Inferred Flow Properties in Modern Mars Gullies Brown University, 2008 Alan D. Department of Environmental Sciences University of Virginia, 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 Alcoves Deeper alcoves expose crater (or scarp) wall bedrock – usually highly fractured. Shallower alcoves can be entirely in “pasted-on” terrain. Deeper alcoves expose crater (or scarp) wall bedrock – usually highly fractured. Shallower alcoves can be entirely in “pasted-on” terrain. Where bedrock exposed, lots of >1m scale boulders on alcove surfaces. Where bedrock exposed, lots of >1m scale boulders on alcove surfaces. Alcoves are a good place for seasonal or longer- term CO 2 snow accumulation. Steep slopes may encourage avalanching of frost accumulation. Alcoves are a good place for seasonal or longer- term CO 2 snow accumulation. Steep slopes may encourage avalanching of frost accumulation. The alcoves may be the least diagnostic feature of the gullies, because almost any triggering mechanism on such steep slopes will generate flows capable of scouring the fragmented bedrock and overlying pasted-on terrain. The alcoves may be the least diagnostic feature of the gullies, because almost any triggering mechanism on such steep slopes will generate flows capable of scouring the fragmented 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 depositional environment, the aprons probably can afford the most direct clues to gully origin and transport mechanics.

Distal Apron Terminations Perhaps the most diagnostic gully landform is the distal end of the aprons. Perhaps the most diagnostic gully landform is the distal end of the aprons. Aprons typically end abruptly at the break of slope at the crater floor or scarp foot. Channels or deposits usually do not continue beyond the slope break. This suggests flows of enhanced but modest mobility. Aprons typically end abruptly at the break of slope at the crater floor or scarp foot. Channels or deposits usually do not continue beyond the slope break. This suggests flows of enhanced but modest mobility. Flows may terminate because: Flows may terminate because: Decrease of gradient reduces stresses below flowing material yield strength Decrease of gradient reduces stresses below flowing material 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

Aprons Almost all Aprons have surfaces with few visible boulders as compared to the very bouldery floors of Alcoves. Almost all Aprons have surfaces with few visible boulders as compared to the very bouldery floors of Alcoves. This suggests that the dominant source of sediment on Aprons is either the pasted-on terrain and/or wind-blown sediment accumulating in Alcoves which is subsequently remobilized. This suggests that the dominant source of sediment on Aprons is either the pasted-on terrain and/or wind-blown sediment accumulating in Alcoves which is subsequently remobilized. The relatively fine-grained nature of the Aprons may contribute to its characteristic surface morphology. The relatively fine-grained nature of the Aprons may contribute to its characteristic surface morphology. Perhaps the most important characteristic of the aprons is the rarity of strongly peaked leveed channels and leveed lobes in comparison to many potential terrestrial analogs. Perhaps the most important characteristic of the aprons is the rarity of strongly peaked leveed channels and leveed lobes in comparison to many potential terrestrial analogs. If volatiles are a dominant component of the formative flows, however, the present morphology of the aprons (e.g. hummocky and lacking high levees) may not reflect the morphology immediately after emplacement because of subsequent volatile loss. If volatiles are a dominant component of the formative flows, however, the present morphology of the aprons (e.g. hummocky and lacking high levees) may not reflect the morphology immediately after emplacement because of subsequent volatile loss.

Pasted-on terrain Gully channels near the Apex often display braided appearance with lenticular bars

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

Measurements of Gully Properties As of December 2007 ~30 HiRISE stereo pairs – considerably more now. As of December 2007 ~30 HiRISE stereo pairs – considerably more now. 17 Have good topographic control from MOLA tracks 17 Have good topographic control from MOLA tracks One set of measurements concerns Apron gradients One set of measurements concerns Apron gradients

HiRISE PSP 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 Slopes Score aprons and mid-slopes separately and add scores. Score aprons and mid-slopes separately and add scores. Mid-slope: Mid-slope: 1: Deeply incised by channels 1: Deeply incised by channels 2: Shallow rills 2: Shallow rills 3: Smooth at multi-meter scale 3: Smooth at multi-meter scale Apron Apron 1: Fan-like, distributary channels and hummocky topography 1: Fan-like, distributary channels and hummocky topography 2: Intermediate 2: Intermediate 3: Smooth at multi-meter scale 3: 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 degrees where slopes are modestly rilled or talus-like (~ angle of repose) – normal dynamic angle of repose.

Measurements on Meandering Channels 9 Stereo HiRISE pairs have well-developed meandering, slightly sinuous mid-slope channels. 9 Stereo HiRISE pairs have well-developed meandering, slightly sinuous mid-slope channels. Measurements on 23 gullies Measurements on 23 gullies Gully gradient, S Gully gradient, S Gully wavelength, λ Gully wavelength, λ Channel width, W Channel width, W

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

Estimating Flow Properties from Channel Dimensions For terrestrial rivers both channel width, W, and wavelength, λ, have been correlated with bankfull discharges, Q. For terrestrial rivers both channel width, W, and wavelength, λ, have been correlated with bankfull discharges, Q. These provide estimates for Martian gullies averaging about 15.9 and 8.5 m 3 /s respectively. These provide estimates for Martian gullies averaging about 15.9 and 8.5 m 3 /s respectively. Due to steep slopes and extrapolation from larger terrestrial rivers, these estimates may be strongly biased. Due to steep slopes and extrapolation from larger terrestrial rivers, these estimates may be strongly biased.

Estimating flow properties from meander wavelength By combining the curvature-driven meander theory of Ikeda, Parker, and Sawai [1981] with the assumption that flow is uniform and steady, flow velocity, V (m/s) can be estimated from meander wavelength, λ (m), and channel gradient, S: By combining the curvature-driven meander theory of Ikeda, Parker, and Sawai [1981] with the assumption that flow is uniform and steady, flow velocity, V (m/s) can be estimated from meander wavelength, λ (m), and channel gradient, S: V 2 ≈1.2 λ S

V (m/s) D (m) & Q (m 3 /s) % Average Minimum Maximum & : 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 Evolution Gully Evolution Images indicate that the gully systems have originated through multiple, perhaps hundreds of, flow events. Images indicate that the gully systems have originated through multiple, perhaps hundreds of, flow events. These flow events are not small trickling flows, but moderate to large flows that have low enough viscosity to generate secondary flows [required to develop meanders] These flow events are not small trickling flows, but moderate to large flows that have low enough viscosity to generate secondary flows [required to develop meanders] The along-flow length of the formative events must have been at least one meander wavelength (~100 m) to develop secondary flows, suggesting total flow volumes exceeding 800 m 3 per event. The along-flow length of the formative events must have been at least one meander wavelength (~100 m) to develop secondary flows, suggesting total flow volumes exceeding 800 m 3 per event. Gullies occur on steep slopes, but not all steep slopes have gullies. Gullies occur on steep slopes, but not all steep slopes have gullies. Some gullies have been active to the present, but others have been modified by wind and other processes, suggesting recent inactivity. Some gullies have been active to the present, but others have been modified by wind and other processes, suggesting recent inactivity. Some gullies show evidence of multiple stages of activity, with intervening episodes of mantling by pasted- on terrain and probably degradation by mass wasting. Some gullies show evidence of multiple stages of activity, with intervening episodes of mantling by pasted- on terrain and probably degradation by mass wasting.

Key Observations Although deeper alcoves expose abundant boulders, aprons are dominated by sub- meter-scale materials, possibly including dust and/or volatiles components. Although deeper alcoves expose abundant boulders, aprons are dominated by sub- meter-scale materials, possibly including dust and/or volatiles components. The pasted-on deposits could be a dominant component of the better- developed aprons. The pasted-on deposits could be a dominant component of the better- developed aprons.

Detachment Mechanisms The large size of channels on the alcoves and aprons suggests the landform-creating events are large and energetic. Slow erosion by, e.g., groundwater seeps or slow snowmelt runoff is not realistic, although these may have served as triggering events and supplied “lubrication”. The large size of channels on the alcoves and aprons suggests the landform-creating events are large and energetic. Slow erosion by, e.g., groundwater seeps or slow snowmelt runoff is not realistic, although these may have served as triggering events and supplied “lubrication”. The main issue in the alcoves is one of detachment. Some possible mechanisms: The main issue in the alcoves is one of detachment. Some possible mechanisms: Marsquakes Marsquakes Decrease in resistance of surface materials by physical weathering – probably an ultimate, not proximal, cause Decrease in resistance of surface materials by physical weathering – probably an ultimate, not proximal, cause Hydrostatic pressure (groundwater), possibly beneath frozen surface layers. Hydrostatic pressure (groundwater), possibly beneath frozen surface layers. Increase in stresses due to saturation of surface layers Increase in stresses due to saturation of surface layers Increase in surface stresses due to accumulation and/or failure of surface accumulations of CO 2 or H 2 O snows Increase in surface stresses due to accumulation and/or failure of surface accumulations of CO 2 or H 2 O snows

Mechanisms to Maintain Flow One possibility is just dry mass wasting (Tremain, 2003). In Valles Marineris slopes and on terrestrial scarps dry debris avalanches typically do not create channels or pronounced lobate deposits. The aprons do not resemble the large landslides with excess mobility seen in Valles Marineris. One possibility is just dry mass wasting (Tremain, 2003). In Valles Marineris slopes and on terrestrial scarps dry debris avalanches typically do not create channels or pronounced lobate deposits. The aprons do not resemble the large landslides with excess mobility seen in Valles Marineris. It has been suggested that the lower gravity encourages a more fluid runout in dry granular material (Shinbrot et al, 2004). It has been suggested that the lower gravity encourages a more fluid runout in dry granular material (Shinbrot et al, 2004). If the debris is water-saturated, the absence of leveed channels, leveed lobate aprons, and runout flows, suggests that the deposits must be coarse gravel allowing sieve deposition. The abundance of pasted-on terrain and atmospheric dust deposition on the Martian slopes may argue against this scenario. If the debris is water-saturated, the absence of leveed channels, leveed lobate aprons, and runout flows, suggests that the deposits must be coarse gravel allowing sieve deposition. The abundance of pasted-on terrain and atmospheric dust deposition on the Martian slopes may argue against this scenario.

More Possible Flow Mechanisms A question is what effect the lower gravity has on flows. Flows will be slower, and presumably deeper. Influence on friction? A question is what effect the lower gravity has on flows. Flows will be slower, and presumably deeper. Influence on friction? What effect does the lower atmospheric pressure (~5-10 mb) have on flow viscosity? It decreases intergranular fluid viscosity but may increase particle frictional interactions by reducing collisional buffering. What effect does the lower atmospheric pressure (~5-10 mb) have on flow viscosity? It decreases intergranular fluid viscosity but may increase particle frictional interactions by reducing collisional buffering. CO 2 (or less likely H 2 O) vaporization might increase pore pressure in the debris flow. The latent heat of vaporization of CO 2 is about the same as that of H 2 O ice melting. CO 2 (or less likely H 2 O) vaporization might increase pore pressure in the debris flow. The latent heat of vaporization of CO 2 is about the same as that of H 2 O ice melting. What effect might silt-to-sand size material have on flow mobitity? – Talcum-powder flows? What effect might silt-to-sand size material have on flow mobitity? – Talcum-powder flows?

Terrestrial Fans that may be Good Analogs Fan head trenches and channels without marked levees Fan head trenches and channels without marked levees Hummocky fan surfaces without well developed leveed lobes Hummocky fan surfaces without well developed leveed lobes No continuing fluvial channels No continuing fluvial channels

Sierra Nevada, U.S. New Zealand (right side)

Death Valley, U.S.

New Zealand

Conclusions about Terrestrial Analogs In terrestrial analog sites, e.g., Iceland and New Zealand, gullies and aprons exhibit a wide range of mobility, ranging from dry rock avalanche to strongly fluvial. This probably in part reflects variability in precipitation intensity and duration. In terrestrial analog sites, e.g., Iceland and New Zealand, gullies and aprons exhibit a wide range of mobility, ranging from dry rock avalanche to strongly fluvial. This probably in part reflects variability in precipitation intensity and duration. Martian gullies did not form from intense precipitation, whereas that is an important factor, at least episodically, for almost all terrestrial analogs. Martian gullies did not form from intense precipitation, whereas that is an important factor, at least episodically, for almost all terrestrial analogs. The Martian gullies show much less variability. Although there are many dry avalanche “spur and gully” scarps, the alcove-channel-apron sequence at the more mobile end of the spectrum is very similar in many settings. This suggests a well defined set of processes forming Martian gullies. The Martian gullies show much less variability. Although there are many dry avalanche “spur and gully” scarps, the alcove-channel-apron sequence at the more mobile end of the spectrum is very similar in many settings. This suggests a well defined set of processes forming Martian gullies.

The End Even the mechanisms acting on potential terrestrial analog slopes are poorly characterized because of site remoteness, stochastic nature of th flow events, and the high intensity of the flows. Even the mechanisms acting on potential terrestrial analog slopes are poorly characterized because of site remoteness, stochastic nature of th flow events, and the high intensity of the flows. Geomorphologists studying terrestrial gullies feel that the term “gully” is inappropriate for the incised martian slopes because of their steepness and origin lacking rainfall runoff. Geomorphologists studying terrestrial gullies feel that the term “gully” is inappropriate for the incised martian slopes because of their steepness and origin lacking rainfall runoff.