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ASTROBIOLOGY Volume 10, Number 3, 2010 ª Mary Ann Liebert, Inc. DOI: 10.1089=ast.2009.0428

To What Extent Does Terrestrial Life ``Follow The Water''?
Eriita G. Jones and Charles H. Lineweaver

Abstract

Terrestrial life is known to require liquid water, but not all terrestrial water is inhabited. Thus, liquid water is a necessary, but not sufficient, condition for life. To quantify the terrestrial limits on the habitability of water and help identify the factors that make some terrestrial water uninhabited, we present empirical pressure-temperature (P-T ) phase diagrams of water, Earth, and terrestrial life. Eighty-eight percent of the volume of Earth where liquid water exists is not known to host life. This potentially uninhabited terrestrial liquid water includes (i) hot and deep regions of Earth where some combination of high temperature (T > 1228C) and restrictions on pore space, nutrients, and energy is the limiting factor and (ii) cold and near-surface regions of Earth, such as brine inclusions and thin films in ice and permafrost (depths less than *1 km), where low temperatures (T < þ408C), low water activity (aw < 0.6), or both are the limiting factors. If the known limits of terrestrial life do not change significantly, these limits represent important constraints on our biosphere and, potentially, on others, since *4 billion years of evolution have not allowed life to adapt to a large fraction of the volume of Earth where liquid water exists. Key Words: Biosphere--Limits of life--Extremophiles--Water. Astrobiology 10, 349­361.

1. Introduction

N

ASA's ``follow the water'' strategy for Mars exploration (e.g., Hubbard et al., 2002) is based on the observation that all terrestrial life requires liquid water during some phase of its life cycle (Rothschild and Mancinelli, 2001). This strategy can be refined by including other fundamental requirements of life. Energy, for example--Hoehler (2004), Hoehler et al. (2007), and Shock and Holland (2007) described how ``follow the water'' can be complemented by a ``follow the energy'' approach. Here, we refine the ``follow the water'' approach by empirically quantifying the boundaries between the habitable and uninhabitable water on Earth. Research on extremophiles has revealed apparent limits of temperature, water activity, nutrient, pore space, and free energy availability (Baross et al., 2007; Jakosky et al., 2007). These factors are implicated in making some water uninhabitable. However, no empirical study has comprehensively quantified the fraction of uninhabited terrestrial water or comprehensively described where these wet uninhabitable environments are. To this end, we have developed a model for presenting Earth and Mars on a pressuretemperature (P-T) diagram (see Jones and Lineweaver, 2008). See Table 1 for the surface P-T conditions of Solar System bodies. Figure 1 is a sketch of our main goal: to identify the boundaries between ``Life'' and ``No Life'' in the regions of Earth where water is liquid. To this end, Fig. 2 presents water in several ways. The bulk of terrestrial ocean water is in the

narrow vertical wedge. The dotted curves mark the phase space boundaries of ``ocean salinity water.'' Similarly, the solid curves mark our estimate of the ``maximum liquid range,'' that is, the largest range of phase space that would be occupied by liquid water (fresh water, salt water, concentrated brines, thin films, and super-heated water) on Earth, if Earth had environments at the full range of temperatures and pressures shown. It does not; see Fig. 3. The melting curve of this ``maximum'' water has a freezing point depression of þ898Cat the triple point, probably due to a combination of thin film effects and high solute (e.g., dissolved salt) concentrations (Clark and Van Hart, 1981). Thin films of unfrozen water can exist at the contact between ice-ice or ice-soil grains at very ¨ low temperatures (Price 2000; Davis, 2001; Mohlmann, 2008). The most soluble common inorganic compounds in water have saturated aqueous molarities less than *11 M (Lide and Haynes, 2009). The magnitude of boiling point elevation due to the molality m of the solute is given by Raoult's equation: DTb ¼ Kb m, where Kb is the ebullioscopic constant for water, 0.5128C=m (Silberberg and Duran, 2002), and m ¼ 11, giving a boiling point elevation of 5.68C, which we use to make the thick vapor curve in Fig. 2. 2. The Pressure-Temperature Phase Diagram of Earth Figure 3 is a P-T phase diagram of all terrestrial environments superimposed on the phase diagram of water from

Planetary Sciences Institute, Research School of Astronomy and Astrophysics and the Research School of Earth Sciences, Australian National University, Canberra, Australia.

349


350

JONES AND LINEWEAVER Table 1. Average Pressures and Temperatures on Some Solar System Bodies

Environment Earth surface Mars surface Venus surface Titan surface Top of Europa's ocean

Pressure (bar) 1 0.06 93 1.5 100

Temperature (8C) 15 þ50 480 þ180 þ3.9

Reference Dziewonski and Anderson, 1981 Barlow, 2008 Petculescu and Lueptow, 2007 Griffith et al., 2000 Reynolds et al., 1983; Melosh et al., 2004

Fig. 2. Conceptually, it is the same as the intersection of the blue and orange circles of Fig. 1; it shows the regions of phase space where there is both Earth and water. It also shows the regions where there is Earth but no liquid water and where there could be liquid water but there is no Earth. The details of our Earth model, including estimates of uncertainties, are described in Table 2 and Appendix A Table A1. The subsurface pressure and temperature gradients of Earth are proportional to each other and are related through density, conductivity, and heat production. This limits the P-T space occupied by Earth. Although the phase diagram labeled ``Maximum liquid range'' is appropriate for the concentrated briny inclusions and thin liquid films in ice and permafrost on Earth's surface, the maximum freezing point depression for water on, and in, aerosols and organic particles in Earth's atmosphere will be intermediate between the ``Ocean salinity water'' and ``Maximum liquid range.'' The phase models in Figs. 3 and 5 include short-term environments that are temporarily at typical mantle temperatures but are at crustal pressures. We have labeled these environments as ``Transient'' in Fig. 3. They include magma chambers in the crust, surface lava, and subsurface lava lakes ( Jonsson and Matthiasson, 1974; Helz and Thornber, 1987). Over timescales of decades, the majority of these environments (Oppenheimer and Francis, 1998) will cool along a constant pressure line to lie within the crustal region. The size of the area occupied by Earth in the P-T space of Fig. 5 is determined by a combination of uncertainty in geothermal

gradients and variations (both spatial and temporal) in geotherms. Thus, we have attempted to include the dynamic nature of Earth in the model by making the shaded Earth region wide enough to include cycling of pressures and temperatures. For example, in Fig. 3 the temperature at the summit of Mount Everest varies between the maximum (point C) and minimum (point D), and atmospheric pressures and temperatures vary within the pink shaded region around the average atmospheric geotherm. Also in Fig. 3 the transient permeation of liquid water to depths of at least 6 km in the crust through permeable rock-cracking (Sleep and Zoback, 2007) is included, as the average crustal geotherm (the solid black line) allows liquid water in the crust to a depth of *30 km. The shallowest crustal geotherm allows liquid water in the crust to a depth of *75 km (green asterisk in Fig. 3). In Fig. 4, the maximum depth of the biosphere is currently determined by the depth of the 1228C isotherm. By ``currently'' we are referring to the temperature of 1228C as a tentative high limit for life, based on the highest temperature environment in which present-day active life has been found to date (Takai et al., 2008). We acknowledge that this presentday limit may be transient, as environments such as deep sea thermal vents are searched for examples of life at higher temperatures. The depth of 1228C in the crust varies with crustal characteristics such as heat flow and thermal conductivity. From Fig. 3, the 1228C isotherm varies between the surface and a maximum depth of *20 km (along a cool geotherm) in crust. 3. The Pressure-Temperature Phase Diagram of Terrestrial Life Figure 4 presents the phase space of life on Earth. We make a provisional distinction between active (green) and dormant (pale green) life. Although spores and other dormant life-forms have evolved to withstand extreme environmental conditions, they could not complete their life cycles in those conditions. Examples of the extreme pressures (P) and temperatures (T) for active and dormant life are given in Tables 3 and 4, respectively (see also Appendix B). These environments include: (i) high P, high T, (e.g., hydrothermal vents on the ocean floor); (ii) high P, low T, (e.g., interstitial environments beneath thick ice sheets); (iii) low P, low T, (e.g., Mt. Everest); and (iv) low P, high T, (e.g., warmest layers of the upper atmosphere). In constructing these inhabited regions of phase space, we assumed that the T < 1228C upper limit to life (Takai et al., 2008) and the T > þ208C lower limit to active life ( Junge et al., 2004) are both valid over a broader range of pressures than has been fully explored by microbiologists. The pressure= pore-space relation and the link between barophilic and thermophilic adaptation is still unclear (Horikoshi, 1998;

FIG. 1. Water, Earth and Life. In the context of P-T phase diagrams, we plot liquid water (blue circle, cf. Fig. 2), all terrestrial environments (orange circle, cf. Fig. 3), and all inhabited terrestrial environments (green wedge, cf. Fig. 4). From these plots we are able to identify where there is both Earth and liquid water (i.e., the intersection of the blue and orange circles) but where ``No Life'' has been discovered (Fig. 5). Color images available online at www.liebertonline.com=ast.


DOES TERRESTRIAL LIFE ``FOLLOW THE WATER''?

351

FIG. 2. Pressure-temperature phase diagram for two types of water. The familiar vapor, sublimation, and melting curves for ocean salinity water (3.5% salt by mass, dotted curves) are indistinguishable on this scale from pure water (Lide and Frederikse, 1996). The dark vertical wedge represents the vast majority of Earth's oceans. We are concerned with the broadest range of pressures and temperatures under which water can remain liquid. This is labeled ``Maximum liquid range'' (solid curves). Due to increasing concentrations of solute and thin film effects at low temperatures, the coldest liquid water on Earth is þ898C and has a triple point pressure of 3.2á10þ7 bar (see Table 2, label E). For reference, the average global surface temperatures and pressures for Earth, Mars, Titan, Venus, and the top of Europa's potential ocean are shown (see Table 1). Color images available online at www.liebertonline.com=ast.

FIG. 3. Superposition of terrestrial environments on the P-T diagram of water from Fig. 2. Earth's core, mantle, crust, and atmosphere are colored separately and centered on the average geotherm (subsurface) and lapse rate (atmosphere). The ocean and crust geotherms meet at point ``I.'' A ``Transient'' region shows mantle that has intruded upwards (e.g., volcanism, geothermal vents). The horizontal thin line at 1 bar is the average sea-level atmospheric pressure of Earth. The parameters of our Earth model (e.g., core, mantle, continental crust, oceanic crust, geotherms, and atmospheric lapse rates) are given in Appendix A. Extreme environments are listed and assigned a letter in Table 2, and they are plotted here. For example, ``C'' and ``D'' represent the hottest and coldest days on the summit of Mt. Everest. As in Fig. 2, the dark vertical wedge identifies the majority of ocean water. The green asterisk represents our estimate of the hottest and deepest water on Earth at T ¼ 4318C and P ¼ 3á104 bar, corresponding to a depth of *75 km. Color images available online at www.liebertonline.com=ast.


352 ´ Garzon, 2004). Thus, our interpolations of the current highand low-temperature limits for life in environments at pressures between *1 and *102­3 bar should be viewed with caution. As a further indicator of caution, the current upper temperature limits of life (Kashefi and Lovley, 2003; Takai et al., 2008) are the results of a limited exploration of high T environments in hydrothermal vents. In the future, these limits will undoubtedly be higher and closer to the real limits of terrestrial life (Horsfeld et al., 2006; Kelley et al., 2007; Holden, 2008). In Fig. 4, the question mark and vertical dashed line at T ¼ 2508C emphasize this uncertainty. Furthermore, in the atmosphere in Figs. 3 and 5, the existence of liquid-water environments are transient. Due to the movement of air currents and the relationship between atmosphere and surface, there are no continual liquid-water environments at low pressures. For example, at the triple point of ``ocean salinity water'' in Fig. 3 at *25 km altitude, the conditions may be dry even though liquid water is stable. 4. Results Figure 5 can be used to quantify the potential limits of the terrestrial biosphere. For example, we can compute what fraction of the volume of Earth is inhabited. The radius of Earth rE ¼ 6378 km. The biosphere extends *10 km above this (e.g., throughout the troposphere, on top of mountains) and extends *10 km down in the oceans and about 5 km down into continental crust (see Table 3, ``6'' and ``7''). Assuming oceans occupy 70% of the surface and continents occupy 30%, the biosphere occupies *1% of the volume of Earth (the 10 km thick troposphere is included in the volume of Earth). Thus, after 4 billion years of evolution, the terrestrial biosphere seems to have been unable to extend into *99% of the volume of Earth. Also of interest is what fraction of the volume of Earth where liquid water exists is known to host life. The computation is analogous to that above except, instead of the denominator in the ratio being the volume of the entire Earth, it is the volume of the shell of Earth that contains liquid water. The thickness of such a shell extends to a depth of *75 km and is marked by the green asterisk in Fig. 5. Here, we ignore the atmosphere and obtain the result that *12% of the volume of Earth where liquid water exists is known to host life. Thus, according to our current state of exploration, 88% of the volume of Earth where liquid water exists is not known to harbor life. We can obtain analogous results for the area in phase space [log P log T] in Fig. 5 by asking: what fraction of the phase space area of Earth where liquid water exists is known to host life? In terms of Fig. 5, this question becomes, what fraction of the yellow area that overlies the blue area is occupied by green (active or dormant life)? Here, for simplicity, we exclude the atmosphere since it is not clear that liquidwater droplets and films on aerosols extend over the full ``maximum liquid range.'' The computation of the relevant areas yields the result that *36% of the phase area occupied by terrestrial water has been found to host life. Approximately 30% is occupied by active life, and the remaining *6% by dormant life. Thus, 64% of the phase space of terrestrial water and 88% of the volume of Earth containing water are not known to harbor life. There are many liquidwater environments on Earth that, as far as we know, do not

JONES AND LINEWEAVER host life. If the known limits of life do not require large modifications, these limits represent a significant constraint on the biosphere, since *4 billion years of evolution have not allowed life to adapt to the 64% of P-T phase space and 88% of the volume of Earth where liquid water exists. 5. Summary and Discussion We have made an empirical description of the phase space and volumetric limits of terrestrial life. Plausible reasons for these limits are combinations of temperature, water activity, energy, and nutrient availability and pore space. However, determining how these factors combine to prevent life will be a challenging part of future extremophilic research. Our analysis can be summarized as follows: (i) High-pressure limit to life. Figure 5 suggests that the apparent high-pressure limit of life is a selection effect since life has been found at the maximum depths for which it has been searched. This conclusion is supported by experimental studies in which barophiles were taken to those pressures encountered deep within the crust, and it was found that the organisms remained alive and viable (Sharma et al., 2002 for pressures at *35 km depth; Margosch et al., 2006 for pressures consistent with *50 km crustal depth). (ii) High-temperature limit to life. The current high-temperature limit for life is 1228C. This limit cannot be verified, as there is a substantial amount of liquid water (*1% of the total terrestrial liquid volume) at higher temperatures that has not been fully searched for life. If 1228C is a real limit, this indicates a severe restriction of the terrestrial biosphere, where only *12% of the volume of Earth that has liquid water supports life. If the hightemperature limit for life is increased to 2508C, the thickness of the biosphere would approximately triple to include *38% of the volume of Earth that has liquid water. However, see Wiersberg and Erzinger (2008) and Ellsworth et al. (2005) for indications that hyperthermophiles--specifically methanogens--are limited to T < 1358C. (iii) Low-temperature limit to life. The current low-temperature limit for life is þ208C. There are thin films of liquid water and potentially briny liquid water that can remain liquid at lower temperatures with a water activity above the proposed limit of 0.6 for life (Grant, 2004). If low water activity is a fundamental limit, rather than low temperature, then active life may occur in water temperatures down to at least þ308C due to the presence of brines. A substantial volume of terrestrial liquid water (>1.5á107 km3) exists in permafrost, and a large fraction of this would persist through temperatures below þ208C. It would be significant if life were found to be excluded from these liquid-water sources. (iv) Low-pressure limit to life. All examples of life found at pressures less than 0.3 bar have been classified as dormant. A low-pressure limit for life may be due to temperature, water activity, or nutrient limits at these altitudes. Liquid water would most likely exist as a thin film on aerosols or organic particles in the atmosphere; however, its existence in low-pressure environments is transient.


DOES TERRESTRIAL LIFE ``FOLLOW THE WATER''?

353

FIG. 4. Inhabited terrestrial environments superimposed on the P-T diagram of water from Fig. 2. Numbers 1­8 circumscribe our estimate of the region occupied by active terrestrial life (green) and are described in Table 3. Numbers 9­14 and the light green area represent our estimate of the region occupied by dormant terrestrial life and are described in Table 4. Atmospheric life is classified as dormant, as most examples are spores and atmospheric non-spore-forming organisms that have extremely low metabolic rates (Appendix B). The question mark and the yellow arrow pointing from the current upper temperature limit of life (Tlife < 1228C) to the vertical dashed yellow line at T ¼ 2508C represents the change that would occur in this diagram if life were found at T ¼ 2508C.

FIG. 5. Superposition of inhabited environments from Fig. 4 and the terrestrial environments from Fig. 3 on the P-T diagram of water from Fig. 2. Life does not seem to inhabit the full range of terrestrial environments where liquid water is available. The upper temperature limit for life, currently 1228C, excludes life from the hottest and deepest water. Color images available online at www.liebertonline.com=ast.


354

JONES AND LINEWEAVER Table 2. Extreme Temperatures and Pressures in Earth's Oceans and Crust

Key A

Environment Surface lava, e.g., Pahoehoe, rhyolite melt, and basalt--hottest surface temperatures (Cotopaxi, Equador) Thermal spring=vent (Pork Chop Geyser, Yellowstone, USA) Mt. Everest summit Minimum land temperature (Vostok, Antarctica) Deep ice (South Pole, Antarctica) Deep ice (Vostok, Antarctica) Marianas trench (Challenger Deep) Mean base of oceanic crust Crust mantle boundary--lowest temperature at thickest point (Himalaya, southern Tibet) Seafloor hydrothermal vents--hottest ( Mid-Atlantic Ridge) Seafloor hydrothermal vents--hottest shallow vents (Tonga arc) Geothermal waters in the crust Lava lakes (Kilaueua Iki, Hawaii; Eldfell volcano, Iceland)

Pressure (bar) Depth (km) 0.4 þ5.897

Temperature (8C) 1200 Harris and and and

Reference et al., 1998; Fagents Greeley, 2001; Stein Spera, 2002; Decker Decker, 1992

B C, D E F G H, K I J L M N O

0.77 þ2.293 0.33 ô 0.01 þ8.848 0.6 þ3.488 72 0.8 2.5á102 2.75 1.32á103 10.896 2.0á03 10 5á104 80 3.5á02 3 49 0.54 28 0.1 102 0.3

97 þ74 to þ26 þ89 þ51 þ40 2­300 200 500 464 265 157 1500

Guidry and Chafetz, 2003 Team Everest (2003) website; West et al., 1983 National Oceanic and Atmospheric Administration (2007) website Price et al., 2002 Abyzov et al., 1998 Todo et al., 2005; The Mariana trench (2003) website Lodders and Fegley, 1998; Huppert and Sparks, 1988 Priestley et al., 2008 Koschinsky et al., 2008 Stoffers et al., 2006 Ellis and Mahon, 1977 Helz and Thornber, 1987; Jonsson and Matthiasson, 1974

Conversions between depth and pressure for atmosphere; land and water used the density values in Table A1 (Appendix A) and the formula P(z) ¼ Po × rgz, where P(z) ¼ pressure at depth z, Po ¼ normalizing pressure, r ¼ density, g ¼ gravitation acceleration. Gravitational acceleration values were taken from Dziewonski and Anderson (1981).

Our phase diagrams can be used to refine and focus searches for life on Earth and elsewhere. Our analysis highlights the need to look for environments that have conditions at the extreme limits known for life so as to quantify the extent to which terrestrial life follows the water. Current limits indicate that the terrestrial biosphere inhabits only a small subset of the full range of liquid-water environments on Earth. More work needs to be done to determine how much of the apparently uninhabited water appears that way due to the difficulty and incompleteness of the observations. A. Appendix A: Details of the Earth Model Shown in Figure 3 A.1. Crust and surface The thickness of the crust varies widely--between 5 to around 80 km (Pavlenkova and Zverev, 1981; Holbig and Grove, 2008) beneath continental crust (averaging 35 km), and only from 2 to 10 km under oceanic crust (averaging 5­ 7 km). The global average depth is 19 km (Dziewonski and Anderson, 1981). As the Moho marks a density transition, it does not have a fixed pressure or temperature. Shallow Moho (beneath thin crust) is typically quite cool due to its

lower heat flow. A large fraction of the crust is <40 km thick ( Mooney et al., 1998) and lies above mantle cooler than 800 K (Blackwell, 1971). With increasing depth the temperature of the crust and Moho typically increases due to increasing heat flow. Deep Moho beneath thick crust can be cool, however, with the current range of deep Moho temperatures being ´ *800 K (Hyndman et al., 2005) to 1400 K ( Jimenez-Munt et al., 2008). Taking a typical average continental crustal density, the 80 km deep Moho (Sokolova et al., 1978) lies at a pressure of around 20 kPa--roughly an order of magnitude greater than the pressure at shallow Moho. Due to these large variations in pressure and temperature at the base of the crust, there is difficulty in representing this boundary simply on a P-T diagram. We have chosen to mark the average conditions at the base of average thickness continental and oceanic crust and to use error bars to encompass the range of crustal thickness (pressures) and the variation in temperature at different Moho depths. A sloped error bar is due to the general trend of deeper Moho being at higher temperature. A more fundamental consideration is Earth's heat flow, which is not explicitly represented in our model. Earth's surface heat flow varies predominantly between 40 and


DOES TERRESTRIAL LIFE ``FOLLOW THE WATER''? Table 3. Active Life Environments Key 1 Description Mt. Everest summit Pressure (bar) Depth (km) 0.33 ô 0.01 þ8.848 Temperature (8C) þ20 Reference Active microbial life has not been searched for at this location. We set 1 at the same minimum temperature at which active life has been found (see 8). Gangwar et al. (2009) report life at þ208C at 5.6 km. Guidry and Chafetz (2003) (Lilypad stromatolites)

355

2 3

Thermal spring=vent (Pork Chop Geyser, Yellowstone National Park, USA) Shallow hydrothermal vents

0.77 þ2.293 2 0.01

97 122

4

Ocean hydrothermal vent where highest-temperature life found (Finn Vent, Mothra Field, Pacific Ocean) Marianas trench, deepest part of ocean (Challenger Deep, Pacific Ocean)

2.1á102 2.27

122

5

1.32á103 10.924

122

6

Deepest active life in crust from borehole (Gravberg, Sweden) Marianas trench (Challenger Deep, Pacific Ocean) Arctic Ice core, lowest-temperature active life

1.55á103 5.278 1.32á103 10.896 102 2.75

75

7 8

2 þ20

Thermophilic life has been found in shallow vents at temperatures between 40 and 1008C (Amend et al., 2003; Prol-Ledesma, 2003; Stetter, 2006). Life has not been found at the maximum temperature of 1228C (see 4); however, temperatures of vents at this depth can exceed 1228C (Prol-Ledesma, 2003). We have therefore placed this marker at a temperature of 1228C. Takai et al. (2008) and Kashefi and Lovley (2003) (Archaea, closely related to Pyrodictium occultum, Pyrobaculum aerophilum) Schrenk et al. (2003) Thermophillic life has not been searched for at this location. Life could exist in this environment, as the maximum temperature for life found thus far is 1228C (see 4). Vents at this depth can exceed 1228C (The Mariana Trench, 2003 website), and life has been found at 2­38C temperatures in this environment (Todo et al., 2005). We have therefore placed this marker at a temperature of 1228C. Szewzyk and Szewzyk (1994) (Thermoanaerobacter, Thermoanaerobium, Clostridium, close relationship to Clostridium thermohydrosulfuricum) Todo et al. (2005), Takami et al. (1997) (Foraminifera: Lagenammina, Nodellum, Resigella) Junge et al. (2004) [Bacteria (EUB338), Cytophaga-Flavobacteria (CF319a), Archaea (ARCH915)]

85 mW mþ2 (Pollack and Chapman, 1977) with an average of * 55 mW mþ2. The heat flow from the Moho beneath the ´ continents is fairly fixed at around 25 mW mþ2 (Cermak et al.,1989) with the discrepancy made up from radiogenic heat production from elements within the crust. There is less radiogenic heat production in the oceanic crust itself (Smith, 1973); thus its heat flow shows much less variation and has an average of 100 mW mþ2 (Sclater and Francheteau, 1970; Davies, 1999). The crustal heat flow and the thermal conductivity of crustal materials together determine the subsurface geotherm, through the relation

qT Q ¼ qz k where qT is the geotherm (K kmþ1); Q ¼ heat flow (mW mþ2) qz and k ¼ thermal conductivity (W mþ1 Kþ1). Hence, this heterogeneity in heat flow (and of course crustal materials) has the consequence that temperatures can still show large regional variation many kilometers beneath the subsurface (Artemieva, 2006), which can continue even into the asthenosphere (Smith, 1973). Thus, oceanic and continental geotherms


356 Table 4. Dormant Life Environments Key 9 Description Vostok ice core life Pressure (bar) Depth (km) 102 2.757 1 Temperature (8C) þ40 Reference Abyzov et al., 1998; Price and Sowers, 2004 Langdahl and Ingvorsen, 1997

JONES AND LINEWEAVER

Metabolic activity Cell activity after incubation; cells interpreted as in situ were able to maintain vital functions but not reproduce Sampled at warmer temperatures so metabolic activity at lowtemperature limit is unknown (Thiobacillus ferrooxidans, Thiobacillus thiooxidans, and Leptospirillum ferrooxidans) Marker 11 is at the minimum temperature at which dormant microbial life has been found in the crust in the presence of liquid water (see 9); the pressure is given by 1. Dormant (spore) in atmosphere but cultured in laboratory (Bacillus simplex, Staphylococcus asteuri, Engyodontium album) Growth after extended incubation ( Micrococcaceae, Microbacteriaceae, Brevibacterium luteolum, Staphylococcus) Dormant (spore) in atmosphere (Circinella muscae, Aspergillus niger, Papulaspora anomala, Penicillium notatum, Mycobacterium luteum, Micrococcus albus)

10

Greenland open mine life

þ30

11

Mt. Everest summit

0.33 ô 0.01 þ8.848

þ40

12

Bacterial spores (atmosphere) Non-spore-forming eubacteria (atmosphere) Bacterial spores (atmosphere)

0.0553* þ20 0.0025* þ41 0.00002* þ77

þ20*

Wainwright et al., 2002 Griffin, 2008 Imshenetsky et al., 1978

13 14

þ56* þ69*

*Temperature and pressure from Hewitt and Jackson (2003).

cannot be guaranteed convergence until below *200 km in the asthenosphere where convective heat transfer dominates (Andersen, 1979; Chapman, 1986). In our model, the oceanic and continental geotherms converge at *10 km into the continental crust. A typical geothermal gradient in the oceanic crust is *35 K kmþ1 (100 mW mþ2 heat flow; thermal conductivity for basalt of 3 W mþ1 Kþ1 from Clauser and Huenges, 1995). The mean continental geotherm is 25 K kmþ1 (Scheidegger, 1976); however, it ranges greatly due to variations in heat flow (10­130 mW mþ2 at the surface) and thermal conductivity of rock (0.5­7 W mþ1 Kþ1 range: Clauser and Huenges, 1995), and is predominantly between 5 and 70 K kmþ1 (Chapman and Pollack, 1975). The extreme environments of Earth are given in Table 2. All known environments in the crust, oceans, and surface lie within the P-T polygon and within the range of the error bars on the average geotherm. The mean continental crustal geotherm shown, averaged over 35 km of crust, is 15 K kmþ1. The brown shaded region accommodates much steeper geotherms (up to *80 K kmþ1) and so is consistent with the range of true geothermal gradients on Earth. A.2. Core Earth's core extends from a depth of 2890 to 6378 km. At the center, the pressure is modeled as 3.6á106 bar (Dziewonski and Anderson, 1981; Davies, 1999). As the core density is not well constrained from seismic models, there is a range of estimates

for the central core pressure between 2.8á106 and 3.8á106 bar (Fowler, 1990; Lodders and Fegley, 1998). Similarly, we have chosen a representative pressure value for the core-mantle boundary (CMB) of 1.4á106 bar (Davies, 1999) within the range of 0.6­3.2á106 bar (Fowler, 1990; Lodders and Fegley, 1998; Davies, 1999). The pressure value for the CMB is not essential for our model, as the transition from core to mantle occurs within a layer *40 km thick (Doornbos, 1983; Williams and Garnero, 1996). At its central depth, the core temperature is taken in our model to be 5700 K (Fowler, 1990). Temperature models for the core are dependent on core composition, and estimates for the central temperature range widely from 3800 to 6800 K ( Jeanloz, 1990; Davies, 1999). Similarly, with the CMB we have chosen a representative temperature of 2700 K (Davies, 1999) within the range of estimates 1700­4500 K ( Jeanloz, 1990; Decker and Decker, 1992; Stein and Wysession, 2003). As stated above, the CMB is thought to be a *40 km thermal boundary layer, with an estimated radial temperature increase of 800 K (Stacey and Loper, 1984) across it. Melting temperatures for the assumed core composition are pressure dependent (Andersen et al., 2002); however, our chosen boundary values and geotherm of 0.85 K kmþ1 leads to a liquid outer region of the core and a solid inner region (from the solidus of Jeanloz, 1990). This is consistent with recent models (Stein and Wysession, 2003). The central core may be liquid if its temperature exceeds 5120 K (Nakagawa and Tackley, 2004). The range of estimates of pressure and temperature conditions in the inner core are represented through vertical and


Table A1. Model Parameters for Earth's Core, Mantle, Crust and Atmosphere
Range Reference

Core Temperature (K)

2700 1:39
2890­6378 1.3á104 (averaged over inner and outer core) 1.27á105 0.6­0.9; our model 0.85
× 1:81 þ 0:76

× 1800 þ 1000

(outer) to 5700
× 0:23 þ 0:77

× 1100 þ 1900

(inner) · 10 (inner)
þ6

Pressure (bar)

· 106 (outer) to 3:57

Depth (km) Density (kg mþ3) Mean pressure gradient (Pa kmþ1) Geothermal gradient (K kmþ1)

Fowler, 1990; Jeanloz, 1990; Lodders and Fegley, 1998; Davies, 1999 Fowler, 1990; Lodders and Fegley, 1998; Davies, 1999; Dziewonski and Anderson, 1981 Dziewonski and Anderson, 1981; Lodders and Fegley, 1998 Dziewonski and Anderson, 1981; Lodders and Fegley, 1998 Lodders and Fegley, 1998; Dziewonski and Anderson, 1981 Ernst, 1990; Saxena et al., 1994; Karato, 2003

Mantle Temperature (K)

800 3:3
5­2890 (average 33 under continents, 7 under oceans) 4.5á103 4.41á104 0.3­1; our model 0.7 184­331 (surface) * 800 (average base temperature; magma) 1 (surface) to 1:3 × 0:9 · 104 (base) þ 1:03 35 (mean) 10­80 range 2.8á103 2.74á104 25 (mean) 271­737 (ocean floor) 4á102 (base ocean) to 1:9 5 (mean oceanic)
× 4:38 þ 0:33 × 26:7 þ 2 :1

× 1380 þ 570

(outer) to 2700
× 1:81 þ 0:76

× 1800 þ 1000

(CMB) · 106 (inner)

Pressure (bar)

· 103 (outer) to 1:39

Depth (km)

Density (kg mþ3) Mean pressure gradient (Pa kmþ1) Geothermal gradient (K kmþ1)

Fowler, 1990; Lodders and Fegley, 1998; Davies, 1999; Decker and Decker, 1992 Fowler, 1990; Lodders and Fegley, 1998; Davies, 1999; Decker and Decker, 1992 Scheidegger, 1976; Dziewonski and Anderson, 1981; Lodders and Fegley, 1998 Lodders and Fegley, 1998 Lodders and Fegley, 1998; Dziewonski and Anderson, 1981 Mason, 1958; Schiano et al. (2006) Lodders and Fegley, 1998; Huppert and Sparks, 1988 Lodders and Fegley, 1998 Dziewonski and Anderson, 1981; Scheidegger, 1976; Burtman and Molnar, 1993 Lodders and Fegley, 1998 Lodders and Fegley, 1998; Dziewonski and Anderson, 1981 Scheidegger, 1982

Continental crust Temperature (K) Pressure (bar) Thickness (km)

Density (kg mþ3) Mean pressure gradient (Pa kmþ1) Geothermal gradient (K kmþ1)

Oceanic crust Temperature (K)

Pressure (bar) Thickness (km)

· 103 ( Moho)

National Oceanic and Atmospheric Administration website, 2007; Koschinsky et al., 2008 Lodders and Fegley, 1998 Dziewonski and Anderson, 1981; Muller et al., 1997; Mooney et al., 1998 Lodders and Fegley, 1998; Dziewonski and Anderson, 1981 Lodders and Fegley, 1998 Scheidegger, 1982 Hewitt and Jackson, 2003; West, 1996; Choi et al., 1998

þ1

4­20 (range) 4 (mean depth under ocean) 2.94á104 3á103 40 6.5 troposphere þ2.0 stratosphere 2.6 mesosphere þ5.8 thermosphere þ

Mean pressure gradient (Pa km ) Density (kg mþ3) Geothermal gradient (K kmþ1) Atmosphere Mean lapse rate temperature (K kmþ1)*

Lapse rate pressure (Pa kmþ1){ below 86 km

þ 0:0012 1 þ

à4:26 z 44307:69

National Oceanic and Atmospheric Administration website, 2007; Pearson and Rugge, 1976

*Temperature lapse rate GT is the rate of decrease of temperature with height: GT(z) ¼ þdT=dz. { Pressure lapse rate GP is the rate of decrease of pressure with height: G P(z) ¼ dP=dz using the equation z ¼ [1 þ (P=10.13)0.19]á44307.69 where z ¼ altitude (km) and P ¼ static pressure (Pa) (Pearson and Rugge, 1976).


358 horizontal error bars that encompass the range of estimates in the literature. The same holds for estimates of conditions across the core-to-mantle transition. A.3. Mantle Earth's mantle extends from a depth of 2890 km at the core-mantle boundary to a range of 4­80 km beneath oceanic and continental crust. Our chosen average of the Mohor´ ´ ovicic discontinuity ( Mohorovicic, 1909) is 3.3á103 bar and 800 K, which is consistent with the average continental crustal depth of 35 km (Lodders and Fegley, 1998). The variation in the Moho will be discussed in more detail below. Our CMB values lead to a modeled mantle geotherm of 0.66 K kmþ1. This temperature profile lies below the assumed mantle solidus (Zerr, 1998; Nakagawa and Tackley, 2004), resulting in the expected solid mantle (Stein and Wysession, 2003). For example, the solidus temperature at the CMB pressure is estimated as 4300 K (Nakagawa and Tackley, 2004). B. Appendix B: Explanatory Details for Tables 3 and 4 Life has been found throughout Earth's atmosphere, to an altitude of 77 km above the surface (Imshenetsky et al., 1978). These organisms can be classified into two types--spore or spore-forming bacteria (e.g., Bacillus simplex; Wainwright et al., 2002) and bacteria that do not form spores (e.g., Micrococcus; Griffin, 2008). This classification is made here, as it allows for a distinction to be made between active and potentially dormant life in the atmosphere. Spore structures allow bacteria to survive for extended periods of time in unfavorable conditions. This means that spores collected from the atmosphere can be dormant and yet viable, so they may be able to be cultivated in the laboratory. It is unclear, however, whether such spores were actively metabolizing, be it at a very slow rate, in atmospheric conditions (personal correspondence, Dr. Yogesh Shouche, April 3, 2009). Non-spore-forming bacteria that are retrieved from the atmosphere, however, are thought to have been active in that environment if they are able to be grown in the laboratory. Using this distinction, some have concluded that active life has been isolated from an altitude of 20 km in Earth's atmosphere (Griffin, 2008) at an ambient pressure and temperature of 0.055 bar and þ568C, respectively. This appears to extend the lower temperature limit of life; however, the result should be viewed with caution, as no measurements of metabolism were made. Other species of non-spore-forming bacteria have been isolated from the atmosphere at higher altitudes and warmer temperatures (between 20 and 70 km altitude) but have been found to be viable yet nonculturable (Wainwright et al., 2004), and are examples of dormant life. The current upper boundary of the biosphere (most likely at the tropopause at *10 km altitude, which is a boundary to the vertical movement of particles; Wainwright et al., 2006) lies at 77 m (ambient pressure and temperature of *10þ5 bar, þ708C respectively) where spores (e.g., Circinella muscae) were isolated (Imshenetsky et al., 1978) and were most likely dormant at in situ conditions. Examples of the potentially lowest temperature, active life, and the highest altitude (dormant) life are included in our life regime in Fig. 4.

JONES AND LINEWEAVER A serious limitation on atmospheric life's capacity to remain active may be the transient nature of low-pressure liquid-water environments. The presence of life in clouds is strongly linked to the availability of liquid water. Studies of low-altitude atmospheric life (samples from <4 km above sea level, Sattler et al., 2001; Bauer et al., 2002; Amato et al., 2005, 2007) have found that the majority of microbes in the clouds are dormant, but viable and remain alive by utilizing cloud water. This suggests that surface life can persist at altitudes up to the triple point of ocean salinity water at *25 km altitude (Fig. 3); however, this life will be largely, or entirely, dormant. Samples isolated at higher altitudes (e.g., *41 km, Wainwright et al., 2002) have a much lower percentage of viable microorganisms, which may be due to the additional stress of almost no cloud water. Cloud water could persist to higher altitudes (lower atmospheric pressures) if it is briny; however, models of liquid-water clouds indicate that water content reaches a minimum at <20 km altitude (Fig. 7 of http:==meteora.ucsd.edu=* iacob=scm_recent=arm3=poster .html; Iacobellis et al., 2003). Acknowledgments We thank Jonathan Clarke, Chris McKay, Geoff Davies, and Andreea Papuc for useful discussions. Eriita Jones acknowledges funding from an Australian Postgraduate Award and the Research School of Astronomy and Astrophysics (ANU). Author Disclosure Statement No competing financial interests exist for either Eriita G. Jones or Charles H. Lineweaver. Abbreviations CMB, core-mantle boundary; P-T, pressure-temperature. References
Abyzov, S., Mitskevich, I., and Poglazova, M. (1998) Microflora of the deep glacier horizons of central Antarctica. Microbiology 67:451­458. Amato, P., Menager, M., Sancelme, M., Laj, P., Mailhot, G., and Delort, A. (2005) Microbial population in cloud water at the Puy de Dome: implications for the chemistry of clouds. Atmos. Environ. 39:4143­4153. Amato, P., Parazols, M., Sancelme, M., Mailhot, G., Laj, P., and Delort, A. (2007) An important oceanic source of microorganisms for cloud water at the Puy de Dome (France). Atmos. Environ. 41:8253­8263. Amend, J., Meyer-Dombard, D., Sheth, S., Zolotova, N., and Amend, A. (2003) Palaeococcus helgesonii sp. nov., a facultatively anaerobic, hyperthermophilic archaeon from a geothermal well on Vulcano Island, Italy. Arch. Microbiol. 179: 394­401. Andersen, D. (1979) The deep structure of continents. J. Geophys. Res. 84:7555­7560. Andersen, D., Pollard, W., McKay, C., and Heldmann, J. (2002) Cold springs in permafrost on Earth and Mars. J. Geophys. Res. 107:4.1­4.7. Artemieva, I. (2006) Global 18á18 thermal model TC1 for the continental lithosphere: implications for lithosphere secular evolution. Tectonophysics 416:245­277.


DOES TERRESTRIAL LIFE ``FOLLOW THE WATER''?
Barlow, N. (2008) Mars: An Introduction to Its Interior, Surface and Atmosphere, Cambridge University Press, Cambridge, UK. Baross, J., Benner, S., Cody, G., Copley, S., Pace, N., Scott, J., Shapiro, R., Sogin, M., Stein, J. Summons, R., and Szostak, J. (2007) The Limits of Organic Life in Planetary Systems, National Research Council, National Academies Press, Washington DC. Bauer, H., Kasper-Giebl, A., Loflund, M., Giebl, H., Hitzenberger, R., Zibuschka, F., and Pauxbaum, H. (2002) The contribution of bacterial and fungal spores to the organic carbon content of cloud water, precipitation and aerosols. Atmospheric Research 64:109­119. Blackwell, D. (1971) The thermal structure of the continental crust. In The Structure and Physical Properties of the Earth's Crust, Geophysical Monograph 14, edited by J.G. Heacock, American Geophysical Union, Washington DC, pp 169­184. Burtman, V. and Molnar, P. (1993) Geological and Geophysical Evidence for Deep Subduction of Continental Crust Beneath the Pamir, Geological Society of America, Boulder, Colorado. ´ Cermak, V., Safanda, J., and Guterch, A. (1989) Deep temperature distribution along three profiles crossing the TeisseyreTornquist tectonic zone in Poland. Tectonophysics 164:151­163. Chapman, D. (1986) Thermal gradients in the continental crust. J. Geol. Soc. London (Special Publications) 24:63­70. Chapman, D. and Pollack, H. (1975) Global heat flow--a new look. Earth Planet. Sci. Lett. 28:23­32. Choi, G., Monson, I., Wickwar, V., and Rees, D. (1998) Seasonal variations of temperature near the mesopause from FabryPerot interferometer observations of OH Meinel emissions. Adv. Space Res. 21:843­846. Clark, B. and Van Hart, D. (1981) The salts of Mars. Icarus 45: 370­378. Clauser, R. and Huenges, E. (1995) Thermal conductivity of rocks and minerals. In Rock Physics and Phase Relations--A Handbook of Physical Constants, edited by T. Ahrens, American Geophysical Union, Washington DC, pp 105­126. Davies, G. (1999) Dynamic Earth: Plates, Plumes, and Mantle Convection, Cambridge University Press, Cambridge, UK. Davis, N. (2001) Permafrost, A Guide to Frozen Ground in Transition, University of Alaska Press, Fairbanks. Decker, B. and Decker, R. (1992) Mountains of Fire: The Nature of Volcanoes, Cambridge University Press, Cambridge, UK. Doornbos, D. (1983) Present seismic evidence for a boundary layer at the base of the mantle. J. Geophys. Res. 88:3498­3505. Dziewonski, A. and Anderson, D. (1981) Preliminary reference Earth model. Physics of the Earth and Planetary Interiors 25:297­356. Ellis, A. and Mahon, W. (1977) Geochemistry and Geothermal Systems, Academic Press, New York. Ellsworth, E., Hickman, S., Zoback, M., Davis, E., Gee, L., Huggins, R., Krug, R., Lippus, C., Malin, P., Neuhauser, D., Paulsson, B., Shalev, E., Vajapeyam, B., Weiland, C., and Zumberge, M. (2005) Observing the San Andreas Fault at depth [abstract #T24B-04]. American Geophysical Union, Fall Meeting 2005, American Geophysical Union, Washington DC. Ernst, W. (1990) The Dynamic Planet, Columbia University Press, New York. Fagents, S. and Greeley, R. (2001) Factors influencing lavasubstrate heat transfer and implications for thermochemical erosion. Bulletin of Volcanology 62:519­532. Fowler, C. (1990) The Solid Earth: An Introduction to Geophysics, 1st ed., Cambridge University Press, Cambridge, UK. Gangwar, P., Alam, S., Bansod, S., and Singh, L. (2009) Bacterial diversity of soil samples from the western Himalayas, India. Can. J. Microbiol. 55:564­577.

359
´ Garzon, L. (2004) Microbial life and temperature: a semi empirical approach. Orig. Life. Evol. Biosph. 34:421­438. Grant, W. (2004) Life at low water activity. Philos. Trans. R. Soc. Lond., B, Biol. Sci. 359:1249­1267. Griffin, D. (2008) Non-spore forming eubacteria isolated at an altitude of 20,000 m in Earth's atmosphere: extended incubation periods needed for culture based assays. Aerobiologia 24: 19­25. Griffith, C., Hall, J., and Geballe, T. (2000) Detection of daily clouds on Titan. Science 290:509­513. Guidry, S. and Chafetz, H. (2003) Anatomy of siliceous hot springs: examples from Yellowstone National Park, Wyoming, USA. Sediment. Geol. 157:71­106. Harris, A., Flynn, L., Keszthelyi, L., Mouginis-Mark, P., Rowland, S., and Resing, J. (1998) Calculation of lava effusion rates from Landsat TM data. Bulletin of Volcanology 60:52­71. Helz, R. and Thornber, C. (1987) Geothermometry of Kilauea Iki lava lake, Hawaii. Bulletin of Volcanology 49:651­668. Hewitt, C. and Jackson, A. (2003) Handbook of Atmospheric Science--Principles and Applications, Wiley-Blackwell Publishers, New York. Hoehler, T. (2004) Biological energy requirements as quantitative boundary conditions for life in the subsurface. Geobiology 2: 205­215. Hoehler, T., Amend, J., and Shock, E. (2007) A ``follow the energy'' approach for astrobiology. Astrobiology 7:819­823. Holbig, E. and Grove, T. (2008) Mantle melting beneath the Tibetan Plateau: experimental constraints on ultrapotassic magmatism. J. Geophys. Res. 113, doi:10.1029=2007JB005149. Holden, J. (2008) Some like it hot: understanding the limits of life using hyperthermophilic microbes [paper 33-0015-08]. In 37th COSPAR Scientific Assembly, Symposium F, Session 33, COSPAR, Paris. Horikoshi, K. (1998) Barophiles: deep-sea microorganisms adapted to an extreme environment. Curr. Opin. Microbiol. 1:291­295. Horsfeld, B., Schenk, H.J., Zink, K., Ondrak, R., Dieckmann, V., Kallmeyer, J., Mangelsdorf, K., di Primio, R., Wilkes, H., Parkes, R.J., Fry, J., and Cragg, B. (2006) Living microbial ecosystems within the active zone of catagenesis: implications for feeding the deep biosphere. Earth Planet. Sci. Lett. 246:55­69. Hubbard, G.S., Naderi, F.M., and Garvin, J.B. (2002) Following the water, the new program for Mars exploration. Acta Astronaut. 51:337­350. Huppert, H. and Sparks, R. (1988) The generation of granitic magmas by intrusion of basalt into continental crust. Journal of Petrology 29:599­624. Hyndman, R., Currie, C., and Mazzotti, S. (2005) Subduction zone backarcs, mobile belts, and orogenic heat. GSA Today 15:4­10. Iacobellis, S., Sommerville, R., McFarquhar, G., and Mitchell, D. (2003) Sensitivity of cloud-radiation interactions to cloud microphysics. In Fourteenth Symposium on Global Changes and Climate Variations, American Meteorological Society, Boston, MA. Imshenetsky, A., Lysenko, S., and Kazakov, G. (1978) Upper boundary of the biosphere. Appl. Environ. Microbiol. 35:1­5. Jakosky, B., Amend, J., Berelson, W., Blake, R., Brantley, S., Carr, M., Fredrickson, J., Keefe, A., Keller, M., McSween, H., Nealson, K., Sherwood-Lollar, B., Steele, A., Summons, R., and Wadhwa, M. (2007) An Astrobiology Strategy for the Exploration of Mars, National Research Council, National Academies Press, Washington DC. Jeanloz, R. (1990) The nature of the Earth's core. Annu. Rev. Earth Planet. Sci. 18:357­386.


360
´ ` ´ Jimenez-Munt, I., Fernandez, M., Verges, J., and Platt, J. (2008) Lithosphere structure underneath the Tibetan plateau inferred from elevation, gravity and geoid anomalies. Earth Planet. Sci. Lett. 267:276­289. Jones, E. and Lineweaver, C. (2008). Identifying the potential biosphere of Mars. In Australian Space Science Conference Series: 7th Conference Proceedings NSSA, National Space Society of Australia, Sydney, pp 60­72. Jonsson, V. and Matthiasson, M. (1974) Lava cooling on Heimaey--methods and procedures. In Proceedings of the Icelandic Association of Chartered Engineers, Vol. 59, Icelandic Association of Chartered Engineers, Reykjavik, pp 70­81. Junge, K., Eicken, H., and Deming, J. (2004) Bacterial activity at þ2 to þ208C in Arctic wintertime sea ice. Appl. Environ. Microbiol. 70:550­557. Karato, S. (2003) The Dynamic Structure of the Deep Earth: an Interdisciplinary Approach, Princeton University Press, Princeton, New Jersey. Kashefi, K. and Lovley, D. (2003) Extending the upper temperature limit for life. Science 301:934. Kelley, D.S., Girguis, P.R., Wheat, G., Cordes, E., Schrenk, M.O., Lin, M., Baross, J.A., and Delaney, J.R. (2007) Towards determining the upper temperature limit to life [abstract V23D02]. American Geophysical Union, Fall Meeting 2007, American Geophysical Union, Washington DC. K o sc h i n s ky , A ., G a r b e - S c ho nb e rg , D ., S a n d e r , S . , S c h m i dt , K ., Gennerich, H., and Strauss, H. (2008) Hydrothermal venting at pressure-temperature conditions above the critical point of seawater, 58S on the mid-Atlantic ridge. Geology 36: 615­618. Langdahl, B. and Ingvorsen, K. (1997) Temperature characteristics of bacterial iron solubilisation and 14C assimilation in naturally exposed sulfide ore material at Citronen Fjord, North Greenland (838N). FEMS Microbiol. Ecol. 23:275­283. Lide, D. and Frederikse, H., editors. (1996) CRC Handbook of Chemistry and Physics, 77th ed., CRC Press, Boca Raton, FL. Lide, D. and Haynes, W., editors. (2009) CRC Handbook of Chemistry and Physics, 90th ed., CRC Press, Boca Raton, FL. Lodders, K. and Fegley, B. (1998) The Planetary Scientist's Companion, Oxford University Press, Oxford, UK. Margosch, D., Ehrmann, M., Buckow, R., Heinz, V., Vogel, R., and Ganzle, N. (2006) High-pressure-mediated survival of Clostridium botulinum and Bacillus amloliquefaciens endospores at high temperature. Appl. Environ. Microbiol. 72:3476­ 3481. Mariana Trench (2003) The Mariana Trench--biology. Available online at http:==www.marianatrench.com=mariana_trenchbiology_001.htm. Mason, B. (1958) Principles of Geochemistry, John Wiley and Sons, New York. Melosh, H., Ekholm, A., Showman, A., and Lorenz, R. (2004) The temperature of Europa's subsurface water ocean. Icarus 168: 498­502. ¨ Mohlmann, D. (2008) The influence of van der Waals forces on the state of water in the shallow subsurface of Mars. Icarus 195:131­139. ´ Mohorovicic, A. (1909) Earthquake of 8 October 1909. Geofizika 9:3­55. Mooney, W., Laske, G., and Masters, G. (1998) A global crustal model at 58á58. J. Geophys. Res. 103:727­747. Muller, M., Robinson, C., Minshull, T., White, R., and Bickle, M. (1997) Thin crust beneath ocean drilling program borehole 735B at the Southwest Indian Ridge? Earth Planet. Sci. Lett. 148:93­107.

JONES AND LINEWEAVER
Nakagawa, T. and Tackley, P. (2004) Effects of thermo-chemical mantle convection on the thermal evolution of the Earth's core. Earth Planet. Sci. Lett. 220:107­119. National Oceanic and Atmospheric Administration. (2007) NOAA satellite and information service. National Oceanic and Atmospheric Administration, Washington DC. Available online at http:==www.ncdc.noaa.gov=oa=. Oppenheimer, C. and Francis, P. (1998) Implications of longeval lava lakes for geomorphological and plutonic processes at Erta `Ale volcano, Afar. Journal of Volcanology and Geothermal Research 90:101­111. Pavlenkova, N. and Zverev, S. (1981) Seismic model of Iceland's crust. International Journal of Earth Sciences 70:271­281. Pearson, J. and Rugge, H. (1976) U.S. Standard Atmosphere, National Ocean and Atmospheric Administration, US Government Printing Office, Washington DC. Petculescu, A. and Lueptow, R. (2007) Atmospheric acoustics of Titan, Mars, Venus and Earth. Icarus 186:413­419. Pollack, H. and Chapman, D. (1977) On the regional variation of heat flow, geotherms, and lithospheric thickness. Tectonophysics 38:279­296. Price, P. (2000) A habitat for psychrophiles in deep Antarctic ice. Proc. Natl. Acad. Sci. U.S.A. 97:1247­1251. Price, P. and Sowers, T. (2004) Temperature dependence of metabolic rates for microbial growth, maintenance, and survival. Proc. Natl. Acad. Sci. U.S.A. 101:4631­4636. Price, P., Nagornov, O., Bay, R., Chirkin, D., He, Y., Miocinovic, P., Richards, A., Woschnagg, K., Koci, B., and Zagorodnov, V. (2002) Temperature profile for glacial life at the south pole: implications for life in a nearby subglacial lake. Proc. Natl. Acad. Sci. U.S.A. 99:7844­7847. Priestley, K., Jackson, J., and McKenzie, D., (2008) Lithospheric structure and deep earthquakes beneath India, the Himalaya and southern Tibet. Geophysical Journal International 172: 345­362. Prol-Ledesma, R. (2003) Similarities in the chemistry of shallow submarine hydrothermal vents. Geothermics 32:639­ 644. Reynolds, R., Squyres, S., Colburn, D., and McKay, C. (1983) On the habitability of Europa. Icarus 56:246­254. Rothschild, L. and Mancinelli, R. (2001) Life in extreme environments. Nature 409:1092­1101. Sattler, B., Pauxbaum, H., and Psenner, R. (2001) Bacterial growth in supercooled cloud droplets. Geophys. Res. Lett. 28:243­246. Saxena, S., Shen, G., and Lazor, P. (1994) Temperatures in Earth's core based on melting and phase transformation experiments on iron. Science 264:405­407. Scheidegger, A. (1976) Foundations of Geophysics, Elsevier Scientific, Amsterdam. Schrenk, M., Kelley, D., Delaney, J., and Baross, J. (2003) Incidence and diversity of microorganisms within the walls of an active deep-sea sulfide chimney. App. Environ. Microbiol. 69:3580­3592. Schiano, P., Provost, A., Clocchiatti, R., and Faure, F. (2006) Transcrystalline melt migration and Earth's mantle. Science 314:970­974. Sclater, J. and Francheteau, J. (1970) The implications of terrestrial heat flow observations on current tectonic and geochemical models of the crust and upper mantle of the Earth. Geophysical Journal of the Royal Astronomical Society 20:509­542. Sharma, A., Scott, J., Cody, G., Fogel, M., Hazen, R., Hemley, R., and Huntress, W. (2002) Microbial activity at gigapascal pressures. Science 295:1515­1516.


DOES TERRESTRIAL LIFE ``FOLLOW THE WATER''?
Shock, E.L. and Holland, M.E. (2007) Quantitative habitability. Astrobiology 7:839­851. Silberberg, M. and Duran, R. (2002) The Molecular Nature of Matter and Change, McGraw-Hill, University of Virginia, Charlottesville, VA. Sleep, N. and Zoback, M. (2007) Did earthquakes keep the early crust habitable? Astrobiology 7:1023­1032. Smith, P. (1973) Topics in Geophysics, MIT Press, Cambridge, Massachusetts. ´ Sokolova, L., Duchkov, A., and Cermak, V. (1978) Crustal thermal models for South Siberia. Studia Geophysica et Geodaetica 22:200­207. Stacey, F. and Loper, D. (1984) Thermal histories of the core and mantle. Physics of the Earth and Planetary Interiors 36:99­115. Stein, D. and Spera, F. (2002) Shear viscosity of rhyolite-vapor emulsions at magmatic temperatures by concentric cylinder rheometry. Journal of Volcanology and Geothermal Research 113:243­258. Stein, S. and Wysession, M. (2003) An Introduction to Seismology, Earthquakes, and Earth Structure, Wiley-Blackwell Publishers, Malden, Massachusetts. Stetter, K. (2006) History of discovery of the first hyperthermophiles. Extremophiles 10:357­362. Stoffers, P., Worthington, T., Schwarz-Schampera, U., Hannington, M., Massoth, G., Hekinian, R., Schmidt, M., Lundsten, L., Evans, L., Vaiomo'unga, R., and Kerby, T. (2006) Submarine volcanoes and high-temperature hydrothermal venting on the Tonga arc, southwest Pacific. Geology 34:453­456. Szewzyk, U. and Szewzyk, R. (1994) Thermophillic, anaerobic bacteria isolated from a deep borehole in granite in Sweden. Proc. Natl. Acad. Sci. U.S.A. 91:1810­1813. Takai, K., Nakamura, K., Toki, T., Tsunogai, U., Miyazaki, M., Miyazaki, J., Hirayama, H., Nakagawa, S., Nunoura, T., and Horikoshi, K. (2008) Cell proliferation at 1228C and isotopically heavy CH4 production by a hyperthermophilic methanogen under high-pressure cultivation. Proc. Natl. Acad. Sci. U.S.A. 105:10949­10954. Takami, H., Inoue, A., Fuji, F., and Horikoshi, K. (1997) Microbial flora in the deepest sea mud of the Mariana trench. FEMS Microbiol. Lett. 152:279­285.

361
Team Everest (2003) Team Everest '03, Austin, TX. Available online at http:==www.teameverest03.org. Todo, Y., Kitazoto, H., Hashimoto, J., and Gooday, A. (2005) Simple foraminifera flourish at the ocean's deepest point. Science 307:689. Wainwright, M., Narlikar, J., and Rajaratnam, P. (2002) Microorganisms cultured from stratospheric air samples obtained at 41 km. FEMS Microbiol. Lett. 10778:1­5. Wainwright, M., Wickramasinghe, N., Narlikar, J., Rajaratnam, P., and Perkins, J. (2004) Confirmation of the presence of viable but non-cultureable bacteria in the stratosphere. Int. J. Astrobiology 3:13­15. Wainwright, M., Alharbi, S., and Wickramasinghe, N. (2006) How do microorganisms reach the stratosphere? Int. J. Astrobiology 5:13­15. Wiersberg, T. and Erzinger, J. (2008) Origin and spatial distribution of gas at seismogenic depths of the San Andreas Fault from drill-mud gas analysis. Appl. Geochem. 23:1675­1690. West, J. (1996) Prediction of barometric pressures at high altitudes with the use of model atmospheres. J. Appl. Physiol. 81:1850­1854. West, J., Lahiri, S., Maret, K., Peters, R., Jr., and Pizzo, C. (1983) Barometric pressures on Mt Everest: new data and physiological significance. J. Appl. Physiol. 54:1188­1194. Williams, Q. and Garnero, E. (1996) Seismic evidence for partial melt at the base of the Earth's mantle. Science 13:1528. Zerr, A., Diegeler, R., and Boehler, R. (1998) Solidus of the Earth's deep mantle. Science 10:243.

Address correspondence to: Eriita G. Jones Research School of Astronomy & Astrophysics Mount Stromlo Observatory Cotter Road Weston, ACT 2611 Australia E-mail: eriita@mso.anu.edu.au Submitted 14 September 2009 Accepted 23 February 2010