H on Mars: Effects of floods, volcanism, impacts, and polar processes

ICARUS

(1990)

87,210-227

D/H on Mars: Effects of Floods, Volcanism, Polar Processes MICHAEL U.S. Geological

impacts,

and

H. CARR

Survey, MS-946, 345 MiddleJield Road, Menlo Park, California 94025 Received January 2, 1990; revised March 30, 1990

Water in the Martian atmosphere is 5.1 times more enriched in deuterium than terrestrial water. The enrichment has been previously attributed to either a massive loss of water early in the planet’s history or the presence of only a very small reservoir of water that has exchanged with the atmosphere over geologic time. Both these interpretations appear inconsistent with geologic evidence of huge floods and sustained volcanism. Large goods are believed to have episodically introduced large amounts of water onto the surface. During a huge flood roughly 1CP7g of water would almost immediately sublime into the atmosphere and be frozen out on the polar layered terrain, to form a new layer several centimeters thick. The long-term effect of a tlood would depend on where the water pooled after the good. ff the water pooled at low latitudes, all the water would slowly sublime into the atmosphere and ultimately be frozen out at the poles, thereby adding several meters to the polar deposits for each good. If the water pooled at high latitude, it would form a permanent ice deposit, largely isolated from further interchange with the atmosphere. Volcanism has also episodically introduced water into the atmosphere. Most of this water has become incorporated into the polar deposits. That released over the last 3.5 Ga could have added a few kilometers to the polar deposits, depending on the amount of dust incorporated along with the ice. Large cometary impacts would have introduced additional huge amounts of water into the atmosphere. The long-term evolution of D/H in the atmosphere depends on the rate of exchange of water between the atmosphere and the polar deposits. If exchange is active, then loss rates of hydrogen from the upper atmosphere are substantially higher than those estimated by Y. L. Yung, J. Wen, J. P. Pinto, M. Allen, K. K. Pierce, and S. Paulsen [Icunrs 76,146159 (1988)]. More plausibly, exchange of water between the atmosphere and the polar deposits is limited, so that after eruptions, floods, and cometary impacts, the atmosphere soon becomes enriched in deuterhtm. According to tbii scenario, the atmospheric D/H is different from the bulk of the planet’s water and so reveals little about the amount of water outgassed. The scenario implies, however, that the polar deposits are older and more stable than formerly thought. o 1990 Academic PM, h.

1. INTRODUCTION

This paper interprets the recently measured D/H ratio in the Martian atmosphere in light of the general model for the geologic evolution of the Martian surface that has evolved over the last two decades, and in light of the fact that water must have been introduced into the Martian atmosphere throughout its history by asteroids and comets. The D/H ratio on Mars was first

measured by Owen et al. (1988) from Doppler shifted HDO lines in the Mars spectrum as observed from Mauna Kea. They showed that the D/H ratio in the Martian atmosphere is 6 t 3 times the terrestrial value of 1.5 x 1O-4for standard mean ocean water (SMOW). Bjoraker et al. (1989) subsequently refined the measurement using the Kuiper Airborne Observatory. This latest estimate for D/H in the Martian atmosphere is 5.15 -+ 0.2 times SMOW. The sim210

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D/H ON MARS ilarity in D/H values between the Earth and most meteorites suggests that the initial D/ H ratio on Mars was similar to that on Earth (Geiss and Reeves 1981). The enrichment is believed to be caused almost entirely by preferential loss of hydrogen with respect to deuterium from the upper atmosphere (Owen et al. 1988, Yung et al. 1988). Owen e t al. (1988) suggested that the high enrichment of deuterium at Mars was due to massive loss of hydrogen from the upper atmosphere early in the planet's history. Their conclusion was based on a general model for isotopic fractionation developed by Hunten (1982), who showed that, to get an enrichment as high as that measured at Mars by isotopic fractionation, almost the entire original inventory of the fractionating species must have been lost. Owen et al. (1988) assumed that the measured D / H ratio was representative of the entire nearsurface inventory and accordingly concluded that over 99% of the water originally outgassed to the surface must have been lost. Because current loss rates fall far short of the rates required to eliminate such a large fraction of the near-surface inventory, for any reasonable estimate of its size, Owen et al. suggested that the hydrogen loss occurred mainly during the first Ga of the planet's history for which warmer climates have been postulated. The warmer climates allowed more water in the atmosphere and, therefore, enhanced the loss rates of hydrogen. An alternative explanation of the deuterium enrichment was pro,posed by Yung et al. (1988). They calculated current H and D loss rates, and assumed that these were sustained throughout geologic time. Given the low loss rates, they suggested that the fractionating reservoir must have been small to get the observed enrichment. They estimated that only 3.6 m of water averaged over the whole planet could have exchanged with the atmosphere over geologic time, and of the 3.6 m only 0.2 m remains. Thus the Yung et al. model, in contrast with the Owen et al. model, assumes that the mea-

211

sured D/H ratio applies only to the water that exchanges with the atmosphere, and that the rest of the water on the planet, if any, is unfractionated. Both the Owen et al. model and the Yung et al. model appear to conflict with the geologic evidence• Early loss of over 99% of the planet's outgassed water as suggested by Owen et al. is difficult to reconcile with evidence of large floods and other indicators of water and ice at the surface, not just early in the planet's history, but throughout its history. The flood features suggest that at least 0.5 km of water was present near the Martian surface when they formed (Carr 1986). Since the features used to make this estimate formed after most of the branching valley networks, which represent the main evidence for an early, warm, wet period on Mars (see, for example, Pollack et al. 1987), there, apparently, was still abundant water on Mars after the postulated, early, warm, wet period. The Yung et al. model appears to conflict with geologic evidence of introduction of significant amounts of water onto the surface at various times in the planet's history. Floods are likely to have introduced sizable amounts of water into the atmosphere. Indeed, many times the entire inventory of water in the atmosphere could have been introduced onto the surface in a single flood (see below). In addition, significant amounts of water may have exchanged between the poles and the atmosphere, particularly during periods of high obliquity (Jakosky, in press), and additional amounts must have been introduced episodically into the atmosphere by volcanic eruptions (Greeley 1987, Tanaka e t al. 1988) and asteroidal and cometary impacts (Chyba 1987, 1990). Jakosky (in press) noted the apparent inconsistency between the Yung et al. model of limited interchange of water between the ground and the atmosphere aad theoretical modeling, which suggests that the north pole is currently losing water to the South pole. He suggested that current loss rates of deuterium and hydrogen apply only to t[ae

212

MICHAEL H. CARR

present position in the obliquity cycle. At higher obliquities the atmosphere may contain much more water than at present, and hydrogen and deuterium loss rates at these times would be correspondingly higher. He suggested, therefore, that Yung el al. had underestimated the amount of water that could have interchanged with the atmosphere over geologic time. This paper presents an alternative model to explain the deuterium enrichment. The model allows for preservation of significant amounts of water near the surface late into Mars history, and for episodic introduction of water into the atmosphere and the ground as the result of floods, volcanic eruptions, and impacts. It is proposed that the measured D/H enrichment of 5.1 applies only to the atmosphere and the small amount of water, mostly at high latitudes, that currently exchanges with the atmosphere. The bulk of the outgassed water is assumed to be less highly enriched. Following an event such as a flood, a volcanic eruption, or a large impact, a relatively large amount of water is introduced into the atmosphere. In a geologically short period of time, this water accumulates at the poles. The newly introduced water would initially have the D/H ratio of the bulk of the outgassed water or that of meteorites. Subsequently, however, the atmospheric water and that which is exchangeable with the atmosphere would become enriched in deuterium, on a geologically short time scale. The atmospheric water would remain enriched until another major event introduced large amounts of water into the atmosphere and reset the D/H ratio. This scenario allows the D/H enrichment of the bulk of the near-surface water to be significantly less than the 5.1 measured for the atmosphere, and so does not require early loss of almost all the outgassed water. However, the model implies that exchange of water between the atmosphere and the polar layered terrains is limited, and that the polar layered terrains are older and more stable than formerly thought, a sup-

position supported by recent crater counts by Plaut et al. (1988). 2. DEUTERIUM ENRICHMENT MECHANISMS

Hunten (1982) developed a generalized model for isotopic fractionation as a result of thermal and nonthermal escape from the upper atmosphere. He showed that for any fractionating species the ratio of the light to the heavy isotope will evolve from its initial value r0 to its present value r according to the expression r/ro = ( M / M o ) x, where M0 and M are the initial and present inventories of the lighter species. The value of x depends, among other things, on the isotope mass and the escape mechanisms. Owen et al. (1988) suggest that for D/H on Mars the appropriate value for x is 0.2. According to this model, for the observed enrichment of D/H of 5.1, less than 0.1% of the initial inventory of exchangeable water remains; 99.9% has been lost. Recognizing that such massive loss is unlikely by mechanisms operating today, Owen et al. concluded that the most of the loss occurred early in the planet's history for which warmer climates have been suggested. Warm climates would have allowed more water in the atmosphere and hence caused higher rates of loss of hydrogen. Yung et al. (1988) constructed a detailed photochemical model to estimate present loss rates of hydrogen and deuterium from the upper atmosphere of Mars. Their model takes into account photochemical production of H2 and HD, transport of H2 and HD to the upper atmosphere, dissociation of HE and HD, and thermal escape of H and D. They find that the present escape rate of hydrogen (~l) is 1.6 x 108 atoms cm -2 sec -1 and that of deuterium (~2) is 8.3 × 103 atoms cm -2 sec -1. They define an efficiency factor (E) that relates the deuterium loss rates to the D/H ratio in water in the ground E = (~J~2)/([HDO]0/2[H20]0)

(1)

where [HDO]0 and [H20]0 are the concentrations of HDO and H20, respectively, in the ground. They estimate the value of E to

D/H ON MARS be 0.32. Yung et al. examined two simple models to explain the observed deuterium enrichment. In the first model new water is introduced into the atmosphere at a rate just sufficient to offset losses at the top of the atmosphere. In this case the isotopic ratio in the atmosphere will evolve in time t according to f ( t ) = f ( O ) [ 1 / E - ([1 - E ] / E ) exp(-EOlt/ao)]

(2)

213

enrichment, the amount remaining is 0.2 m. Yung et al. concluded, therefore, either that Mars has outgassed very little water (3.4 m) or that very little of the outgassed water has been in contact with the atmosphere. Both the Owen et al. (1988) and Yung et al. (1988) models are somewhat idealized. In the next few sections we examine how different geologic processes might have affected interchange of water between the ground and the atmosphere, and how meteoritic infall might have added water to the atmosphere. At the end of the paper we return to examine how these processes would have affected D/H in the atmosphere.

where f ( t ) and f(0) are the D/H ratios in the atmosphere and ground respectively, and a0 is the amount of water in the atmosphere. The enrichment reaches a limit of approximately 3f(0) in time tUm, approximately ao/ECbl. For the present atmosphere and loss rates, tlim is approximately 105 3. FLOODS years. However, the enrichment that is The evidence for large floods on Mars is possible by this mechanism falls short of the observed enrichment, if the bulk water well known and has been summarized elsehas a D/H close to the terrestrial value. where (see, for example Baker 1982, Carr Clearly, if the loss rates change in the obli- 1981). Here we focus on how much water quity cycle, as suggested by Jakosky (in might be injected into the atmosphere durpress), or the atmosphere actively (season- ing a large flood. Most of the flood features ally) exchanges with some reservoir of wa- are in the northern hemisphere around ter in the ground, such as the poles or the Chryse Planitia and northwest of Elysium. regolith, in addition to the slow leakage to Much of the water from these floods apoffset losses, then tlimwill be affected corre- pears to have pooled in low-lying areas at spondingly. high northern latitudes. Flow was roughly In the second model Yung et al. assumed northward, down the regional slope. that, throughout the history of the planet, Sculpted landforms, indicative of flowing water in the atmosphere actively ex- water, are found as far north as 35° latitude changed with a reservoir of crustal water. for the Chryse channels and as far north as This reservoir was either isolated from the 43 ° for the Elysium channels. Close to the rest of the planet's water or constituted all termini of the channels and extending from the water available at the surface. In this 35 to 65°N are a variety of landforms that case have been attributed t o t h e presence of ground ice (Carr and Schaber 1977, Rossbaf ( t ) = f(O)[c(O)/(c(t)] ~l-e) (3) cher and Judson 1981, Lucchitta 1981, where c(0) is the amount of water that has McGill 1985, Lucchitta et al. 1986). Other interacted with the atmosphere over the life large flood channels around Hellas termiof the planet and c(t) is the amount of water nate in the center of the Hellas basin, which in the reservoir at time t. This is the same is also at a high latitude. Thus, the water formula used by Owen et al. (1987) except from many of the largest floods appear to that the value of the exponent is different. have pooled at relatively high latitudes The amount of water lost from the reservoir where ground ice is stable (Farmer and over the 4.5 × 109-year life of the planet is Doms 1979). We first examine, therefore, O~t, or 3.4 m. From (3) and the observed the fate of the water introduced onto the

214

MICHAEL H. CARR

surface during a flood that terminates at high latitude. To estimate the amount of water injected into the atmosphere during a flood, a number of assumptions must be made. 1. The climate was assumed to be similar to the present-day climate. This is based on the following observations: (a) Extremely low erosion rates have been maintained throughout most of Mars' history (Arvidson et al. 1979). (b) Cold surface conditions and a thick permafrost may be required for many large floods to occur (Carr 1979). (c) Models of the exchange of volatiles between the regolith, atmosphere, and poles suggest wide excursions of atmospheric pressure are unlikely in the second half of Mars history (Fanale et al. 1982). (d) Crater ages of flood features (Masursky et al. 1977, Carr and Clow 1981) indicate that most of the recognizable floods postdate the early era when most of the branching valleys formed and warmer conditions have been postulated. 2. The temperature of the floodwater is assumed to have been 273°K. This is a reasonableassumption since the water probably erupted from just below the permafrost zone or, if volcanically induced, from within the permafrost zone. The temperature could have been lower if the water was saline and higher if the water was erupted from well below the base of the permafrost. 3. After a flood the water is assumed to have ponded and formed a lake 10 m deep and 106 km 2 in area. The 106 km 2 is based on the distribution of the uniquely mottled plains with polygonal fractures that occur at the ends of both the circum-Chryse and Elysium outflow channels (Lucchitta et al. 1986, McGill 1985). The presence of these features in low areas at the ends of large channels suggests that formation of the features was somehow connected with accumulation of water and eroded debris from the floods. There is clearly great uncertainty here. The size of the terminal lake would have depended on size of the individ-

ual flood. While the total volume of water that passed through the flood channels can be roughly estimated from the eroded volumes (Carr 1986), the channels we presently observe are likely to have formed by multiple events and the size of each discrete flood is unknown. The assumption of a 10-m depth is also somewhat arbitrary. Fortunately, as will be demonstrated below, for these high-latitude deposits, the amount of water sublimed into the atmosphere is almost independent of the depth of the terminal lake, provided it is more than several meters deep, because when the ice cover becomes several meters thick sublimation rates fall to extremely low values. 4. The floods are assumed to have lasted 106 sec, or about 2 weeks. This is a reasonable assumption. Irrespective of origin, floods are always short-lived because of the difficulty of sustaining high discharges at the source. The Channeled Scablands floods, for example, are estimated to have had peak discharges for a few days, and much lower discharges for a week or more (Baker 1973). This assumption affects only the amount of water almost instantaneously introduced into the atmosphere at the time of the flood. It does not affect long-term interchange of water between flood deposits and the atmosphere. For convenience, the events associated with a flood are divided into three stages. S t a g e 1: A c t i v e P h a s e

During this stage water travels through the channels from the source regions to the sinks in the northern plains. Because of the scale of the floods and their expected turbulence (Baker 1982), the flood streams are unlikely to have developed a continuous ice cover. The sublimation rate from the surface of the flowing water was estimated following the method of Toon et al. (1980), using their Eq. (1 I). For a water surface at 273°K, an atmosphere of 7 mb, and a wind speed of 10 m sec -I, the evaporation rate is 3 × 10 -5 g cm -2 sec -~. The area of Ares

D/H ON MARS Valles is roughly 2 x 1015 cm z. Thus for an Ares Valles flood lasting 106 sec, 6 x 1016g of water is estimated to have evaporated into the atmosphere while the flood was in progress. This is equivalent to 0.5 mm spread over the whole planet or a 4-cmthick layer on the polar layered terrain. The estimate is very approximate and could be off by an order of magnitude. The evaporated water would rapidly condense in the atmosphere, possibly to be precipitated downwind from the channel. Once the flood was over, the precipitated ice would resublime into the atmosphere and be trapped out at the poles on a geologically short time scale, probably a few years at most. S t a g e 2: I c e - C o v e r e d L a k e

Following a flood, the water would have pooled in low areas at the end of the flood channel and the resulting lake would have frozen over immediately. At this stage, therefore, we first had an ice-covered lake and the lake progressively froze. When the lake became completely frozen a thermal anomaly would have remained and taken several years to dissipate. On basis of the photogeologic evidence referred to under the assumptions listed above, the water was assumed to have pooled at a mean latitude of 50°N. The rate of sublimation from the lake was estimated using a one-dimensional model similar to that described in Carr (1983). The sublimation rate depends largely on the surface temperature T of the ice, which was determined from the heat balance at the surface of the ice - e o r T 4 + (1 - a)Si + Qc - Qs + LW = 0

(4) where tr is the Stefan-Boltzman constant, e is the infrared emissivity, a is the albedo, Si is the incident solar flux, Qc is the heat conducted up through the ice, Qs is the heat lost through sublimation, and LW is the long wave radiation incident on the surface. The geothermal heat flux into the lake is negligible for the short times being consid-

215

ered. Following Kieffer et al. (1977), an emissivity of 1 was assumed and the longwave radiation was taken as 0.02 times the noontime insolation or surface emission, whichever was greater. The incident solar flux could be modified for different optical depths of the atmosphere. The atmosphere was assumed to be well mixed and to contain I0 precipitable microns of water, which implies a frost point temperature of 197°K. Accordingly, when the surface temperature was below 197°K, Qs was set to zero. When temperatures were above 197°K sublimation rates were determined following Toon et al. (1980). Qc was obtained from temperatures within the ice, which were derived by standard relaxation methods, keeping the base of the ice at 273°K and changing the surface temperature according to Eq. (4). Growth of the ice cover was controlled by the heat lost at the surface and by conduction into the lake floor. After the lake had completely frozen, the decay of the temperature anomaly was followed for 2 x I0 a sec, or about 6 years, by which time the temperature anomaly had largely dissipated. The conductivity, density, and heat capacity assumed for the ice were 2.22 × 105, 0.92, and 2.1 x 107 (cgs), and for the ground surface 2.32 × 105, 1.8, and 1.1 × 107 (cgs). A typical result is shown in Fig. 1. A 10m-deep lake takes about 1 year to freeze, by which time the sublimation rate has fallen from an initial 10-5 to close to 10-8 g cm -2 sec -1. For an albedo of 0.5, a wind speed of 10 m sec -1, and aflood a t L s = 0, 2-3 g cm -2 of water would have sublimed after 6 years, most of it in the first 6 months while the ice cover was thin. If the ice was dirty or had a thin covering of dust that lowered the albedo to 0.25, the total sublimed after 6 years would be 6 to 12 g cm -2, depending mainly on the timing of the flood. Thus, assuming the terminal lake had an area of 106 km 2, approximately 1017g would have evaporated from the lake by the time the anomaly had dissipated. This is roughly the same amount that evaporated while the flood was

216 10-5

MICHAEL H. CARR . . . . . . . .

,

. . . . . . . .

,

. . . . . . . .

,

. . . . . . . .

105 years). However, the accumulation of dust on top of the ice would inhibit sublimation, and this is treated in the next section.

10-6

Stage 3: Dust-Covered Ice Deposit

10-9

........

.......

;;7

Time (seconds)

li

10s

.

.

.

.

.

.

FIG. 1. Sublimation rate at the surface of a 10-mdeep covered lake that formed at 50°N at an Ls of 180°. Ice aibedo is 0.5, wind speed is 10 m s e c - L Figures over the c u r v e s indicate the a m o u n t o f water sublimed (g cm 2 sec-~) in each year.

in progress. Thus, if all the water evaporated by this time had frozen out on top of the polar layered terrains, it would have formed a layer roughly 10 cm thick at each pole. When the thermal anomaly had completely dissipated sublimation rates would have been very low, ranging from essentially zero, for an albedo of 0.5 and an optical depth of the atmosphere of 0.3, to as high as 0.3 g cm -2 year -1, for an albedo of 0.15 and an optical depth of 0.1. The low sublimation rates result because the high latitude and high thermal inertia allow surface temperatures to climb above the frost point of 197°K for only a small part of the year. The atmosphere is likely to have been dusty so that after several years the ice should have been covered with a thin layer of dust, thereby lowering its albedo. For the most probable range of albedo (0.15-0.2) and an atmospheric optical depth of 0.2, sublimation after dissipation of the thermal anomaly should have been in the range of 0.01 to 0.1 g cm -2 year-L At these rates, the entire 10-m-thick ice deposit would dissipate in a geologically short period ( 1 0 4 to

As is evident from the Viking lander pictures and the frequent dust storms on Mars, any newly exposed surface is likely to become covered with dust. This is probably particularly true at high latitudes (Soderblom et al. 1973) where most of the large flood features appear to terminate. In addition, the flood water is likely to have carried a large sediment load. Because the upper surface would have frozen quickly, fine-grained sediment would have been trapped in the ice. Thus, as the ice sublimed, a sediment lag would have accumulated on the surface. We explore here the effect of a dust cover on the rate of sublimation from an ice deposit. Since the rate of accumulation of dust is unknown, we will simply examine what effect dust layers of different thickness would have. The dust affects the sublimation mainly in two ways. First, burial of ice below a dust layer, particularly one that has a low thermal inertia, will reduce daily temperature excursions, so that temperatures in the ice stay close to the daily mean. Second, the water vapor is inhibited from reaching the atmosphere because it must diffuse through the dust layer. The loss of water from the ice through the overlying dust cover was determined by first estimating the temperature, and hence the vapor pressure of water at the soil/ice interface, and then applying the techniques developed by Clifford and Hillel (1983) to determine the rate of diffusion of water upward through the soil. The surface temperature was determined from Eq. (4), setting the sublimation term to zero and using thermal properties appropriate for a soil (thermal inertia of 1.2 to 2.5 × 105 erg cm -z sec -1/2 °K -1, density of 1.2 g cm -3, and heat capacity of 4.4 x 106 erg °K -1 g-~). The temperature profile in the soil and ice was determined using the same relaxation technique used in stage 2.

D/H ON MARS To estimate the rate of diffusion of water vapor through the soil, the diffusion coefficient of water vapor through the CO2 in the pores of the soil had to be determined. Following Clifford and Hillel (1983) the molecular diffusion coefficient of H20 (component A) through CO2 (component B) is given by DAB = 0 . 1 6 5 4 ( T / 2 7 3 . 1 5 ) 3 / 2 ( 1 . 0 1 3

× 106/p)

(5) and the Knudsen coefficient for straight cylindrical pores of radius r is given by DKA = (2r/3)(8TR/zrMA) 1/2.

--DABDKA

To determine the water vapor pressure within the soil profile, the water vapor pressure at the solid/ice interface was kept at saturation for the determined temperature. The vapor pressure at the surface was assumed to be that for a well-mixed atmosphere containing 10 precipitable microns of water or the saturation vapor pressure for the surface temperature, whichever was smaller. The vapor pressure in the profile was then determined by standard relaxation techniques. From Eq. (8) the flux into the atmosphere is given by

(6)

T and P are the temperature and pressure, R is the gas constant, MA is the molecular weight of H20. Following the suggestion of Clifford and Hillel (1983), an effective diffusion coefficient was derived by dividing their Eq. (5) by (1 + DAB/DKA) to give

dnA

JA = DKA +DAB-- nAOtDxA/n dz " (7) JA is the flux of component A, nA the molecular concentration of component A, n is the total molecular concentration of gases in the pores, z is the depth, and a is 1 - (MA/ M s ) t/z, where Ms is the molecular weight of CO2. The third term in the denominator is very small at the low concentrations of water vapor being considered here and can be ignored. Equation (7) is for straight parallel capillaries. Taking into account the tortuosity (q) and porosity (p), we have

217

JA = Dell(P(2) - e ( 1 ) ) / k T z .

(9)

P(1) and P(2) are the vapor pressures at the surface and at depth z, respectively, and k is the Boltzmann constant. Typical results are shown in Figs. 2-5. The curves in these figures shift vertically according to the assumed porosity and tortuosity of the soil, and the horizontal axis is extended or compressed according to the thermal inertia. The rate of loss of water vapor from covered ice deposits depends mainly on the thickness of the soil, the soil

10-3

10-4

~

10-5

v CP

--DABDKAP dnA

cc

(8)

.~10 -6

Following Clifford and Hillel (1983) a tortuosity of 5 was assumed. The effective diffusion coefficient (Deff), the first term on the right-hand side of Eq. (8), was determined for the soil by assuming a log normal distribution of pores. Two cases representative of terrestrial soils were examined in which the log of the pore size was - 7 + 1.5(log m) and - 6 -+ 1.5(log m) [pore size distributions a and h of Clifford and Hillel (1983)].

~1o-7

JA =

(DKA + DAB)q dz "

J

10-8

I

I

20

40

i

[

60 80 Soil Depth (cm)

i

I

100

i

I

120

i

140

FIG. 2. S u b l i m a t i o n rate o f ice at 40°N as a f u n c t i o n

of the thickness of overlying dust. Different curves are for different obliquities as indicated. The conditions assumed are given in the text. The curves steepen abruptly at the ordinate, where the dust cover is thinner than the diurnal skin depth.

218 10-3

MICHAEL H. CARR ,

i

,

,

,

i

,

i

,

,

,

,

,

10-4

~ 10-5 m

.io

g~ -

10-7

10-8

as

/

-~-10-6

20

40

60 80 100 Soil Depth (cm)

I

120

140

FIG. 3. Same as in Fig. 2 but for latitude 50°N. No sublimation occurs at 15° obliquity,

properties, the opacity of the atmosphere, and the obliquity. If the soil c o v e r is thinner than the depth o f penetration of the diurnal wave (typically a few centimeters), the loss rates approach those determined in the previous section for which there is no soil cover. The effect of the diurnal wave causes the curves in Figs. 2-5 to steepen sharply as they approach the ordinate.

When the soil c o v e r exceeds the depth of the diurnal thermal wave, sublimation from the buried ice is controlled by the annual thermal wave, which is sensitive to obliquity. At low obliquity, there is essentially no annual wave so temperatures below the depth of penetration of the diurnal wave are at the mean annual temperature for the assumed latitude. When the soil c o v e r exceeds the depth of the annual thermal wave, typically a few tens of centimeters, the rate of sublimation depends on the mean annual temperature, and sublimation occurs only if mean annual temperature is above the frost point. At 50 ° latitude, for the conditions assumed in Figs. 2-4, this condition is met only at obliquities close to 35 °. H o w e v e r , if the albedo were as low as 0.15 and the optical depth of the atmosphere were as low as 0.1, then sublimation could occur at obliquities as low as 30° . Figures 2 - 4 indicate that if an ice deposit at 50 ° latitude were covered with a soil a few tens of centimeters thick, then at high obliquities water would sublime into the atmosphere at a rate of roughly 10 -6 to 10 -7 g cm -2 year -t. Approximately 10% of the obliquity cycle is spent near 35 ° obliquity (Jakosky, 1990); thus the average sublima-

10-3 10-2

I

I

'

I

I

I

I

~

l

I

I

I

~. 10-4 -~10- 3 ~o~10-5 n"

10-6

3 5 ~

~c10-4

NlO-S

10-7I lo%

, 20 ' ,,:o ' ~o

~

'16o't~o

Soil Depth (cm)

~4o 10-6

' 2b ' , b

' ~

'sb

'1~o'1~o't4o

Soil Depth (cm) FIG. 4. Same as in Fig. 2 but for latitude 60°N. No sublimation occurs at obliquities less than 30°.

FIG. 5. Same as in Fig. 2 but for 0° latitude.

I

D/H ON MARS tion rate over the obliquity cycle is 10-7 to 10-8 g cm -2 year -l. The curves in Figs. 2-4 are for an albedo of 0.22 and an optical depth of 0.2. In the unlikely event of an albedo as low as 0.15 and an almost continuously clear atmosphere (optical depth of 0.1), the deep sublimation rates would increase by almost a factor of 10. On the other hand, if the albedo were 0.27 or the optical depth were 0.3, the present average for when there are no dust storms, the sublimation rate would be zero. Thus, although conditions can be conceived that would result in higher loss rates, and upper bound of 10-7 to 10-8 g cm -2 year -1 for sublimation rates seems reasonable. Although most large floods terminate at high latitudes, some such as Mangala Vallis terminate at low latitude. Figure 5 shows sublimation rates expected for a dust-covered ice deposit at the equator. The assumptions are the same as for Figs. 2-4. If covered with a few tens of centimeters of soil, ice at the equator is expected to sublime at a rate close to 10-s g cm -2 year -1 at all obliquities. Thus the average sublimation rate over the entire obliquity cycle is 100 times higher than at 50 ° latitude. This may be an underestimate of the latitude difference since it ignores the apparent preferential accumulation of debris at high latitudes. At the estimated rate, a 10-m-thick ice deposit would sublime in 108 years. If the debris cover were less, the time to sublime away would be correspondingly shorter as indicated by Fig. 5. In summary, in a typical large flood that terminates at high latitude we should expect a pulse of roughly 1017 g of water vapor into the atmosphere followed by a long period of slow sublimation into the atmosphere. The current atmosphere contains almost 1015 g so the initial pulse of water would reset the D / H ratio in the atmosphere to that of the groundwater. Most of the water initially introduced into the atmosphere would have migrated very rapidly to the poles, become incorporated in the polar layered terrain, and affected the atmo-

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spheric D/H, only insofar as there was exchange of water vapor between the atmosphere and the poles. How a flood subsequently affected the evolution of D/H in the atmosphere would have depended on the rate at which water from the terminal ice deposits sublimed into the atmosphere. If the flood terminated at high latitude, sublimation rates are expected to be in the range 10-8 to 10 -7 g c m -2 year -l. For the nominal terminal lake area of 10 6 k m 2, the loss would be 108 to 109 g year -1, as compared with 2.4 x 10l° g year -1 currently being lost from the upper atmosphere (Yung et al. 1988). Thus, seepage from a high-latitude terminal ice deposit is unlikely to have been sufficient to offset losses from the upper atmosphere, and if this were the only replenishing source of atmospheric water, the atmosphere would have soon become enriched in deuterium. For a low-latitude terminal deposit, the sublimation rate is estimated to be roughly l0 II g year -1. Seepage at this rate would suppress deuterium enrichment in the atmosphere until the ice deposits had fully sublimed. 4. VOLCANISM

Since volcanic activity introduces water into the atmosphere, it will affect the D/H ratio in the atmosphere. How the D/H has been affected by volcanism depends on a variety of factors such as the total volume of material extruded, the timing of the eruptions, the water content of the magmas, the extent to which the magmas are degassed during an eruption, the size of individual eruptions, the episodicity of the eruptions, and any interaction of the erupted products with water-rich materials at the surface. Since these factors are almost completely unknown, we cannot precisely assess how volcanism has affected the D / H ratio. We can, however, make some judgments based on terrestrial analogies and on what little we know about the timing and volume of erupted products. The best estimate of the volume of volcanic materials extruded onto Mars is that

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by Greeley (1987). On the basis of stratigraphic studies and different absolute cratering chronologies, he estimates that 110 to 180 × 106 km 3 of volcanic materials has been erupted onto the planet's surface in the last 3.5 Ga, and that this volcanism introduced 25 to 41 m of water averaged over the whole planet. Greeley's figures suggest that the rates of volcanism have been declining with time, and that during the last 1 Ga, 3-20 x 10 6 km 3 of volcanic materials has accumulated. Although there are exceptions, most of the activity appears to have involved eruptions of fluid lavas to form plains or shield volcanoes. The landforms, and what little we know of the chemistry of the surface materials, suggest that most of the volcanism has been basaltic. We have little direct evidence of the original water content of Martian basalts. Although the SNC meteorites contain little water (Yang and Epstein 1985, Kerridge 1988), we do not know the extent to which they were degassed during eruption. Presence of hydrous amphiboles indicates that the melts contained at least 0.2% water (Treiman, 1985). Greeley assumed that Martian basalts contain 1 wt% water, by analogy with terrestrial basalts. [For a summary of water in basalts see Basaltic Volcanism Study Project (1981, p. 393, and references cited there).] The 25-41 m of water introduced onto the surface by basaltic volcanism in the last 3.5 Ga is likely to have originated deep within the mantle. It almost certainly would have had a juvenile D/H ratio and not suffered enrichment in deuterium during the early warm period postulated by Owen et al. (1988). Thus, if the near-surface water was enriched during an early warm period, the effects of this enrichment must have been subsequently diluted by addition of juvenile volcanic water. Introduction of water into the atmosphere as a result of volcanism is likely to have been episodic. Direct measurements of water contents of lavas during basaltic eruptions show that most of the water is

lost during fountaining and flow (Swanson and Fabbi 1973). Zimbelman (1985) showed that flows near the summit of Ascreus Mons have an average volume of 0.4 km 3. Much larger flows occur on the plains around Arsia Mons, where flow volumes may be as high as 2000 km 3 (Schaber and Dial 1978). If the lava contained 1 wt% water then one eruption could introduce 1013 to 1017 g of water into the atmosphere. The present atmosphere contains 10 ~5 g of water. Thus, the larger eruptions would rapidly introduce into the atmosphere substantially more water than it normally holds. As in the case of the large floods, the D/H ratio in the atmosphere would be reset to that of the outgassed water, and most of the water would rapidly migrate to the poles and become incorporated into the polar deposits. Thus, a substantial amount of the water outgassed is likely to have been incorporated into the polar deposits, retaining its juvenile D/H ratio, since there would have been little time for atmospheric processes to modify the ratio. Not all Martian eruptions are the quiet effusion of basaltic lava. Other forms of volcanism may have involved surface water that could have had a modified D/H ratio. Pyroclastic eruptions have been proposed in some cases, as, for example, Tyrrhena Patera (Carr 1981), Alba Patera and Hecates Tholus (Mouginis-Mark et al. 1984, Wilson and Mouginis-Mark 1987), and the Medusae Fossae Formation (Scott and Tanaka 1982). These events are likely to have been sporadic, each introducing substantial amounts of water into the atmosphere. Wilson and Mouginis-Mark (1987) estimate that pyroclastic eruptions on Alba Patera introduced 7 to 70 × 1017 g of water into the atmosphere over a short period. In addition, water may have been released into the atmosphere as a result of interaction of lava with ground ice (MouginisMark et al. 1984, Squyres et al. 1987). These events would have affected the atmosphere in the same way as the floods, treated in the previous section. Indeed,

D/H ON MARS

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related to their mass by a 1.25 power law, whereas the short-period comets are related by a 1.7 power law. Assuming these power laws, a 2 x 1015-g long-period comet should hit the planet every 5 Ga and a similar-sized short-period comet every 0.2 to 0.7 Ga, thereby substantially perturbing the D/H in the atmosphere. Thus the effect of long-period comets is negligible but the effect of short-period comets is likely to have been significant. Estimation of the effects of asteroid collisions is more difficult because of uncertainties in the relative impact rates on Mars and the Earth and because of uncertainties in 5. IMPACTS the compositions of the asteroids that imAsteroidal and cometary impacts must pact Mars. However, some simple calculaepisodically introduce water into the Mar- tions suggest that the effects of asteroidal tian atmosphere (Chyba 1987, 1990). Com- water on D/H in the atmosphere are likely ets are estimated to contain roughly 50% to be small. The Basaltic Volcanism Study water (Delsemme 1988) and C-type carbo- Project (1981) estimated that the cratering naceous chondrites contain roughly 5-10% rate on Mars due to asteroids was about 1.5 water (Mason 1971). Weissman (1982) esti- times that on the Earth but could be as mated the cometary impact rates for the much as four times. Taking into account the Earth from the observed distribution of smaller area, the lower impact velocity, and comets and their orbits. Assuming impact the higher gravity on Mars (correction facvelocities of 56 and 29 km/sec for long- and tors in Hartmann 1977), the impact rate for short-period comets, respectively, he con- Mars should range from 0.5 to 1.4 times cluded that a long-period comet of mass 1.9 that on the Earth. Wetherill and Shoemaker X 1013 g impacts the Earth to form a 10-km (1982) estimate that an asteroid with diamediameter crater every 34 million years, and ter 0.52 km or larger hits the Earth every a short-period comet of mass 7.2 x 10 ~3 g 0.08 to 0.2 Ma. Taking the clearly erroneimpacts the Earth to form a 10-km diameter ous assumption that all the asteroids hitting crater every 1.6 million years. Impact rates Mars are carbonaceous chondrites containon Mars are estimated to be 0.7 times the ing 7% water, then a 3 x 10T6-g asteroid terrestrial rate for long-period comets, and would be required to deliver the amount of 2 to 3.6 times the terrestrial rate for short- water presently in the atmosphere. Using period comets (Basaltic Volcanism Study 1.8 scaling between frequency and mass, Project 1981). Correcting for the relative we find that an asteroid of this size should surface areas of Mars and Earth, we find hit Mars every 0.2 to 3.5 Ga. This is clearly that a 1.9 x 1013-g long-period comet im- a large overestimate, since only a small pacts Mars every 17.4 million years and a fraction of the asteroids hitting Mars are 7.2 × 1013-g short-period comet impacts likely to contain 7% water. The effect of Mars every 1.4 to 2.6 million years. If it is asteroid impacts on D/H in the atmosphere, assumed that a comet contains 50% water, therefore, appears small. then a comet of mass about 2 x 1015g would 6. POLAR PROCESSES deposit as much water as is already in the atmosphere. According to Weissman Polar processes are crucial for under(1982), numbers of long-period comets are standing D/H in the atmosphere. If (I) the some floods may be caused by volcano-ice interaction. In summary, significant amounts of juvenile water have been introduced onto the surface in the last 3.5 Ga. The introduction of water would have been episodic, and each episode would have reset the atmospheric D/H to the value of the erupted water, which in most cases is likely to have been the juvenile value. Much of the water would have been rapidly removed from the atmosphere by being frozen out at the poles without having its D / H ratio significantly changed.

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polar layered deposits contain tens of meters of water, as has been suggested (Carl" 1981, Rossbacher and Judson 1981, Jakosky 1990), (2) hydrogen and deuterium loss rates are close to those estimated by Yung e t al. (1988), and (3) the water in the layered deposits actively exchanges with the atmosphere so that it has the same D/H ratio as the atmosphere, then the present enrichment of deuterium in the atmosphere must be inherited from an earlier era, as suggested by Owen et al. (1988). (Present-day loss rates of hydrogen and deuterium fall far short of those required to produce a 5.1 enrichment of such a large reservoir of water.) Each of the three assumptions just listed is examined to see whether there are plausible alternatives to early massive hydrogen loss. The polar layered deposits are believed to be composed of dust and ice, and the deposition of the layers is believed to have been in some way modulated by variations in the oribtal and rotational parameters of the planet. These suppositions are based on (1) the rhythmic nature of the layering, (2) the relatively young age of the deposits as indicated by the scarcity of impact craters, (3) theoretical modeling of the effects of obliquity, eccentricity, and precession on surface conditions, (4) the probable role of condensation of CO2 in scavenging dust from the atmosphere, (5) the observation of relatively large amounts of water over the northern summer cap, (6) the apparent low density of the layered deposits, and (7) the recognition that the poles must act as a cold trap for water introduced into the atmosphere (Murray et al. 1972, 1973, Kieffer et al. 1976, Cutts et al. 1979, Toon et al. 1980, Pollack and Toon 1982, Malin 1986). The polar deposits cover an area of 8 x 105 km 2 at each pole, and appear to be at least 3 km thick (Dzurizin and Blasius, 1975), giving a volume of at least 5 x 106 km 3. The fraction of water present is unknown. Malin (1986) suggested, on the basis of density estimates, that the deposits are primarily water ice. If so, then the polar

layered deposits contain the equivalent of at least 34 m of water averaged over the whole planet. This number could be an overestimate if the density estimate is in error, or an underestimate if the average depth of the deposits is greater than 3 km. As we saw earlier, most of the volcanic water outgassed during the last 3.5 Ga is likely to have been trapped at the poles, in addition to significant amounts of the water from floods and cometary impacts. While the estimate is only approximate, it appears geologically implausible that only 0.2 m of water, or less, is present at the poles as is required by the Yung et al. (1988) model of progressive enrichment of D/H in the atmosphere. Either Yung et al. underestimated current loss rates, or the water at the poles is not isotopically identical to that in the atmosphere, or the deuterium enrichment was inherited from early in the planet's history. In the Introduction we discussed reasons why inheritance of the enrichment from early in the planet's history is unlikely. This conclusion was reinforced by the subsequent discussion which showed that much of the polar water is likely to be juvenile (i.e., volcanic origin), and so could not have been enriched by surface processes. Jakosky (1990) explored the possibility, that Yung et al. (1988) had underestimated the average loss rates from the upper atmosphere. Jakosky suggested that the water content of the upper atmosphere, and hence the hydrogen loss, is sensitive to obliquity. The Yung et al. (1988) estimates of the size of the reservoir interacting with the atmosphere are small because current hydrogen loss rates are small and the D/H enrichment is large. If the loss rates were larger, the reservoir would be proportionately larger to produce the same enrichment. Jakosky estimates that the water content of the atmosphere could be 100 times higher at an obliquity of 30°. Since 20% of the time is spent at obliquities higher than 30° , he suggested that the time-averaged water content of the atmosphere, and by inference the hydrogen loss

D/H ON MARS rates, could be 20 times the current value. If so, then the reservoir of water that has interacted with the atmosphere must be increased from the Yung et al. estimate of 3.6 m to 72 m and the amount remaining increases from 0.2 m to 4 m. However, while enhanced losses may occur at high obliquity, the magnitude of the enhancement may not be as large as Jakosky suggests. The estimates of the enhanced water content of the atmosphere at high obliquities are based on estimates of sublimation rates at the poles (Toon et al. 1980, Jakosky and Carr 1985). But the water content of the entire atmosphere may not be proportionately enhanced, for there is little evidence that at high obliquity the equatorial atmosphere could hold much more water than it does at present. In addition, the 4 m of water estimated to be remaining is still significantly less than that estimated to be present in the polar deposits. Thus, if the atmosphere and polar deposits are isotopically identical, the polar deposits must contain much less water than estimated, or the hydrogen loss rates are even higher than Jakosky's estimates. An alternative is that part of the polar inventory has been protected from interacting with the atmosphere and so is isotopically distinct from the atmosphere. The stability of the layered terrains gives some indication of the extent to which the water they contain has interacted with the atmosphere. Until recently the polar deposits were thought to be extremely young compared with the age of the planet. This was based mainly on the observation by Cutts et al. (1976) that there is less than one crater 300 m or larger in an area of 800,000 km 2, which implies an age of less than 106 years, and on estimates of current depositional rates (4 x 10-2 cm year-l), which would result in the entire polar deposits having accumulated in less than 10 7 years. Given these young ages, the deposits must have either accumulated or been extensively reworked recently (less than 107 years ago). In either case, the polar deposits and the atmosphere would recently

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have had identical D/H ratios. However, recent crater counts on the south polar layered terrain by Plaut et al. (1988) suggest that the layered deposits are older and more stable than formerly thought. They concluded that the surface of the layered terrains has a crater age of 0.12 - 0.04 Ga, that the deposits are at least several hundred million years old, and that they have accumulated debris and ice at a rate no greater than 8 -+ 2 km/Ga. The age of 0.12 Ga is a crater retention age of the surface. The materials that constitute the layered terrain could have been slowly accumulating for much of Mars' history. The main reason for believing that the present atmosphere is actively interchanging with a much larger reservoir of water is a postulated net annum transfer of water from the north to the south pole, as evidenced by a steep north-to-south gradient in water vapor during southern summer (Jakosky 1985). Clearly, if the same unidirectional net flux from pole to pole were maintained from year to year, then the water in the atmosphere would be continually replaced and deuterium enrichment would be suppressed. The evidence for this net annum flow is, however, equivocal. Although the concentration of water over the northern summer pole is high, recent circulation modeling by Haberle and Jakosky (1990) shows that circulation at the poles lacks the intensity and scale to move water far from the poles in the same year. Nevertheless, there could be a net north-to-south flow via temporary storage in the regolith. During the northern winter, flow may be in the opposite direction, aided in part by dust storms (Davies 1981, Pollack et al. 1979), so much so that Davies (1981) argues that this counterflow is so large that there is no net annual exchange between the two hemispheres. Unfortunately, this crucial issue remains unresolved. Yung et al. (1988) showed that if the water in the atmosphere were almost isolated from the rest of the water on the planet, being replenished only to offset losses from the upper atmosphere, it would rapidly un-

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dergo a roughly threefold enrichment. The time over which this occurs is a o / E ~ , where a0 is the reservoir undergoing enrichment, E is 0.32, and ~1 is the time-averaged loss rate from the upper atmosphere (see Section 2). For the present amount of water in the atmosphere and present hydrogen loss rates, the time required is 105 years. This time would be increased in proportion to the size of the reservoir actively exchanging with the atmosphere, and decreased in proportion to the ratio of the time-averaged hydrogen loss rates to the present loss rates. For example, if the atmosphere were interacting with a reservoir containing 100 times the amount of water present in the atmosphere and the time-averaged hydrogen loss rates were 10 times the present rates because of the obliquity effect, the atmosphere would undergo a threefold enrichment in 106 years. A reservoir with 100 times the present atmospheric inventory of water would form a layer of ice roughly 10 cm thick at the poles. We have no way of estimating the amount of water that is currently exchanging with the atmosphere, but the D/H ratio may be telling us that exchange is limited, and that the exchangeable reservoir is so small that the water in the reservoir has become significantly enriched since the last flood, major volcanic eruption, or large cometary impact. If exchange between the polar layered deposits has been limited, as is implied by the relatively old age of their surface, then the evolution of D/H in the atmosphere would have been discontinuous. After large volcanic events, floods, and cometary impacts, its D/H ratio would have been repeatedly reset to the value of the groundwater or juvenile water. Much of the water involved in these events would have frozen out at the poles, thereby adding to the layered deposits. If exchange of water between the atmosphere and poles were limited to the upper few centimeters, the atmosphere would have become enriched, and an enriched veneer would have devel-

oped on top of each newly deposited layer at the pole, but much of the water in the layered deposits would have been isolated from the atmosphere, thereby retaining its initial D/H ratio. According to this scenario, the water in the polar layered terrain is partly enriched and partly unenriched, and the present D/H ratio of the atmosphere is typical of quiescent periods between floods and eruptions. Such a sequence of events cannot account for the full enrichment observed if the exchangeable reservoir is replenished by water with a juvenile D/H to offset upper atmosphere losses. According to Eq. (2), if the water content of the atmosphere is maintained by addition of groundwater to offset losses from the upper atmosphere, D/ H enrichment in the atmosphere will reach a limit of 3.2 over the value in the replenishing water. The observed enrichment is 5.1 so that the replenishing water must have an enrichment of at least 1.6. If the analysis in Sections 3 and 4 is correct, most of the water that periodically resets the atmosphere is volcanic, which would mostly have the juvenile D/H ratio, and is presumably not enriched. However, the water that replenishes the atmosphere between floods and eruptions to offset upper atmosphere losses could be enriched since it could be from a variety of secondary sources, such as ground ice, water adsorbed in the regolith, or polar terrain. This 1.6 enrichment could be inherited from early in the planet's history since it requires loss of a much smaller fraction of the original inventory than the 5.1 enrichment. [Alternatively, the estimate of the efficiency factor in Eq. (1) is in error.] A 1.6 enrichment is consistent with 1.5 to 2.0 enrichment of near-surface water in basalts as determined from analysis of SNC meteorites (Kerridge 1988). The scenario just outlined does not address the origin of the valleys incised into the layered deposits. If these are old, as is implied by the Plaut et al. (1988) crater counts, then the scenario is consistent with the presence of the valleys. The erosional

D/H ON MARS episodes that resulted in the valleys would have reset the D/H ratio in the atmosphere to that of the water in the layered deposits. When erosion was over, the D/H ratio in the atmosphere would have evolved as after floods and eruptions. If the valleys are currently forming, as implied by the Howard (1978) and Cutts and Lewis (1982) models, substantial interchange must be taking place between the atmosphere and the polar deposits, and they have identical D/H ratios. As already indicated, however, such a model is difficult to reconcile with the small size of the reservoir that Yung et al. (1988) estimate is being depleted. In summary, geologic evidence suggests that the polar layered terrains contain a few tens of meters of water averaged over the whole planet. Much of this water, being of volcanic origin, is likely to have originally had a juvenile D/H ratio. If the Yung et al. (1988) estimates of hydrogen loss rates are valid, then this water must have been largely isolated from the atmosphere, at least in the geologically recent past, to account for the present deuterium enrichment of the atmosphere. One possibility is that the polar layered deposits accumulated episodically over geologic time and that, at any one time, only water in the upper few centimeters at most interchanges with the atmosphere. Alternatively, because of obliquity effects, hydrogen loss rates are substantially higher than the Yung et al. estimates, substantial exchange occurs between the polar deposits and the atmosphere, and the atmosphere and polar deposits are isotopically identical. 7. C O N C L U S I O N S

The intent of this paper was to determine if the recently measured D/H ratio in the Martian atmosphere demands a revision of the model for the geological evolution of the planet that has evolved over the last two decades. Of particular interest is the history of the action of water, for previous interpretations of the D/H enrichment implied that very little water had interchanged

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with the atmosphere, contrary to the geologic evidence. While the uncertainties are many, the discussion above suggests that the D/H enrichment in the atmosphere is compatible with previous geologic arguments that large amounts of water have outgassed from the planet, that volanic action periodically introduced juvenile water onto the surface throughout most of the history of the planet, and that the surface has been extensively sculpted by running water. The following are additional conclusions. 1. The atmospheric enrichment in D/H of 5.1 times the terrestrial value is not necessarily that of the bulk of the near-surface water. It is that of the exchangeable nearsurface water. 2. Floods, volcanic eruptions, and cometary impacts have periodically introduced large amounts of water onto the surface. Each such event would have immediately reset the D/H ratio in the atmosphere to that of the groundwater or juvenile water. 3. Most of the water introduced onto the surface by volcanism would have almost immediately frozen out at the poles, thereby adding to the polar layered deposits. 4. The fate of the floodwater would depend on the latitude of the terminal lake. Almost all the water involved in floods that end at low latitudes would ultimately become incorporated into the polar layered deposits. Most of the water from floods that terminate at high latitudes would form permanent ice deposits at these latitudes. 5. Evolution of D/H in the atmospheric between floods and eruptions would depend on the amount of interchange between the atmosphere and the poles. If interchange between the poles and the atmosphere is limited, the atmospheric water becomes enriched in deuterium within a geologically short period (105 to 10 7 years), but the water frozen out at the poles remains largely unenriched. If water has actively exchanged between the poles and the atmosphere throughout geologic time, then water in the two reservoirs must be isotopi-

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cally identical, and the average rates of hydrogen loss from the upper atmosphere must have been substantially greater than the present rates. 6. T h e D / H e n r i c h m e n t o f t h e b u l k o f t h e s u r f a c e w a t e r m a y b e i n t h e r a n g e 1.5 t o 2.0, that measured in SNC meteorites. If so, then the planet may have lost 50-90% of its originally outgassed water early in its history. 7. T h e D / H r a t i o i n t h e a t m o s p h e r e r e veals little, if anything, about the total amount of water that has outgassed from the planet. ACKNOWLEDGMENTS I would like to thank Steve Clifford who provided valuable advice on how to treat the diffusion of water vapor through a soil. The paper also benefited from two thoughtful reviews by Bruce Jakosky and Chris Chyba. I am particularly indebted to Chris Chyba who rightly pointed out the importance of cometary infall in periodically adding water to the atmosphere, thereby changing its D/H ratio. REFERENCES ARVlDSON, R. E., E. A. GUINESS, AND S. W. LEE 1979. Differential aeolian redistribution rates on Mars. Nature 278, 533-535. BAKER, V. R. 1973. Paleohydrology and sedimentology of Lake Missoula flooding in eastern Washington. Geol. Soc. America Spec. Paper 144. BAKER, V. R. 1982. The Channels o f Mars. University of Texas, Austin. Basaltic Volcanism Study Project 1981. Basaltic Volcanism on the Terrestrial Planets. Pergamon, New York. BJORAKER, G. L., M. J. MUMMA, AND H. P. LARSON 1989. The value of D/H in the Martian atmosphere: Measurements of HDO and H20 using the Kuiper Airborne Observatory. Proc. 4th Int. Conf. Mars, Tucson, Jan. 10-13, 1989, pp. 69-70. CARR, M. H. 1979. Formation of Martian flood features by release of water from confined aquifers. J. Geophys. Res. 84, 2995-3007. CAm~, M. H. 1981. The Surface o f Mars. Yale Univ. Press, New Haven, CT. CARR, M. H. 1983. The stability of streams and lakes on Mars. Icarus 56, 476-495. CARR, M. H. 1986. Mars: A water-rich planet? Icarus 68, 187-216. CARR, M. H., AND G. D. CLOW 1981. Martian channels and valleys: Their characteristics, distribution and age. Icarus 48, 91-117.

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