Gullies on Mars Geology and Origin Allan H

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!

Water on Mars One of the most natural human curiosities – related to the ult Viking 1 Orbiter MGS although many images suggest SIGNS of water, NO LIQUID WATER Solar System - C. C. Lang ever been observed! or RAIN has 5 Sept 2003 4

Geological Evidence: Outflow Channels Ares Valles MGS data Outflow channels suggest that massive floods happened: - water discharge rates ~10, 000 times Mississippi River flood 5 Sept 2003 Solar System - C. C. Lang - did these floods form into a massive ocean? 5

Geological Evidence: Martian Channels Martian Gullies: left (Mars) and right (Earth). In the Earth picture, rain water flowing under and seeping along the base of a recently-deposited volcanic ash layer has created the gully. For Mars, water is not actually seen but is inferred from the landforms and their similarity to examples on Earth. 5 Sept 2003 Mars Global Surveyor Image (June 2000) Solar System - C. C. Lang Earth (Mt. St. Helens) 6

Geological Evidence: Sustained Water Flow 5 Sept 2003 Solar System - C. C. Lang 7

5 Sept 2003 Solar System - C. C. Lang MGS 8

Geological Evidence: Possible Ancient Streambeds and Erosion Channels 5 Sept 2003 Solar System - C. C. Lang 9

Summers in the Martian southern hemisphere are shorter than summers in the northern hemisphere. Why? (1) The northern hemisphere is chilled by a larger polar cap (2) There are more volcanoes in the southern hemisphere (3) The southern hemisphere is at much lower elevation on average the Martian equatorial bulge makes the sun shine more directly on the Southern hemisphere (4)This is a consequence of Mars’ elliptical orbit. 5 Sept 2003 Solar System - C. C. Lang 10

The physics of finding water on Mars When/how does water (H 20) exist in its liquid state? LIQUID water exists over a very NARROW range in TEMPERATURE - if T is too high gas - if T is too low solid (ice) The TEMPERATURE of water will depend on the ATMOSPHERIC PRESSURE - if Pressure is too low – water will vaporize (evaporate) - if Pressure is too high – water will stay liquid! Mars’ atmospheric pressure is ~1% of Earth’s - LOW pressure - LOW temperature FROZEN WATER (ice!) 5 Sept 2003 Solar System - C. C. Lang 11

Mars Global Surveyor TES instrument 5 Sept 2003 Solar System - C. C. Lang 12

Mars Global Surveyor TES instrument 5 Sept 2003 Solar System - C. C. Lang 13

Martian Atmosphere Carbon Dioxide (CO 2): 95. 32% Nitrogen (N 2): 2. 7% Argon (Ar): 1. 6% Oxygen (O 2): 0. 13% Water (H 2 O): 0. 03% Neon (Ne): 0. 00025 % • 1/1, 000 as much water as our air • only enough to cover the surface of the planet with 0. 4 inches if melted out of the atmosphere!! • Martian atmosphere does not regulate the surface temperature 5 Sept 2003 Solar System - C. C. Lang on the planet as Earth’s 14

Mars Global Surveyor TES instrument 5 Sept 2003 Solar System - C. C. Lang 15

History of Mars’ atmosphere: first like Earth’s Atmospheres of terrestrial planets created by out-gassing of volcanoes - ~4 Billion years ago (Mars/Earth = same atmosphere) - volcanic gasses: nitrogen, CO 2, water vapor • H 2 O in atmospheres condensed to create oceans on Earth • CO 2 cycles through our atmosphere: volcanoes/carbonate rocks 5 Sept 2003 Solar System - C. C. Lang 16

What happened to the atmosphere on Mars? • CO 2 and H 2 O may have condensed out and onto the surface – got so cold, that the water froze into permafrost • End of volcanic activity, however, would stop the cycle 5 Sept 2003 Solar System - C. C. Lang 17 • Strong UV light from the Sun will break up chemicals on surface

Storage place for Water: some in N Polar Cap! • Caps are frozen carbon dioxide CO 2 (‘dry ice’) and some frozen water • notes on frozen CO 2 - CO 2 freezes at T~150 K (~ -200 F) - block of ‘dry ice’ T~ -110 F - sublimes directly ice to gas • Southern residual cap is ~300 km across, T ~150 K. 5 Sept 2003 • Northern cap is ~1, 000 km, T~200 K (implies mostly water ice b/c CO 2 ice Solar System - C. C. Lang 18 would be sublimating)

Chemical Evidence: Looking for presence of minerals which suggest water • Small amounts of water vapor in the atmosphere indicated that much of the planet’s water lost to space – evaporated • Recent Mars missions have looked for evidence of carbonates - large seafloor beds resulting from consumption of atmospheric CO 2 - finding little or none – this is problematic, controversial • Recent Mars missions have found Hematite – a signature of dried-up lakes on a few places on the planet from TES instrument on MGS 5 Sept 2003 Solar System - C. C. Lang 19

Recent Chemical Evidence for water on Mars: Detection of Hydrogen (Mars Odyssey - 2003) 5 Sept 2003 Solar System - C. C. Lang 20

Recent chemical Evidence for water on Mars: Detection of Hydrogen (Mars Odyssey - 2003) 5 Sept 2003 Solar System - C. C. Lang 21

Theories for Water on Mars: Current Data & Ideas… • Mars probably did have a “warmer, wetter” past, but not extreme - studies show water can exist in ice-covered lakes - as long as temperatures get above freezing for a few days - clues from glacial regions on Earth have helped Geological Evidence: Use Ancient Lakebeds/Glacial Lakes as analogy 5 Sept 2003 Solar System - C. C. Lang 22

Geological Evidence: Layered Sedimentary Features 141 km (88 miles) Holden Crater 5 Sept 2003 Solar System - C. C. Lang 23

The pasted-on terrain sometimes displays reticulate cracks, which may indicate dessication of volatile-rich materials or development of iceor sand-wedge polygons. The pasted-on deposits are 10’s of meters thick.

Mars impact Crater: Polygon Formations -Each section slightly larger than a football field (http: //www. msss. com/mars_images/moc/7_19_99_fifth. Mars/07_npolys)

• (A) Polygon troughs highlighted by frost as the south polar cap retreats during spring. The circular features are the locations of buried craters that were originally formed by meteor impact. This image was acquired on 1 October 2001. • (B) Summertime view of polygons, highlighted by dark, windblown sand, on the floor of a crater. Obtained on 21 January 2002. • (C ) Polygon troughs highlighted by the retreating south polar frost cap during southern summer. This picture was taken on 13 January 2000. • (A) (B) (C ) •

Martian Meteorites Martian data without going there! What are these “meteorites”? How did they get to Earth? Unusual rocks found in Antarctica An impact on Mars (crater size: 10 -100 How do we know these meteorites are from Mars? part of the Martian surface Chemical composition does not match u Only 1. 3 billion yrs old (most asteroid. Why aren’t they orange – the color of Mars surface? MUCH older); Higher content of volati 5 Sept 2003 Solar System - C. C. Lang 27 Has to do with how the rock

What DO they tell us? -Physical processes on Mars Crust/core developed early in So Volcanism until < 1 Billion Yea - Chemical composition What DON’T they tell us? - Location of origin Interaction with water - Enough about Mars’ water & atmo Martian atmosphere composition - Need to RETURN ROCKS from M Case Study: Martian Rock ALH 84001 5 Sept 2003 Mass = 1. 9 kg Igneous Rock Discovered in Antarctica (easier to f Solar System - C. C. Lang 28 Formed on Mars 4. 5 Billion yr ago

Controversial – microbial presence in meteorites? ? Globules of carbonate minerals (the yellow-orange grains) are scattered along cracks in this small chip of ALH 84001. The rims contain iron oxides (including magnetite) and iron sulfides--incompatible minerals that on Earth would suggest microbial action Close up views reveal structure similar to Earth microbes! 5 Sept 2003 Solar System - C. C. Lang 29

Mars has a very thin atmosphere and no magnetosphere. If humans populated the Martian surface, what environmental problems will they be concerned about? (1) Global Warming (2) Solar flare particles and ultraviolet radiation (3) Nitrogen poisoning (4) Lead contamination from volcanoes (5) Magnetic anomalies in the interior causing brain Solar System - C. C. Lang disorders 5 Sept 2003 30

Monday: Mars Surface Exploration 5 Sept 2003 Solar System - C. C. Lang 31

Monday: Mars Surface Exploration 5 Sept 2003 Solar System - C. C. Lang 32

Theoretical Ice Table Depth Today Continuous permafrost Discontinuous, episodic permafrost No shallow ice 6 counts/second isopleth from GRS instrument (summer data only) Discontinuous, episodic permafrost Continuous permafrost Ice will be buried to a depth such that the average temperature at that depth is at the frostpoint, ~196 K today. • As climate warms or frostpoint falls, ice sublimates 33 Source: Mellon and Feldman (2005) 04/19/06 • As climate cools or frostpoint rises, ice condenses

Example Geothermal Gradient 04/19/06 Ice stable w. r. t. atmosphere. Operates like a cold trap. Addition of heat (from any source) would cause ice to sublime. Environments T = 196 warmer than this will K become progressively 34 dessiccated.

Biology-Geology Relationship Propagation Possible 1 Cu su rre bs nt ur fac Mar co e eq s sh nd a itio uilib llow ns riu m 0. 1 0. 001 0. 0001 -90 -80 -70 -60 -50 -40 Proposed Threshold Water Activity Proposed Threshold -30 Temperature (deg. C) 35 04/19/06 -20 -10 0

Semi-permeable Crusts • Some types of desert crust on Earth have formed by processes that could have operated on Mars, and duricrust has been observed on Mars. • Could crust create conditions that exceed the. Atmosphere values? threshold Regolith Ice 36 04/19/06

Desert Crusts as Vapor Barriers • Desert crusts are semi-permeable, not impermeable – Unfractured hydraulic conductivity typically ranges from 0. 5 to 0. 75 cm/hr. Permeability to gas is typically higher than permeability to liquid. – In natural settings, a wide range of processes result in the formation of voids, pores, and fractures that prevent continuous sealing. • At T = -20 C, the water vapor pressure is relatively high, and the water will slowly be driven out unless the recharge rate exceeds the loss rate. 37 04/19/06

Water on Mars: The Bottom Line 1. Mars today is a desert l Plenty of places warm enough for transient water to exist l Plenty of water in the form of ice in cold places l No way to get the ice from the cold places to the warm places! 2. Mars in the past was likely slightly wetter (104 -107 years – outside our time horizons) l Orbital forcing drives climate change l Gullies are primary indication of occasional transient water l Snow was a likely transport mechanism 3. Speculative areas where water has survived in disequilibrium would be obvious special regions today l Vestigial water sources from past epochs (e. g. snowpacks on crater walls)? l Recent impacts or volcanism? l There is no evidence for any of these phenomena producing liquid water today (+ 100 years). 4. The only other plausible way to make water today would be through the influence of the spacecraft itself. 38 04/19/06

Possible Glaciers • The topic of glaciation—even at equatorial latitudes—has been discussed and debated for more than 3 decades. • Huge possible glacial deposits on Tharsis volcanoes. • Eskers, drumlins, and other indications of classic wet-based glaciation are absent. This suggests that cold-based glaciation (typical of polar latitudes on Earth) is a more appropriate analog. • Although we cannot rule out that there may be some residual ice at depth in equatorial deposits, because of their age it would certainly be below a thick sublimation till—residual shallow ice is highly unlikely. Promethei Terra at the eastern rim of the Hellas Basin, 38º S, 104º, ESA/DLR/FU Berlin Deuteronilus Mensae region (40 N, 25 E, THEMIS v 12057009). 40 04/19/06

Map of Possible Equatorial Glaciers 180 150 120 90 60 30 0 330 300 270 240 210 180 90 60 V-2 30 V-1 MPF 0 MER-B 30 60 MER-A Possible glacial deposits shown in yellow. 90 FINDING. Although glacial deposits may be present at different latitudes, there is no evidence for melting. 41 04/19/06 Source: Head (various)

Craters with Residual Heat • A crater could retain heat to the present if: – Very young (heat lost with time) – Big (more energy with bigger impacts) Crater Size (diameter) 3 km 100 years 10 km 1, 000 years 30 km 04/19/06 Time for which crater environment has potential to retain enough heat to exceed threshold condition 100, 000 years 42

Large, Fresh Craters Identification of the most recent large craters. 1. Sharp rim, depth approximates that expected for a pristine crater. 2. No superposed features on either crater or ejecta blanket (dunes, floor deposits, tectonic/fluvial features, or small impact craters). 3. Ejecta blanket and interior morphologies are sharp and well preserved. 4. Crater and ejecta blanket display thermally distinct signatures in daytime and/or nighttime infrared views. THE TOP NINE LATITUDE (N) 04/19/06 LONGITUDE (E) DIAMETER (KM) 7. 03 117. 19 18. 0 7. 16 174. 41 9. 6 8. 93 43. 82 12. 10 CENTRAL STRUCTURE EJECTA* Central Peak MLERSRd Floor Pit MLERS 10. 9 Summit Pit MLERS 169. 24 5. 9 Summit Pit SLERS 13. 70 29. 52 11. 5 Central Peak MLERS 16. 95 141. 70 13. 6 Floor Pit MLERSRd 19. 51 141. 18 9. 2 Floor Pit MLERS 20. 01 246. 68 7. 9 None SLERSRd 23. 19 207. 76 28. 3 Central Peak MLERSRd 43

Young Volcanics Distribution of the youngest volcanic rocks on Mars (map unit AEC 3 from Tanaka et al. 2005). 45 04/19/06

Young Volcanics (cont. ) • Since volcanic heat is lost with time, only extremely young volcanics have the potential to exceed the temperature threshold for propagation. • Simple calculations show that the temperature at the surface drops to less than -20°C within about 1000 y. • Age can be estimated from albedo and crater density—none are thought to be as young as FINDING. We do not have evidence for volcanic rocks on Mars of an age young enough to retain enough heat to qualify as a 1000 years. modern special region. • Estimates of the volcanic recurrence interval in 46 04/19/06 the youngest volcanic provinces suggest that

The Non-discovery of Geothermal Vents • An important objective of the THEMIS infrared investigation has been the search for temperature anomalies produced by – evaporative cooling associated with near-surface water – heating due to near-surface liquid water or ice, or hydrothermal or volcanic activity. • FINDING. Despite a deliberate and systematic search spanning THEMIS has mapped virtually all of Mars at several years, no evidence for the existence of near-surface liquid night in the infrared at 100 -m per pixel water close enough to the surface to be capable of producing resolution, and has observed portions of the measurable thermal anomalies has been found. 47 surface a second time up to one Mars’ years Source: Phil Christensen 04/19/06

Polar Ice Caps North Pole • Mentioned in current COSPAR definition, HOWEVER: – Maximum summer temperatures typically reach about 200 K at the north pole – The south polar cap, despite receiving more summer sunlight, is protected by a layer of highly reflective CO 2 ice, which holds the surface temperature at a constant 145 K. South Pole – Contributing to the perpetual low temperature is not only the latitude (hence low sun angle) but also the high conductivity of solid ice. FINDING. The martian polar caps are too cold to be naturally occurring special regions. 48 04/19/06

Dark Slope Streaks • Some water-related hypotheses are in the literature, HOWEVER: – At these equatorial latitudes very nearsurface is unstable, and – There is evidence that wind is a controlling factor in the streak occurrence in some cases. 49 04/19/06 Source: Phillips, Aharonson

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 CO 2. – CO 2 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 CO 2.

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, CO 2) – 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.