Inferred Flow Properties in Inferred Flow Properties in Modern Mars GulliesModern Mars Gullies

Brown University, 2008

Alan D. Howard@

Department of Environmental SciencesUniversity of Virginia, USA

@ With help from: Jeffrey Moore, William Dietrich, and Taylor Perron

Exposed Rock on crater wall. Generally highly Fragmented.

“Pasted-On” Terrain. Generally smooth surface, but HiRISE shows scattered >1m boulders

The pasted-on deposit accumulated on the interior walls of the crater after formation of the crater but before and, in some cases, episodically during gully formation.

The pasted-on terrain sometimes displays reticulate cracks, which may indicate dessication of volatile-rich materials or development of ice- or sand-wedge polygons.

The pasted-on deposits are 10’s of meters thick.

The gullies on the right side are developed mostly in pasted on terrain, whereas bedrock is exposed in floors of gullies to left.

Pasted-on terrain appears to be absent here and the aprons show little of the hummocky texture or channeling as where such terrain is present.

Conjecture: Flow structures and channels on aprons are best developed where pasted-on terrain is present.

In general alcoves reveal coarse bedrock.

Some Observations about Some Observations about AlcovesAlcoves

Deeper alcoves expose crater (or scarp) wall Deeper alcoves expose crater (or scarp) wall bedrock – usually highly fractured. Shallower bedrock – usually highly fractured. Shallower alcoves can be entirely in “pasted-on” terrain.alcoves can be entirely in “pasted-on” terrain.

Where bedrock exposed, lots of >1m scale Where bedrock exposed, lots of >1m scale boulders on alcove surfaces.boulders on alcove surfaces.

Alcoves are a good place for seasonal or longer-Alcoves are a good place for seasonal or longer-term COterm CO22 snow accumulation. Steep slopes snow accumulation. Steep slopes may encourage avalanching of frost may encourage avalanching of frost accumulation.accumulation.

The alcoves may be the least diagnostic feature The alcoves may be the least diagnostic feature of the gullies, because almost any triggering of the gullies, because almost any triggering mechanism on such steep slopes will generate mechanism on such steep slopes will generate flows capable of scouring the fragmented flows capable of scouring the fragmented bedrock and overlying pasted-on terrain.bedrock and overlying pasted-on terrain.

As with terrestrial alluvial fans, the apex is commonly the site of alcove-head trenching.

The degree of apex trenching often varies from fan to fan, suggesting autocyclic cycles of avulsion, entrenchment, back-filling, and new avulsion, as on terrestrial alluvial fans.

The middle portion of aprons typically displays numerous hummocks that are lenticular or downstream-bifurcating. Levees are low or absent.

APRONS

Being a Being a depositional depositional environment, environment, the aprons the aprons probably can probably can afford the most afford the most direct clues to direct clues to gully origin and gully origin and transport transport mechanics.mechanics.

Distal Apron Distal Apron TerminationsTerminations

Perhaps the most diagnostic gully landform is Perhaps the most diagnostic gully landform is the distal end of the aprons.the distal end of the aprons.

Aprons typically end abruptly at the break of Aprons typically end abruptly at the break of slope at the crater floor or scarp foot. Channels slope at the crater floor or scarp foot. Channels or deposits usually do not continue beyond the or deposits usually do not continue beyond the slope break. This suggests flows of enhanced slope break. This suggests flows of enhanced but modest mobility.but modest mobility.

Flows may terminate because:Flows may terminate because: Decrease of gradient reduces stresses below flowing Decrease of gradient reduces stresses below flowing

material yield strengthmaterial yield strength Water freezes or boils away.Water freezes or boils away.

Older aprons typically have smoother surfaces and a larger fraction of exposed rocks. This suggests reworking of the surface, possibly by wind scour, sublimation, or creep

YOUNG

OLD

ApronsAprons Almost all Aprons have surfaces with few visible Almost all Aprons have surfaces with few visible

boulders as compared to the very bouldery floors of boulders as compared to the very bouldery floors of Alcoves.Alcoves.

This suggests that the dominant source of sediment on This suggests that the dominant source of sediment on Aprons is either the pasted-on terrain and/or wind-blown Aprons is either the pasted-on terrain and/or wind-blown sediment accumulating in Alcoves which is subsequently sediment accumulating in Alcoves which is subsequently remobilized.remobilized.

The relatively fine-grained nature of the Aprons may The relatively fine-grained nature of the Aprons may contribute to its characteristic surface morphology.contribute to its characteristic surface morphology.

Perhaps the most important characteristic of the aprons Perhaps the most important characteristic of the aprons is the is the rarityrarity of strongly peaked leveed channels and of strongly peaked leveed channels and leveed lobes in comparison to many potential terrestrial leveed lobes in comparison to many potential terrestrial analogs.analogs.

If volatiles are a dominant component of the formative If volatiles are a dominant component of the formative flows, however, the present morphology of the aprons flows, however, the present morphology of the aprons (e.g. hummocky and lacking high levees) may not reflect (e.g. hummocky and lacking high levees) may not reflect the morphology immediately after emplacement because the morphology immediately after emplacement because of subsequent volatile loss.of subsequent volatile loss.

Pasted-on terrain

Pasted-on terrain

Gully channels near the Apex often display braided appearance with lenticular bars

MeandersMeanders

Another Another diagnostic feature diagnostic feature is low-sinuosity is low-sinuosity meandering in meandering in some gullies and some gullies and apron distributary apron distributary channels.channels.

Measurements of Gully Measurements of Gully PropertiesProperties

As of December 2007 ~30 HiRISE stereo As of December 2007 ~30 HiRISE stereo pairs – considerably more now.pairs – considerably more now.

17 Have good topographic control from 17 Have good topographic control from MOLA tracksMOLA tracks

One set of measurements concerns Apron One set of measurements concerns Apron gradientsgradients

• HiRISE PSP 1792-1425• Image width 1.4 km• Blue: Depositional Apron• Yellow: Thick Pasted-On Terrain• Red Arrow: Typical Measured Apron Slope• White Arrow: Mid-Slope Location• Asterisk: Meandering Gullies

Characterizing Gullied Characterizing Gullied SlopesSlopes

Score aprons and mid-slopes separately and Score aprons and mid-slopes separately and add scores.add scores.

Mid-slope:Mid-slope: 1: Deeply incised by channels1: Deeply incised by channels 2: Shallow rills2: Shallow rills 3: Smooth at multi-meter scale3: Smooth at multi-meter scale

ApronApron 1: Fan-like, distributary channels and hummocky 1: Fan-like, distributary channels and hummocky

topographytopography 2: Intermediate2: Intermediate 3: Smooth at multi-meter scale3: Smooth at multi-meter scale

• Bimodal nature of apron slopes:• 8-18 degrees where aprons fan-like and mid-slopes are deeply channeled indicates flows of enhanced mobility.• 25-40 degrees where slopes are modestly rilled or talus-like (~ angle of repose) – normal dynamic angle of repose.

Measurements on Measurements on Meandering ChannelsMeandering Channels

9 Stereo HiRISE pairs have well-developed 9 Stereo HiRISE pairs have well-developed meandering, slightly sinuous mid-slope meandering, slightly sinuous mid-slope channels.channels.

Measurements on 23 gulliesMeasurements on 23 gullies Gully gradient, Gully gradient, SS Gully wavelength, Gully wavelength, λλ Channel width, Channel width, WW

The meander wavelength to width ratio averages about 10.2, similar to terrestrial meandering channels

Estimating Flow Properties Estimating Flow Properties from Channel Dimensionsfrom Channel Dimensions

For terrestrial rivers both channel width, For terrestrial rivers both channel width, WW, and wavelength,, and wavelength, λλ, have been , have been correlated with bankfull discharges, correlated with bankfull discharges, QQ..

These provide estimates for Martian These provide estimates for Martian gullies averaging about 15.9 and 8.5 mgullies averaging about 15.9 and 8.5 m33/s /s respectively.respectively.

Due to steep slopes and extrapolation Due to steep slopes and extrapolation from larger terrestrial rivers, these from larger terrestrial rivers, these estimates may be strongly biased.estimates may be strongly biased.

Estimating flow properties Estimating flow properties from meander wavelengthfrom meander wavelength

By combining the curvature-driven By combining the curvature-driven meander theory of meander theory of Ikeda, Parker, and Ikeda, Parker, and Sawai [1981] Sawai [1981] with the assumption that flow with the assumption that flow is uniform and steady, flow velocity, is uniform and steady, flow velocity, VV (m/s) can be estimated from meander (m/s) can be estimated from meander wavelength, wavelength, λλ (m), and channel gradient, (m), and channel gradient, SS::

VV22≈1.2 ≈1.2 λλ S S

V(m/s)

D(m)&

Q (m3/s)%

Average 3.8 0.71 27.9

Minimum 2.1 0.19 0.62

Maximum 6.7 1.92 198.2

&: D=Depth; Assumes W/D=8 %: Q=WDV

These estimated flow properties are in the range of terrestrial debris flows in alpine environments or water floods in steep channels

Gully EvolutionGully Evolution Images indicate that the gully systems have originated Images indicate that the gully systems have originated

through multiple, perhaps hundreds of, flow events.through multiple, perhaps hundreds of, flow events. These flow events are not small trickling flows, but These flow events are not small trickling flows, but

moderate to large flows that have low enough viscosity moderate to large flows that have low enough viscosity to generate secondary flows [required to develop to generate secondary flows [required to develop meanders] meanders]

The along-flow length of the formative events must have The along-flow length of the formative events must have been at least one meander wavelength (~100 m) to been at least one meander wavelength (~100 m) to develop secondary flows, suggesting total flow volumes develop secondary flows, suggesting total flow volumes exceeding 800 mexceeding 800 m3 3 per event.per event.

Gullies occur on steep slopes, but not all steep slopes Gullies occur on steep slopes, but not all steep slopes have gullies.have gullies.

Some gullies have been active to the present, but others Some gullies have been active to the present, but others have been modified by wind and other processes, have been modified by wind and other processes, suggesting recent inactivity.suggesting recent inactivity.

Some gullies show evidence of multiple stages of Some gullies show evidence of multiple stages of activity, with intervening episodes of mantling by pasted-activity, with intervening episodes of mantling by pasted-on terrain and probably degradation by mass wasting.on terrain and probably degradation by mass wasting.

Key ObservationsKey Observations

Although deeper alcoves expose abundant Although deeper alcoves expose abundant boulders, aprons are dominated by sub-boulders, aprons are dominated by sub-meter-scale materials, possibly including meter-scale materials, possibly including dust and/or volatiles components.dust and/or volatiles components.

The pasted-on deposits could be a The pasted-on deposits could be a dominant component of the better-dominant component of the better-developed aprons.developed aprons.

Detachment MechanismsDetachment Mechanisms The large size of channels on the alcoves and aprons The large size of channels on the alcoves and aprons

suggests the landform-creating events are large and suggests the landform-creating events are large and energetic. Slow erosion by, e.g., groundwater seeps or slow energetic. Slow erosion by, e.g., groundwater seeps or slow snowmelt runoff is not realistic, although these may have snowmelt runoff is not realistic, although these may have served as triggering events and supplied “lubrication”.served as triggering events and supplied “lubrication”.

The main issue in the alcoves is one of detachment. Some The main issue in the alcoves is one of detachment. Some possible mechanisms:possible mechanisms: MarsquakesMarsquakes Decrease in resistance of surface materials by physical Decrease in resistance of surface materials by physical

weathering – probably an ultimate, not proximal, causeweathering – probably an ultimate, not proximal, cause Hydrostatic pressure (groundwater), possibly beneath Hydrostatic pressure (groundwater), possibly beneath

frozen surface layers.frozen surface layers. Increase in stresses due to saturation of surface layersIncrease in stresses due to saturation of surface layers Increase in surface stresses due to accumulation and/or Increase in surface stresses due to accumulation and/or

failure of surface accumulations of COfailure of surface accumulations of CO22 or H or H22O snowsO snows

Mechanisms to Maintain Mechanisms to Maintain FlowFlow

One possibility is just dry mass wasting (One possibility is just dry mass wasting (Tremain, 2003Tremain, 2003). ). In Valles Marineris slopes and on terrestrial scarps dry In Valles Marineris slopes and on terrestrial scarps dry debris avalanches typically do not create channels or debris avalanches typically do not create channels or pronounced lobate deposits. The aprons do not pronounced lobate deposits. The aprons do not resemble the large landslides with excess mobility seen resemble the large landslides with excess mobility seen in Valles Marineris.in Valles Marineris.

It has been suggested that the lower gravity encourages It has been suggested that the lower gravity encourages a more fluid runout in dry granular material (a more fluid runout in dry granular material (Shinbrot et Shinbrot et al, 2004al, 2004). ).

If the debris is water-saturated, the absence of leveed If the debris is water-saturated, the absence of leveed channels, leveed lobate aprons, and runout flows, channels, leveed lobate aprons, and runout flows, suggests that the deposits must be coarse gravel suggests that the deposits must be coarse gravel allowing sieve deposition. The abundance of pasted-on allowing sieve deposition. The abundance of pasted-on terrain and atmospheric dust deposition on the Martian terrain and atmospheric dust deposition on the Martian slopes may argue against this scenario.slopes may argue against this scenario.

More Possible Flow More Possible Flow MechanismsMechanisms

A question is what effect the lower gravity has on flows. A question is what effect the lower gravity has on flows. Flows will be slower, and presumably deeper. Influence Flows will be slower, and presumably deeper. Influence on friction?on friction?

What effect does the lower atmospheric pressure (~5-10 What effect does the lower atmospheric pressure (~5-10 mb) have on flow viscosity? It decreases intergranular mb) have on flow viscosity? It decreases intergranular fluid viscosity but may increase particle frictional fluid viscosity but may increase particle frictional interactions by reducing collisional buffering.interactions by reducing collisional buffering.

COCO22 (or less likely H (or less likely H22O) vaporization might increase pore O) vaporization might increase pore pressure in the debris flow. The latent heat of pressure in the debris flow. The latent heat of vaporization of COvaporization of CO22 is about the same as that of H is about the same as that of H22O ice O ice melting.melting.

What effect might silt-to-sand size material have on flow What effect might silt-to-sand size material have on flow mobitity? – Talcum-powder flows?mobitity? – Talcum-powder flows?

Terrestrial Fans that may Terrestrial Fans that may bebe

Good AnalogsGood Analogs Fan head trenches and channels without Fan head trenches and channels without

marked leveesmarked levees Hummocky fan surfaces without well Hummocky fan surfaces without well

developed leveed lobesdeveloped leveed lobes No continuing fluvial channelsNo continuing fluvial channels

Sierra Nevada, U.S.

New Zealand (right side)

Death Valley, U.S.

New Zealand

Conclusions about Terrestrial Conclusions about Terrestrial AnalogsAnalogs

In terrestrial analog sites, e.g., Iceland and New In terrestrial analog sites, e.g., Iceland and New Zealand, gullies and aprons exhibit a wide range Zealand, gullies and aprons exhibit a wide range of mobility, ranging from dry rock avalanche to of mobility, ranging from dry rock avalanche to strongly fluvial. This probably in part reflects strongly fluvial. This probably in part reflects variability in precipitation intensity and duration.variability in precipitation intensity and duration.

Martian gullies did not form from intense Martian gullies did not form from intense precipitation, whereas that is an important factor, precipitation, whereas that is an important factor, at least episodically, for almost all terrestrial at least episodically, for almost all terrestrial analogs.analogs.

The Martian gullies show much less variability. The Martian gullies show much less variability. Although there are many dry avalanche “spur Although there are many dry avalanche “spur and gully” scarps, the alcove-channel-apron and gully” scarps, the alcove-channel-apron sequence at the more mobile end of the sequence at the more mobile end of the spectrum is very similar in many settings. This spectrum is very similar in many settings. This suggests a well defined set of processes forming suggests a well defined set of processes forming Martian gullies.Martian gullies.

The EndThe End

Even the mechanisms acting on potential Even the mechanisms acting on potential terrestrial analog slopes are poorly terrestrial analog slopes are poorly characterized because of site remoteness, characterized because of site remoteness, stochastic nature of th flow events, and the high stochastic nature of th flow events, and the high intensity of the flows.intensity of the flows.

Geomorphologists studying terrestrial gullies feel Geomorphologists studying terrestrial gullies feel that the term “gully” is inappropriate for the that the term “gully” is inappropriate for the incised martian slopes because of their incised martian slopes because of their steepness and origin lacking rainfall runoff.steepness and origin lacking rainfall runoff.