The extraordinary outflow length and morphology of Addams crater deposits are compared with the Saharan submarine debris flow off Northwest Africa. Vapor cloud modelling and comparison of sonar data with radar images suggest a possible similar origin for the Addams crater outflow deposits

Petford, Nick

Purdie, P.

English

Bournemouth University Research Online

ADDAMS CRATER, VENUS: OUTFLOW ANALOGOUS WITH A SUBMARINE DEBRIS FLOW?Philip Purdie1 and Nick Petford1, 1Centre for Earth and Environmental Science Research, Kingston University, Sur-rey, KT1 2EE, UK (n.petford@kingston.ac.uk)Introduction: The Magellan mission to Venus(1989 to 1994) recorded over 900 impact features onthe surface of the planet [1]. Due to the relative lack oferosion, recent volcanism and other tectonic activity,the mostly complete impact record allows for a detailedstudy of crater and outflow mechanics. From an initialvapour cloud formation scenario, the origin of the out-flow as a cometary impact event sems unlikely. Vapourcloud modelling for iron and stony meteorite impactscorrelate well with predicted source cloud emplace-ment and point to catastrophic emplacement of the dis-tal outflow deposits. The extraordinary outflow lengthand morphology of Addams crater deposits are compa-rable to that of the Saharan submarine debris flow offNorthwest Africa. Comparison of sonar data with ra-dar images of the Addams crater outflow lead us topostulate that both flows were formed from a similartwo-tiered flow regime.Addams Crater (56.1ºS, 98.9ºE): Addams crateris situated on an unnamed corona-chain complex (east-ern edge) between Lada Terra and Aino Planitia [2]. Ithas the largest crater outflow feature associated withany Venusian impact [3]. New surface area and linearmeasurements of the outflow features have been madeto better understand its formation. The extraordinaryoutflow length and morphology of Addams crater iscomparable to that of the Saharan submarine debrisflow off Northwest Africa. In this article we draw com-parisons between both flows, and speculate on the pos-sible mode of emplacement of the Addams crater out-flow deposits.89 kmFig. 1. Addams crater, showing proximal (A), medial (B) anddistal (C) ejecta outflows.Crater outflows: The thick Venusian atmosphereaccounts for the lack of impact craters below approxi-mately 2 km in diameter, as small impactors tend to becatastrophically disrupted and vaporized before reach-ing the surface [4]. Crater formation on Venus is char-acterized a combination of ejecta deposits, classifiedinto three distinct morphologies (proximal, medial anddistal ejecta flows [3,5], and associated lava flows[1,2]. Observational evidence suggests the crater out-flows navigate fluidly around obstacles and that theirdistant margins have a rough nature, indicated by in-creased (brighter) radar backscatter. In comparisonwith the other terrestrial planets, the Venusian ejectadeposits appear relatively easy to define, possibly re-flecting emplacement from a cloud rather than purelyballistic processes [6].Vapour cloud modelling: Early impact experi-ments [8] on vapour/debris clouds produced from im-pacts show the atmosphere clearly limits cloud devel-opment. Instead of expanding to many times the vol-ume of the impactor in a largely vertical direction (asin a vacuum), atmospherically-emplaced vapour/debrisclouds assume a ground hugging, lateral downstreamdisplacement. Due to the dense atmosphere on Venus,most of the vaporised impactor material should be re-tained [7].We now calculate the total vapour cloud travel dis-tance (L) for Addams crater from:1/ 30.41/ 33 31 1/ 31367 / 3 /,50 / 10 /vap vapairp projvap trMLr Mkg m g cmVMJ kg km s                     (1)where, L is the total travel distance of the first wave ofthe vapour cloud, rp is the radius of the impactor, ρair is the ambient air density (65 kg/m3), ρvap is impactvapour density, Mvap is the mass of impact vapour, Mprojis the mass of the impactor, εvap is 25 MJ/kg (foroblique impacts), and Vtr is downstream translationalvelocity (12 km/s). In order to gauge impact vapourdensity, the volume of a hemisphere was calculatedusing a base diameter of 237 km, which is scaled fromthe centre of the interfered ejecta blanket of the out-flow deposit [7]. It is important to note that the dis-tance (L) is taken to mean the distance that the cloudtravels in its first wave and where its velocity reaches 0km/s and not the terminal cloud position. The equationalso takes into account the mass of vapour produced, aswell as any melt from the impactor. Impactor types ofvarying mass and density (iron, stony, comet) wereABCLunar and Planetary Science XXXVI (2005) 1044.pdfchosen for comparison. Both the impacts from the me-teorites modelled closely the terminal position of thefirst wave of lateral cloud movement, the comet less so.A Saharan debris flow emplacement mechanismfor the outflow deposits at Addams crater? The Sa-haran debris flow off Northwest Africa, noted for itslong runout length of c. 700 km, provides an interestingterrestrial counterpart to the Addams crater deposits.Using sonar images [8], it is possible to compare themorphology and geometries of both flows. A simpleanalysis involves a comparison of the height (H) of theoutflow source region (vertical difference betweensource elevation and final deposit elevation) with theflow length (L), measured horizontally from the sourceregion to the flow’s distal limits [9]. The resultingdivision (L/H) is the efficiency.Narrowing of flow89 kmFig. 2. Radar image of the distal end of Addams crater out-flow indicating flow through a constriction.For Venus, the relevant topographic information isgleaned from the altimetry data collected by the Magel-lan probe. However, this data is relatively course andthe best approximation of vertical height from the cra-ter rim to the distal end elevation is 2 km. The lengthhowever is more easily constrained at 640 km.Bathymetry data for the Saharan debris flow [10] isbetter constrained and the height difference (H) hasbeen calculated also as 2.0 km. The accompanyingflow length (L) is 700 km. The resulting efficiency(L/H) values are 320 for Addams crater and 300 for theSaharan debris flow. When compared to other effi-ciency readings taken from a range of debris flows [9],both the Saharan and Addams crater flows show re-markably high runout efficiency values. Although theorigin of the two flows is different, it is the contentionof this report that they are the result of the same flowprocesses.Discussion: Superfical similarities exist between theAddams outflow and the Saharan debris flow. Bothwere required to overcome geometrical constrictions[e.g. 10 and Fig. 2]. Lateral margin deposits appear tobe characteristic of both flows, and their width dimen-sions and the presence of onlap sequences correlatewell (Fig. 3). Both flows can be judged to be flowingthrough high pressure “atmospheres”, with atmospheric pressure on Venus at 92 bar and the Saharan debrisflow under approximately 2 km of water proximallyand nearly 5 km distally. How these two atmospheresinteracted with the flows, and whether or not entrain-ment acted to prolong them, is an open question. If theAddams crater outflow started as a vapour/debriscloud, rapid collapse and condensation may havehelped provide the high pore fluid pressures requiredfor long runout lengths in terrestrial settings.6 kmPossible onlap depositsLateralmargindepositsFig. 3. Lateral margin and possible onlap deposits (darker)in the distal end of Addams crater outflow (yellow square,Fig. 2).References: [1] Herrick et al. (1997) Venus II.University of Arizona Press, Tuscon. [2] Stofan et al.(1993) J. Geophys. Res. 97, 13347-13378.[3] Chadwick D. J. and Schaber G. G. (1993) J. Geo-phys. Res. 98, 20891-20902. [4] Phillips et al. (1991)Science, 252, 288-296. [5] Schultz P.H. (1992) J. Geo-phys. Res. 97, 16183-16248. [6] Herrick R. R. andPhillips R. J. (1994) Icarus 112, 253-281. [7] Sugita S.and Schultz P.H. (2002) Icarus 155, 265-284 [8] Geeet al. (1999) Sedimentology, 46, 317-355. [9] IversonR.M. (1999) Rev. Geophys. 35, 3 245-296. [10]. Gee etal (2001) Sedimentology 48 1389-1409.Lunar and Planetary Science XXXVI (2005) 1044.pdf

Addams Crater, Venus, Venus: outflow analogous with a submarine debris flow?

Abstract

Similar works

Full text

Available Versions

Bournemouth University Research Online