Geomorphic Evidence for the Distribution of Ground Ice on Mars

High-resolution Viking orbiter images show evidence for quasi-viscous relaxation of topography. The relaxation is believed to be due to creep deformation of ice in near-surface materials. The global distribution of the inferred ground ice shows a pronounced latitudinal dependence. The equatorial regions of Mars appear to be ice-poor, while the heavily cratered terrain poleward of � 30� latitude appears to be ice-rich. The style of creep poleward of � 30� varies with latitude, possibly due to variations in ice rheology with temperature. The distribution suggests that ice at low latitudes, which is not in equilibrium with the present atmosphere, has been lost via sublimation and diffusion through the regolith, thereby causing a net poleward transport of ice over martian history.

H.HfPso2PcH2o x dt lKl 02 kZ (1 + Kh) [H] [H+]2 9. Values for the Henry's law constants for SO2 and CH20 at 298 K are H, = i.2+sM atm ' and Hf = 6.3 x io'M atm' (in terms of hydrated CH20). Pi is the partial pressure of species i. The acidity constants for S( lV) are Ka. = 1.29 X IO2M and Ka2 = 6.0I x io'M. The hydration constant for CH20 is Kh = 2.53 X IO'. The original references and enthalpy data are given by Munger et al. (i).
The rate constants are k, = 7.9 X io2M' sec-(activation energy = 5.9 kcal molP'); k2 = 2.48 X io7M sec' (activation energy =   tion of subsurface ice on Mars is critical for understanding the planet's volatile inventory and will be a major concem for future martian exploration. Many martian landforms suggest the former presence of ground ice or water, including fretted and chaotic terrain (1), valley systems (2), outflow channels (3), and, with less certainty, various types of pattemed ground (4) and rampart craters (5). None of these landforms provides strong evidence for the present distribution, however. If sufficient ice is present now, the regolith should undergo quasi-viscous flow due to creep deformation of the ice. Accordingly, to determine where ice may be present, we have examined the approximately 24,000 Viking Orbiter images taken within 5,000 km of the surface dard solutions. Analytical error was within s percent. I6. Total CH20 was determined in a separate aliquot of sample to which Nash reagent was added imnmediately after collection [T. Nash, Biochem J. 55, 4I6 (I953)]. Prior to color determination, I2 was added to destroy the S(IV) in the sample [R. V. Smith and P. W. Erhardt,Anal. Chem. 47, 2462(1975]. Recovery of NaCH20HSO3 standards was 90 to I00 percent compared to CH20 standards. Minimal losses of CH20 were observed during storage. Analytical error was less than s percent. U7. HMSA was determined by ion-pairing chromatography on a polystyrene divinylbenzene column (Dionex MPIC) followed by a suppressor column containing cation-exchange resin in the Ag+ form (Dionex ISR), and a conductivity detector. The mobile phase contained 2 mM tetrabutylamrnmonium chlorideg s percent (by volume) CH30H, and 2 X I0-M HCI. Analytical error was less than S percent. Samples to be analyzed for sulfonic acids were refrigerated, but no preservatives were used. I8. California Air Resources Board, unpublished data.
Lehrman, Atmos. Environ., in press. 2o. Supported by the California Air Resources Board (contract A4-075-32) and a summer undergraduate research fellowship from California Institute of Technology (C.T.). We thank D. Jacob, J. Waldman, D. Bucholz, and S. Hawes for their contributions to this work.
Lobate debris aprons are accumulations of erosional debris at the base of steep escarpments (7). Surface lineations and compressional ridges adjacent to obstacles indicate that the constituent materials have flowed away from the escarpment. Distinct convex-upward topographic profiles suggest creep deformation throughout the entire thickness of the debris apron. They are morphologically similar to terrestrial rock S. W. Squyres

REPORTS 249
Geomorphic Evidence for the Distribution of Ground Ice on Mars STEVEN W. SQUYRES AND MICHAEL H. CARR High-resolution Viking orbiter images show evidence for quasi-viscous relaxation of topography. The relaxation is believed to be due to creep deformation of ice in nearsurface materials. The global distribution of the inferred ground ice shows a pronounced latitudinal dependence. The equatorial regions ofMars appear to be ice-poor, while the heavily cratered terrain poleward of ±300 latitude appears to be ice-rich. The style of creep poleward of ±30°varies with latitude, possibly due to variations in ice rheology with temperature. The distribution suggests that ice at low latitudes, which is not in equilibrium with the present atmosphere, has been lost via sublimation and diffusion through the regolith, thereby causing a net poleward transport of ice over martian history. widespread toward the south pol.
It has been postulated that movement Of lobate debris aprons is aided iby creep of interstial ice- (7). Creep of fini surface debris can occur in ice-fire ials under some circumances. On the Moon, small debris aprons (<1 kin) com occur at the base ofsteep slopes (10).
ever, the debris aprons on the moonr moe xhn an order of m u than those on Mars and do sho t flow incations charactic of t lbate debris aprons and terial ro a ciers. Furthermore, tie distbutionof iim lobate debris aprons is not,unifr that on the moon but is stgl d on latitue. The lattudinal dependenceagests that climatic facto are i rnt dtir development, as would bec WC were involved. Tlh pstulatd i the debris apron was probably largely d rived fiom the escarpmen that was eroded to provide the debris and may hive been supplemete-d by condensation from t atmosphere (7).
We also considered different processes that could lead to terran softening. Atmospheric obscuration is negligible because of our screenig of the imagos. Tis is shown by Fig. 1B, in which veryfinc surface texhures are visible in the sned tein; :dely the perceived softening is not due to a spheri efcts. The two Wmost lly causes of the softening are buril by colian 'debris and creep of the near-surface materials. We regard creep asthc probable cause.
Sedimntiion wold leave m teild over the terrain, producing domncantlycave upward slopes and hiding sharp basal breaks in slope. The rvese is obseed: slopes on crater walls tend to be convex upward, and the sharpest breaks in slope occur at the bottom of slopes rathr,than at rin crests, which is what is e fiom aeep. Also, olia maing wol p taly obscure small-scale knobs, pits, and impact craters, yet such -fatures arc very com_m in many areas of highly softeed terrainEolian mantli does occur in many rgin on Mars, as has been inferred from lowto moderate-resolution Mariner 9 im-V (11). We conclude, however, that coan mling clearly cannot account for the morphology observed here at high resolution. Thereore, acrep is probably the dominant process; contribting to terrain soften-W0ar: gue ithat the ai variations in c jpho are due to the presence or :bsenoe of g cnd and to changes in the r: o y of ce 0with latide. Fanale and co-workers (15) have developed a model of diffusional nsport of H20 inthe marnan regolith. Their calculations predict long-term desiccation of quatorial regions and rnn ofground ice at middle and high la . The trsiion zone is predicte to undergo dessicition near the surfce, producing a wlike ;tnning Ofthe desiccated zone and thicken-ing of tie icc-rich zonc with dsnce fr. the cquator. The q predictnsof their model ae in good agreement with obsevations if the features wc havem pp ar the tesult of ice-abett crep. agreemntthefor lnds firther c to the suppsition that ice was involved At latitudeaboe ±55, flowfatur a less de n at latitudes fim 30ti 55" (Fig. 2) (17). The most imo tant fthiese is the variation in b ty which has a dominant priodicty f-1.2 x 105 years. As judged fnr t characteristic of terrestrial p f the skin depth. afcctted by warming over e obliquty cycle is approximtely:00.
-D.ZQ X It is useful to compare our observed distribution with the distribution of other features that provide evidence for former ground ice and ground water on Mars. The most definitive such evidence comes from valley systems, chaotic terrain, oufflow channels, and fretted terrain. Valley systems are believed to have formed by sapping and are found only in the ancient cratered terrain (2). They are present over a wide range of latitudes and may be weakly concentrated toward equatorial regions. Chaotic terrain and outflow channels are thought to result from melting and rapid release of large amounts of ground ice (3), while fretted terrain may result from more gradual release of ground ice by either melting or sublimation (1). All are most common at low to middle latitudes. Chaotic and fretted terrain dissect the highland materials; outflow channels, commonly arising from chaotic terrain, debouch onto the lowland plains. Crater age dating shows that outflow channel formation took place after the formation of the ancient highlands but still fairly early in martian history (18), while valley systems formed during the very earliest part of the planet's history (2). Their distributions are therefore consistent with the concept that the equatorial regions were once rich in water and ice and have subsequently been desiccated.
The origin of the ice in the martian regolith is unclear. The many lines of evidence implying that ice was common in the cratered uplands early in martian history suggest that the ice was emplaced during an early period of intense outgassing. The alternative extreme is continuous outgassing throughout the planet's history at a rate substantially lower than the low-latitude depletion rates in order to keep the low latitudes ice-free. In either case, intense early meteoritic brecciation was probably largely responsible for the apparent capability ofthe deep regolith to hold large amounts of water. We cannot determine what fraction of ice must be present in the near-surface materials to produce the features we have mapped. Even if only modest fractions (5 to 10 percent by volume) are present, then the data presented here suggest that the total inventory of water at the surface is close to the upper limit of the post-Viking estimates of 10 to 100 m spread evenly over the surface of the planet (19).
The human T-celi -y chain genes have been characterized in an attempt to better understand their role in immune response. These immunoglobulin-like genes are encoded in the genome in variable, joining, and constant segments. The human 'y genes include at least six variable region genes, two joining segments, and two constantregion genes in germline DNA. Variable and joining segments recombine during the development of T cells to form rearranged genes. The diversity of human y genes produced by this recombinational mechanism is greater than that produced by the murine genome but is more limited than that of other immunoglobulin-like genes. N ACTIVE y GENE IS FORMED BY immunoglobulin-like rearrangeent of variable (V) and joining (J) segments that occurs during somatic differentiation of T cells (1)(2)(3). It has been suggested that the -y chain plays a role as a cell surface receptor because of its similarities to immunoglobulin and T-cell antigen receptor a and 13 chains (2)(3)(4)(5)(6)(7)(8)(9). However, the polypeptide chain has not yet been identified and its precise function remains uncertain. One approach to understanding the function of this molecule is to characterize the diversity ofexpressed -y chains. The mouse expresses a very limited number of -y chains because its genome encodes only three variable region genes. Only one of these participates in the formation of a functional gene (3).
There are two -y chain constant regions in the human genome that reside on the short arm of chromosome 7 (10). One of these constant (C) region gene segments is deleted from the genome of most human T cells. Here we report that the two C region segments are encoded about 12 kilobases (kb) apart, with at least one J region segment 5' of each constant region. These J region segments recombine with multiple (at least three) variable region segments, suggesting that the human genome encodes more y chain diversity than the murine genome.
To determine the structure of the human -y chain polypeptide and to obtain probes to the human V and J region segments we isolated a y chain complementary DNA (cDNA) clone (11). The cDNA clone, pT-y-1, was isolated by screening a cDNA library made from the cell HPB-MLT with a DNA probe specific for the human -y chain C region (10). The nucleotide sequence of pT-y-1 was determined (12) and V, J, and C regions were identified by homology to the mouse -y gene (Fig. iB). The two y chain C region genes were