Rain infiltration and crust formation in the extreme arid zone of the Atacama Desert, Chile

ARTICLE IN PRESS Planetary and Space Science 58 (2010) 616–622

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Rain infiltration and crust formation in the extreme arid zone of the Atacama Desert, Chile Wanda L. Davis a,b,c,, Imke de Pater b, Christopher P. McKay a a

NASA Ames Research Center, Space Science & Astrobiology Division, Moffett Field, CA 94035-1000, USA Earth & Planetary Science, University of California, Berkeley, Berkeley, CA 94270, USA c Carl Sagan Center for Life in the Universe, SETI Institute, 515 N. Whisman Road, Mountain View, Ca 94043, USA b

a r t i c l e in f o

a b s t r a c t

Article history: Received 11 March 2009 Received in revised form 22 August 2009 Accepted 26 August 2009 Available online 8 September 2009

A key question in understanding life on Mars under dry(ing) conditions is how arid soils respond to small levels of liquid water. We have conducted a series of simulated rain experiments in the hyperarid core region of the Atacama Desert. Rain amounts from 0.24 to 3.55 mm were applied in the early evening to the soil. We conclude that rain events of less than 1 mm do not saturate the surface, and the soil humidity at the surface remains below 100%. Rain events of 2 mm or more generate free water in the pore space of the soil surface, which may be necessary to support biological activity in the soil. The crust on the surface of the soil is a strong barrier to the diffusion of subsurface moisture and subsequent evaporation. Our results show that once the relative humidity in hyperarid soils begins to fall below 100% the rate of decrease is quite rapid. Thus, the precise value assumed for the limits of life or water activity, do not appreciably change the time of water availability resulting from small desert rains. The Atacama Desert results may be applied to models of (H2O) wetting in the upper soils of Mars due to light rains, melting snow and heavy precipitating fog. & 2009 Elsevier Ltd. All rights reserved.

Keywords: Soil moisture Soil crust Hyperarid Experimental rainwater infiltration Atacama Desert Mars analogue

1. Introduction The availability of water limits life in deserts (Connon et al., 2007; Lester et al., 2007). As aridity increases, plants can no longer survive and photosynthesis only occurs by hypolithic cyanobacteria growing under diaphanous rocks or by chasmolithic cyanobacteria where direct sunlight enters cracked rocks (Potts and Friedmann, 1981; Nienow et al., 1988; Caiola et al., 1993; Cockell and Stokes, 2004; Warren-Rhodes et al., 2006, 2007; Wierzchos et al., 2006; Davila et al., 2008). In the world’s most arid deserts, such as the Atacama Desert, conditions become so arid that even the hypolithic and chasmolithic cyanobacteria communities are rare (Warren-Rhodes et al., 2006) yet soil bacteria are still present (Navarro-Gonza lez et al., 2006; Drees et al., 2006; Connon et al., 2007; Lester et al., 2007). The arid core region (221S to 261S) of the Atacama Desert has ´ been studied (Dose et al., 2001; Cockell et al., 2007; Gomez-Silva et al., 2008) as an analogue for Mars (Friedmann, 1993; Cabrol et al., 2001; Skelley et al., 2006, 2007; Warren-Rhodes et al., 2007)  Corresponding author at: NASA Ames Research Center, Space Science & Astrobiology Division, MS 245-3, Moffett Field, CA 94035-1000, United States. Tel.: + 1 650 604 3186; fax: + 1 650 604 6779. E-mail addresses: [email protected] (W.L. Davis), idepater@astron. berkeley.edu (I. de Pater), [email protected] (C.P. McKay).

0032-0633/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2009.08.011

and reported to contain ‘‘Mars-like’’ soil with respect to soil bacteria, organic material and a non-biological soil oxidant (Navarro-Gonza lez et al., 2003; Quinn et al., 2005). There are locales with virtually no culturable soil bacteria (Bagaley, 2006) and only a small number of non-lichenized fungi have been cultured (Conley et al., 2006). DNA extraction yields low levels (Connon et al., 2007; Lester et al., 2007) compared to the soils in the southern, wetter, parts of the Atacama Desert (Skelley et al., 2005) or along the coast where marine fog provides moisture for hypolithic cyanobacteria, lichen and even cacti (Rundel et al., 1991; Moore, 1998; Ca ceres et al., 2004). There are very low levels of organic material and the organics, which are present, are refractory (Navarro-Gonza lez et al., 2003; Ewing et al., 2006). An oxidizing agent present in the soil equally oxidizes L and D amino acids and L and D sugars (Navarro-Gonza lez et al., 2003; Quinn et al., 2005, 2007). Indeed, if the Mars Viking Landers had sampled in the driest regions of the Atacama Desert they would have returned results similar to what they returned from Mars ´ (Navarro-Gonza lez et al., 2003; Gomez-Silva et al., 2008). To grow and reproduce, bacteria require liquid water as well as a source of organic material and energy. The presence of organic material in the Atacama soil has been documented (Navarro-Gonza lez et al., 2003, 2006; Lester et al., 2007; Connon et al., 2007). The variable that thus appears to control the ability of microorganisms to grow and reproduce in the Atacama

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Desert is the occasional availability of liquid water (Davila et al., 2008). Moisture in the Atacama Desert is highly variable and often in minute quantities. Time between rains can be long. McKay et al. (2003) present a record of 20 years of rain data from Baquedano, Chile, a site in the central Atacama Desert about 100 km north of Yungay, Chile. During the 20 years interval reported, there were only eight months with recorded rain. Five of these eight have monthly total rains of 1.5 mm or less. McKay et al. (2003) also reported on a detailed four-year record at the Yungay site in the Atacama Desert. Using precision rain monitoring equipment, they reported five rains in the four years. Three of the rains were at the limit of detection (0.1 mm), one at 0.3 mm and one at 2.3 mm over a many hour period. High nighttime RH and dew did not produce measurable moisture in the soil or under rocks. Only rain resulted in moisture below the rock. Large rains do occur in the Atacama Desert but they are rare. In the 20 years record at Baquedano (McKay et al., 2003) there was only one heavy rain month. Small rains are much more frequent. Thus, a key question for understanding the biological potential of the hyperarid core of the Atacama Desert is the ability of light rains to produce liquid water in the top surface of the soil at levels sufficient to allow for microbial growth. Low levels of moisture may be insufficient to raise the water potential to the level needed for life. In the Atacama Desert, as would be expected on Mars, the surface crust of soil particles cemented by salt is an important factor that influences water penetration into the soil (Jury and Horton, 2004; Birkeland, 1999) and, in turn, is produced and modified by water movement. Soil bacteria have been reported from layers below the crust (cf., Navarro-Gonza lez et al., 2003; Dong et al., 2007), and at depth (Ewing et al., 2006). Thus, the role of the crust in inhibiting water flow into the soil or trapping water already present, may be key to understanding the distribution of soil bacteria in the arid core of the Atacama. By analogy, the role of crust in evaporation and infiltration has implications for understanding water activity in the upper soils of Mars and any possibility for potentially habitable regions (Amundson et al., 2008). In this paper, we report on experiments to determine water infiltration, subsequent evaporation and crust formation in the extreme arid zone of the Atacama Desert as an analogue for Mars. We do not investigate soil biology per se, but rather the water activity that is required to support biology in soil. In this study, we have taken the limit for microbial life in the soil to correspond to a water activity of greater than 0.95—corresponding to that required for cyanobacteria (see Section 5 below). We focus on small rain events to determine the minimum rain that can provide for sufficient water in the soil to allow bacteria to grow and reproduce. In addition, we seek to quantify the extent that a soil crust inhibits water penetration into the soil and subsequent evaporation.

2. Site description The Atacama Desert is a coastal desert that extends from Chile to Peru along the Pacific coast 171S to 281S between 691W and 711W ¨ (Borgel, 1973; Caviedes, 1973). This desert owes its aridity to the persistent temperature inversion associated with the cool northflowing Humboldt Current and the generally stable position of the strong Pacific anticyclone. The Andes Mountains prevent moisture from the east (Houston and Hartley, 2003). Our study site is located near the University of Antofagasta Desert Research Station located in Yungay, Chile (2414.1130 S, 69152.0640 W) at approximately 1030 m elevation. Yungay is in the basin of an east–west trending valley flanked by low hills that are approximately 500 m higher than the basin floor (Fig. 1). This minimizes any infiltration of fog from the coastal areas. The surface of the soil is covered with an adhesive soil

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Fig. 1. Satellite image of the Yungay, Chile area in the Atacama Desert. The study site is shown. The hill known informally as the ‘‘rock garden’’ is also marked.

Fig. 2. The desert soil at Yungay is inflated with fine grain gypsum, which results in a compressible surface.

matrix that binds soil grains, salts and small rock fragments to form a slight crust of about 5 mm. A few millimeters below the surface, the soil is inflated with fine grain gypsum and as a result, the surface is compressible (Fig. 2). A cemented halite and nitratite soil horizon is present at about 1 m (Ewing et al., 2006), which impedes vertical movement of water owing to the lower permeability of the finer textured sediment beneath the coarse sediment above (Jury and Horton, 2004; Birkeland, 1999). Below about 1.5 m little porosity is evident owing to compression (Ewing et al., 2006). The salts of the Atacama are hygroscopic and as such, they influence the water vapor retention in the soil and more importantly the subsequent evaporation from the soil (Davila et al., 2008). Ewing et al. (2006) and Amundson et al. (2008) provide an in depth discussion of the volumetric abundances of elements for each of the soil horizons in the area of our experiments, and the potential relevance to soil processes on Mars.

3. Methods 3.1. Instrumentation To monitor water activity level during the course of our TM experiments we employed Honeywell (HIH 3610 series) relative humidity (RH) sensors placed at 2 mm and 5 cm depths on a

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4-quadrat plot (each 1 m2). These probes are well suited for low relative humidity conditions because their output is linear with RH. Each consists of a thermoset polymer, three-layered capacitance, platinum electrodes and an on-chip silicon integrated voltage output signal. The polymer provides mechanical protection from dust. Relative humidity was chosen as the method to monitor soil water for three reasons. First, relative humidity is a direct measure of water activity, which is the factor that controls biological growth (e.g. Beaty et al., 2005). Second, measurement of relative humidity is well suited to monitor the moisture content in hyperarid desert soils where water activity between rains can fall to 0.1 and lower (McKay et al., 2003). Third, RH sensors are small and easily implanted into the soil with minor disturbance and can be monitored continuously. A similar RH sensor was placed on the Phoenix Lander arm at the Phoenix landing site (68.221N, 234.251E) on Mars (Smith et al., 2009). We also employed conductivity probes at 2 mm and 5 cm depth, to indicate the presence of liquid water. The sensors are based on the voltage drop across two bare wires 5 mm apart, referenced to a 2.2 kO resistor. Excitation provided by an alternating 5-V current eliminated any polarization effects. The conductivity probes responded in the presence of the liquid water conduction channels between the two electrodes (as described in McKay et al., 2003). When liquid water was present the voltage was measured, as the liquid water receded the drop in the voltage was recorded. This method does not provide an amount for the relative humidity but does indicate the presence or absence of TM liquid water in the soil (Edlefsen and Anderson, 1941). An Onset HOBO 4-channel external sensor measured and recorded the soil TM temperatures at 2 mm depth in each plot. A Campbell 21  data logger recorded the output from the relative humidity chips and conductivity probes, and provided excitation sources. A solar panel was used to maintain battery supplies at full potential for the duration of the experiments. When the RH sensors were exposed to bulk liquid water, the reading exceeded 100% RH. However, tests indicated that the sensors returned to normal operation when dried during ambient conditions. We have replaced RH readings over 100% with that value. The accuracy of the temperature measurements is 71 1C, the RH measurement accuracy is 75%, and the accuracy of the depth, height and thickness measurements are 5%. 3.2. Experimental rain TM

An application of Avian distilled water was applied to each plot (l/m2) since there was no supply of local rainwater. Natural rainwater is not distilled but any choice of salts or minerals added to the distilled water would have been arbitrary. In any case, the salt content of the surface would be expected to dominate over any dissolved solids in natural rainwater. Water was added to the experimental plots in the early evening to minimize immediate evaporation from the desert soil surface during higher temperatures and to avoid dispersion and evaporation

of spray in strong daytime winds. The application device was a compressed air pressure pump sprayer that simulated a light rain as a fine spray. Water was applied from a height of 1.5 m. The amount of water applied was equivalent to rains from 0.24 to 3.55 mm. Successive applications over 5 consecutive days resulted in cumulative simulated rains of 3.6 to 7.1 mm. The frequency and duration of the water applications did not cause surface runoff or ponding at the surface of the plots. Fig. 3 shows the study site with the instrumentation in the 4-quadrat plot, with some portions of the plots scraped. Of note is that the surrounding work areas that have been trampled (not part of the experiment) show a dramatically changed appearance. In both cases the gravel that form the desert pavements are removed from the surface. In our experiment, they are removed mechanically and in the trampled areas they are pushed below the softer soil. 3.3. Surface crust After two water applications, approximately 2 mm of the surface soil was removed from half of the plot above the sensors (sector A). The upper 2 mm of the surface soil was removed from the rest of the plot surface (sector B) on subsequent days. This allowed us to determine the extent of crust formation on the remaining non-scraped areas and to monitor evaporation and infiltration for a fixed amount of rain for soil areas with and without crust as a function of time. A straight edge was used to gently move the surface layer away without disturbing the sensors. The scraped surfaces were more homogeneous than the non-scraped surfaces of the plots since gravel fragments on the surface of the crust were carried away during scraping which resulted in a smoother surface (Fig. 3). Surface soil removal was performed only after the moisture content of the upper surface returned to the pre-rain value, about 40% soil relative humidity. A metal ruler was used to determine the thickness of the crust. The extent of the crust was visibly apparent owing to the cementation of the particles above the loose soil below. Following multiple rain applications, multiple crusted areas were apparent. The multi-rain crust measurements were of the total amount of the culminated crust. A control plot, C, was located 10 m from the four experimental plots. Approximately 2 mm of the surface crust was removed from half of the control plot (sector CA) during the morning of the fourth day (am 4). During the morning of the fifth day (am 5), the crust on sector CA was measured to determine the amount of crust that forms naturally during a day/night period. The balance of the plot was not disturbed (sector CB) until the fifth morning (am 5) when the crust was measured to determine the steady state crust thickness for the area. Even without any rain application, the surface crust was visibly discernible from the loose soil below. Table 1 gives the soil treatments and crust measurements for each of the plots. The cumulative amount of rainwater (mm) that

Fig. 3. Photograph of the study site with instrumentation of the 4-quadrat areas. There is a clear difference between sites that have had the crust removed and the original surface.

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Table 1 Experimental Soil Treatment. Plot (1 m2)

1

Sector (0.5 m2)

1A

am 1: Deploy instruments

Soil Relative Humidity, Soil Soil Temperature: 0.2 cm 1.95 mm No treatment 1.55 mm 2 mm 1.87 mm 2 mm 1.77 mm 5.0 mm 6.0 mm 7.14 mm

pm 1: Apply H2O am 2: pm 2: Apply H2O am 3: Scrape Soil pm 3: Apply H2O am 4: Scrape Soil pm 4: Apply H2O am 5: Measure Crust Total H2O Applied

2 1B

2A

3 2B

3A

4 3B

4A

Control C 4B

CA

CB

Conductivity: 0.2, 5 cm 0.97 mm

0.49 mm

0.24 mm

No treatment

0.77 mm 2 mm 0.93 mm

3.09 mm No treatment No treatment 2 mm 3.55 mm 2.5 mm 7.0 mm 7.13 mm

3.09 mm No treatment No treatment 2 mm 0.43 mm 1.0 mm 3.76 mm

No treatment No treatment No treatment 2 mm No treatment 1.0 mm 5.0 mm 0.0 mm

2 mm 0.89 mm 2.0 mm 3.56 mm

1.5 mm

Fig. 4. Penetration of moisture as a function of rain amount. Soil relative humidity at the surface (gray line) and at 5 cm depth (thin solid line) and conductivity at 2 mm (thick solid line) for 24 h after varying amounts of initial rain from 0.24 to 1.9 mm added to dry unaltered soil.

corresponds to a crust measurement is the sum of the added water from the start of the experiments for each sector of the plots. Periodic experimental rains and the total cumulative amount of rains that were applied to each plot are listed. For example, Plot 1 Sector A received 1.95 mm of water on the first evening (pm 1), then 1.55 mm of water the following evening (pm 2). On the third morning (am 3), we scraped 2 mm from the soil surface. As shown in Table 1, the control plot C received no water and sector CA had 2 mm of crust removed on the forth morning (am 4). At the end of the experiment (am 5) the crust thickness in both sectors (CA, CB) of the control plot was measured.

4. Results and analysis In general, we added water to the soil at sunset and the soil remained wet through the night. The penetration of the water into the subsurface depended both on the amount of water added and the presence of a surface crust. During the day, the surface of the soil quickly dried out although water was retained at depth in some instances. In the cases of the heaviest rain application, the subsurface water rewetted the surface the following night. The effectiveness of the rain to wet the soil surface is shown in Fig. 4. Initial rain amounts correspond to 0.24, 0.49, 0.97 and 1.95 mm. Fig. 4 shows the relative humidity in the soil surface and at a depth of 5 cm as a function of time for 24 h after the initial wetting for all four cases. In all cases, the relative humidity in the

soil surface increases but only reaches 100% for water amounts of 0.97 and 1.95 mm. The conductivity of the soil surface at 2 mm shows a sharp increase – indicating capillary liquid water – only for the highest wetting level, 1.9 mm. This spike persists for only a few hours, presumably due to the water infiltration to lower depths. Only the heaviest rain of 1.95 mm increased soil relative humidity at 5 cm depth and then only from 40% to 50% for about 4h, 15 h after the application. Fig. 5 shows the effect of surface crust on the infiltration and evaporation of water for night 2 and day 3 for the four plots (see Table 1 for soil treatments). Plots 1, 3 and 4 received approximately 3.5 mm total rain and plot 2 received 1.75 mm total rain. In all cases, immediately after the rain was applied the surface had capillary water as indicated by the spike in conductivity at the surface. These surfaces remain saturated during the night and into the early morning as indicated by the 100% relative humidity. The end of the high surface humidity was the same for all cases even though total rains varied by a factor of 2 and the level of the most recent rain application varied by a factor of 4. We interpret this to be due to the heat of the day completely controlling the surface conditions. At the depth of 5 cm however, the relative humidity began to increase 11 70.5 h after the rain application and the maximum relativity humidity occurred 16 72 h after the rain application. For all rain amounts, the increase in relative humidity at 5 cm depth was linearly proportional to the amount of rain (with a slope corresponding to a change in relative humidity of 9% for each mm of water added and with a correlation coefficient of 0.91).

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Fig. 5. Effect of surface crust. Soil relative humidity at the surface (gray line) and at 5 cm depth (thin solid line) for 24 h after varying amounts of rain from 1.5 to 3 mm, added to sites that had previously been wetted but were dry by the time of application of these rains. The conductivity of the surface is shown as a thick solid line. The build up of moisture at 5 cm depth takes about 12 h independent of the amount of rain.

Fig. 7. Water rebound. Surface humidity (gray line), humidity at 5 cm depth (thin solid line) and conductivity of the surface (thick solid line) are shown for a 48 h period following the two heaviest rains in the experiment. The night after the rain the surface humidity is 100% and then falls to low values in the day. The next night moisture returns to the surface from below (see the high RH at 5 cm depth) and the surface again has 100% humidity.

Fig. 8. Soil temperatures in 1C as a function of time during the experiment.

Fig. 6. Penetration of water with crust removed. Soil relative humidity at the surface (gray line) and at 5 cm depth (thin solid line) and surface conductivity (thick solid line) for 24 h after varying amounts of rain from 1.7 to 3.5 mm were added to soil after the upper 2 mm of crust were removed. The build up of moisture at 5 cm depth takes less than 3 h independent of the amount of rain—significantly faster than when a crust is present.

In Fig. 6 a similar set of experimental rains 1.77, 1.87 and 3.55 mm is shown. These values are within the range of rains shown in Fig. 5 (see Table 1 for listing of the rain events). This time the surface crust has been removed as described in method Section 3.3. In this case the increase in relative humidity at 5 cm occurred more rapidly—less than 371 h after the rain application. We can construct a simple model of this process by assuming diffusion through a uniform material. In that case the timescale t to diffuse a distance x is given by (e.g., Satterfield, 1970), t =x2/D, where D, is the diffusion coefficient, corrected for porosity and tortuosity. Using the timescales for moisture penetration to 5 cm depth in the rain experiments shown in Fig. 5, we can compute the ratio of the value of diffusion for the crust Dc and the value of diffusion of the underlying 5 cm of soil D given thickness x. Using the timescales for diffusion through the crust tc and the underlying

bulk soil material tb we obtain; tc =3 h= (2 mm)2/Dc and tb = 11 h= (50 mm)2/D. We have ignored the time to diffuse through the bulk soil under the crust compared to the time to diffuse through the crust in evaluating Dc. We find that Dc D/170. The diffusion through the crust is more than two orders of magnitude reduced compared to diffusion through the underlying substrate. Fig. 7 shows the RH values at the surface and at 5 cm depth for two days after the heaviest application of water in the experiment. Each frame shows the results of a rain of 3.1 mm followed by no rain on the next night. In both cases, after wetting, saturation was maintained for approximately 12 h and then the soil relative humidity decreased immediately to 40% with the increased temperature at sunrise. As the temperature decreased the next evening the relative humidity at the soil surface increased gradually over several hours into night reaching 100% even without an additional application of rain. This saturation was maintained until high daytime temperatures dried the soil. Both frames show that the relative humidity increased at 5 cm depth approximately 15 h after the rain applications and remained above steady state (  40%) for a period of 12 h. The soil temperatures for each plot are shown in Fig. 8. In general, temperatures are lowest at 5:30 am and highest at 1:30 pm local time with a diurnal variation of 50 1C. The temperatures of the four plots are similar for the low temperatures and except for two exceptions (plot 3 and 4) are similar for the high

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temperatures. From the data (Table 1) we see that rains over 3 mm at evening reduce the high temperature approximately 10 1C during the following daytime period, with no effect on subsequent days. Plot 2 which received about 2 mm of rain each evening, had a reduction in the low temperature of a few degrees.

5. Discussion It is clear from the results that very small rains do not result in free water on the surface. The soil is generally so dry that some moisture is needed just to bring the soil up to high humidity. The RH of these soils before any rain is typically 30–0% (McKay et al., 2003) as also seen in our data (Figs. 4–7). This is well below the wilting point of plants (water potential, c, of  1.5 MPa which equates to 98.5% RH, where c = RT ln(RH), R is the gas constant and T the temperature) and the minimal water activity of even the most xerotolerant microbes (Kushner, 1981). Cyanobacteria, the primary producers for most dry desert areas require water activity levels of essentially 100% (Kushner, 1981 ( 494%); Potts and Friedmann, 1981 (  98%); Palmer and Friedmann, 1990 ( Z90%)). It is therefore not surprising that Warren-Rhodes et al. (2006) report that hypolithic cyanobacteria, which are present under translucent rocks in virtually all other deserts, are not present in the arid core region of the Atacama Desert. Wierzchos et al. (2006) and Davila et al. (2008) however, have shown that cyanobacteria and associated heterotrophic bacteria can grow inside saline endoevaporitic habitats where moisture is condensed by halite when relative humidity values are above 75%. Thus, liquid water, albeit salty, is generated from nighttime humidity even when there is no rain, dew, or fog. Under the dry conditions of the Atacama Desert, water added to the soil by the very light rain is mainly transported in the vapor phase where the driving forces are the moisture gradient and the temperature gradient, as shown in this study (Fig. 8) and previous work (McKay et al., 2003). In considering the implications of our results for biological systems it is important to note that for microbial life it is not enough that the soil is near 100% RH, but that there must be actual liquid water. This does not necessarily mean that the pore spaces are all filled with water or that the soil is at field capacity. However, it does require more water than just bringing the soil to 100% RH. Our results suggest that twice the rain level is needed to add enough water to fill some of the pore spaces in the soil (indicated by conductivity probes) compared with just raising the soil humidity to 100% (indicated by the RH sensors). For simulated rains above 3 mm, we observed the return of water to the surface from the subsurface due to upward migration of water vapor 24 h after the rain was applied. This storage and rebound of water vapor is probably not of biological significance, since although the RH climbs back up to 100% there is no free capillary water that would be indicated by saturation of the conductivity probes. However, water vapor may be important for the salt transport process that results in the crust formation. Indeed, it is observed that after desert rains there is a white coloring on the surface of the desert due to salts. This capillary movement of water and vapor may also be important in explaining the salt distribution and organics seen at depth (Ewing et al., 2006; Amundson et al., 2008).

6. Conclusions We have conducted a series of simulated rain experiments in the arid core region of the Atacama Desert. Rain amounts up to

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3.55 mm were applied to the soil surface. Based on our results we draw the following conclusions. 1. Rain events of less than 1 mm do not saturate the surface, and the soil RH remains below 100%. 2. Rain events of 2 mm or more generate free water in the pore space of the soil, but may not fill all pores. It is likely that this represents the minimum rain that will support biological activity in the soil. 3. The slight crust on the surface of the soil is a strong barrier to the diffusion of moisture into the soil. The diffusion coefficient through the crust is about a factor of 170 lower than in the subsurface soil. 4. For rain events larger than 2 mm, moisture is retained at depth. On subsequent nights, water vapor can diffuse upward, mobilize salts and rewet the dry surface. Based on these conclusions we suggest that it is useful to categorize the light rains of the Atacama Desert in the following way. Rain amounts less than 1 mm do not saturate the surface. Rain amounts more than 1 mm but less than 2 mm do wet the surface but the pore spaces in the soil are not filled with water. Rain amounts greater than 2 mm result in the filling of pore spaces and the capillary movement of liquid water into the subsurface. Only rains in this latter category generate moisture levels high enough for microbiological activity. In our study, we have taken the limit for microbial life in the soil as that required for photosynthetic cyanobacteria, corresponding to a water activity greater than 0.95 (see Section 5 for details). For planetary protection purposes however, NASA and COSPAR have defined ‘‘special regions’’ on Mars based on the temperatures greater than  15 1C and then with a margin set to  20 1C, and water activity greater than 0.62 and then with a margin set to 0.5 (Beaty et al., 2005). Our results show that once the water activity level in a hyperarid soil begins to fall below 0.95, the fall is quite rapid, so that the precise value for the lower limit of water activity for life does not significantly change the amount of time that habitable conditions are produced by small desert rains. Therefore, our Atacama Desert results can be used as a guide to estimate how water moves in the upper soils of Mars, suggesting how much water as rain, snowmelt, or precipitating fog must reach the surface to create conditions suitable for life with respect to water activity.

Acknowledgements We acknowledge support from NASA’s Astrobiology Science and Technology for Exploring Planets Program, NASA-Ames/SETI Institute Cooperative Agreement (NCC2-1408; 6CA9304), National Science Foundation (DEB 971427) and the University of Antofagasta, in Chile. References Amundson, R., Ewing, S., Dietrich, W., Sutter, B., Owen, J., Chadwick, O., Nishiizumi, K., Walwoord, M., McKay, C., 2008. On the in situ aqueous alteration of soils on Mars. Geochim. Cosmochim. Acta 72, 3845–3864. Bagaley, D.R., 2006. Uncovering Bacterial Diversity on and Below the Surface of a Hyper-arid Environment, The Atacama Desert, Chile. M.S. Thesis, Louisiana State University, Baton Rouge, LA, 158 pp. Beaty, D., Buxbaum, K., Meyer, M., Co-Chairs MEPAG Special Regions-Science Analysis Group, et al., 2005. Findings of the Mars Special Regions Science Analysis Group. News & Views, Astrobiology, Vol. 6(5). Navya, pp. 677–732. Birkeland, P.W., 1999. Soil and Geomorphology, 3rd ed, Oxford University Press, New York, NY 430 pp. ¨ Borgel, R.O., 1973. The coastal desert of Chile. In: Amiran, D.H., Wilson, A.W. (Eds.), Coastal Deserts, Their Natural and Human Environments. University of Arizona Press, Tucson, AZ, pp. 111–114.

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