valley networks on venus

Ž .Geomorphology 37 2001 225–240www.elsevier.nlrlocatergeomorph

Valley networks on Venus

Goro Komatsu a,), Virginia C. Gulick b, Victor R. Baker c

a International Research School of Planetary Sciences, UniÕersita’ d’Annunzio, Via Pindaro 42, 65127 Pescara, Italyb Space Sciences DiÕision, MS 245-3, NASA-Ames Research Center, Moffett Field, CA 94035, USA

c Department of Hydrology and Water Resources, UniÕersity of Arizona, Tucson, AZ 85721-0011, USA

Received 1 June 1994; accepted 13 January 1999

Abstract

Valley networks on Venus are classified as rectangular, labyrinthic and pitted, or irregular. The venusian valley networksare structurally controlled, as indicated by the morphological patterns of valley branches, consistency between valley andfracture orientations, and associations with the deformed terrains. The morphologies resemble those of terrestrial and martiansapping valleys. Valley networks on Venus probably formed initially from fracture systems and became enlarged by lowviscosity lava sapping processes. Subsurface flow of lava may locally have been assisted by surface flows. The lavasprobably moved through permeable media and fractures. Venusian valley networks have a higher degree of networkintegration than do lunar sinuous rilles, but they are less integrated than martian and terrestrial sapping valleys. The viscosityof valley-forming lavas must have been very low, but was not low enough to exploit the permeable media so extensively asto attain a high degree of network integration. The compositions of these lavas may have been mafic to ultramafic or maficalkaline. Alternatively, the lavas could have had more exotic compositions, such as carbonatite and sulfur. Valley networksare often associated with corona and corona-like features, which are hypothesized to be the surface expressions of mantleplumes. A plume association could mean that the lavas came from the mantle. q 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Venus; Valley networks; Lavas

1. Introduction

The discovery of valley networks on Venus by theŽ .Magellan radar mapping mission 1990–1994 was

) Corresponding author. Tel.: q39-85-453-7507; fax: q39-85-453-7545.

Ž .E-mail addresses: [email protected] G. Komatsu ,Ž [email protected] V.C. Gulick ,

Ž [email protected] V.R. Baker .

completely unexpected for a planet devoid of liquidwater. These valleys have integrated systems of trib-utaries suggesting that they formed by fluid flow andnot purely by tectonic processes. Although tectoni-

Ž .cally formed valleys e.g. graben are numerous onVenus, we do not include them in this study. Themorphology of valley networks on Venus is some-what similar to that of sapping valleys on Earth and

ŽMars Baker et al., 1992; Gulick et al., 1992; Ko-.matsu et al., 1992, 1993 . In this paper, we describe

the morphologic characteristics of the fourteen valley

0169-555Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-555X 00 00084-2

( )G. Komatsu et al.rGeomorphology 37 2001 225–240226

Table 1Valley networks on Venus

Location Type Magellan images

1 2.1N70.4 Rectangular F 00N070–2 0N69.5 Rectangular F 00N070–3 54.5N204.5 Labyrinthic F 55N208–4 34.5N261.5 Labyrinthic Cl 30N261–5 33N268.5 Labyrinthic Cl 30N261–6 7S142.5 Labyrinthic Cl 00N146–7 8.5S87 Labyrinthic F 10S087–8 15S85 Labyrinthic Cl 15S077–9 16S57 Labyrinthic Cl 15S060–

10 19S196 Rectangular to Cl 15S197–labyrinthic

11 28S50 Labyrinthic Cl 30S045–12 57.5S166 Rectangular to Cl 60S153, F 60S164– –

labyrinthic13 68.5S1 Labyrinthic Cl 75S023–14 4N304 Pitted or irregular Cl 00N300, F 05N307– –

Ž .networks discovered to date Table 1 , and evaluatetheir origin.

2. Classification

The valley networks are distinct from the venu-sian channels in that they lack bedforms on thefloors that are direct indicators of fluid flow. Theseintegrated valley systems are somewhat similar inmorphology to those produced by groundwater sap-ping processes on Earth and Mars. Like the terres-trial and martian counterparts, venusian valley net-works probably did not form by fluids beingconveyed through conduits at bank-full stage. In-stead, these landforms appear to have formed by the

Ž .Fig. 1. A typical rectangular valley network lat. 2.18N, long. 70.48E in Ovda Regio. Note the lack of evidence for fractures in theplains-forming material. Radar illumination from left. Magellan image Cl 00N300. Scale bar equals 5 km.–

( )G. Komatsu et al.rGeomorphology 37 2001 225–240 227

undermining and subsequent collapse of surface ma-terial by outflow of low-viscosity subsurface fluidsaugmented by surface flow.

The venusian valley networks are divided intoŽ . Ž .three morphological classes: 1 rectangular; 2

Ž . Žlabyrinthic; and 3 pitted or irregular Gulick et al.,.1992 . Some networks display a morphology that is

Žtransitional between rectangular and labyrinthic Ta-.ble 1 . Here, we describe the morphologic character-

istics of each class and present examples of eachvalley type.

Rectangular networks, which are up to 100 kmlong and less than a kilometer wide, form the mostintegrated valley systems on Venus. The geometricdrainage patterns of the rectangular networks suggeststructural control, even though fractures are mostlyabsent from the dissected rock materials; the originof the networks may be best explained by buried

Ž .Fig. 2. Typical labyrinthic valley networks A: lat. 8.58S, long. 878E in Ovda Regio. Radar Illumination from right. Magellan imageŽ .F 10S087. Scale bar equals 100 km. The valleys occur in the vicinity of lava channels B , which are located on a domal rise. The domal–

Ž .rise is partly surrounded by a series of depressions C ; this rise may represent a corona-like structure. These geological settings imply agenetic connection among valley networks, lava channels, and the rise.


fracture systems, which were re-exposed and ex-ploited by subsurface fluid flow. The drainage pat-

Žtern e.g. right-angled bends of the tributaries and.unusually straight valley segments of a valley net-

Ž .work located at 2.18N, 70.48E Fig. 1 , for example,

indicates structural control. The network, which isover 50 km long, less than a kilometer wide, andcontains at least 15 first-order and five second-ordertributaries, occurs on radar dark smooth plains mate-rial.

Ž . Ž . Ž .Fig. 3. a Icebarg Canyon lower left and Slick Rock Canyon upper middle on the Colorado Plateau. Icebarg Canyon is approximately 6Ž . Ž .km long. These canyons are theater-headed, which is indicative of sapping processes. b Valley networks lat. 178N, long. 728W associated

Ž . Ž . Žwith Kasei Vallis, Mars. Viking mosaic quadrangle MC-10NW. Scale bar equals 50 km. c Valley networks lat. 278S, long. 448W Nigel.Vallis , which are located in the cratered southern highlands, Mars. Viking image 466A54. Scale bar equals 10 km.


Ž .Fig. 3 continued .

Labyrinthic Õalley systems are the most commonŽ .type of valley networks on Venus Fig. 2 . Similar to

the rectangular networks, labyrinthic systems exhibita pronounced structurally controlled pattern, exceptwidths and depths of valleys appear significantlyenlarged. Tributary heads are stubby and theater-headed, and the overall pattern is less integratedwhen compared with rectangular systems. Thesemorphological characteristics are similar to terrestrial

Ž .and martian sapping valleys Fig. 3a,b and c .Labyrinthic valley networks seem to have formed

either within or near tectonically deformed terrainsŽ .Figs. 2 and 4 or tectonic and volcanic landformsŽ .Figs. 5a and b, 6 and 7 . Tectonically deformedterrains, however, are complex and clear associationsbetween the orientations of valley segments andstructures cannot always be determined. Fig. 5a andb shows examples of valley networks occurring nearor on the surface of a corona. Coronae are concentric

structural landforms characterized by internal andŽ .external volcano-tectonic features Head et al., 1992 .

The orientations of individual valley segments areconsistent with the trends of the corona-centeredconcentric and radial fractures. In places, labyrinthic

Ž .valley networks connect to sinuous rilles Fig. 6Ž .Komatsu et al., 1993 , so named because of themorphological similarity to lunar sinuous rilles, or

Ž .merge with scalloped troughs Fig. 7 . Scallopedtroughs are ubiquitous on Venus and are interpreted

Ž .to be graben Solomon et al., 1992 .Pitted or irregular Õalley systems occur in wrin-

Ž .kle ridged plains material Fig. 8 . These systemshave lengths of 200 km or more and widths ofseveral tens of kilometers; main valley reaches aretypically 50–75 km wide. Pitted or irregular valleysappear be a series of coalesced pits or scallopeddepressions with diameters up to several kilometers.This particular subclass of venusian valley networks



appears similar in morphology to dissected rockoutcrops on Mars, which are interpreted to be

Ž .thermokarst Fig. 9 , and to valleys in permafrostregions on Earth. Mechanisms that allow for pittedor irregular valleys to form on Venus may be similarto those that drive such valley erosion on Earth and

ŽMars e.g. a mechanism that utilizes a fluid or.sublimation of a solid . Pitted features are also ob-

served on terrestrial lava fields such as those thatŽ .mark the McCartys’ Basalt Flow Fig. 10 . Even

though the pits on the McCarty’s Flow are verydensely spaced, they do not coalesce as much as thevenusian pitted or irregular valley systems; they havebeen interpreted as lava collapse depressionsŽ . ŽHeacock et al., 1966 or inflation features L.

.Keszthelyi, personal communication .


Ž .Fig. 4. Valley networks lat. 168S, long. 578E , which are located in tectonically deformed terrains. Radar illumination from left. Magellanimage Cl 15S060. Scale bar equals 10 km.–


3. Origin of the valley networks on Venus

3.1. Discussion of Õalley morphology

Valley networks tend to occur in or near thetectonically deformed terrains where structural con-

Žtrol of the morphologic patterns is evident Figs. 2,.4, 5 and 7 . Valley morphologic patterns are inte-

grated and often follow fractures, while in otherinstances the integrated patterns may connect to scal-

Ž .loped troughs interpreted as graben . Whereas it ispossible that valleys were enlarged and connected

Ž . Ž . Ž .Fig. 5. a Valley networks A: lat. 57.58S, long. 1668E associated with coronae B: Fotla Corona and C . Radar illumination from left.Magellan image Cl 60S153. Scale bar equals 100 km. The orientations of valleys are generally consistent with the orientations of the–

Ž . Ž .fractures. b Valley networks lat. 198S, long. 1968E associated with a corona. Radar illumination from left. Magellan image Cl 15S197.–Scale bar equals 10 km.



together by regional tectonic processes, the degree oftributary integration is too high to be of a purelystructural origin. Integrated valley patterns, there-fore, reflect formation by a fluid moving on thesurface or in the subsurface. For instance, if fracturesare encountered by the migrating fluid, the fluid willpreferentially exploit those fractures, because thoseare the regions of highest permeability. Surface

andror subsurface erosion is enhanced along thesefractures because fluid flow becomes concentratedalong these zones of weakness.

At least one of the valley networks contains aŽ .meandering reach Fig. 6 . Although meandering

generally indicates fluvial processes, some lavachannels may also exhibit this characteristic. Lunarsinuous rilles, which are thought to have a lava


Ž . Ž .Fig. 6. Valley networks A: lat. 158S, long. 858E connected with a sinuous channel B . Radar illumination from left. Magellan imageCl 15S077. Scale bar equals 10 km.–

Ž .origin Hulme, 1973; Carr, 1974; Strom, 1966 , forexample, display meandering patterns in places.

Some venusian lava channels also meander; thoseŽclassified as canali Baker et al., 1992; Komatsu et


Ž . Ž .Fig. 7. Valley networks A: lat. 54.58N, long. 204.58E connected with a scalloped trough B . Radar illumination from left. Magellan imageF 55N208. Scale bar equals 30 km.–

. Žal., 1993 even have oxbows abandoned meander.bends . Such features indicate that the channel has

eroded laterally into its walls, and subsequently mi-grated across the surface by cut and fill processes.

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Ž .Fig. 8. Pitted or irregular valley networks A and B: lat. 48N, long. 3048E . Radar illumination from left. Magellan image F 05N307. Scale bar equals 50 km. The general–southwest trend of valleys indicate structural control.


Ž .Fig. 9. Thermokarst feature lat. 238N, long. 368W on Mars. Viking image 008A70. Scale bar equals 5 km.

Whereas most venusian valley networks appear en-larged, no evidence exists for lateral migration.

Although the venusian valley networks appearmorphologically somewhat similar to martian andterrestrial valley systems, they do not approach thedegree of complexity or diversity normally attained

Ž .by fluvial-formed water valleys on either Earth orŽ .Mars Gulick et al., 1992 . Venusian valleys do,

however, have a higher degree of network integra-tion than that obtained by lunar sinuous rilles. The

morphologic patterns of the valley networks on Venusindicate that pre-existing fractures strongly con-trolled formation. The integrated nature of thesepatterns suggests fluid flow was involved. Therefore,based on morphology, we find that the fluid thatformed the valleys on Venus was probably morewater-like than the lavas that formed the lunar sinu-ous rilles, and yet more lava-like than the water that

Žformed the Martian valley networks Gulick et al.,.1992 .


Ž .Fig. 10. A field of lava collapse depressions or lava inflation features on Earth McCartys’ Basalt Flow in New Mexico, USA . Scale barequals 100 m.

3.2. Nature of Õalley-forming fluids

Because the Venusian valley networks are mor-phologically most similar to sapping valleys on Earth

Ž .and Mars Gulick et al., 1992 , a lava-sapping pro-Ž .cess was proposed Komatsu et al., 1992 for the

origin. Although terrestrial analogs of lava sappingdo not exist, the process is thought to be somewhatanalogous to groundwater sapping, except that lowviscosity lava is the eroding fluid. In this hypotheti-cal scenario, outflow of low viscosity lava from thesubsurface might remove substrate, causing a col-lapse of overlying material and subsequent enlarge-ment of existing valleys in a manner similar to

Žgroundwater sapping on Earth and Mars Baker et

.al., 1990 . Low viscosity lava, having properties thatapproach those of water, would flow preferentiallyalong buried fracture systems, because those wouldbe the avenues of highest permeability. Thermalerosion along the fracture walls, in a manner similar

Ž .to that proposed by Carr 1974 for the formation oflunar sinuous rilles, may have resulted in melting ormechanical plucking of wall material, and subse-quent incorporation of this material into the flowinglava. The process probably ceased, either when sub-surface outflow of lava was insufficient to maintainsapping, or when overall viscosity of the lava in-creased because of cooling. Venusian valley net-works probably formed by a combination of theseprocesses.


Although the valley networks on Venus somewhatresemble those formed by sapping, these valleysprobably did not form by groundwater outflow. OnVenus, liquid water is unstable on the surface and inthe crust. For water to be stable, surface tempera-tures would have to be at least 170 K lower than atpresent for the 90-bar atmosphere on the planet. Apossible alternative to liquid water, however, is sili-cate lavas. Silicate lavas have very low viscosities athigh temperatures, particularly the mafic to ultra-

Žmafic, or mafic alkaline varieties see Fig. 19 of.Baker et al., 1992 ; therefore, these lavas may have

thermally and mechanically eroded and removed ma-terial along the fractures. Such lavas may have pro-duced erosion if the discharge rates were high. A fewproblems occur with the silicate hypothesis. At lowrates of discharge, silicate lavas would have cooledtoo quickly to sustain fluidity and would have solidi-fied before becoming effective in removing sub-strate. Furthermore, no clear example exists ofsilicate lavas forming valley networks on Earth. Al-though silicate lavas have been invoked as a likelyfluid in the formation of lunar sinuous rilles, andpossibly also for venusian sinuous rilles, such fea-tures have a simpler morphology than the venusianvalley networks.

Other possible alternatives are carbonatite or sul-fur, because of the low viscosities and low meltingtemperatures. Under present surface conditions, sul-fur remains a liquid and carbonatite magma forms attemperatures slightly higher than ambient surface

Ž .temperatures on Venus Baker et al., 1992 . Whereasthese fluids cannot thermally erode silicate host rocks,they can physically remove unconsolidated material.Exotic lavas are especially attractive candidates forthe pitted or irregular valley networks where sulfurmay have evaporated in a manner similar to sublimi-nating ice in thermokarst landforms on Earth andMars.

Rheologically, carbonatite and sulfur are attrac-tive candidates for the lava sapping processes. Thenecessary volumes of these magmas, however, areproblematic. In terrestrial sapping processes, theamount of water required to erode valleys in theColorado Plateau would be on the order of 102

Ž . 5 Ž .unconsolidated sediments –10 consolidated rockstimes more than the amount of eroded materialŽ .Howard et al., 1988 . Similarly, Gulick and Baker

Ž .1993 concluded that the amount of water requiredŽto erode valleys in the volcanic terrains mostly

.permeable basalt flows with some ash interbeds ofHawaii is approximately 3000:1. By analogy, if lowviscosity magmas on Venus can mechanically andchemically erode material in a manner similar togroundwater sapping processes, then we can estimatethe volumes of magma required to form valley net-works on Venus. Assuming the volume of the valley

Ž .networks at lat. 57.58S, long. 1668E Fig. 5a isabout 1010 –1011 m3, with depths of 10–100 m, thenthe volume of magma required to form the valleys isabout 1012 –1016 m3. The production of such largevolumes of exotic lava has not been documented onEarth.

If the lava was carbonatite or sulfur, then it couldŽhave originated either as a primary magma Kaula,

.1993; Kargel et al., 1993 or have been derived fromremobilized materials stored in the crust. If the lavawas silicate, then it must have originated as a pri-mary magma. The association between valley net-works and coronae suggests that some valley-for-

Žming magma may have had a mantle source Komatsu.et al., 1993 .

4. Conclusions

Valley networks on Venus are classified as rect-angular, labyrinthic, and pitted or irregular. Thevenusian valley networks are structurally controlledas indicated by the morphological pattern of valleybranches, consistency between valley and fractureorientations, and associations with deformed terrains.The morphologies resemble terrestrial and martiansapping valleys. The valley networks probably origi-nated as structurally formed fracture systems, whichwere enlarged by low viscosity lava sapping pro-cesses, possibly assisted by surface lava flow insome cases. Lavas probably moved through perme-able substrate and fractures. Venusian valley net-works have a higher degree of network integrationthan lunar sinuous rilles, but lower than the degree ofnetwork integration of martian and terrestrial sappingvalleys. The viscosity of lavas involved must havebeen very low, but was not low enough to exploitpermeable media extensively to attain a high degreeof network integration. Compositions of these low-


viscosity lavas may have been mafic to ultramafic ormafic alkaline. Alternatively, these lavas could havehad exotic compositions, such as carbonatite andsulfur. The venusian valley networks are often lo-cated near corona and corona-like features, that areconsidered to be surface expressions of mantleplumes. If this is true, then the low-viscosity lavasthat formed the valley networks on Venus may havea mantle origin.

Acknowledgements

Support for G. Komatsu and V.R. Baker providedby National Aeronautics and Space Administrationthrough the Venus Data Analysis Project, grantNAGW-3515, and the Planetary Geology and Geo-physics Program, grant NAGW-285. Support for V.C.Gulick was provided by the NASA Graduate StudentResearchers Program and by the National ResearchCouncil’s Research Associateship Program.

References

Baker, V.R., Kochel, R.C., Laity, J.E., Howard, A.D., 1990.Spring sapping and valley development. In: Groundwater Geo-

Ž .morphology. Higgins, C.G., Coates, D.R. Eds. , Geol. Soc.Am., Spec. Pap., vol. 252, pp. 235–265.

Baker, V.R., Komatsu, G., Parker, T.J., Gulick, V.C., Kargel, J.S.,Lewis, J.S., 1992. Channels and valleys on Venus: preliminaryanalysis of Magellan data. J. Geophys. Res. 97, 13421–13444.

Carr, M.H., 1974. The role of lava erosion in the formation ofLunar rilles and Martian channels. Icarus 22, 32–43.

Gulick, V.C., Baker, V.R., 1993. Fluvial erosion on Mars: impli-cations for paleoclimatic change. In: Lunar and Planet. Sci.Conf., vol. XXIV. Lunar and Planet. Inst., Houston, TX, pp.587–588.

Gulick, V.C., Komatsu, G., Baker, V.R., 1992. Integrated valleysystems on Venus: a comparative morphological study. In:Lunar and Planet. Sci., vol. XXIII. Lunar and Planet. Inst.,Houston, TX, pp. 467–468.

Heacock, R.L., Kuiper, G.P., Shoemaker, E.M., Urey, H.C.,Whitaker, E.A., 1966. Ranger VIII and IX: Part II. Experi-menters’ analyses and interpretations. National Aeronauticsand Space Administration, Technical Report No. 32-800.

Head, J.W., Crumpler, L.S., Aubele, J.C., Guest, J.E., Saunders,R.S., 1992. Venus Volcanism: classification of volcanic fea-tures and structures, associations, and global distribution fromMagellan data. J. Geophys. Res. 97, 13153–13197.

Howard, A.D., Kochel, R.C., Holt, H.E., 1988. Sapping featuresof the Colorado Plateau. NASA SP-491, 108 pp.

Hulme, G., 1973. Turbulent lava flow and the formation of Lunarsinuous rilles. Mod. Geol. 4, 107–117.

Kargel, J.S., Komatsu, G., Baker, V.R., Strom, R.G., 1993. Thevolcanology of Venus landing sites and the geochemistry ofVenus. Icarus 103, 253–275.

Kaula, W.M., 1993. Compositional evolution of Venus. In: Taka-Ž .hashi, E. Ed. , Chemical Evolution of the Earth and Planets.

Am. Geophys. Union, Washington, DC.Komatsu, G., Gulick, V.C., Kargel, J.S., Baker, V.R., 1992.

Venus lava sapping valleys. Lunar and Planet. Sci. Conf., vol.XXIII. Lunar and Planet. Inst., Houston, TX, pp. 719–720.

Komatsu, G., Baker, V.R., Gulick, V.C., Parker, T.J., 1993.Venusian channels and valleys: distribution and volcanologicalimplications. Icarus 102, 1–25.

Solomon, S.C., Smrekar, S.E., Bindschadler, D.L., Grimm, R.E.,Kaula, W.K., McGill, R., Phillips, J., Saunders, R.S., Schu-bert, G., Squyres, S.W., Stofan, E.R., 1992. Venus Tectonics:an overview of Magellan observations. J. Geophys. Res. 97,13119–13255.

Strom, R.G., 1966. Sinuous rilles. In: Heacock, R.L., Kuiper,Ž .G.P., Shoemaker, E.M., Urey, H.C., Whitaker, E.A. Eds. ,

Ranger VIII and IX: Part II. Experimenters’ Analyses andInterpretations, JPL Tech. Rept. 32-800, pp. 199–210.

valley networks on venus

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