Gullies on Mars: Geology and
Origin
Allan H. Treiman
Lunar and Planetary Institute
Houston, TX
Why is Water Important?
• NASA Mars Exploration Strategy: “Follow
the Water.”
• Crucial geological agent: erosion, transport,
sedimentation, chemical reaction.
• Crucial resource for humans: metabolic
requirement, rocket propellant.
• Crucial to Life as-we-know-it.
Water on Mars
• Liquid water.
– Outflow channels
– Valley networks
• Water vapor – clouds
• Polar ice
• Ground ice
– Softened terrain
– Neutron, gamma ray data!
MGS/MOC Sees “Gullies”
• Unusual, unexpected
landforms on pole-facing
walls of craters and chasms.
(Malin and Edgett (2000) Science 288,
2330.)
• Material flowed down the
walls; landforms like waterrich debris flows.
• Recent liquid water at Mars’
surface! And there was
great rejoicing!
• Astrobiology target!!
What are Gullies?
• Alcoves high on walls,
sources of flow.
• Channels: sinuous,
tributaries, distributary,
commonly with levees.
• Depositional cones, cut
by channels, commonly
with lobate toes and
sharp raised margins.
Nirgal Vallis, fluvial
channel
Impact Crater wall, near
Newton Basin
How did they form?
• Look like terrestrial
debris flows: water +
rock + soil.
• So, are inferred to be
debris flows.
• On Mars, liquid water is
not stable near the
surface, so liquid water
must form at depth, and
gully deposits must form
rapidly before it freezes
or evaporates!
Pat Rawlings
How to Stabilize Liquid Water?
– to add heat to allow
liquid;
– to isolate liquid from
surface P;
– to overpressure liquid
to produce catastrophic
eruption.
2
• Can’t form at surface
(T, p, water pressure
too low).
• Must be mechanisms
How to Pressurize?
• Freezing of liquid water.
– Liquid water trapped, bounded by impermeable layers of
rock, ice, frozen soil.
– During cooling climate or annual cycle, water begins to
freeze.
– Volume increase on freezing pressurizes liquid.
– Tensile failure of rock/soil permits rapid ejection of liquid.
• None needed: water, injected from depth, erupts.
• Melting of solid CO2.
– CO2 trapped as solid in rock/soil.
– Melting inside impermeable trap increases pressure
– Tensile failure of rock/soil permits rapid ejection of liquid.
How to Heat?
• Heat from sunlight on pole-facing slopes.
– Greater obliquity in past?
• Normal geothermal heat, with highly insulating
surface materials (e.g., dust?).
• Heat from magma.
• No heat needed: briny water has low freezing T.
• No heat needed: liquid water intrudes from depth
and erupts before freezing.
• No heat needed: liquid is CO2.
Geologic Data to Test Theories
1. What is global distribution of gullies?
2. Are they in preferred orientations, e.g. polefacing?
3. Are they associated with ground ice?
4. Are they associated with a particular host
rock? (e.g., layered sediments)
5. Are they associated with particular types of
slopes or geologic terranes?
6. What flowed down the gullies?
1. Distribution of Gullies
• Circum-global.
• Most in mid-latitudes, southern hemisphere.
• Most abundant on south-facing slopes,
• = pole-facing in southern hemisphere.
• Clustered.
2. Orientations of Gullies: South
• Originally cited as
dominantly pole-facing:
• The motivation for solar
heating hypotheses.
• New data show a
preference for southfacing, in both
hemispheres,
• With orientations in all
directions.
Edgett et al. (2003) LPSC XXXIV,
Abs. #1038.
3. Near-Surface Water
• Abundances & distribution of near-surface water
measured by Mars Odyssey GRS.
– LANL neutron spectrometer and IKI high-energy
neutron detector subsystems
• Hydrogen shallower than a few meters depth
thermalizes neutron population.
– Yields high ratios of thermal/epithermal &
thermal/fast neutrons.
• Unknown if water at a few meters can be linked
to gully source water at a few hundred meters.
Water abundances
within a few meters
depth of the Martian
surface. Wm. Feldman.
AAAS talk & Los
Alamos Nat’l. Lab.
Press Release, 15 Feb.
2003.
(SPACE.com report, 16
Feb. 2003)
Distribution of Gullies
Malin & Edgett (2001)
JGR 106, 23429.
4. Host Rock Preferences?
• Layered, competent rock.
• Layered rock, but broken
and shattered at impact
craters.
– Poor candidate for water
trap
• Unlayered rock in impact
crater central peaks.
– Poor candidate for water
trap.
Gorgonium Chaos
Central Peak, Hale
impact crater
5. Specific Geologic Setting?
• All types of slopes: • All types of ‘bedrock:’
– impact crater
• walls
• central peaks, rings
–
–
–
–
collapse pits,
fluvial channels,
volcanic calderas,
polar pits.
• All types of surfaces:
– layered rock,
– massive rock,
– ‘dust’ mantles.
–
–
–
–
–
–
–
ancient cratered highlands,
young volcanic plains,
shield volcanos,
chaotic terrain,
sedimentary basins,
northern plains (ocean?),
peri-polar etched terrain.
Volcanic Caldera,
Pavonis Mons
Pit in sedimentary fill,
Rabe crater
Collapse pit, Dao
Vallis,
Mesa in S. peri-polar
pitted terrane
6. What Flowed?
• Fine grained solids.
• Rocks rare in deposits.
• Deposits are same
‘color’ as wind-blown
sediment (silt-sized).
• Deposits erode away,
leaving no rocks.
Dark Boulders
500 m
Eroded away, no boulder ‘lag’
Evaluate Hypotheses
• Pressurized in sealed aquifers? (Water, brine, CO2)
– Most gullies on broken and/or unlayered rock.
– How could water enter aquifers on isolated peaks?
• Heat from solar heating on slopes?
– Gullies on slopes of all orientations.
• Heat from normal geothermal gradient?
– Gullies beneath dusty surfaces and bare rock.
• Heat from magma intrusions?
– No correlation with known volcanos.
– Why only in mid-latitudes, and not nearer poles?
• Explosive breakouts?
– Few rocks in gully deposits.
Another Hypothesis: Silt!
• Flows of granular materials, silt- or dust-sized.
• Lots of fine granular material available on Mars.
– Global dust storms.
– Dunes.
– Mantling deposits.
• Easily eroded by wind.
• Does not require pressure or heat, or any sort of
substrate or bedrock geology.
• Granular materials flow as Bingham fluids (yield
strength), consistent with levees and toe lobes.
Granular Flows
• Silt-sized sediment dropped
by wind at obstructions
(crater walls, hills).
• Sediment accumulates to,
and beyond, angle of
repose. Fails of own weight,
impact shock, etc., and
flows.
• Flowing silt incises
meandering channels into
its own and earlier deposits.
• Bingham rheology yields
levees and lobate toes.
Adventdalen
• Few examples of
large-scale dry
granular flows on
Earth (too wet).
• Dry snow sluff gives
landform similar to
last season’s wet
debris flow (2002).
Adventdalen,
Spitsbergen Island.
• Photo by H.E.F. Amundsen.
All rights reserved.
Avalanche
• Climax snow
avalanche, Ashcroft
CO, 1998. Wall
~200 m tall, ski
track shows scale.
• Headscarp (H),
alcove (A), bedrock
(B), and cone of
deposed snow (C).
Arrow shows
channels on cone.
• Photos by R. Day. All rights
reserved.
Implications of Gullies as Silt
• Gullies will form where silt lies thick on
steep slopes.
• No relationships between gullies and:
–
–
–
–
–
–
Cause of slope (impact, fluvial, collapse);
Age of bedrock (ancient through recent);
Geology of bedrock (volcano, sediments, etc.);
Physical state of rock (broken, layered, etc.);
State of surface (bare rock vs. dust-covered);
Abundance of subsurface water.
• Expect relationship between gullies & wind.
Silt Deposition and Wind
• Expect deposition of wind-blown silt where
wind decelerates – decreased carrying
capacity.
• Local control – obstructions:
– Downwind walls of hills, craters, & chasms
– Provided they are steep enough to disrupt flow.
• Regional controls:
– Boundaries of orographic/katabatic flows;
– Boundaries of Hadley circulation cells.
Surface Wind Speeds
and Directions.
Fenton & Richardson
(2001) JGR 106, 32855.
Distribution of Gullies.
Malin & Edgett (2001)
JGR 106, 23429.
Conclusions, and Conclusion
• Gully landforms are widespread,
– On steep walls of all sorts,
– Concentrated in southern mid-latitudes.
• Their occurrence is not correlated with:
–
–
–
–
Character of land surface (except for steep slope);
Subsurface geology (e.g., volcano, sediments);
Characteristics of rock (layered, massive, broken);
Abundance of subsurface water.
• Gullies’ characteristics and distribution are
consistent with formation as flows of eolian silt.
• No need to invoke near-surface liquid water; no
clear astrobiology target.