Dry granular flows can generate surface features resembling those seen in Martian gullies Troy Shinbrot*†, N.-H. Duong‡, L. Kwan‡, and M. M. Alvarez§ Departments of *Biomedical Engineering and ‡Chemical and Biochemical Engineering, Rutgers University, Piscataway, NJ 08854; and §Instituto Tecnologico y de Estudios Superiores de Monterrey, Monterrey, 52926, Mexico Edited by Henry J. Melosh, University of Arizona, Tucson, AZ, and approved April 7, 2004 (received for review December 11, 2003)
Over the past decade or more, contradictory evidence of Martian climate, indicating that surface temperatures seldom if ever approach the melting point of water at midlatitudes, and geomorphic features, consistent with liquid flows at these same latitudes, have proven difficult to reconcile. In this article, we demonstrate that several features of liquid-erosional flows can be produced by dry granular materials when individual particle settling is slower than characteristic debris flow speeds. Since the gravitational acceleration on Mars is about one-third that on Earth, and since particle settling speeds scale with gravity, we propose that some (although perhaps not all) Martian geomorphological features attributed to liquid flows may in fact be associated with dry granular flows in the presence of reduced gravity.
A
number of recent studies have investigated evidence for liquid water on Mars. Much of the available evidence has concerned recent, or even contemporary (1), geomorphological features, such as eroded channels and gullies that are common signatures of terrestrial water flow (2–4). This evidence is difficult to reconcile (5–7) with surface temperatures on Mars, which seldom exceed ⫺50°C at latitudes and locations where these features are often found (6, 8). This paradox persists despite recent Mars Rover data supporting the case for ancient surface water. Several authors have proposed possible resolutions to this contradiction (9–12), notably a recent analysis suggesting that some liquid-like flow features in Martian gullies may instead be associated with dry granular flow (1). In this article, we observe that gravity on Mars is 38% that on Earth (13), and behaviors of dry grains at reduced gravitational acceleration have never, to our knowledge, been catalogued. As we will show, reduced gravity has the effect of prolonging fluidization of particle flows by decreasing particle settling speeds as compared with debris flow speeds. We present data demonstrating that many features that have been attributed to liquid flow in Martian gullies can indeed be reproduced in terrestrial laboratory experiments designed to mimic reduced gravity. Our point of departure is the observation that the speed at which small dry particles (14) settle in a gas can be approximated by the Stokes settling relation: Vs ⫽
共s ⫺ g兲d2g , 18g
[1]
whereas the terminal speed that large debris flows can reach while flowing through a gas is bounded by (15): Vt ⫽
冑
4Dsg . 3Csg
[2]
Here g and g are the gas density and viscosity, d, s, and Cs are characteristic particle diameter, density, and drag coefficient, g is gravitational acceleration, and D gives the scale of the debris flow (16). These relations are only correct to the lowest order; however, they illustrate that, all other things being equal, particle settling speeds go as g, whereas debris flow speeds go as 公g. We 8542– 8546 兩 PNAS 兩 June 8, 2004 兩 vol. 101 兩 no. 23
Fig. 1. Liquid-like flow patterns in dry grains under conditions of reduced settling-to-flow speed ratio. (a) Features found in laboratory flows by using lightweight, hollow beads immediately after an initial collapse, exhibiting liquid-like behaviors such as rivulets near the steep upstream edge (black arrow), narrowed incipient channel (white arrows), and reflected waves (emanating from white arrows). (b) Later development of waves from a separate experiment. (c) Superficially similar wave-like dunes downhill from central peaks (top of snapshot) of Hale crater, Mars.¶
stress that nothing is profound about the difference between these relations, which simply reflect the fact that particles, being small, settle at a low Reynold’s number, whereas debris flows, being large, travel at a high Reynold’s number. Because of this difference, however, it is possible that Martian gravity being about one-third of Earth’s could permit particles on Mars to settle more slowly, and therefore remain fluidized longer, than on Earth. Indeed, arguably particle fluidization on Mars may be further amplified both by wind (17) and by the fact that the atmospheric density, g, is two orders of magnitude smaller on Mars than on Earth (18). This raises the possibility, analyzed in subsequent sections, that grains on Mars may collectively flow more rapidly than on Earth while at the same time they individually settle more slowly. Since particle flows persist until individual particles settle against one another (19), it follows that reducing the ratio of settling to flow speeds, Rst ⫽ Vs兾Vt, can produce distinctly liquid-like appearances (Fig. 1). As a first example, in Fig. 1 a and b we show low-density, hollow ceramic beads (PQ Corp., Augusta, GA), with bulk density 1兾15 that of solid glass and diameters ranging from 4 to 90 m. The beads are deposited into a simple acrylic box 40 cm This paper was submitted directly (Track II) to the PNAS office. Abbreviation: NASA, National Aeronautics and Space Administration. †To
whom correspondence should be addressed. E-mail:
[email protected].
¶Malin,
M. C., Edgett, K. S., Carr, M. H., Danielson, G. E., Davies, M. E., Hartmann, W. K., Ingersoll, A. P., James, P. B., Masursky, H., McEwen, A. S., et al. (2000) Autumn Afternoon in Hale Crater, NASA’s Planetary Photojournal MOC2–257, November 17, 2000. Available at www.msss.com兾mars㛭images兾moc兾nov㛭00㛭hale.
© 2004 by The National Academy of Sciences of the USA
www.pnas.org兾cgi兾doi兾10.1073兾pnas.0308251101
grain density does carry the potential to generate spurious effects associated with complex air-particle interactions during settling (21, 22), and as we discuss shortly, some of the effects that we report are indeed significantly air-mediated. Although no earth-bound experiment can fully capture Martian conditions, we find aspects of numerous different morphologies that have been attributed to liquid-borne flow in Martian gullies.
Fig. 2. Alcoves. (a) Rounded hillocks separating flowing gullies appear immediately after the initial collapse of a flat surface of a bed of hollow beads. (b) Alcove features in Martian polar pits cited elsewhere (see footnote ‡‡ and ref. 5) as evidence for surface water flow. (c) Laboratory alcoves with characteristic ‘‘spur-and-gully’’ structure seen also in Martian landforms (e.g., Fig. 3b). (d) A filled alcove from Dao Vallis on Mars discussed elsewhere†† (8): this alcove is filled with subsequent deposits, but has a similar outline to that shown in c. (e) Upstream precipice of gullies in laboratory with cohesive glass beads. To produce the jagged upstream feature shown, we perform the same gradual tipping experiment as in a, but we use solid-glass particles sieved between 45 and 90 m that have been exposed to high humidity (90% RH for several weeks), thus producing aggregates held together by liquid bridges (25). ( f) Detail of upstream edge of channel shown in Fig. 3c. This snapshot shows an eroded gap between upper container wall and granular bed (see text): gap between arrows is ⬇3 mm wide.
on a side, and the bed of beads is then tipped until the beads begin to flow. The low mass of the beads slows settling in air, producing visible waves, sloshing, and reflection from boundaries that strongly resemble liquid flows (20) (movies of these flows and additional data are published as supporting information on the PNAS web site). Once the particles come to rest, this liquid-like flow leaves signatures in the frozen state; thus, waves that reflect from the narrowed region of the incipient channel (Fig. 1a, white arrows) are visible, as are fine rivulets upstream (black arrow). As time progresses, the downstream waves advance, and the upstream rivulets accentuate to form mountainous features as shown in Fig. 1b (see also Figs. 2c and 3a). By comparison, in Fig. 1c, we display wave-like (20) sand dunes in the Hale crater on Mars. Like our experiments, these dunes emanate downhill from steep peaks: based on estimates using shadow length and time of day,储 the peak visible at the top of this figure is ⬎150 m above the crater floor, which is visible at the bottom of the figure. Similar fluid-like features are seen in Newton Basin in Sirenum Terra,** again traveling downhill from steep peaks. Evidently, dry grains can flow in a liquid-like manner when their settling speed is sufficiently reduced—in our experiments by a simple reduction in grain density. We caution that reducing 储The
largest of the central peaks has been estimated (www.msss.com兾mars㛭images兾moc兾 nov㛭00㛭hale) to be 630 feet from the crater floor; the peak shown in Fig. 1c casts a shadow one quarter as long. This estimate neglects the height and inclination of the surface on which the shadow falls, but sets a reasonable approximation for the height in the figure.
**Malin, M. C., Edgett, K. S., Carr, M. H, Danielson, G. E., Davies, M. E., Hartmann, W. K., Ingersoll, A. P., James, P. B., Masursky, H., McEwen, A. S., et al. (2002) Gullies in crater at 42.4°S, 158.2°W, NASA’s Planetary Photojournal MOC2–320, October 7, 2002. Available at www.msss.com兾mars㛭images兾moc兾e7㛭e12㛭captioned㛭rel.
Shinbrot et al.
Upstream Features In Fig. 2, we compare some upstream features of laboratory experiments with Martian gully features. In Fig. 2a, we show a laboratory pattern of gullies in which grains flow separated by static hillocks. In the experiment shown, the hollow beads of Fig. 1 are deposited in a simple rectangular box that is tipped until flow begins; the originally flat bed surface is visible at the top of the figure. By comparison, in Fig. 2b, we show a photograph‡‡ of Martian gullies (5), separated by superficially similar hillocks. As time progresses in our experiments, the hillocks erode, giving rise to a characteristic ‘‘spur-and-gully’’ morphology, shown in Fig. 2c. Comparable spur-and-gully formations are seen in innumerable locations both in terrestrial and in Martian geology. For example, in Fig. 3b we see a portion of such a formation on Mars, and in Fig. 2d, we show a Martian alcove that has been filled with subsequent deposits, leaving an outline that suggests an originally scalloped alcove similar in shape to that seen in Fig. 2c. Since a variety of particle types are doubtless present at the Martian surface (14), we performed separate experiments using round, angular, and weakly cohesive grains. We found that, provided the grains remain sufficiently small (ⱕ60 m), many of the features that we describe for hollow beads were retained, but channels became irregular and sinuous for angular and weakly cohesive grains [termed Geldard type C in the powder literature (23)]. Tipping a bed of these grains causes aggregates to dislodge in chunks (24), leaving jagged upstream outcroppings, such as in Fig. 2e (possibly comparable with Martian outcroppings as in Fig. 3d). Midstream Features As with upstream features, midstream channels can develop a rich variety of patterns, depending on elapsed time (compare Fig. 1 a and b), particle properties (Fig. 2 e vs. f ), and bed preparation. The most common midstream feature seen is a broad channel emanating from a spur-and-gully, as shown in Fig. 3a alongside a comparable Martian channel in Fig. 3b. Such channels are often straight, but can be sinuous as in this example. In this experiment as in those shown in Fig. 1, a bed of hollow ††Malin, M. C, Edgett, K. S, Davis, S. D, Caplinger, M. A, Jensen, E., Supulver, K. D, Sandoval,
J., Posiolova, L. & Zimdar, R. (2000) MOC image M03– 04950, Malin Space Science Systems Mars Orbiter Camera Image Gallery, October 16, 2000. Available at www.msss.com兾 moc㛭gallery兾ab1㛭m04兾images兾M0304950.html. ‡‡Malin,
M. C., Edgett, K. S., Carr, M. H., Danielson, G. E., Davies, M. E., Hartmann, W. K., Ingersoll, A. P., James, P. B., Masursky, H., McEwen, A. S., et al. (2000) Polar Pit Wall, NASA’s Planetary Photojournal MOC2–237, June 22, 2000. Available at www.msss.com兾 mars㛭images兾moc兾june2000兾sp㛭pit.
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Morphological Comparisons To compare laboratory and Martian morphologies, we note that Martian geological features that have been identified as indicating liquid surface water on Mars have been classified into three categories (5): upstream features (e.g., alcoves), midstream features (channels), and downstream features (aprons). In the next three sections, we briefly compare features of our terrestrial experiments in each of these flow regions in turn with Martian analogues. After these qualitative comparisons, we analyze the applicability of terrestrial, laboratory-scale, experiments for the simulation of Martian, geological-scale, systems.
Fig. 3. Channels. (a) Hollow spheres after prolonged flow, exhibiting a sinuous stream and multiply branched inlet patterns. (b) Mountainous gully in Candor Chasma on Mars.§§ (c) Narrow channel under conditions of increased bed cohesion described in text. (d) Channels in gullies described in previous studies of Martian surface water (6).¶¶
beads was aerated (by recently avalanching the grains) and then tipped rapidly to produce comparatively wide flows of beads (1–2 cm or 500–1,000 bead diameters across). To better understand how our laboratory granular flows carve channels and to explore a sampling of the spectrum of structures that can be produced by granular flows at reduced Rst, we performed variants of our flow experiments. Notably, by comparison with earlier results, in Fig. 3c we prepared the bed (details follow) so as to generate a well settled mass that was then tipped more slowly, producing the long narrow channels shown (about 20 diameters across) with steep upstream boundaries (magnified in Fig. 2f ) and conical downstream debris deposits (Fig. 4a). In detail, for Fig. 3c we first allowed the beads to settle overnight; this preparation is necessary because the small size and density of the beads implies that they will only very slowly settle and expel air from interstices. If we instead tilt a bed of beads that has been recently agitated, we obtain broad channels as in Fig. 1b. In Fig. 3c, after overnight settling, we slowly stirred the bed with a rod to break up aggregates, and then tipped the box onto a corner and gently tapped it to move the beads into the corner and to smooth the bed surface. Finally, we slowly tilted the box until flow began. We repeat the experiment by tapping and then tilting the box again. In Fig. 3d, we show comparable Martian gully channels discussed in investigations of the case for liquid water on Mars (1, 5). Downstream Features The final category of flow features that have been attributed to surface water flow on Mars is downstream depositional patterns, two of which are shown in Fig. 4. As we have mentioned, Fig. 4a displays a detail of the downstream conical pile of material from the channel of Fig. 3c. Alongside this pile, we show the conical (1, 5) deposit from the Martian channel of Fig. 3d, suggesting §§Malin, M. C, Edgett, K. S, Davis, S. D, Caplinger, M. A, Jensen, E., Supulver, K. D., Sandoval,
J., Posiolova, L. & Zimdar, R. (2000) MOC image M11– 02514 Malin Space Science Systems Mars Orbiter Camera Image Gallery, October 16, 2000. Available at www.msss.com兾 moc㛭gallery兾m07㛭m12兾images兾M11兾M1102514.html. ¶¶Malin, M. C., Edgett, K. S, Davis, S. D, Caplinger, M. A, Jensen, E., Supulver, K. D, Sandoval,
J., Posiolova, L. & Zimdar, R. (2000) MOC image M03– 02290 Malin Space Science Systems Mars Orbiter Camera Image Gallery, October 16, 2000. Available at www.msss.com兾 moc㛭gallery兾ab1㛭m04兾images兾M0302290.html.
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Fig. 4. Deposits, aprons, and swales. (a) Detail of conical depositional pile downstream of channel shown in Fig. 3c. (b) Detail of comparable depositional cone from Martian gully channel shown in Fig. 3d. (c) Depositional apron of hollow beads, showing a fine swale that formed by erosion from a fluidized stream. (d) Later evolution of downstream deposit, showing extensive variegated erosion. (e) Comparative variegated aprons downstream of channel described elsewhere (5).
that dry granular flows may account for simple depositional structures. More complicated features are also seen downstream of Martian gully streams, however; so in Fig. 4c we show a portion of an apron containing hollow beads at the downstream edge of a flowing region (the base surface of the tipped container is visible at the bottom of the snapshot). As indicated by the arrow, subsequent flows of beads cut a thin swale into the apron; similar swales develop into striated channels as shown in a later evolution of the hollow beads in Fig. 4d. Such multiply striated regions are not common in our experiments, but they can be found at the tail end of flowing regions as shown in the figure, especially when the flow encounters an obstacle: here, the vertical end wall of the container, which is located at the very bottom of this snapshot. By comparison, swales cut into aprons on Mars as identified by the arrow in Fig. 4e have been attributed to water flow (5). Analysis Evidently, reducing Rst by changing particle properties has the effect of producing remnant structures that are similar to some geomorphic features believed to imply water flows in Martian gullies. A unifying feature of all of the liquid-like flow patterns that we have seen in laboratory dry grains is that, for the considerations of the interplay between settling and flow speeds that we have summarized to come into play, particles need to become fluidized to start with. The conditions imposed by using hollow and small beads permit the grains to remain fluidized, but without a source of fluidized grains, the patterns that we have shown would not form or endure. Shinbrot et al.
Table 1. Data for settling comparisons using Eqs. 1 and 2 Flow type Terrestrial laboratory Terrestrial debris Martian debris
s
g
d
D
g
g
Cs
Rst ⫽ Vs兾Vt
0.15 2 2
1.3䡠10⫺3 1.3䡠10⫺3 1.9䡠10⫺5
0.002 0.2 0.2
1 104 104
980 980 360
1.8䡠10⫺4 1.8䡠10⫺4 1.8䡠10⫺4
100 100 100
0.005 2.6 0.2
In our experiments, we have found that fluidization of slowly settling particles can appear spontaneously in one of two ways. First, a region of steep slope may abruptly collapse, releasing enough energy to fluidize grains downstream. Such a case is seen in Fig. 2a, where downstream flows emanate from steep-walled upstream alcoves after the initial collapse of the bed. Fluid-like flows are readily observed to be generated after a region of the upstream bed collapses, producing an avalanche of rapidly flowing grains. Similarly, we find liquid-like flows typically originate in steep mountainous regions (1). An example of this is shown in Fig. 3a, where we show results of an experiment with hollow beads in which jagged surfaces, such as those shown previously in Fig. 1b, are permitted to mature through prolonged flow into sinuous and multiply branched channels. Alongside this snapshot from our laboratory, in Fig. 3b we display an actual mountainous region that supplies a Martian gully channel (5, 6). The scenario of dry granular mass flow initiated by an upstream collapse at a steep wall is consistent with available Mars data; in Martian gullies, it has been remarked that a universal feature associated with gully morphologies that indicate water flow is steep upstream walls (1); moreover, collapses that generate significant mass flows have been seen in recent Mars Express photographs (www.esa.int兾export兾externals兾images兾016090204㛭2-0037㛭02-6p.jpg). In our experiments [both the single collapse events (Fig. 2a) and prolonged flow (Figs. 2c and 3a)], the fluidized granular cascades that we observe leave upstream edges that are invariably steep and multifaceted. A second mechanism that spontaneously generates fluidization in our experiments appears when fluidized flow initiates at an already steep surface such as the container wall, where unobstructed air velocities are presumably highest relative to the substrate material. Such a situation is seen in the rivulets in Fig. 1a. Air flow near the container wall visibly erodes material, as evidenced by the fact that a substantial groove is produced between the wall and the granular bed: this groove is highlighted (arrows) in Fig. 2f, which presents an enlarged view of the upstream edge of the channel of Fig. 3c. The groove is smoothly polished and is observed to enlarge steadily over time through erosion. Evidently, either in flow initiated by a collapse or in steady flow originating near a smooth boundary, the resulting upstream structures exhibit steep walls similar to those that have been remarked (1) to be a signature of Martian gully flows. From our investigations of low Rst materials, it appears that steep upstream walls may be far from coincidental to Martian gullies, but rather may serve as a source of energized particles without which fluidization could not begin. Scaling Conditions and scales are enormously different between laboratory and Martian geological dimensions. Moreover, Martian Shinbrot et al.
geological substrates seem certain to be more robust than our simple compacted bed, and so the processes, whether water- or airborne, that could erode them are bound to be enormously slower and more incremental than in our model laboratory system. It is therefore surprising that Figs. 1–4 indicate that some geomorphological features associated with liquid flows may be duplicated on the laboratory scale with dry grains by suitably reducing Rst. Similar scaling arguments have been applied to extrapolate laboratory fluid experiments to geophysical scales (26) and to relate laboratory granular experiments to extraterrestrial craters (27). Even more germane, scaling using a ratio between settling and debris flow speeds has been reported in terrestrial particle-laden oceanic and volcanic flows (28, 29). In those systems, direct comparisons between laboratory and geological scales have been made, including identification of a transition between liquid-like, suspended, and solidified, sedimented, phases that is seen when settling and flow speeds become comparable. To assess the extent of applicability of our terrestrial experiments using reduced–density grains to Martian geological flows, in Table 1 we display values that appear in Eqs. 1 and 2 alongside the ratio Rst ⫽ Vs兾Vt for our laboratory experiments using (i) hollow beads, (ii) some terrestrial debris flows, and (iii) some Martian debris flows. We reiterate that Eqs. 1 and 2 are intended only as scaling relations, and the values provided in the table are only representative approximations. Nevertheless, the data illustrate that both our laboratory beads and Martian geological grains can settle more slowly than they flow (i.e., Rst ⬍ 1), whereas terrestrial geological grains typically do not (Rst ⬎1). In conclusion, several features in Martian gullies that are suggestive of liquid water flow have been revealed in past and ongoing Mars missions. We have not attempted to comprehensively duplicate all of these features in the laboratory; however, from our limited studies, it appears that some features that have been attributed to liquid flow may in fact also occur in dry grains under conditions that prolong fluidization, as can be anticipated under reduced gravity. In particular, when we reduce the settling ratio, Rst, by using low-density beads, we have found (i) that grains can flow, slosh, and reflect from boundaries in a liquidlike manner, (ii) that flowing grains can erode elongated channels, and (iii) that remnant structures left by these flows can include hillocks, steeply banked alcoves, sinuous streams, multiply branched inlets, depositional cones, and variegated aprons that resemble those reported in Martian gullies. The duplication of these features under dry conditions does not imply that liquid water does or does not exist in Martian gullies, nor does it speak to growing evidence of subsurface water, either liquid or solid (4, 33–36). In addition, in light of the magnificent images and data recently returned by National Aeronautics and Space AdminPNAS 兩 June 8, 2004 兩 vol. 101 兩 no. 23 兩 8545
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All units are cgs, and these data apply at above 5–10 Torr: below that value, air viscosity scales (15) with pressure2兾3, and settling speeds become ballistic (see Analysis). Moreover, these are upper bounds. For example, particle settling speeds on Earth never exceed debris flow speeds, and as emphasized in the text, these data are only presented as scaling relations. Particle densities, s, for hollow beads are reported by the manufacturer, and for debris flows are measured in refs. 30 and 31. Gas densities, g, and viscosities, g, are a matter of record, see for example, ref. 4. Hollow bead sizes, d, are measured in our laboratory from digitized microscope photographs, and geological particle sizes are chosen to be representative of coarse sand. Debris sizes, D, are approximations from Figs. 1 and 2 in our experiments and are from recorded data (e.g., ref. 31 and A. Mashiyama and K. Kurita, unpublished work) for geological flows. The drag coefficient, Cd, is obtained from a recent computational study (32) and represents an overestimate of drag across a geological surface.
son, atmospheric pressures on Mars, although variable, average close to 8 Torr (39, 40). Thus, it appears that Mars’ atmosphere serendipitously lies very close to the transition pressure at which the behaviors we describe begin to come into play (17). We speculate that the tendency for liquid-like f low features to be seen on colder, poleward, sides of midlatitude craters and not on warmer, opposing, sides (5, 7) may in fact be a ref lection of reduced atmospheric densities and pressures on the warmer sides rather than being due to enhanced liquid formation on the colder sides. Definitive testing of this speculation and of the presence of liquid water itself must await future explorations focused on distinguishing between true liquid-based f lows and granular surrogates on Mars.
istration’s (NASA’s) Mars Rovers and the European Space Agency’s Mars Express, some of which has provided significant new evidence of the former presence of liquid water on Mars, we hasten to reiterate that our results deal specifically with gully features known to be geologically recent, for which the discordance between geological and climatic evidence is especially problematic (1, 17). For these features, our results imply that surface landforms that are diagnostic of liquid water under terrestrial conditions may not by themselves be diagnostic on Mars (17). Interestingly, laboratory investigations of granular f lows at reduced pressure performed elsewhere (37, 38) indicate that granular f lows are significantly inf luenced by their surrounding atmosphere until pressures fall below 5–10 Torr; at much below this pressure, grains travel nearly ballistically and do not settle in a Stokes-like manner (i.e., as in Eq. 1), whereas, above this value, grains remain f luidized as described. By compari-
We thank Carlos Caicedo and Cyrus Chi for dedicated technical assistance and Conway Leovy, Allan Treiman, and Jerry Gollub for insightful remarks. This work was supported by the National Science Foundation, Division of Chemical and Transport Systems.
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