Inferred Flow Properties in Modern Mars Gullies

1. Inferred Flow Properties in Modern Mars Gullies Alan D. Howard@ Department of Environmental Sciences University of Virginia, USA @ With help from: Jeffrey Moore, William Dietrich, and Taylor Perron Brown University, 2008

2. 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.

3. 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.

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

5. 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.

6. In general alcoves reveal coarse bedrock.

7. Some Observations about Alcoves • 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. • Alcoves are a good place for seasonal or longer-term CO2 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.

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

9. 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.

10. APRONS Being a depositional environment, the aprons probably can afford the most direct clues to gully origin and transport mechanics. The middle portion of aprons typically displays numerous hummocks that are lenticular or downstream-bifurcating. Levees are low or absent.

11. Distal Apron Terminations • 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. • Flows may terminate because: • Decrease of gradient reduces stresses below flowing material yield strength • Water freezes or boils away.

13. YOUNG 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 OLD

14. Aprons • 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. • 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. • 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.

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

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

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

19. 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

20. Characterizing Gullied Slopes • Score aprons and mid-slopes separately and add scores. • Mid-slope: • 1: Deeply incised by channels • 2: Shallow rills • 3: Smooth at multi-meter scale • Apron • 1: Fan-like, distributary channels and hummocky topography • 2: Intermediate • 3: Smooth at multi-meter scale

21. 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.

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

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

24. Estimating Flow Properties from Channel Dimensions • 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 m3/s respectively. • Due to steep slopes and extrapolation from larger terrestrial rivers, these estimates may be strongly biased.

25. 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: V2≈1.2 λ S

26. &: D=Depth; AssumesW/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

27. Gully Evolution • 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] • 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 m3 per event. • 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 show evidence of multiple stages of activity, with intervening episodes of mantling by pasted-on terrain and probably degradation by mass wasting.

28. Key Observations • 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.

29. 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 main issue in the alcoves is one of detachment. Some possible mechanisms: • Marsquakes • Decrease in resistance of surface materials by physical weathering – probably an ultimate, not proximal, cause • Hydrostatic pressure (groundwater), possibly beneath frozen 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 CO2 or H2O snows

30. 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. • 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.

31. 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? • 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. • CO2 (or less likely H2O) vaporization might increase pore pressure in the debris flow. The latent heat of vaporization of CO2 is about the same as that of H2O ice melting. • What effect might silt-to-sand size material have on flow mobitity? – Talcum-powder flows?

32. Terrestrial Fans that may beGood Analogs • Fan head trenches and channels without marked levees • Hummocky fan surfaces without well developed leveed lobes • No continuing fluvial channels

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

34. Death Valley, U.S.

35. New Zealand

36. 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. • 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.

37. 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. • 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.