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A Model for the Hydrologic and Climatic Behavior of Water on Mars

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A Model for the Hydrologic and Climatic Behavior of Water on Mars JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. E6, PAGES 10,973-11,016, JUNE 25, 1993 A Model for the Hydrologicand Climatic Behavior of Water on Mars STEPHEN M. CLIFFORD Lunar and Planetary Institute,Houston, Texas Paststudies of the climaticbehavior of wateron Mars haveuniversally assumed that the atmosphereis the sole pathwayavailable for volatileexchange between the planet'scrustal and polar reservoirs of H20. However,if the planetaryinventory of outgassedH20 exceedsthe pore volume of thecryosphere by morethan a few percent,then a subpermafrostgroundwater system of globalextent will necessarilyresult. The existenceof sucha systemraises the possibilitythat subsurface transport may complementlong-term atmospheric exchange. In thispaper, the hydrologic responseof a water-richMars to climatechange and to the physicaland thermal evolution of its crustis considered. The analysisassumes that the atmosphericleg of the planet'slong-term hydrologic cycle is reasonablydescribed by current models of insolation-drivenexchange. Under the climatic conditionsthat have apparentlyprevailed throughoutmost of Martiangeologic history, the thermalinstability of groundice at low- to mid-latitudeshas led to a netatmospheric transport of H20 fromthe "hot"equatorial region to the colderpoles. Theoretical arguments and variouslines of morphologicevidence suggest that thispoleward flux of H20 hasbeen episodically augmented by additionalreleases of water resultingfrom impacts,catastrophic floods, and volcanism.Given an initially ice- saturatedcryosphere, the deposition of materialat thepoles (or any otherlocation on the planet'ssurface) will result in a situationwhere the local equilibrium depth to the meltingisotherm has been exceeded, melting ice at thebase of the cryosphereuntil thermodynamicequilibrium is once again established.The downwardpercolation of basal meltwaterinto the globalaquifer will resultin the riseof the local watertable in the form of a groundwatermound. Given geologicallyreasonable values of large-scalecrustal permeability (i.e., >•10 -2 darcies),the gradientin hydraulichead created by the presenceof the moundcould then drive the equatorwardflow of a significantvolume of groundwater(>• 108 km 3) overthe courseof Martiangeologic history. At temperateand equatorial latitudes, the presenceof a geothermalgradient will then result in a net dischargeof the systemas water vapor is thermally pumpedfrom the highertemperature (higher vapor pressure)depths to the colder (lower vapor pressure)near- surfacecrust. By thisprocess, a gradientas smallas 15 K km-1 coulddrive the verticaltransport of 1 km of water to thefreezing front at thebase of the cryosphereevery 106-10 7 years,or theequivalent of -102-103 km of water overthe courseof Martian geologichistory. In thismanner, much of the H20 that hasbeen lost from the crustby the sublimationof equatorialground ice, impacts,and catastrophicfloods may ultimately be replenished.The validity of this analysisis supportedby a detailedreview of relevantspacecraft data, discussionsof lunar and terrestrialanalogs, and the use of well-establishedhydrologic models. Among the additionaltopics discussed are the thermaland hydrologic properties of the crust,the potentialdistribution of groundice and groundwater,the thermal evolutionof theearly cryosphere, the rechargeof thevalley networksand outflowchannels, the polarmass balance, and a reviewof severalimportant processes that are likely to drive the large-scalevertical and horizontaltransport of H20 beneaththe Martian surface.Given a geologicallyreasonable description of the crust,and an outgassed inventoryof waterthat exceeds the porevolume of the cryosphereby just a few percent,basic physics suggests that thehydrologic model described here will naturallyevolve. If so, subsurfacetransport has likely playedan important rolein the geomorphicevolution of theMartian surface and the long-term cycling of H20 betweenthe atmosphere, polarcaps, and near-surface crust. CONTENTS The Stability and Replenishment of Equatorial H20 ........... 10,985 The Stability of Equatorial Ground Ice ....................... 10,985 Introduction ................................................................ 10,974 Other Crustal Sources of Atmospheric Water .............. 10,987 The Geophysical Basis for a Global Groundwater Evidence of Ground Ice Replenishment ...................... 10,988 System on Mars ........................................................... 10,974 Processes of Replenishment ..................................... 10,989 Porosity Versus Depth ............................................ 10,975 Transport Through the Cryosphere ............................ 10,993 Thermal Structure .................................................. 10,976 Polar Deposition, Basal Melting, and the Physical The Distribution of Groundwater ............................... 10,978 Requirementsfor Pole-to-Equator Groundwater Flow ......... 10,995 Terrestrial Analogs: The Occurrence of Groundwater Polar Basal Melting ................................................. 10,995 and the Permeability of the Earth's Crust ......................... 10,980 Growth of a Polar Groundwater Mound ...................... 10,997 The Occurrence of Groundwater ............................... 10,981 The Effect of Crustal Permeability on Pole-to-Equator The Large-Scale Permeability of the Earth's Crust ....... 10,981 Groundwater Flow .................................................. 11,000 Local and Regional Groundwater Flow ...................... 10,983 Examples of Large-Scale Terrestrial Groundwater The Large-Scale Permeability of the Martian Crust ...... 11,001 Flow Systems ........................................................ 10,983 The Early Evolution of the Martian Hydrosphere and Climate ................................................................. 11,002 The Evolution of the Early Martian Climate and the Copyright1993 by the AmericanGeophysical Union Initial Emplacementof Crustal H20 .......................... 11,002 Papernumber 93JE00225. The Hydrologic Response of Mars to the Thermal 0148-0227/93/93JE-00225505.00 Evolution of Its Early Crust ..................................... 11,003 10,973 10,974 CLIFFORD:HYDROLOGIC AND CI_IMATIC BEHAVIOR OF WATER ON MARS Further Considerations .................................................. 11,006 meltwaterbeneath the polar capswill then result in the rise of Recharge of the Valley Networks and the localwater table in the form of a groundwatermound. Given Outflow Channels ................................................... 11,007 geologicallyreasonable values of crustalpermeability, the gra- The Polar Mass Balance and High Obliquities ............. 11,008 dientin hydraulichead created by the presence of themound will Qualifications and Potential Tests .................................... 11,009 then drive the flow of groundwateraway from the poles and Conclusions ................................................................. 11,010 towardsthe equator.At equatorialand temperatelatitudes, the Notation ..................................................................... 11,011 presenceof a geothermalgradient will result in the local dis- chargeof the systemas water vapor is thermally pumpedfrom the higher temperature(higher vapor pressure)depths to the 1. INTRODUCTION colder(lower vaporpressure) near-surface crust. In this fashion, Virtually all past studiesof the climatic behaviorof water on much of the groundice and groundwaterthat has been removed Mars have focused on the potential for insolation-drivenex- from the crust by impacts, catastrophicfloods, and long-term changebetween the regolith, atmosphere,and polar caps [e.g., sublimation,may ultimatelybe replenished. Leighton and Murray, 1966; Toon et al., 1980; Fanale et al., Variousaspects of this model, includingthe inherentinstabil- 1982, 1986]. Implicit in this work has been the assumptionthat ity of equatorialground ice [Clifford and Hillel, 1983], and polar the atmosphereis the sole pathway available for volatile ex- basal melting [Clifford, 1987b], have been discussedin detail change.However, if the planetaryinventory of outgassedH20 elsewhere.The focusof this paperwill thereforebe on thoseas- exceedsby morethan a few percentthe quantityrequired to satu- pectsof the hydrologicresponse of Mars to climate changethat rate the pore volumeof the cryosphere(that regionof the crust have so far receivedlittle or no attention,with particularempha- where the temperatureremains continuously below the freezing sis on the potentialrole of subsurfacetransport. Among the top- point of water), then a subpermafrostgroundwater system of ics discussedare the thermal and hydrologicproperties of the global extent will necessarilyresult. As discussedby Clifford crust, the potentialdistribution of groundice and groundwater, [198lb, 1984], the existenceof sucha systemmay have playeda the stability and replenishmentof equatorialground ice, basal criticalrole in the long-termclimatic cycling of wateron Mars. meltingand the polar massbalance, the thermalevolution of the This paper examinesthe potentialhydrologic response of a early cryosphere,the rechargeof the valley networksand outflow water-richMars to both climate changeand to the physicaland channels,and a review of several processesthat are likely to thermalevolution of its crust.It is basedon the ad hoc assump- drivethe large-scalevertical and horizontaltransport of H20 tion that Mars possessesa global inventoryof water sufficientto within the crust. Throughoutthis
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