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IntroductionDry permafrost is ground that never warms above 0°C and has negligible ice content. Dry permafrost overlying ice-cemented ground is rare on Earth, but is widespread on Mars. In the polar regions of Mars, the dry permafrost layer begins at the surface and ice-cemented ground is found below it (Mellon & Jakosky 1993, Mellon et al. 2009, Smith et al. 2009). On Earth, dry permafrost over ice-cemented ground has been reported in the arid upland regions of the Antarctic Dry Valleys (Campbell & Claridge 2006, Bockheim et al. 2007). For example, in an early year-round monitoring study of dry permafrost at Linnaeus Terrace in Upper Wright Valley at an elevation of 1600–1650 m, McKay et al. (1998) found that maximum soil temperatures exceeded 0°C to a depth of 12.5 cm, dry permafrost extended from that depth to 25 cm and ice-cemented soil was present below that level.Motivated by the connection to Mars, as well as by the questions of the stability and age of the ground ice, there have been extensive studies of dry permafrost over ice-cemented ground in University Valley (77°52'S, 163°45'E; 1700 m a.s.l.), one of the upper valleys in the Quatermain Range in the Dry Valleys (e.g. Ugolini 1964, Pringle et al. 2003, Bockheim et al. 2007, Vieira et al. 2008, McKay 2009, Lacelle et al. 2011, 2013, 2016, Marinova et al. 2013, Heldmann et al. 2013, Mellon et al. 2014, Lapalme et al. 2017). Similarly, there have been many studies of dry permafrost over massive ground ice in Beacon Valley, located 300 m below University Valley (e.g. Sugden et al. 1995, Marchant et al. 2002). Massive ground ice can form from buried glaciers or segregated ice. However, here we are focused on ice-cemented ground, in which the ice just fills the pore spaces of sandy soils. Other ice-free regions of Antarctica are also likely to contain dry permafrost over ice-cemented ground, although no detailed studies had been reported when we initiated this survey.Here we report on a survey for these conditions in Ellsworth Land, Antarctica, thousands of kilometres distant from the Dry Valleys. Our results now follow the paper by Schaefer et al. (2017) that reported thermal measurements in the ground near this site in Ellsworth Land that clearly indicate dry permafrost and ice-cemented ground. The Schaefer et al. (2017) paper is the first detailed confirmation of the thermal state of dry permafrost outside of the Dry Valleys. Our results complement the Schaefer et al. (2017) paper by including relative humidity (RH) as well as temperature measurements. We identify and characterize with year-round data a site with dry permafrost over ice-cemented ground. We present an initial analysis of this site and assess its possible habitability for microorganisms.MethodsTo assess for dry permafrost sites, a survey in the area was conducted in December 2016, recording summer air temperature, surface temperature and depth and temperature of the ice table. Note that we follow the convention of defining the year in Antarctica as beginning 1 December and summer as the months of December, January and February (e.g. Doran et al. 2002, Andersen et al. 2015). The depth to the ice table was determined by trenching. Temperatures were measured using a Signstek 6802 II Digital Thermometer with a K-type thermocouple probe.The monitored site (79°49.213'S, 83°18.860'W, 718 m elevation) is located on the north flank of Mount Dolence in Ellsworth Land close to a large outcrop informally called Elephant Head, as shown in Fig. 1. The location was selected based on the survey described above and is within 10 m of one of the survey sites. It is expected that a deep ice table with summer temperatures below freezing can indicate dry permafrost over ice-cemented ground. Our site was selected independently but is only c. 1.5 km west of the sites studied by Schaefer et al. (2017) and at approximately the same elevation (Fig. 1). Our site faces generally north; comparing the maximum summer soil temperatures (at 3.5 cm for our site and at 5 cm for the Schaefer et al. 2017 paper) suggests that the solar heating at the two sites is approximately equivalent. The geological setting is well described by Schaefer et al. (2017); the soils are primarily derived from quartzites with occasional phyllites.sFig. 1.a. Map, b. satellite image, and c. photograph of data station at the dry permafrost site overlying ice-cemented ground near Elephant Head, 79°49.213'S, 083°18.860'W, 718 m elevation. The site monitored for the year is marked on the map and the photograph with a red X. The site studied in Schaefer et al. (2017), c. 1.5 km to the east, is marked with an S.Three identical Onset U23 Pro v2 External Temperature/Relative Humidity data loggers were installed at this site that recorded temperature and RH hourly starting on 28 November 2017. The sensors were emplaced on site more than 24 hours before recording began. The combined air temperature and RH sensor was located 82 cm above the ground inside an Onset Solar Radiation Shield RS3-B. The subsurface sensors were placed at a depth of 3 cm in dry, loose material and at the top of the ice table at 49 cm depth. Data from the units were retrieved on 4 December 2018. The air and ice table units successfully recorded for the entire period, but the unit at 3 cm depth stopped recording on 18 March 2018. The complete raw data and the corrected values are given in the supplemental online material.Error estimates for the temperature sensor are listed as ± 0.21°C from 0°C to 50°C and we assume the same for temperatures below 0°C. The resolution of temperature data is 0.02°C at 25°C. Drift is < 0.1°C per year. Because the temperature-sensing element is a 10 kΩ thermistor, we assume that the main source of error is due to systematic offsets and not random noise. Hence the error in average values is not significantly decreased with increased sample size. The accuracy of the RH sensor is typically stated as ± 2.5% in the 10–90% RH range and ± 5% below 10% or above 90% RH. The resolution of the sensor is 0.05% and its drift is < 1% per year.Relative humidity sensors record humidity with respect to liquid water even for subfreezing temperatures. In addition, RH sensors often have offsets at high humidity and low temperature. The RH readings were corrected for these effects as described in the Appendix, using the following formula:1$$RH_i = RH_w - 2 - 0.65T$$where RHw is the RH with respect to water and is the reading from the sensor in percent, T is the temperature and RHi is the RH with respect to ice and is the corrected value. For determining the activity of water, aw, for use in habitability comparison, the direct reading from the sensors was used (aw = RHw). The water vapour density and the frost point were calculated from temperature and corrected RH values using the formulae in McKay (2009).ResultsThe location, air temperature, ground ice temperature and ice depth data resulting from the survey are shown in Table I. The distance between sites was less than c. 1 km. The vicinity of Site #3 was selected for further monitoring because it had the deepest and coldest ice table. At Site #3, the air, surface and ice table temperatures were -4.5°C, +2.8°C and -4.8°C, respectively, and the ice table was 45 cm below the surface. The location for the long-term monitoring was within 10 m of Site #3.sTable I.Survey results from the Elephant Head area.SiteS latitudeW longitudeAltitude (m)Depth to ice table (m)Temperature (°C)Local timeAirSurfaceIce table179°49.106'83°18.139'7000.17−1.76.7−1.516h32, 9 Dec 2016279°49.129'83°18.355'6900.20−1.05.2−1.517h17, 9 Dec 20163a79°49.211'83°18.835'7230.45−4.04.0−4.018h10, 9 Dec 20167230.45−4.52.8−4.820h04, 9 Dec 2016479°49.324'83°20.727'8350.10−6.03.0−2.219h17, 9 Dec 2016aThe monitored site, 79°49.213'S, 83°18.860'W, is within 10 m of Site #3.Our monitored site showed seasonal trends and weather patterns that are similar to those of the Dry Valleys (Figs 2 & 3). As expected, the variation in air temperature exceeded the variation in temperature at the ice table, 49 cm below the surface. Neither the temperature of the air nor the temperature of the ice table rose above 0°C. The temperature at 3 cm depth was warmer and more variable than the air temperature due to solar heating of the soil surface. The high atmospheric RH and attenuated diurnal temperature cycles showed that the loggers were emplaced during a period when clouds and snow were present. The moisture levels in the soil surface (red line in Fig. 3) remained high for a few days until clear atmospheric conditions resulted in higher soil temperatures and a gradual drying of the soil (evident by 11 December 2017 data points in Fig. 2). Similar cycles continued through the summer, notably at the end of December 2017 and the end of January 2018. The key statistics for the summer and yearly datasets (as well as the derived parameters of frost point and hours of habitability) are listed in Table II.sFig. 2.Temperature of the air and the ice table over the climate year from 1 December 2017 to 30 November 2018.sFig. 3.Temperature and relative humidity corrected for ice for the summer months of December, January and February. Data shown include air, 3 cm below surface and the ice table located 49 cm below the surface. Temperature and relative humidity curves are presented on the same abscissa to show correlations with time.sTable II.Summary of temperature and moisture conditions at the monitored site.ParameterAirIce table, 49 cm depth3 cm depthYearly values: 1 Dec–30 NovT max−0.14°C−5.2°C−T min−42.3°C−30.3°C−T avg−20.3°C−19.2°C−RHw max (aw)95.3%97.8%−RHw min (aw)21.6%82.1%−RHw avg (aw)57.5%89.2%−RHi max98.7%100.7%−RHi min23.8%95.4%−RHi avg68.7%99.7%−Frost point−22.7°C−17.0°C−Summer values: 1 Dec–28 FebT avg summer−10.3°C−8.6°C−5.4°CRHi avg summer51.1%99.1%46.2%Hours T > −10°C, aw > 0.8−35165Hours T > −5°C, aw > 0.8−080The temperature error (± 0.21°C) combined with the RH error for the air (± 2.5%) imply an error in the air frost point of c. 1°C, dominated by the RH error. For the ice table, the RHi is set to 100% and the only error is in the temperature. Considering the entire year, the frost point in the air was -22.7 ± 1°C, and at the ice table it was -17.0 ± 0.2°C. This difference is larger than the error in these averages.DiscussionWe can estimate the depth to dry permafrost – the depth at which maximum summer temperatures are < 0°C - by interpolation between summer maximum temperatures at the ice table and at 3 cm depth. This gives a depth to permafrost of 13.5 cm. Thus, the region from 13.5 cm depth to 49 cm depth is dry permafrost overlying ice-cemented ground. We note that Schaefer et al. (2017) determined the depth to permafrost at their site as c. 48 cm, which is considerably deeper than at our site. We do not use the term ‘active zone’, which is often used in studies of Arctic permafrost for describing the surface zone that melts in the summer with a lower boundary that is both the top of the permanently frozen ground and a change from liquid water to ice. In Antarctic dry permafrost locations, the top of the permanently frozen zone is not associated with any change in composition.Compared to the conditions at Linnaeus Terrace (McKay et al. 1998), the ice table at the Elephant Head site is deeper (49 cm here compared to 25 cm at Linnaeus Terrace) and has a larger temperature difference in frost point between the ice and the atmosphere (5.7°C compared to 4°C at Linnaeus Terrace). To first order, these differences cancel each other out, and hence the vapour loss rate from the ice table at the two sites should be similar. McKay et al. (1989) report a net loss rate of 0.4–0.6 mm yr−1 of ice, computed using atmospheric conditions as the upper boundary. McKay et al. (1989) suggest that the ice is either decreasing with time or there is episodic recharging of the ice table.However, as pointed out by Fisher et al. (2016), the relevant boundary condition for computing the flux of vapour from the ice table is the water vapour density at the surface of the soil, not the vapour density of the atmosphere. In general, the moisture at the soil surface exceeds that in the atmosphere, and thus the loss rate from the ice table computed using the water vapour density of the air as the upper boundary condition would give much higher flux than a calculation using the water vapour density of the soil surface. Williams et al. (2015) showed that this same effect occurs on Mars, with ground frost at the Viking 2 site enhancing the stability of the ice-cemented ground. The ratio of the average water vapour density at the surface divided by the average water vapour density in the atmosphere is the water vapour enhancement factor, and we can use the dataset to compute its value for the monitored site. As is shown in Table II, the average summer RH values at the 3 cm depth are lower than those of the air, but because the temperatures are higher, the average water vapour density at 3 cm is actually 1.5× higher than the water vapour density in the air. Because the water vapour density values are highest in the summer, this enhancement is likely to represent the year-round average.The variation in the frost point that corresponds to an enhancement factor of the water vapour density of the air of between 1.0 and 1.7 is shown in Fig. 4. Over that range, the frost point increases from -22.7°C to -16.9°C, crossing the value of the ice table frost point (dotted line in Fig. 4). Given the uncertainties in the frost point and the fact that our data are from only one year, Fig. 4 shows that the Fisher et al. (2016) mechanism indicates that the ground ice at this site may be stable and in equilibrium with the water vapour density at the soil surface - this sets the depth to the ground ice. Note that in University Valley, Fisher et al. (2016) found that the observed enhancement factor of the water vapour density at the surface (compared to the air) of 1.7 would explain the ice table depth observed. In their calculation, this enhancement factor was due entirely to higher RH in the soil (85%) compared to the atmosphere (c. 50%). At Elephant Head, we find a similar value of the enhancement factor of the water vapour density at the surface compared to the air, but it is entirely due to a higher mean temperature, with little change in RH. As is shown in Fig. 4, if the enhancement factor was < 1.4, then the frost point of the upper boundary would be colder than the frost point of deep ice-cemented ground (c. -19°C) and there would not be an ice table, with dry permafrost extending to all depths. Conversely, if the enhancement factor was greater than c. 2.5, the ice table would be just at the depth where maximum temperatures exceed 0°C and there would be no dry permafrost layer. Thus, dry permafrost overlying ice-cemented ground occurs only over a specific range of surface moisture conditions.sFig. 4.Frost point of the atmosphere as the average water vapour density is scaled by an enhancement factor. The frost point of the ice table is shown as a dotted line.The data in Figs 2 and 3 would suggest that the water vapour density of the soil surface exceeds that of the atmosphere due to episodic snow (McKay 2009) or frost (Williams et al. 2018). Snow on the ground is indicated in the data (Fig. 2) and images of the site (Fig. 1), but a systematic study of the effect of snow would require several years of observation, given the year-to-year variability in snow cover. The presence of hygroscopic salts (Williams et al. 2018) could also enhance surface moisture and would require analysis of the soil composition to test.Temperature and moisture availability are key parameters in determining habitability in cold, dry environments. On Mars, Special Regions are locations that are considered potentially habitable, and they are defined (Rummel et al. 2014) as follows: ‘Special Regions on Mars continue to be best determined by locations where both of the parameters (without margins added) of temperature (above 255K) and water activity (aw > 0.60) are attained.’ Here the definition of habitability is in support of planetary protection and is justifiably conservative. Studies of metabolism in permafrost samples (Rivkina et al. 2000) show that activity is high at -5°C, lower at -10°C and difficult to discern below that temperature. With respect to water activity, Palmer and Friedmann (1990) report that, at + 8°C, Antarctic cryptoendolithic lichens begin to photosynthesize when the RH exceeds 70%. Extreme halophiles are active in saturated NaCl solutions, aw ≈ 0.75 (e.g. Kushner 1981). Yeasts and fungi can be active for aw ≈ 0.6 (e.g. Kushner 1981). Thus, we consider two cases in order to characterize the habitability of the dry permafrost and the ice table: a moderate case of T > −5°C and RHw (aw) > 0.8 and an extreme case of T > −10°C and RHw (aw) > 0.8.As discussed in the Appendix, we take aw to be the direct RHw reading without the correction for ice and ignoring any sensor offsets. At the ice table, the water activity is always high, RHi = 1 and RHw (aw) varies from 0.85 to 0.95 (see Appendix). However, temperatures are low, never exceeding -5°C. There are 35 hours in the summer (1 December to 29 February) when T > −10°C and RHw (aw) > 0.8 at the ice table. In the dry soil at 3 cm depth, the temperatures are higher but the RH values are generally lower. However, there are 80 hours in the summer when T > −5°C and RHw (aw) > 0.8 and 165 hours when T > −10°C and RHw (aw) > 0.8. Thus, the ice table and the dry ground above it are potentially habitable and both would be considered ‘Special Regions’ on Mars. Further microbial investigations of this site would be of interest. These results are comparable to those of Goordial et al. (2016), who found that in the dry permafrost in University Valley there were 74 hours per year when T > −15°C and RHw (aw) > 0.95.ConclusionWe have measured year-round temperature and humidity in the air and at the ice table at a site in Ellsworth Land in Antarctica over 2000 km distant from the Dry Valleys. From these data we draw four conclusions. Firstly, dry permafrost over ice-cemented ground in Antarctica is not found only in the upper elevations of the Dry Valleys. It may be present in many locations in Antarctica. Secondly, a survey using summer measurements of the depth and temperature of deep ice tables can efficiently indicate locations of dry permafrost over ice-cemented ground. Thirdly, dry permafrost at the site meets the temperature and water activity requirements for habitability. Fourthly, the ice table at this site in Ellsworth Land could be stable despite the low atmospheric frost point due to a c. 50% enhancement factor of the water vapour density at the surface compared to the air. These results motivate a more systematic search for locations in Antarctica with dry permafrost over ice-cemented ground and a search for microbial activity within such sites.
Antarctic Science – Cambridge University Press
Published: Oct 1, 2019
Keywords: frost point; habitability; ice table; Mars; Mount Dolence
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