, 0.3 ppm Kr, 0.13 ppm
CH2O, 0.08 ppm Xe, 0.04-0.02 ppm O3, 10.5
ppb CH4
Mean Molecular Mass
43.49 g/mole
Pressure
6.1 mbar (average) - 0.2-12 mbar (range)
Temperature
215 K (average) - 140-310 K (range)
Density
0.010-0.020 kg/m3
Wind Speed
0-30 m/s
Atmospheric visible optical depth
0.1-10
Atmospheric dust mass density
1-100·103 kg/m3
Atmospheric dust particle number density
1-100 cm-3
Atmospheric dust grain size
0.010-10 μm
[0.1-0.3 under clear conditions]
[ if ≥1 increased dust;
if >5 major dust storm]
Martian atmosphere – T and P
Atmospheric temperature-height profile on Earth
and Mars (Zurek, R.W., 1992)
Temperature (top) and pressure (bottom) daily
variations measured in-situ by Viking Lander-1
(Chryse Planitia, 22.70°N 48.22°W) and Mars
Pathfinder rover (Ares Vallis, 19.13°N 33.22°W)
Martian atmosphere – global circulation
•
•
•
•
•
Global Martian atmosphere dynamics ruled by the Hadley circulation
Warmer air raises at equator, cools at higher altitudes, flows towards the
poles and closes the circulation towards the equator => meridional (SouthNorth) movements
The Coriolis effect (generated by planet’s rotation) causes strong lateral zonal
winds (East-West).
A large Hadley cell forms, involving both hemispheres and straddling the
equator
Vertical movements are not prevented due to the lack of a stratosphere
Meridional winds and Hadley circulation on Earth and on Mars (Forget, F., et al., 2006)
Martian atmosphere – surface interaction
•
•
•
•
•
Martian dust comes from clastic alteration, chemical weathering and erosion
(wind/water/ice) of rock beds, as well as regolith (impacts of all sizes) + ID
Dust is permanently present in the atmosphere with variable abundance
It is a key player in the dynamic and thermodynamic evolution of the
atmosphere (large scale circulation at diurnal-seasonal-annual time-scales)
Wind effects on dust at various scales: dust cycle, dust storms and dust devils
are typical phenomena involving atmospheric dust
Aeolian erosion, dust redistribution on surface and weathering couple surface atmosphere (wind intensity; grain properties)
Dune field, Proctor
crater (MGS-MOC)
Southern Highlands
wind
Dust-blown deposits in Syrtis Major (MEX-HRSC)
Dust devil (MRO-HiRISE)
Martian atmosphere – dust cycle
Dust Devil
Dust Cycle
Dust cycle in the Martian atmosphere and dust devil mechanism (Forget, F., et al., 2006)
Saltation: loose particles carried by fluid (wind) back to surface
Winds
Suspension in
Atmosphere
(dp < 20 μm)
Dust Storms and
Dust Devils
Surface Volatile
Outgassing
Dust Grains
Lift Off
and Transport
(dp ≈ 1 μm – 1 cm)
Creep
Gravitational
Deposition
Baroclinic Waves
Lift Off and
Saltation
at Ground Level
Sandblasting
(S. Ventura, PhD Thesis, 2011)
Martian atmosphere – dust properties
•
•
•
•
•
•
•
•
•
Current knowledge about the Martian dust derives mainly from optical remote sensing
measurements
Mainly made of silicon dioxide (SiO2), ferric oxide (Fe2O3) and aluminium oxide (Al2O3)
structures
MER Microscopic Imager observations have shown that surface dust occurs as fragile,
low-density, sand-sized aggregates (Sullivan et al. 2008)
Typical radius: 10-3 – 10 μm
Average mass density: 2.73 x 103 kg/m3
Dust number density close to the Martian surface (Moroz et al. 1993) :
n = 1 - 2 cm-3 in constant haze
Dust mass density (Metzger et al. 1999):
1.8 x 10-7 kg m-3 in standard conditions
7 x 10-5 kg m-3 during a dust devil.
This mass densities correspond to number densities
(Esposito et al. 2011):
n = 2 cm-3 and n = 1500 cm-3
for particles with r = 1.6 µm and ρ = 2.6 g cm-3.
Size distribution of atmospheric dust
(e.g., Toon et al. 1977; Drossart et al. 1991;
Pollack et al. 1995; Tomasko et al. 1999;
Esposito et al. 2011)
Martian atmosphere – dust dynamics
Northern Spring and Summer/
Southern Autumn and Winter
Northern Autumn and Winter/
Southern Spring and Summer
•
•
•
•
Threshold velocity (uτ) and wind velocity (Vwind) required to
move dust grains on Mars surface (Greeley et al. 1980).
• Few dust storms
• Low concentration of suspended dust
• Global and/or regional dust storms
(statistically speaking, 33% probability)
• Growing from the Southern hemisphere
(Mars at perihelion)
• Lasting for several days
Saltating sand-size particles responsible for
dust entrainment via impacts (Iversen and
White 1982)
Recent
observations
show
that
sand
movements are rare with respect to dust
lifting (e.g., Fenton 2006; Bourke et al.
2008; Chojnacki et al. 2010; Silvestro et al.
2010).
Fragile low-density sand-sized aggregates at
the surface could be the primary source of
airborne dust (Sullivan et al. 2008).
The dynamics of aggregates is linked to grain
electrification (Merrison et al. 2004; Merrison
et al. 2007).
Martian atmosphere – water vapour
•
•
•
•
•
•
•
•
•
Minor abundance (0.03% volume) of water in
atmosphere, but plays a fundamental role in climate
and appearance of life
Indicator of seasonal climate changes
Mean pressure at surface level:
6 - 7 mbar
Variation with elevation:
Hellas Basin => 13 mbar
Olympus Mons => 0.2 mbar
Typical atmospheric pressure at surface ~ triple point
(6.1 mbar) => water condenses/sublimates directly
to/from the solid state
Caution: diagram for pure water system; on Mars,
partial pressure of water vapour instead of
atmospheric pressure
Martian dust: future studies
In-situ measurements will obviously provide a more accurate determination of the
dust properties
In order to characterise the Martian environment close to the surface, in-situ
quantitative measurements must address the following physical quantities:
•
•
•
•
•
•
•
•
Atmospheric dust particle-size distribution
Density number vs. size / concentration of suspended dust
Dust deposition rate
Suspended grain electrification
Wind speed
Dust – water vapour relative concentration
Time evolution of the former quantities vs. long term variations and short
term / local events (e.g. wind, dust devils, dust storms)
Humidity variations
Improvement of Martian climate evolution models
The Role of Dust in the Martian Climate