WHITE MARS: A GLOBAL EVOLUTION MODEL FOR MARS ... - NASA

WHITE MARS: A GLOBAL EVOLUTION MODEL FOR MARS’ SURFACE BASED ON CO 2 . N.Hoffman, WNS GeoScience, 22 Marlow Place, Eltham VIC 3095, Australia (nhoffman@vic.bigpond.net.au).Introduction: A model is presented for theevolution of the Martian surface and atmospherelocated in pressure and temperature conditions broadlyaround the triple point of carbon dioxide. Mars’surface features and evolution are shown to fit betterthan any model based on liquid water. TheAmazonian outburst “floods” are reinterpreted as anew class of cold, gas-supported density flowanalogous to Terrestrial pyroclastic surges andsubmarine turbidites, here termed Cryoclastic flows.Studies of the present day surface of Marsdemonstrate recurrent processes best explained by thepresence of liquids at various times in its evolution.However, atmospheric models of Mars have greatdifficulty in achieving the stability criteria for liquidwater [1]. This paradox is resolved here by modellingMars’ evolution with CO 2 as the active fluid, anddemonstrating that many otherwise obscure features ofthe Planet can be explained by interactions of CO 2around its solid / liquid / vapour triple point, ratherthan that of water. In particular, the early Noachianand Hesperian erosive valley features are shown to bedue to local liquid CO 2 cycling driven by precipitationof CO 2 snow and rain within a moderately dense (~5Bar) early atmosphere, locally warmed by impactheating and volcanism, whilst later Amazoniancatastrophic outburst features are explained by largelysolid subsurface deposits of CO 2 being released in selfsustainingcollapse/flow events at the solid/vapourphase boundary with long-distance transport of the“flood” deposits sustained by internal release of CO 2vapour. The closest terrestrial analogues to thisprocess are Pyroclastic volcanic flows, submarineturbidity currents, or air-supported long-runoutlandslides and snow avalanches.Pressure and Temperature on Mars:compared to the phase diagram of the H 2 O/CO 2system (after Miller [2]). Early Mars is close to theCO 2 triple point just as Earth is now situated aroundthe H 2 O triple point.Fig. 2. A simulation of Mars’ mean surfacetemperature through geological time based on asteadily warming sun and impact cratering into an icyregolith with repeated cycles of ephemeralatmospheric generation and collapse during theNoachian and early Hesperian.This extends the model of Kasting [1] for earlyatmospheric collapse post-bombardment to one ofrepeated cycles of collapse during the closing phase ofthe bombardment itself. Mean temperatures do notreach the crucial 273 K required for liquid water atsurface, suggesting that liquid water would have beena rare occurrence in local warm spots, at best. In theAmazonian, solar warming brings equatorial regionsinto unstable regimes where CO 2 ices in the regolithare prone to sublimation. Collapse events lead to theAmazonian outburst “floods” (actually resulting fromgas-supported flows, not aqueous floods). Eachcollapse leads to temporary atmospheric support, butfar below Noachian pressures. After the vulnerableterrain is consumed, the “floods” diminish and Marsevolves to the present thin, dry atmosphere.Fig. 1. Present and Past Martian P, T conditions

WHITE MARS: N HoffmanElsewhere an icy carapace protects the surface fromthe climate.Fig. 3. Mean atmospheric pressure within thesame simulation. Note that during Noachian times,occasional occurrences of P>5 bars occurs, permittingliquid CO 2 at surface. Liquid CO 2 would be a commonoccurrence during these times, except in local hotspots where it would be too volatile. CO 2 cycles wouldconsist of rain in the periphery of hotspots, drainingthrough river systems into local warm lakes thatevaporated strongly and resupplied the rain.Ice and Layering: The ubiquitous layering ofMars (craters cutting into layered terrain, andthemselves filled with layered deposits), calls for anew insight. Standard sedimentary models of thelayering cannot explain the infill of large closedcraters and basins, without coexisting degradation oftheir margins, or breaching of their rims by majorsediment transport systems. Volcanic explanations faildue to a lack of sufficient vents or fissures and dearthof flow morphologies. An ignimbrite origin is possiblebut again, vents and calderas on sufficient scale areclearly lacking. Instead, an interaction between impactsheets and episodic atmospheric collapse and icylayering leads to a thick layered regolith of rocky andicy layers. On early Mars, water ice is a refractorycrustal component and is spread with other lithicfragments in the impact sheet. CO 2 is the activeatmospheric volatile.Noachian fluid erosion by liquid CO 2 : In localareas, atmospheric cycles involving solid and vapourCO 2 ephemerally include a liquid CO 2 phase and leadto fluid erosion of the Noachian valley systems byliquid CO 2 .Fig. 5. At later times, liquid CO 2 disappears and acold iceworld sets in. Eventually, atmosphericpressure decreases enough for solid CO 2 to sublimateaway entirely at high altitude.Outburst “Floods”: The origin of Martianoutburst “floods” in zones of chaos is entirely naturaland obvious now that we understand that the terrain islargely layered ices and rock debris. Collapse of theterrain on weak volatile-rich layers is encouraged bysteep local slopes such as craters or canyon walls, andfault scarps. Tectonic activity and volcanic swellingencourage slippage as does the global warming ofsurface temperatures into the Amazonian. Lambert &Chamberlain [3,4] introduced this style of collapse butconcentrated on shallow instability. Instead, we nowsee that the instability extends to considerable depthand involves coherent collapse and slippage of kmscaleregolith blocks.The initial collapse leads to a partially-liquefiedslurry of regolith blocks, loose fragments of rock andice, and liquid CO 2 as the lubricant. As this headsdownslope it loses integrity and transforms into achaotic avalanche of blocks and fragments, lubricatedby CO 2 vapour, and transforms finally into a truedensity flow. This is capable of travelling downslopefor hundreds to thousands of km, and transportinglarge boulders and fragments within the debris cloud.Fig. 4. The surface of White Mars in theNoachian. Local CO 2 cycling in the vicinity of warmcraters leads to erosion and valley networks.Fig. 6. Generation of icy regolith slurries fromcollapse of layered terrain leads to flow transition intocold gas-supported density flows that form theAmazonian outburst “flood” channels and theirassociated topographic and textural enigmas.Surrounding areas receive additional airfall of finedebris, blanketing small craters and adding to theuppermost layers/

WHITE MARS: N HoffmanFig. 7. Comparison of density flow in motion andthe resulting morphology and texture after flowceases. Note stranded blocks with pyramidal or mesaforms, and minor local slumping. Transition zonesbetween flow regimes are preserved as surface chutesleading from flat plains to incised channels.Terrestrial Analogues:morphological link is with submarine density flows(turbidites) [5]. The reason why this analogue holds isthat the Martian flows were also fluidised densityflows.Another type of density flow on Earth is thevolcanic pyroclasic flows and surges. These aresubaerial gas-supported flows of huge speed andenergy that emerge either directly from an eruptivevent, from the collapse of an eruptive column, or froman avalanche of unstable fresh hot lava. It is this latteranalogue that we turn to in explaining the transitionfrom a collapsing sector of chaotic terrain, progressingthrough an incoherent avalanche and becoming a fullyfluidised gas-supported density flow. On Earth thepressure support comes from volatiles in the hotmagma, and from the expansion of entrained air. Dueto the high temperatures and low volatile content, thisis initially explosive, but short lived on Earth. OnMars, due to large volume % of ices in the layeredregolith, the support can continue for a long time,although the expansion pressure is much lower so theprocess is not explosive.Note that I am not suggesting that the outburst“floods” on Mars are volcanic. Instead, I am invokingtransport analogues from equivalent gas-supportedvolcanic flows, as I also do from terrestrial submarineturbidites where water is the fluid phase. The flowconditions on Mars would have been cold (

WHITE MARS: N HoffmanFig. 9. An analogous view of Martian outflowchannels showing the same broad, flat-bottomedstriated channels, and pointed islands.Fig. 11. Surface boulders deposited by apyroclastic flow from the Soufriere hills volcano,Martinique. Note the striking similarity in scale andappearance to the rock-strewn surface around thePathfinder Lander site.During evolution of the flow, it may separate intodistinct high and low density flows with differentparticle sizes, velocities, and flow behaviourdepending on details of its flow history, the local slopeand roughness of the ground, and the degree of lateralconfinement. Low density flows can ascendconsiderable hills (>1 km), and travel long distancesacross flat plains, generally in wide and unconfinedflow fronts. High density flows can be strongly erosiveand travel in confined channels downslope. It is theseconfined flows that define the Martian outflowchannels.Fig. 10. A subaerial pyroclastic channel sourcedfrom Unzen volcano, Japan. The flow cuts acrosscultivated land.Surface textures and deposits: Density flowdeposits depend on the grain sizes involved in theflows. Ignimbrite plains result when largely finegrainedmaterial is dumped over a wide area. Rockysurfaces result from coarser grained flows.Fig. 12. A surface deposit at the margin of apyroclastic flow from Mount St Helens. Note theuniform blocky debris at the surface, looking likecrushed rock dumped from trucks. Much of Mars’surface will consist of rocky plains like this, often witha surficial coating of fine airfall debris from othernearby flows. The dust of Mars is the fine debris fromthe flows, being slowly reworked in the present thinatmosphere.References: [1] Kasting J.F. (1991) Icarus, 94, 1-13. [2] Miller S. L. (1974) In Natural gases in marinesediments (I. R. Kaplan, Ed.) 151-177. Plenum Press,N.Y. [3] Lambert R. St J. and Chamberlain V. E.(1978) Icarus, 34, 568-580. [4] Lambert R. St. J. andChamberlain V. E. (1992) Abstract - NASAMSATTMA Workshop. [5] Kornar P. D. (1979) Icarus37, 156-181.