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Planetary and Space Science 56 (2008) 289–302

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Periods of active permafrost layer formation during the geologicalhistory of Mars: Implications for circum-polar

and mid-latitude surface processes

Mikhail A. Kreslavskya,b,�, James W. Heada, David R. Marchantc

aDepartment of Geological Sciences, Brown University, Providence, RI 02912, USAbEarth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA 95065, USA

cDepartment of Earth Sciences, Boston University, Boston, MA 02215, USA

Received 18 July 2005; accepted 2 February 2006

Available online 29 August 2007

Abstract

Permafrost is ground remaining frozen (temperatures are below the freezing point of water) for more than two consecutive years. An

active layer in permafrost regions is defined as a near-surface layer that undergoes freeze–thaw cycles due to day-average surface and soil

temperatures oscillating about the freezing point of water. A ‘‘dry’’ active layer may occur in parched soils without free water or ice but

significant geomorphic change through cryoturbation is not produced in these environments. A wet active layer is currently absent on

Mars. We use recent calculations on the astronomical forcing of climate change to assess the conditions under which an extensive active

layer could form on Mars during past climate history. Our examination of insolation patterns and surface topography predicts that an

active layer should form on Mars in the geological past at high latitudes as well as on pole-facing slopes at mid-latitudes during repetitive

periods of high obliquity. We examine global high-resolution MOLA topography and geological features on Mars and find that a

distinctive latitudinal zonality of the occurrence of steep slopes and an asymmetry of steep slopes at mid-latitudes can be attributed to the

effect of active layer processes. We conclude that the formation of an active layer during periods of enhanced obliquity throughout

the most recent period of the history of Mars (the Amazonian) has led to significant degradation of impact craters, rapidly decreasing the

steep slopes characterizing pristine landforms. Our analysis suggests that an active layer has not been present on Mars in the last �5Ma,

and that conditions favoring the formation of an active layer were reached in only about 20% of the obliquity excursions between 5 and

10Ma ago. Conditions favoring an active layer are not predicted to be common in the next 10Ma. The much higher obliquity excursions

predicted for the earlier Amazonian appear to be responsible for the significant reduction in magnitude of crater interior slopes observed

at higher latitudes on Mars. The observed slope asymmetry at mid-latitudes suggests direct insolation control, and hence low

atmospheric pressure, during the high obliquity periods throughout the Amazonian. We formulate predictions on the nature and

distribution of candidate active layer features that could be revealed by higher resolution imaging data.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Mars; Past climate; Permafrost; Cryoplanation

1. Introduction

Permafrost occurs on the Earth in vast high-latituderegions and local high-elevation areas (e.g., Washburn,1973; Pewe, 1991), where the year-average surface tem-perature is below the water freezing point (e.g., Williams

e front matter r 2007 Elsevier Ltd. All rights reserved.

s.2006.02.010

ing author. Department of Geological Sciences, Brown

vidence, RI 02912, USA.

ess: kreslavsky@Brown.Edu (M.A. Kreslavsky).

and Smith, 1989). Muller (1947) defined permafrost onEarth as a soil or rock layer in which temperatures arebelow the freezing point of water for a continuous periodof two years. The layer of frozen ground can be up tohundreds of meters thick (e.g., Brown, 1970). The uppermeter(s)-thick layer of the ground in permafrost regionscan undergo a yearly freezing and thawing cycle; this isknown as the active layer (Fig. 1) (Miller and Black, 2003).In the spring, a thawing wave propagates downwards intothe permafrost and in the fall and early winter, a freezing

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Fig. 1. Diagrams illustrating different configurations of permafrost and active layers. (a) Permafrost: the situation in which the day-average temperature is

below the freezing point of water all year around, and no active layer is present. (b)–(c) Active layer: the situation in which there is seasonal melting of

water ice. During the warm season (b), surface temperatures exceed the freezing point of water, and a thawing front propagates down through the active

layer. Then, during the cold season (c), surface temperatures fall below the freezing point and a freezing front (the frost table) propagates downward from

the surface and the active layer progressively freezes. A second freezing front slowly propagates upward from the permafrost table. Later in the cold season

the entire thickness of the active layer becomes frozen. (d) Active layer with talik: under some conditions the freezing front (frost table) does not reach the

permafrost table in the cold season, and a confined layer of unfrozen ground (known as talik) is present year round. This could be due to surface

temperature trends over short (year to year) or longer (millennial) time periods.

M.A. Kreslavsky et al. / Planetary and Space Science 56 (2008) 289–302290

wave propagates down from the surface, sealing andprogressively freezing the active layer. Occasionally, freez-ing during early fall also commences upward from thepermafrost table (Mackay, 1981, 1983). The frost tabledelimits the seasonal base of the active layer (the limit ofthe freezing front as it descends downward in the winter)and this usually, but not always corresponds to thepermafrost table (the top surface of the underlyingperennially frozen ground) (Martini et al., 2001). In caseswhere it does not reach the permafrost table, soil withliquid water can exist year-round in the intervening spaceand is known as talik. This situation occurs when the year-average surface temperature is close to 0 1C, and thepermafrost is metastable or marginally stable. Thepermafrost table forms an impermeable barrier at the baseof the active layer. The thickness of an active layer dependsprimarily on the temperature of the atmosphere, andsecondarily on substrate heat conduction. Typically, activelayer thicknesses are greater away from the pole; they arethinnest under insulating snow and thicker in areas ofhigher heat conduction. In some permafrost areas, such asthe high-elevation parts of the Antarctic Dry Valleys(Bockheim and Hall, 2002), the day-average surfacetemperature never exceeds the freezing point of water,and a traditional active layer does not form (e.g., Marchantand Head, 2004, 2005). Dry permafrost is the special casewhere neither free water nor ice is present, although somemoisture in the form of interfacial water may occur. Drypermafrost is thaw stable and accompanying dry activelayers, if present, lack cryoturbation; thus, neither drypermafrost nor dry active layers will be further consideredin detail here.

A water-containing active layer in permafrost regionscauses a number of specific surface modification processesand produces a variety of distinctive geomorphic features.These include cryoturbation (frost heaving), patternedground formation (including sorted and unsorted circles,nets, steps, stripes and polygons), solifluction and gelifluc-tion (gelifluction is associated with frozen ground; lateralflow produces sheets, lobes, benches), cryoplanation(reduction of the topography of a terrain surface by iceand active layer processes), thermokarst (variety of surfacedepressions caused by melting of ground ice), etc. (e.g.,Washburn, 1956, 1973; French, 1976; Williams and Smith,1989; Krantz, 1990). The scale of these individual featuresranges from sub-meter to many hundreds of meters. Somespecific types of features form on permafrost surfaceswithout an active layer, for example, sublimation polygonsobserved in the Antarctic Dry Valleys (Marchant et al.,2002; Marchant and Head, 2005).Geomorphologic features characteristic of active layer

processes are abundant on Mars (Fig. 2). High-resolutionimages taken by the MOC camera onboard Mars GlobalSurveyor (Malin and Edgett, 2001) have revealed a varietyof features and patterns, including polygonal ground. Mostof the polygonal ground occurs at high latitudes. A numberof researchers have analyzed these features and pointedto permafrost-related and active layer-related origins(e.g., Carr and Schaber, 1977; Rossbacher and Judson,1981; Lucchitta, 1981, 1983, 1985; Squyres and Carr, 1986;Squyres et al., 1992; Carr, 1996; Mellon, 1997; Seibertand Kargel, 2001; Masson et al., 2001; Kuzmin et al.,2002; Kuzmin, 2005; Leverington, 2003; Mangold et al.,2002; Mangold, 2005). Among the variety of polygon

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Fig. 2. An example of martian polygons morphologically similar to

terrestrial ice-wedge polygons suggestive of the active layer processes.

Small-scale knobby pattern superposed over the large polygons is

interpreted to be sublimation polygons formed in the absence of an active

layer. This place is located on the floor of a crater at 621N 221E. Portion of

MOC NA image E21/01593, illumination is from lower left.

M.A. Kreslavsky et al. / Planetary and Space Science 56 (2008) 289–302 291

morphologies on Mars, there are examples strikinglysimilar to terrestrial ice-wedge polygons (Fig. 2). Suchpolygons occupy the floors of many high-latitude impactcraters. These morphological similarities suggest a similarorigin that appears to require formation of an active layer.Similarly, Mangold (2005) noted slope morphologiesstrikingly similar to terrestrial solifluction lobes which alsorequire seasonal thawing of an outer layer. A distinctiveand widespread variety of patterned ground at highlatitudes may not be related to the formation of an activelayer. A small-scale polygonal pattern (dubbed ‘‘basketballterrain’’ by Malin and Edgett, 2001; Fig. 2) was probablyformed as sublimation polygons that do not require anactive layer, but instead require thermal oscillation attemperatures constantly below 0 1C (Marchant et al.,2002).

On Mars, the year-average surface temperature is wellbelow the water freezing point (0 1C) everywhere on theplanet and the resulting permafrost covers all the planetand is currently several kilometers thick (e.g., Clifford,1993, and references therein). The surface temperature onMars under the present climate conditions routinelyexceeds 0 1C in some places during some time of the day.This occurs in the equatorial zone throughout the wholeyear, and in mid-latitudes in summer, especially onequator-facing slopes. Thawing or sublimation of ice, if itexists at or very near the surface, can potentially occurunder these transient conditions. Current knowledgesuggests that there is no ice available for melting in theimmediate vicinity of the surface in these regions (e.g.,Mellon and Jakosky, 1993; Jakosky et al., 1993, 1995).Recent calculations show that if ice/snow/frost is present,true melting of small amounts of ice (rather thansublimation) will probably occur only under favorable

local conditions (Hecht, 2002). Such melting, however,would not initiate process characteristic of an active layerbecause the maximum possible amount of liquid water istiny (not more than a few millimeters) and exists for only avery short time (hours). Due to the high evaporation rateon Mars a diurnal freeze–thaw cycle cannot currently leadto active layer processes typical of moist soils on Earth.Under different conditions on Mars in the past or in thefuture, active layers could potentially form due to seasonalthermal (thaw) waves, which imply positive temperatures(above freezing) to a depth comparable to the yearlythermal skin thickness, at least tens of centimeters. Thisrequires that the day-average (‘‘day-average’’ meansaverage over the whole spin period of the planet) surfacetemperature exceeds 0 1C during a warm season. Presentlythe day-average surface temperature is below 0 1C every-where on Mars throughout the entire year (Fig. 3);therefore, formation of an active layer as defined heredoes not occur.In addition to the presence of permafrost and positive

warm-season temperatures, active layer processes thatproduce geomorphic change require the availability ofwater that can freeze and melt. On the Earth, water ispresent virtually everywhere in the permafrost regions. OnMars, theoretical considerations (Mellon and Jakosky,1993) predict ground ice beneath a thin dry soil layer athigh latitudes and beneath a much thicker dry layer at lowlatitudes. Recent Odyssey GRS experiment data generallyconfirmed the current distribution of near-surface groundice predicted by these models (e.g., Boynton et al., 2002;Feldman et al., 2002; Mitrofanov et al., 2002). Longer terminsolation variations due to evolving spin and orbitalparameters cause the latitudinal migration of near-surfacewater ice stability (Mellon and Jakosky, 1993, 1995;Mellon et al., 2004).Since there is presently no active layer on Mars, the

numerous geological features characteristic of active layerprocesses are likely to have formed in the geological pastunder different climate conditions. The climate history ofMars is an exciting and controversial topic. In earliestMars history (the Noachian), valley networks have beencited to support a ‘‘warm and wet’’ climate period duringwhich conditions may have permitted pluvial activity (e.g.,Jakosky and Phillips, 2001; Carr, 1996). In the HesperianPeriod, the emplacement of large aqueous outflow channelsfollowing cryospheric breaching and groundwater dis-charge may have created regional standing bodies of waterthat in themselves influenced global atmospheric andclimate conditions (e.g., Baker et al., 1991; Gulick et al.,1997). During the youngest Amazonian Period, postulated‘‘ice ages’’ affecting mid-latitudes (e.g., Head et al., 2003)and tropical and mid-latitude mountain glaciers (e.g.,Lucchitta, 1981; Head and Marchant , 2003; Shean et al.,2005; Head et al., 2005) provide evidence for largefluctuations in climatic conditions. These factors, togetherwith recent studies of spin/orbital parameters predictingthat the current insolation conditions are not typical

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Fig. 3. Year-maximum day-average surface temperature on Mars. Surface temperature data are taken from the European Martian Climate Database

available at http://www-mars.lmd.jussieu.fr/. This data set has been generated with a GCM (Forget et al., 1999), but represents well the observed Martian

climate system.

M.A. Kreslavsky et al. / Planetary and Space Science 56 (2008) 289–302292

(e.g., Laskar et al., 2004), suggest the possibility that anactive layer has been present on Mars in its past historyand may have had an important influence on the geologicaland geomorphological record.

Under what conditions will an active layer form onMars, and when in the past history of Mars might this haveoccurred? We approach the problem of predicting thepresence of an active layer by starting with the present,where conditions are most well known. We then examineclimate conditions different from the present climate thatare necessary to change conditions sufficiently to producean active layer. Then we examine the statistics anddistribution of steep slopes on Mars, which are mostsensitive to change during periods of active layer forma-tion. On the basis of these data, we infer the distributionand onset of active layer formation. Finally, we presentevidence for the plausible timing of the most recent possibleepisodes of active layer formation and when it might occuragain in the future.

2. Climate conditions necessary for active layer formation

‘‘Astronomical forcing’’ is the term used to describe thechange of spin and orbit parameters that strongly influencethe climate system of Mars (e.g., see the review by Kiefferand Zent, 1992). Climate change due to astronomicalforcing is the most probable cause for the appearance anddisappearance of an active layer. The orbital eccentricity ofMars and the position of its spin axis in space change withtime due to gravitational perturbations from other planets.These changes cause fluctuations in insolation patterns andin this manner strongly influence the surface temperatureand the behavior of volatiles and the atmosphere–surfacevolatile exchange system. The insolation pattern iscompletely defined by three spin-orbital parameters: orbiteccentricity e, obliquity y (the angle between the orbitalplane and the equatorial plane), and season of perihelion

quantified as the areocentric longitude of perihelion fromthe moving equinox LP. Although all three parameters areessential in determining the characteristics of the climatesystem, obliquity has long been recognized as the mostpowerful climate driver (e.g., Kieffer and Zent, 1992).Obliquity oscillates with a period of �125 ka, and has an

amplitude modulation with a period of �1.3Ma. Themaximum oscillation amplitude is �201. In addition tothese quasi-periodic oscillations, there are chaotic changesof mean obliquity that occur at time scales of �5Ma andgreater (Laskar and Robutel, 1993). Due to the chaoticnature of these oscillations, and of solar system dynamicsat longer time scales, predictive calculations of the spin/orbital parameters are possible only back to �10–20Mabefore the present. The calculations show that the present-day oscillations around �251 obliquity were preceded by aperiod of generally higher obliquity (�357101) prior toabout 5Ma ago. Recently, Laskar et al. (2004) performedan extensive series of calculations of spin/orbital para-meters for much longer (250Ma) time spans. The calcula-tion runs differed due to tiny variations of the assumedmoment of inertia of the planet. These calculations are notpredictive for these time scales for the reasons describedabove, but they do provide a plausible picture of obliquityvariation patterns that are likely to have existed in the LateAmazonian (Fig. 4). During this period, the obliquity ispredicted to have varied from almost 01 to �651 with themean value over the range of all calculations of �351. Ofcourse, this mean value should not be considered as anestimate of the actual mean obliquity for the last 250Ma,because the mean obliquity varies widely from run to run(Fig. 4). This mean value illustrates well, however, the factthat the present-day obliquity (�251) is almost certainlymoderately low compared to that of the recent geologicalpast.Spin/orbital parameters control the surface temperature

through a very complex climate system. The whole system

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Fig. 4. Selected possible scenarios for the history of obliquity during the

last 250 million years. Fifteen (15) of 1001 runs taken from Laskar et al.

(2004). The obliquity history for the last 20Ma is very similar for all 15

plots, strong differences for earlier epochs appear in calculations due to

tiny differences in the assumed moment of inertia of Mars and are caused

by chaotic nature of the solar system dynamics. Green line marks 451

obliquity considered as a rough proxy for active layer formation

threshold.

M.A. Kreslavsky et al. / Planetary and Space Science 56 (2008) 289–302 293

can be modeled with global climate models (GCMs)(Forget et al., 1999; Haberle et al., 2003; Richardson etal., 2003; Mischna et al., 2003), which reproduce well thepresent-day climate. Recently a number of studies haveanalyzed the behavior of the GCMs under higher obliquity(e.g., Richardson and Wilson, 2002; Mischna et al., 2003;Levrard et al., 2004). Since the models have a large numberof adjustable parameters that are not fully constrained, theGCM predictions for different past climate conditionsshould be applied with these uncertainties in mind.

The most uncertain GCM parameter is the atmosphericpressure at high obliquity. First, it is not clear how muchCO2 was available for atmospheric cycling during pastperiods of high obliquity (for example, an unknownportion of the CO2 inventory may currently be storedwithin the polar layered deposits). Second, it is not clearwhether additional available CO2 would cause self-sustained greenhouse warming, or permanent solid CO2

deposits would form instead. In this paper we consider theclimate system qualitatively, rather than using specificclimate models; with this approach we inevitably lose someof the accuracy of the more complex models, but we gainthe ability to provide an overview of the principal trendsregardless of specific model parameter variations. In thismanner we can also make general predictions that can befurther tested with more refined GCMs.

The response of summer day-average surface tempera-ture to variations in insolation patterns depends stronglyon the atmospheric pressure. For example, for a denseatmosphere, such as that on the Earth, there is a strong

thermal coupling between the atmosphere and the surface;the day-average surface temperature is close to the day-average air temperature. Therefore, for a given latitude,there is a strong elevation-related zonality in surfacetemperature caused by atmospheric lapse, and surfaceslopes have little effect on the surface temperature. On theother hand, for a thin atmosphere, such as that on present-day Mars, the surface temperature is primarily controlledby insolation. Therefore, there is no elevation-relatedtemperature trend for a given latitude, and topographyinfluences the temperature as long as it influences insola-tion. During periods of very low obliquity (o10–151), theatmosphere may completely collapse, because almost allatmospheric CO2 condenses in the coldest areas at highlatitudes (Kreslavsky and Head, 2005).Unlike the full global climate system, insolation itself can

be accurately and readily calculated, especially if oneassumes a transparent atmosphere. If we take surfaceslopes into account, the insolation has a complexdependence on season, latitude, and slope orientationreflecting a complex interplay between how high the sunrises and how much time it spends above the local horizon;several illustrative examples are shown in Fig. 5. Generally,as obliquity increases, the yearly insolation is redistributedfrom the equatorial zone to the polar regions. At theequator, the insolation pattern varies weakly with obliquitychanges. Seasonal changes are minor and the overalldifferences between slopes of different orientation are notgreat unless obliquity is extremely high (4�601). At highobliquity, polar regions receive higher insolation duringspring and summer seasons, while slope orientation haslittle effect. In mid-latitude zones the seasons are morestrongly expressed at high obliquity, and the insolationregime of slopes depends strongly on slope orientation. Thepole-facing slopes at higher obliquity (4�351) are morewell illuminated in the summer than equator-facing slopes.The general dependence of summer day-average insolationon obliquity, latitude, and north–south slopes is schema-tically summarized in Fig. 6a. Analysis of the full range ofvariations illustrated shows that the greatest insolation isreached at high obliquity at high latitudes, as well as onpole-facing slopes at mid-latitudes. For a thin atmosphere,such as that on present-day Mars, these are the regionswhere we might expect summer day-average temperaturesto exceed 0 1C, and therefore, for an active layer to form.Insolation is of course not directly mapped into surface

temperatures. Generally, however, an increase in insolationproduces an increase in temperature, and we will deriveapproximate estimates of the threshold of active layerformation in terms of insolation. Climate models can beused to quantify these considerations (e.g., Paige, 2002).Costard et al. (2002) presented the results of calculatedsummer day-average surface temperatures with a modifiedGCM from Forget et al. (1999) for a range of obliquities,latitudes, and surface slopes. From these results wedetermined that the 0 1C isotherm corresponds to anabove-atmosphere, day-average insolation in the range of

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Fig. 5. Year-maximum day-average insolation for horizontal surfaces (bold line) and 301-steep slopes of different orientations (thin lines: blue marked

with N is for the north-facing slopes, red marked with S is for the south-facing slopes, and green not marked is for the east- or west-facing slopes). The

insolation (horizontal axis) is scaled as parts of the solar constant at the mean Mars distance form the sun, and is plotted as a function of the latitude

(vertical axis). The three plots correspond to three different spin/orbit configurations: (1) the present, (2) the most recent well-pronounced obliquity peak

0.64Ma ago, (3) the maximal extent of the active layer 9.45Ma ago.

Fig. 6. (a) Dependence of insolation on obliquity, latitude and north–south facing slopes, shown in a qualitative scheme. (b) The global trend of steep

north- and south-facing slope occurrence, shown in a qualitative scheme.

M.A. Kreslavsky et al. / Planetary and Space Science 56 (2008) 289–302294

0.68–0.75, where insolation is measured in parts of thesolar constant at the mean Mars distance from the Sun.The model uses the present available CO2 inventory.

We also applied a second approach by using the publiclyavailable results of the GCM by Forget et al. (1999) as agood proxy for the present observed Mars climate. Thesedata clearly show (Fig. 3) that the surface temperature isnot solely a function of insolation: under the same above-atmosphere, day-average insolation, but for different areasand seasons, the day-average temperature varies within arange as wide as �20 1C. These variations are mostlycaused by surface albedo. Despite these wide variations,the general global trend of increasing temperature withincrease of insolation is clearly seen (compare thelatitudinal insolation trend in Fig. 5(l), bold curve, andthe map in Fig. 3). For regions and seasons of the highestday-average temperatures on Mars (�60 to �25 1C) we

determined the regression of the temperature on the above-atmosphere insolation (for horizontal surfaces). Then weextrapolated the regression line to 0 1C and obtained aninsolation of 0.62. This is, of course, an extrapolation anddoes not have a well-established physical basis, but it doesnot involve the use of any poorly defined parameters.Below we use geologic observations to make an indepen-dent estimate of the onset and presence of active layerformation.

3. Properties of an active layer on Mars

How do active layer properties and processes on Mars inthe past differ from those currently observed on the Earth?On the basis of general considerations we expect a numberof significant differences. Unless there is a thick atmo-sphere and strong self-sustaining greenhouse effect on

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Fig. 7. Left: a typical crater (6.5 km diameter) at mid-latitudes (43.51S,

194.81E). Portion of MOC NA image M23/00404, north is approximately

on the top; illumination is from the upper part of the upper-left. Boxes

show locations of two enlargements presented on the right. Vertical line

shows location of MOLA profile shown on the upper right. Southern,

equator-facing crater wall is steep and shows bedrock outcrops and mass-

wasted debris. Northern, pole-facing wall has been modified by formation

of an active layer and transport of debris down onto the crater floor,

smoothing the crater rim and modifying the crater interior. Dissected and

partly eroded layered mantle is clearly seen everywhere except on the

steepest slopes.

M.A. Kreslavsky et al. / Planetary and Space Science 56 (2008) 289–302 295

Mars, the year-average temperature is low, well below 0 1C,even if in the summer the temperature is high and activelayer forms. This causes the permafrost below thepermafrost table to be cold in comparison to typicalterrestrial cases. Cold permafrost would cause moreintensive upward freezing during the cold season(Fig. 1c); this will reduce effects related to the confinementof liquid water between freezing fronts. For the samereason, taliks (Fig. 1d) are not likely to occur on Mars.

A number of factors influence the thickness of an activelayer. The greater distance from the Sun is balanced byhigher sun and long summer days at high obliquity. Thelonger martian year causes a longer warm season, whichfavors deeper penetration of the thawing front. On theother hand, the much lower cold-season temperatures slowthe propagation of the thawing wave, which favors athinner active layer. Generally, we would expect the sameorder of magnitude (tens of centimeters) for the thicknessof the active layer on Earth and Mars. Due to weakthermal coupling between the atmosphere and the surfaceon Mars, the surface albedo causes great variations in thesummer temperatures, which, in turn, have a strong effecton the existence of an active layer and its thickness. Thus,we expect very wide lateral variations of active layerproperties related to local conditions.

The nature and intensity of active layer processes dependnot only on surface temperature, but also on theavailability of water ice to melt and freeze, and on itsabundance. As shown recently by GCM calculations,obliquity changes cause migration of water ice betweenpolar regions and mid- and/or low-latitudes (e.g., Mischnaet al., 2003; Levrard et al., 2004; Forget et al., 2006). Thusunder some conditions even surface ice can be available forsummer melting. Mellon and Jakosky (1995) show that theground ice stability zone expands from high latitudes tomid-latitudes with an increase in obliquity, and there areno published predictions of ground desiccation in the polarregion. Thus, it is quite probable that at least some ice isalways available for active layer processes at medium tohigh latitudes.

Another consequence of the thin and generally coldatmosphere on Mars is high evaporation rates. Theavailability of water in the soil depends on re-supply ofwater through condensation and precipitation, which isthought to be strongly controlled by local microclimateconditions. This would also contribute to the lateralvariability of active layer properties.

4. The occurrence of steep slopes and implications for active

layer formation

One of the major manifestations of the presence of anactive layer is the modification and degradation of slopesand a consequent reduction in slope magnitude (e.g.,cryoplanation) (Fig. 7). Initial steep slopes are produced bya variety of processes, such as tectonic scarps associatedwith faulting and caldera walls associated with volcanism.

Fresh impact craters provide widespread examples oflandforms characterized by steep interior walls and rimcrests (e.g., Pike, 1980; Garvin, 2005). Thus, globalaltimetry data sets can be very useful in locating landformswith the steepest slopes (e.g., Kreslavsky and Head, 1999)and areas that lack steep slopes (Kreslavsky and Head,2003) (Fig. 8), and thus identifying candidate regions orlocations that might have been shaped by active-layer slopedegradation.We have statistically examined north–south slopes on

Mars at a baseline of 0.3 km. The slopes were derived fromprecise MOLA topographic profiles (Smith et al., 2001).We have found a strong latitudinal trend in the occurrenceof steep slopes on Mars (Kreslavsky and Head, 2003). Thefrequency of steep (4201) slopes drops more than threeorders of magnitude from equatorial to high-latituderegions (Figs. 8 and 9). The boundary between steep slopesthat are preserved and those that are reduced occurs atlower latitudes for pole-facing slopes and at higherlatitudes for equator-facing slopes (Fig. 9). This producesa strong asymmetry in steep slopes in the mid-latitude zonearound 451 latitude in both hemispheres (Fig. 10). Thenumber of steep pole-facing segments in MOLA profiles in

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Fig. 8. Distribution of slopes steeper than 301. High concentration of steep slopes at Valles Marineris walls is clearly seen. Note the paucity of steep slopes

at mid-to-high latitudes in both hemispheres.

Fig. 9. Abundance of N-facing and S-facing slopes relative to that of the

typical equatorial highlands plotted against latitude for Terra Cimmeria

(180–2201W). The actual quantity plotted is the proportion of MOLA

profile segments of proper steepness and slope direction within a narrow

latitudinal zone normalized by the same proportion for 10–201S zone

representing ‘‘typical equatorial highland’’ terrain. The different curves

correspond to different ranges of slopes in degrees, as shown, black curves,

N-facing slopes, gray curves, S-facing slopes.

Fig. 10. Asymmetry of differential slopes calculated within 150-km wide

latitudinal zones in Terra Cimmeria (180–2201W) and plotted against

latitude. The measure of asymmetry is A ¼ (NN�NS)/(NN+NS), where

NN and NS are numbers of steep N- and S-facing segments in all MOLA

profiles. Different curves correspond to different ranges of differential

slopes in degrees, as shown.

M.A. Kreslavsky et al. / Planetary and Space Science 56 (2008) 289–302296

these zones is almost a factor of three smaller than thenumber of steep equator-facing slopes.

We interpreted these observations (Kreslavsky andHead, 2003) in the following way. Among the geologicalprocesses producing steep slopes, impact cratering operatescontinuously, producing slopes near the angle of repose(�30–351) everywhere on the planet. In the equatorial zonethere are many craters with walls steeper than 301. Thevirtual absence of very steep slopes (4301) and the strongdeficiency of steep slopes (4101) at high latitudes is due tothe operation of efficient processes of steep-slope removal.The universal latitude trend of these distributions points toa climate-related slope removal mechanism, and thedependence on slope orientation suggests that insolationhad played a major role. Slopes are removed from exactlythe same places that experience the highest day-average

surface temperatures through obliquity variations, that is,all slopes at high latitudes and pole-facing slopes at mid-latitudes (see Figs. 6a and b). We infer that active-layerprocesses are the major slope-degradation and removalmechanism. Depending on ambient conditions and groundice abundance, the following active layer processes canoperate and contribute to slope removal: (1) segregation ofmeltwater and slope erosion by transient streams;(2) erosion by wet debris flows, the mechanism proposedby Costard et al. (2002) to explain the recent gullies;(3) solifluction and gelifluction, the latter being downslopemovement of wet material over a frozen substrate and theformer operating without an icy substrate; (4) cryoturba-tion, responsible for slope degradation according to Perronet al. (2003); and (5) cryoclastic erosion, the disintegrationof rocks by water freezing in cracks.

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These slope degradation processes operate duringrepeated and geologically short obliquity peaks (eachlasting several tens of thousands of years). These processesmay be intense enough to effectively remove slopes overgeologically short periods of time, leaving few steep slopesat high latitudes. We performed a detailed study of thetopography and morphology of all 130 craters in the10–25 km diameter range within the typical northern plains(Kreslavsky and Head, 2006). Even the freshest crater inthis population has walls 25–281 steep, less steep than30–351 that should be expected for a fresh crater. Theentire crater population was formed during the whole ofthe Amazonian (Head et al., 2002; Tanaka et al., 2005).Detailed analysis of the slope frequency distribution in thispopulation indicated that intensive steep-slope removaloperated (perhaps in repeated short episodes) at leastduring the last 200–400Ma (Kreslavsky and Head, 2006), ifwe assume the Hesperian/Amazonian boundary to be at�3.1Ga (Hartmann and Neukum, 2001).

The occurrence of steep slopes is not the only globallatitudinal trend observed in the statistical characteristicsof topography. Among others, the most pronounced trendis a high-latitude decrease in subkilometer-scale roughness(Kreslavsky and Head, 2000). This trend occurs at similarlatitudes, as does the deficiency of steep slopes; thesmoothing boundary is diffuse and located at slightlyhigher latitudes that the onset of removal of equator-facingslopes. This smoothing is manifested as a ‘‘softened’’appearance of topographic features in some medium-resolution (hundreds of meters scale) images of the surface.This effect was first noted in Mariner 9 images (Soderblomet al., 1973) and was initially attributed to eolian mantling.Squyres and Carr (1986) studied the same phenomenon of‘‘terrain softening’’ in Viking images and attributed it tocreep of a near-surface ice-rich layer. We avoid the term‘‘terrain softening’’ because in earlier work (Squyres andCarr, 1986; Jankowski and Squyres, 1992; Squyres et al.,1992) this term was applied to two very differentphenomena: (1) the soft appearance of topographicfeatures at high latitudes mentioned above, and (2)latitude-dependent trends in crater depth, concentric craterinfill, etc. The former is caused by smoothing oftopography with a characteristic vertical scale on the orderof meter(s), while the latter is related to creep of ahundreds-of-meters-thick layer; hence, the mechanisms,even if they are related genetically, are very different.

A global latitudinal trend of roughness (Kreslavsky andHead, 2000) is observed for typical surface slopes atsubkilometer scales. At these scales the typical slopes aregentle, and the vertical scale associated with subkilometer-baseline slopes is on the order of a meter. Thus, theobserved high-latitude smoothing is caused by meter-scalechanges in topography. Subkilometer-scale steep slopes areassociated with hundred(s)-of-meters topographic varia-tions, and meter-scale changes of topography are unable toremove them. Thus, the observed smoothing at highlatitudes is not due to the same process as the removal of

steep slopes. These processes, however, may be relatedgenetically. The presence of active layer processes mobilizessurface material and should lead to surface smoothing(cryoplanation) by the diffusion creep mechanisms dis-cussed above. Certainly, the active layer contributes to theroughness trend observed. Morphological observationswith high-resolution images, however, clearly indicate thedeposition of a smooth icy mantle at high latitudes as apowerful smoothing mechanism (Kreslavsky and Head,2002; Head et al., 2003). This smooth mantle withsublimation polygons on its surface covers all small-scalesurface features and obscures the potential morphologicalsigns of active layer processes almost everywhere at highlatitudes. At least the upper layer(s) of the mantle wereprobably emplaced later than the episodes of active layerformation had occurred, as discussed below. Thus, themost recent active layer is not the only contributor toterrain smoothing. Additional morphological observationsare necessary to understand the relative roles being playedby (1) active layer processes and (2) emplacement of a thin,latitude-dependent mantle in the creation of small-scaletopography smoothing at high latitudes.In addition to the active layer processes listed above,

flowing ice at the ground surface and ice-assisted creep atdepth can and perhaps did act as eroding agents. A widevariety of small-scale ice flow features are observed inassociation with slopes at mid-latitudes (e.g., Milliken etal., 2003; Head et al., 2005). Erosion by flowing ice doesnot require temperatures above the melting point and doesnot imply active layer formation. Fast flow of thin ice,above or below the ground surface, is only possible whenthe temperatures are near the melting point and slopes areof sufficient steepness. Insolation conditions for truemelting and for bringing near-surface ice close to themelting point are generally similar. This means that activeflow of thin sheets of surface and subsurface ice could showgeographic trends similar to those of active layers, that is,all slopes at high latitudes and pole-facing slopes at mid-latitudes. Thus, flowing ice could contribute to theobserved removal of steep slopes. We believe, however,that this contribution is generally less important thanactive layer processes for the following reasons. First, theboundary of steep-slope preservation is very sharp andconsistent through the globe. This appears to be moreconsistent with a sharp threshold caused by a phasetransition (solid ice to liquid water) rather than withgradational acceleration of ice flow. Second, the steepslopes of old geological features in the equatorial zone arewell preserved, even though traces of regionally distributedglacial features are observed in this zone (e.g., Head andMarchant, 2003; Head et al., 2005).We now use the observed zonality of the occurrences of

steep slope (e.g., Figs. 8–10) to independently derive theonset insolation for active layer formation from thesegeological observations (Kreslavsky and Head, 2004). Thelow-latitude onset of slope asymmetry occurs at �351latitude for 201 steep slopes and at �421 latitude for 101

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steep slopes (Fig. 10). This suggests that the onset of activelayer formation corresponds to the insolation that occurs onthese steep pole-facing slopes at these latitudes under the mostfavorable but still repeatedly occurring spin/orbital condi-tions. Favorable conditions are: high eccentricity ( ¼ closerproximity of the sun at perihelion), high obliquity, andperihelion close to the solstice. This latter condition occursregularly. The recent calculations of the spin/orbit parametersof Mars (Laskar et al., 2004) showed that for the time spancorresponding to the last 1Ga, the maximum frequent valueof eccentricity is �0.12, and that for obliquity is �54.51.Using the approach outlined above, we calculated that thesevalues correspond to an onset day-average insolation of�0.83. This value is higher than the insolation correspondingto the 0 1C isotherm from GCMs (see discussion above). Thisdifference is not surprising, because intensive active layerprocesses require somewhat warmer conditions than 0 1C onthe hottest day in the summer.

One important implication of the observed slopeasymmetry at mid-latitudes is that its formation requiresa thin atmosphere such as that on Mars today. As we notedabove, if the atmospheric pressure is high enough, thermalcoupling occurs between the atmosphere and the surface,and slope orientations have little effect on active layerformation. The observed very strong steep-slope asymme-try suggests that even at �501 obliquity the atmosphericpressure was not high. Furthermore, if we accept theinterpretation that periods of active layer formationoccurred during the entire Amazonian, then the observedstrict zonality of these slopes is a long-term feature of theplanet and this strongly implies that polar wandering (e.g.,Schultz and Lutz, 1988) did not occur during the last 3Ga.

5. Timing of an active layer in the recent past

How often and over what time duration might an activelayer have formed in the Late Amazonian? Laskar et al.(2004) have presented an extensive series of calculations ofplausible obliquity variation patterns over the last 250Ma(Fig. 4) that are not explicitly predictive due to the chaoticchanges of mean obliquity at long time scales. Nonetheless,these patterns provide a framework in which to considerthe role of an active layer in each of the sample histories.For simplicity, if we assume a value of 451 obliquity todelineate periods in which an active layer would form(4451 obliquity) or would not be predicted to form (o451obliquity) we can examine the family of patterns to assesscandidate scenarios. Obliquity patterns in which an activelayer would be predicted to occur for extensive periods(4150Ma) over the majority of the last 250Ma areillustrated in Fig. 4e, f, i, m. On the basis of theseassumptions, all but a few of these patterns (Fig. 4g, k, l, o)would have produced a period of active layer formation inexcess of 10Ma, a time interval certainly sufficient toproduce significant surface modification.

If an active layer is likely to have occurred repeatedlyduring the Amazonian Period, as we argue above on the

basis of the nature and distribution of the steepest slopes,but does not currently occur on Mars due to presentclimate conditions, when in the recent past were the lasttimes that an active layer was likely to occur? We usedcalculations from Laskar et al. (2004) of the spin/orbitalparameters for the last 10Ma to find when and where theformation of an active layer could occur in the recent past.If we formally apply the threshold of 0.83 for the day-average insolation we obtain the following results (Fig. 11).During the last 10Ma period the insolation has never

exceeded the threshold value in the southern hemisphere.The maximum insolation in the southern hemisphere,however, was very close to our inferred onset during fourobliquity peaks (A in Fig. 11), reaching 0.823 at the southpole �5.30Ma ago. In the northern hemisphere, four peaksof summer insolation exceeded 0.83 for the period from7.75–8.09Ma ago (B in Fig. 11). The second peak (7.86Maago) was the highest, at �0.866 maximal insolation. Fivemore peaks occurred from 9.12–9.57Ma ago, the fourthone yielded an absolute summer-time insolation maximum(0.872) on Mars during the last 10Ma. This happened9.46Ma ago. For this highest peak, the summer insolationat the pole exceeded the onset limit for a period of 17 ka(�9000 summers). At the very maximum part of the periodthe insolation exceeded the onset value down to 741N forhorizontal surfaces. For 101 steep pole-facing slopes thiszone extended down to 631N. For 201 steep pole-facingslopes, the active layer is predicted to be in the 871N–521Nzone. Of course, for the reasons we describe above, thesenumbers should not be viewed as firm specific determina-tions. Instead, they provide an assessment of how thetemporal and spatial distribution of the occurrence of anactive layer might appear. The formation of an active layer,especially on a slope, is strongly controlled by the localenvironment. For example, at mid-latitudes alcoves onsteep walls can shield the surface from the cold sky duringthe nights and can raise the day-average surface tempera-tures. The same topographic features at high latitudes cancast shadows during the day and lower the temperature. Ifthe surface at high latitude were covered with bright ice, theformation of an active layer would not occur even at 901obliquity.The nature and intensity of active layer processes depend

not only on surface temperature, but also on theavailability of water ice to melt and freeze, and on itsabundance. As shown recently by GCM calculations,obliquity changes cause migration of water ice betweenpolar regions and mid- and/or low-latitudes (e.g., Mischnaet al., 2003; Levrard et al., 2004; Forget et al., 2006). Thusunder some conditions even surface ice can be available forsummer melting. Mellon and Jakosky (1995) show that theground ice stability zone expands from high latitudes tomid-latitudes with an increase in obliquity, and there areno published predictions of ground desiccation in the polarregion. Thus, it is quite probable that at least some ice isalways available for active layer processes at medium tohigh latitudes. The proximity to the sun in summer due to

ARTICLE IN PRESS

Fig. 11. Upper panel, the history of obliquity and eccentricity of Mars for the last 10Ma, data from Laskar et al. (2004). Lower panel, year-maximum

day-average insolation at the poles of Mars calculated from spin/orbital evolution for the last 10Ma. Thin horizontal line marks the estimate of the active

layer threshold. (a) Four insolation peaks in the southern hemisphere when insolation approaches the estimated threshold. (b)–(c) Four and five insolation

peaks in the northern hemisphere, when the insolation exceeded the estimated threshold. Vertical lines 2 and 3 mark points that correspond to panels 2 and

3 in Fig. 5.

M.A. Kreslavsky et al. / Planetary and Space Science 56 (2008) 289–302 299

the eccentric orbit also plays an important role inproducing a high insolation level. For example, the highestinsolation peak 9.45Ma ago occurred at 441 obliquity,noticeably lower than the maximum over 10Ma (481).

Recently, detection and mapping of a latitude-dependentsmoothing of high latitude topography (Kreslavsky andHead, 2000, 2002), together with evidence for a latitudin-ally distributed dissected mantle (Mustard et al., 2001) andrelated features (Milliken et al., 2003) have led to thehypothesis that these deposits were emplaced during an‘‘ice age’’ characterized by high amplitudes of obliquityoscillations that took place from about 0.4–2.1Ma ago(Head et al., 2003). Subsequent to this time, as theamplitude of the obliquity of Mars decreased, the deposithas undergone desiccation in the 301 to 501N–S latituderange. Could an active layer have been formed during thistime period? This deposit is characterized by a series ofdistinctive morphological features that could be cited asevidence for the presence of an active layer. The mostwidespread is a type of pattern ground that resemblespolygonal ground. This distinctive mounded terrain has

been called ‘‘basketball terrain’’ due to its close similarityto the texture of the surface of a basketball (e.g.,Kreslavsky and Head, 2003). Most polygons associatedwith active layers are characterized by angular patterns ofice wedges, sometimes with raised rims. The basketballterrain, however, widespread in the regions from �601north and south latitudes to the polar deposits, consists ofpolygonal mounded patterns. These are very similar inmorphology and scale to ‘‘sublimation polygons’’ found inthe hyperarid, Mars-like polar deserts of the Antarctic DryValleys (Marchant et al., 2002). These features form fromthermal cycling 50 1C in the absence of an active layer andderive their rounded morphology from sublimation pro-cesses operating preferentially at the cracked polygonmargins. These sublimation polygons form in the ‘‘uplandfrozen zone’’ of the Antarctic Dry Valleys (Marchant andHead, 2005), and are very distinct in morphology andstructure from the ice wedge and sand wedge polygonsforming in the coastal thaw zone and mixed zone of theAntarctic Dry Valleys where active layers are present(Marchant and Head, 2005). Thus, the basketball texture

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on Mars could have formed by thermal cycling in theabsence of an active layer, as predicted by non-active layerconditions in the last 5Ma (Fig. 11). Other features relatedto the latitude-dependent layer and attributed to formationin association with the ice age and its aftermath (Millikenet al., 2003; Head et al., 2003) include dissected terrain,seen in the 301 to 501N and S latitude regions. This consistsof irregular pits and depressions in the mantle that havebeen described as ‘‘cryokarst’’ and interpreted to be due toeolian deflation and sublimation of the ice-rich layer duringthe interglacial period when obliquity decreased and icebegan to migrate from the lower latitude portions of themantle, where it was now unstable, back to the polarregions. Lack of evidence of melting associated with thesefeatures suggested that they represented ‘‘cryokarst’’ ratherthan ‘‘thermokarst’’, where melting is implied. Viscous flowfeatures and gullies were also observed concentrated in the30–501N and S latitude bands and are thought to be due tolocal microenvironments (largely impact crater interiors)that accumulated greater-than-average accumulations ofice and snow, which underwent localized flow and minormelting during and subsequent to the recent ice age period.Thus, none of the major features observed in associationwith the recent ice ages and their aftermath require thepresence of an areally extensive active layer, an observationthat is consistent with the predictions of the lack of anactive layer in the last 5Ma (Fig. 11).

Geologically recent intra-crater ice-wedge polygons areobserved in 50–751 latitude zones in both hemispheres(Mangold, 2005). The low-latitude boundary of these zonesis at somewhat lower latitude than the extent of thegeologically recent (in the 5–10Ma period; Fig. 11) activelayer predicted above. This minor difference is notsurprising, bearing in mind the general nature of ourestimates. Small-scale sublimation polygons are formed ontop of the larger ice-wedge polygons (Fig. 2) (Mangold,2005), which is consistent with the predicted absence of theactive layer in the most recent epoch (in the period0–5Ma). It is not clear why the ice-wedge polygons appearonly in craters and have not been observed outside craters.Insolation conditions in the center of a flat crater floor andon the surrounding plains are the same. Elevation itselfcould not play a major role, because similar polygons are inboth hemispheres at very different absolute elevation. Thelocal environment in the crater interiors may favoraccumulation of larger amounts of water (ice/snow) readyfor a seasonal thawing/freezing cycle, or water mightpreferentially flow to crater interiors in previous activezone periods. An alternative explanation could be relatedto albedo control of the surface temperature. Migration ofwater ice during times of obliquity change are predicted toform high-latitude icy mantles (Head et al., 2003). Aspreviously discussed, traces of such mantles are observed inmorphology (Kreslavsky and Head, 2000; Mustard et al.,2001) and predicted by atmospheric general circulationmodels (Levrard et al., 2004). Such a freshly formed mantlemay have high-albedo ice at its surface, which would

protect the surface from heating and prevent the formationof an active layer. Craters may also act as traps for wind-blown dust and migrating sand and their floors could havea relatively low albedo and be susceptible to increasedheating. Finally, the apparent superposition of the basket-ball terrain interpreted to be sublimation polygons on theice-wedge polygons (Fig. 2) suggests that these patternswere formed in different periods of recent geologicalhistory and that the period of active layer formationhas been superceded by a period without active layerformation.The specific predictions concerning the formation and

evolution of an active layer in the past history of Marsdescribed in this paper can be tested with existing data andthe distinctive landforms can be distinguished from thoseproduced by other processes. High-resolution MOC imagedata (e.g., Malin and Edgett, 2001) provide the ability todistinguish landforms at several meters-scale resolution,and thermal emission spectrometer (TES) data andTHEMIS data permit the mapping of the thermal inertiaproperties at several resolutions. Mars Express high-resolution stereo camera (HRSC) image and stereo data,as well as OMEGA imaging spectrometer data, providekey contributions. Features identified and characterizedwith these data can be compared with the topography andslope data obtained by the Mars Orbiter Laser Altimeter(MOLA), to test these hypotheses. Furthermore, upcomingorbital missions (e.g., Mars Reconnaissance Orbiter) willprovide even higher surface image resolution and support-ing data, and landers (e.g., Phoenix) and future increasedsurface mobility will permit in situ analysis of surfaceconditions and landforms.When will conditions conducive to active layer forma-

tion recur in the future? Projections by Laskar et al. (2004)to 10Ma into the future predict that obliquity will be at orbelow the range typical of the past few million years, andthus conditions are not likely to be conducive to activelayer formation in the foreseeable future.

Acknowledgments

Discussions with N. Mangold were extremely helpful.The work was partly supported by NASA Grant NAG5-12286 (MK) and NASA Grant NNG04GJ99G (JH), whichare gratefully acknowledged. Thanks are extended to AnneCote for help in manuscript preparation.

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