Planetary and Space Science 50 (2002) 675 – 683
www.elsevier.com/locate/pss
Search parameters for the remote detection of extraterrestrial life Dirk Schulze-Makucha; ∗ , Louis N. Irwinb , Huade Guanc a Department
of Geological Sciences, University of Texas at El Paso, 500 W University Avenue, El Paso, TX 79968-0555, USA b Department of Biological Sciences, University of Texas at El Paso, EL Paso, TX 79968, USA c New Mexico Institute of Mining and Technology, Socorro, NM, USA Received 29 November 2000; received in revised form 11 July 2001; accepted 29 September 2001
Dedicated to Philip Haddad, who died suddenly at the age of 31 before he had a chance to achieve a promising scienti5c career.
Abstract Direct consequences of biological activity (biosignatures) and alterations of the geological environment due to biological processes (geosignatures) are currently known only for the planet Earth. However, geoindicators remotely detectable by robotic technology have revealed a number of sites in the solar system where conditions compatible with the support of life may exist. By focusing on a search for energy gradients, complex chemistry, liquids that may act as solvents, atmospheres, and indicators of geological di9erentiation, robotic exploration of the solar system and beyond should lead to fruitful targets in the search for extraterrestrial life. An analysis of all major solar system bodies for these parameters suggests that Mars, Titan, and the Galilean satellites should be given the highest priority in the search for extraterrestrial life in our solar system. Extending them to other bodies in the solar system, however, draws attention to Io, Triton, Titania, Enceladus, and Iapetus, among others, as worthy of greater attention. ? 2002 Elsevier Science Ltd. All rights reserved.
1. De nitions of life and the problem of remote detection The distinguishing characteristics of life usually referred to in vernacular (e.g. Random House Dictionary, 2nd Edition, 1987) de5nitions are metabolism, growth, reproduction and adaptation to the environment. However, these characteristics are inadequate as guides for the detection of extraterrestrial life, because they involve dynamic processes that occur on (1) a spatial scale too small to be detected by current remote technology, and (2) a time scale too prolonged to be sampled feasible. In addition, many properties of minerals show life-like characteristics such as growth, mutability, adaptation (Table 1) that cannot be distinguished from those of biogenic origin except by direct analysis of samples. Since retrieval of samples for direct analysis for years to come will be limited to meteorites, comet material returned to Earth by the Stardust Mission in 2007, and Martian surface samples possibly retrievable by the third decade of this century, the search for extraterrestrial life will rely primarily on remote detection methods for the foreseeable future.
∗
Corresponding author. Tel.: +1-915-747-5168; fax: +1-915-747-5073. E-mail address:
[email protected] (D. Schulze-Makuch).
More sophisticated and abstract de5nitions of life point to characteristics such as disequilibrium thermodynamics (e.g. Schroedinger, 1944; Szent-GyBorgyi, 1972), information storage and replication (e.g. Lwo9, 1962; Moreno et al., 1990), and environmental transformation (Chyba and McDonald, 1995) whose consequences can be detected if they occur on a large enough scale. Examples include (1) atmospheric gas composition, such as high O2 and CH4 , resulting from biogenic processes, (2) rocks and sediments produced by biogenic processes such as the banded-iron formation (BIF) and stromatolite deposits of early Earth, (3) rate and type of erosion consistent with biological processes, (4) known biogenic substances such as chlorophyll not explicable by naturally occurring inorganic chemical processes, (5) structural complexity, such as geometric regularity (roads, canals) or unnatural local aggregates (insect colonies, cities) not explicable by natural geophysical processes, (6) energetic emissions, such as radiowaves, which are neither highly regular, as from a pulsar, or highly random, as in the universal background radiation.
0032-0633/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 0 6 3 3 ( 0 1 ) 0 0 1 2 1 - 0
676
D. Schulze-Makuch et al. / Planetary and Space Science 50 (2002) 675–683
Table 1 Properties of life as de5ned in the random house dictionarya , basic requirement and mechanisms (from Schulze-Makuch et al., 2001)
Life property
Basic requirement
Organic mechanism
Inorganic parallel
Metabolism
Yield of energy by electron transfer
Various types of biochemical pathways including photosynthesis
Energy uptake via heat or light, uplift of electrons into the higher energy bands, where they can absorb energy at speci5c frequencies Energy storage by luminescence or in interlayers of clay minerals due to high heat capacity of water and OH-groups
Energy storage as ATP or GTP
Growth
Increase in size of single unit
Cell growth as long as nutrients are available and environmental conditions are favorable until reproduction occurs Self-organizing as development proceeds
Crystal growth as long as favorable environmental conditions prevail and a suHcient ion source is present Local surface reversibility makes it possible to correct certain mistakes during the growth of silicate or metal oxide crystals from aqueous solution
Reproduction
Multiplication of information
Various mechanisms of reproduction (most commonly by binary 5ssion). The genetic code is preserved and duplicated between di9erent generations
Crystals often break up during their growth because of cleavage. For example, if a mineral has a strong preference for cleaving across the direction of growth and in the plane in which the information is held, new “individuals” may form and each of the new “individuals” may expose the information on the new surface (Cairns-Smith, 1982)
Adaptation to Environment
Reaction to, compensation for, and development of new abilities in response to various types of environmental changes
Genetic adaptation from generation to generation: mutations, transposition of genetic material, transformation, conjugation, transduction
Adaptation from generation to generation: changes in mineralogy due to changes in environment and ion source (e.g. in extreme case transformation from one (clay) mineral to an other) Adaptation within individuals: adaptation to outside environment via water release or adsorption in interlayer (clay minerals), development of outer, weathering-resistant layers such as Al2 O3 or silicate layers in tropical soils
Homeostatic adaptation within individuals: examples are movement of individual to nutrient source, cellular shrinkage or formation of spores in nutrient-poor environment, enzyme induction a Second
Edition, 1987.
These features constitute biosignatures and geosignatures of life as we know it and are readily detectable on Earth from space, validating their correlation with the existence of life (Sagan et al., 1993). 2. Biosignatures and geosignatures of life The atmospheric composition of planet Earth is a prime example of a signature (consequence) of life. The Earth’s
atmosphere is the peculiar product of a particular biological process: photosynthesis. Oxygen by itself would not be considered a signature, but the high amounts of oxygen (ca. 21%) together with the presence of hydrogen (H2 ), methane (CH4 ), ammonia (NH3 ), methyl iodide (CH3 I), methyl chloride (CH3 Cl) and various sulfur gases that are detectable can only be explained because of the continuous production by waste-producing life faster than the compounds can react with each other (Sagan, 1994). These gases, highly reactive when mixed, could not coexist at such high concentrations
D. Schulze-Makuch et al. / Planetary and Space Science 50 (2002) 675–683
unless the levels were being actively maintained. It is this type of disequilibrium associated with high amounts of oxygen that can be used as an indicator for oxygen producing photoautotrophs. The atmospheric composition of a planetary body can be identi5ed with relative ease by remote sensing of its absorption spectrum. Particular geosignatures also exist for chemoautotrophs. An example are the limestones and ironstones produced by biological activity on the early Earth. Both types of rocks can form from inorganic processes. The huge quantities produced during early Earth history, however, can hardly be explained by abiotic processes. Chemosynthesis generates various chemical end-products depending on the exact metabolic process (other than oxic photosynthesis where the end-product is always oxygen). Nevertheless, the chemical end-product may provide a useful indicator, especially if produced in a large enough amount over an extended period of time and in thermodynamic disequilibrium with its surrounding environment, making it a signature of chemotrophic life. The biochemical end products often exhibit large-scale geomorphological characteristics such as stromatolite colonies and coral reefs, some of them large enough to be observed with the naked eye from the Moon (e.g. Great Barrier Reef). Radar is especially useful for identifying topographic and geomorphologic characteristics, including even surface roughness. Stereoscopic methods can be used with images of either visible wavelength or radar, and thus are often a useful tool for enhancing the detection limit of surface features. Russell et al. (1999) claimed to possibly have identi5ed a hydromagnesite stromatolitic microbialite on Mars, the so-called “White Rock” deposit. If con5rmed, this possible geosignature should be given priority for further study. The high rates of erosion and types of erosion observed on Earth due to biological and chemical weathering induced by living organisms provide another example of a geosignature. Fungus-lichen rock dwellers on Earth are estimated to weigh about 13 × 1013 tons, a biomass greater than all life in the ocean (Margulis, 1998). Thus, their e9ect on chemical weathering from metabolic by-products is immense. Rates and types of erosion can best be inferred from the visible and microwave wavelengths of the electromagnetic spectrum. Among the most powerful biosignatures are macromolecules that are directly linked to biogenic metabolism or other cellular functions. Chlorophyll is the prime example and can be identi5ed by radiance spectra in the visible region (Hovis, 1980; Gordon et al., 1980) and by advanced very high resolution radiometer (AVHRR) measurements (Gervin et al., 1985; Tucker et al., 1985). Signatures of life also include structural complexity produced by biogenic processes ranging from termite mounds to arti5cial constructions such as streets and evidence of an advanced civilization such as a Dyson Sphere, that would imply the presence of designed, dynamic activity known only to living systems.
677
Some biosignatures such as fossil remnants (e.g. McKay et al., 1996) and isotopic fractionation caused by biotic processes (Schidlowski, 1988; Anbar, 2001) are not detectable by remote sensing. However, they may be detectable on other worlds that have or can be visited robotically, or in the rare fortunate circumstance that a meteorite from that world can be analyzed by in-situ methods. Chirality, or non-racemic handedness, is a fundamental property of terrestrial biogenic molecules and thus may be used as an indicator of possible extraterrestrial life detectable by remote sensing in the near future. Asher and Winebrenner (2000) reported that left- and right-hand circularly polarized light interacts di9erentially with chiral molecules, especially at blue–green and shorter wavelengths. Di9erent types of organisms appear to cause a di9erent amount of optical scattering. Thus, there is some evidence that the signal in eukaryotes is about 10 times larger than in prokaryotes because of chiral tertiary structure in the genome (Winebrenner, 2001). Living systems maintain their highly structured and organized (low entropic) state in thermodynamic disequilibrium by continually drawing energy from their environments, some of which is converted to unorganized energy in the form of heat. Thus, another possible biosignature of life is the remote detection of biogenic heat, though current technologies are still far above the detection limit required. Both biosignatures and geosignatures may be found eventually in other solar systems, but have not been found on any world in our solar system other than Earth. Therefore, there is presently no available evidence for life as we know it elsewhere in the solar system. 3. The problem of life unknown to us Life could exist nonetheless, either in a form known or unknown to us, without giving rise to any of the biosignatures or geosignatures indicated above, if it (1) occurs beneath an opaque surface, (2) is too small to cause environmental transformations extensive in magnitude or spatial extent, or (3) is insuHciently complex to generate complex phenomena, such as roads or radiowaves. Under these circumstances, the more sophisticated and abstract de5nitions of life alluded to above must be used to provide the basis for an alternative set of parameters that could point to conditions favorable for generic forms of life, either known or unknown to us. Speci5cally, (1) the maintenance of disequilibrium from the environment requires the availability of energy Pow (hence gradients of energy) for sustaining low entropy states; (2) the level of chemical complexity required to transform and store energy appears to require a Puid medium where concentrations can be high but molecular mobility can be maintained; and (3) the storage and transmission of information appears to require polymeric chemistry that can involve the making and breaking of covalent bonds with relative ease.
678
D. Schulze-Makuch et al. / Planetary and Space Science 50 (2002) 675–683
4. Geoindicators and the possibility for the existence of life Geoindicators are parameters that are consistent with the presence of life but do not con5rm its existence. In light of the above argument, primary geoindicators of life would include evidence of (1) energy Pow or gradients, (2) a liquid medium, and (3) complex polymeric chemistry. The presence of an atmosphere provides a medium for dynamic energy gradients, a9ords a stabilizing and protective shield, and may rePect the presence of liquid — hence it is another geoindicator favorable for the existence of life. Finally, planetary bodies large enough to have undergone di9erentiation have an inherent source of energy in the form of radiogenic heating and cooling from the core outward. Thus, geological di9erentiation serves as an additional geoindicator of conditions favorable for life. Remote sensing can detect all of these conditions in principle. (Table 2). 5. Presence of an atmosphere or ice shield It is diHcult to envision the presence of life on any planetary body that is not shielded by an atmosphere. Without
an atmosphere any liquid or gaseous compound will vaporize into the vacuum of space. The origin and persistence of life under these conditions is diHcult to imagine. Relatively dense atmospheres appear to exist only (aside from the gas giants) on Earth (1 atm), Venus (∼ 90 atm) and Titan (∼ 1:5 atm). However, temperatures on Venus (very hot) and Titan (very cold) would require a biochemistry with properties unfamiliar in life forms on Earth. The surface of both Venus and Titan is obscured from visual light penetration by a thick atmosphere. On Titan, organic compounds such as methane and ethane are present in the atmosphere (Lorenz, 1993; Coustenis and Lorenz, 1999), and Titan is also the only planetary body known to us aside from Earth having nitrogen as the most abundant atmospheric gas. The high nitrogen content in Earth atmosphere has been interpreted to be caused by biological processes (Lovejoy, 2000). Since the atmosphere can only be penetrated in the narrow windows between methane bands with current remote sensing technology (GriHth et al., 1991; Lorenz and Lunine, 1997), probes have to be sent to explore surface conditions. The Jovian moons Europa, Ganymede, and possibly Callisto, and Neptune’s moon Triton, do not have signi5cant atmospheres, but suitable conditions for life in a planetary
Table 2 Remote detection of energy gradients
Type of energy gradient
Examples within the solar system
Examples of remote detection
Light
Mercury to Jovian system in suHcient intensity
Directly measurable
Chemical Cycling
Io, Europa, Iapetus, Triton
Molecular absorption spectra, surface rePectance spectra, imaging spectroscopy, polarimetry, radar measurements, detection of alteration minerals, gradients in surface coloration
Heat
Titan, Jovian satellites, Triton
Gradients of infrared radiation, thermal radiometry, infrared to visible spectral imaging, distance to sun, mass suHcient for internal di9erentiation of planetary body (gravitational measurements), microwave radiometry to detect geothermal heat Pow
Motion
Venus, Mars, Enceladus, Jovian satellites, Triton, Titan
Doppler imaging, radar interferometry electromagnetic indications of a conducting liquid (e.g. Europa), thermal and infrared imaging (volcanically induced movement)
Gravitational Tides
Jovian satellites, Triton
Visible evidence of surface fragmentation and resurfacing, microwave radiometry
Pressure
Venus, Titan, Jupiter
Visible clouds, changing atmospheric patterns (e.g. Red Spot on Jupiter)
Eletromagnetism
Jovian and Saturnian system
Measurement of electromagnetic 5eld Puctuations, detection of energetic particles
D. Schulze-Makuch et al. / Planetary and Space Science 50 (2002) 675–683
ocean, if it exists, would be shielded by an ice crust. This ice crust would act as a shield for possible subsurface compounds to not evaporate into space and would also provide a shield against cosmic rays.
6. Internal di&erentiation Life on any planetary body is diHcult to envision without internal di9erentiation into a radioactive core, a mantel and a crust. The internal di9erentiation is a sign of endogenic activity that is powered by radioactive decay, thus a much needed condition for the origin of life. Without internal differentiation a planetary body would be a “cold, dead rock” in space such as asteroids on which life at least in an active phase is diHcult to envision. The mass of a planetary body can be measured directly by its inPuence on orbiting or passing probes and by the gravitational attraction it exerts on other planetary bodies or light. Beyond a speci5c mass limit, internal di9erentiation is usually inferred by contemporary models. Remote sensing can only measure surface compositions and thus no direct measurements about internal di9erentiation can be obtained. However, some information can be obtained about the silicate content of the surface from the Reststrahlen band using the thermal infrared wavelength (Sabins, 1997), and about the moisture content in the very shallow subsurface using synthetic aperture radar (SAR).
7. Polymeric chemistry Chemical complexity is presumed to be based at the molecular level on polymeric molecules joined by covalent bonds (Lwo9, 1962). Carbon appears to be the only element capable of forming polymers that readily undergo chemical alterations under the physical conditions prevailing on Earth. Since organic carbon molecules are now known to be pervasive throughout the observable universe; life wherever it occurs is often assumed to be carbon-based. However, other polymer-forming elements are conceivable under exotic physico-chemical conditions (Sagan, 1994); for example, silicon-based polymers can exist under high pressure and high or low temperatures. Polymeric organic compounds are in general detected by their absorption spectra. On an active planet, chemical cycling must be associated with polymeric organic compounds. Chemical cycling can be inferred from spectra and gradients in surface coloration, and appears to be widespread in our solar system (e.g. Io, Europa, Enceladus, Iapetus, Triton). On Earth, chemical cycling is known to occur though oxidation–reduction cycles that are actively maintained by organisms or that occur inorganically. Detection of alteration minerals from weathering (e.g. hydrothermal changes) is a particularly relevant indicator of geochemical cycling.
679
To the extent that the evolution of life on Earth is a typical example, plate tectonics or some other e9ective recycling mechanism for minerals and nutrients appears to be a requirement for the persistence of living systems. Nutrients and minerals would otherwise be quickly exhausted and evolving life, especially when still at a metabolically primitive stage, would not be able to meet its nutrient demands within a relatively short time frame (on a planetary time scale). On Earth and probably early Mars the recycling mechanism was plate tectonics (Sleep, 1994; Connerney et al., 1999). Plate tectonics on Earth also constantly produces greenhouse gases that acted as a global thermostat providing stability for the evolution of life (Ward and Brownlee, 2000). The presence of plate tectonics can be identi5ed with remote sensing methods based on measured magnetic properties of the rock, visible symmetry along a speading axis, and speci5c patterns in fracture orientation and propagation. 8. Energy source Life requires a Pow of energy to organize its material substance and maintain its low entropic state (Morowitz, 1968), thus an external energy source is a minimal requirement for life. Light and oxidation of inorganic compounds fuel the biosphere on Earth, so wherever light and a means for sustaining oxidation–reduction cycles can be demonstrated, the possibility for maintaining life is present. Light is a highly e9ective form of energy on Earth, and phototrophic organisms are responsible for the high oxygen content in Earth’s atmosphere. Light from the sun could serve as the principle energy source for living systems on all the inner planets of our solar system, and possibly as far as the Jovian system. Light is directly measurable using remote sensing and thus a good indicator for the possible existence of life elsewhere. Heat is a commonly available energy source throughout the inner solar system, and also among some larger satellites as shown from gradients of infrared and thermal infrared radiation (e.g., Io and Titan). Heat can be derived from solar emissions or from radioactive heating if the mass of a planetary body is suHcient for di9erentiation into a radioactive core, as in the major Jovian satellites, Titan, and Triton). Multispectral remote sensing methods are suitable for detecting rocks altered by hydrothermal heat and solutions, because their rePectance spectra di9er from those of unaltered host rock. Thermal radiometry has been used to determine that night-time temperatures on Europa are colder at the equator than at mid-latitudes for some longitudes, apparently due to latitude-dependent thermal inertia (Spencer et al., 1999, 2001). Motion as an energy source is possible wherever gas or Puids exist. Motion can be detected directly from visible clouds that move in an atmosphere, such as those of Mars, Venus, the gas giant planets, and Titan from visible and rePected infrared images. Doppler imaging at various wavelengths can be used to determine speed and direction of
680
D. Schulze-Makuch et al. / Planetary and Space Science 50 (2002) 675–683
moving objects in an atmosphere or a planetary surface. Active faults on which earthquakes may occur can be identi5ed by observation of topographical features from space using radar (Tapponnier and Molnier, 1977). For example, topographic ridges may be identi5ed on Mars that have been o9set laterally to shut o9 drainage channels, while scarps may be identi5ed on Europa that cut a topographic ridge to form a faceted ridge. Changes in the shape of a volcano caused by an expanding or contracting magma chamber can be determined by radar interferometry. Increased emissions of gas and heat of volcanoes can be identi5ed with thermal infrared images and movements of plumes by images in the visible or infrared wavelengths. Electromagnetic measurements with the magnetometer instrument on board the Galileo orbiter have recently been used to infer a conducting liquid in Europa’s interior (Khurana et al., 1998; Kivelson et al., 2000). Together with the observed fragmentation and apparent geomorphological processes of Europa’s ice crust (Sullivan et al., 1998; Prockter and Pappalardo, 2000), a good case can be made for the motion of subsurface liquids. Gravitational tides are exhibited by planetary bodies and major satellites that are in periodic alignments such as the Earth–Moon system, the Neptun–Triton system, and Jupiter and its four major moons. Signi5cant tidal Puctuations in these suHciently massive bodies are visible by the evidence of surface fragmentation and resurfacing of the planetary or lunar surface. For example, arcuate lineaments of vast extension on Europa have been interpreted as surface expressions of these enormous tidal forces (Hoppa et al., 1999). Pressure gradients in an atmosphere can be inferred from banded cloud patterns and measurements of their rotational velocities, by large storm systems such as Jupiter’s famous Red Spot and less dramatic but similar examples on the other gas giants. Also, strati5cation as measured at Jupiter and anticipated to be measured at Saturn by the Cassini orbiter and on Titan by the Huygens probe in 2005 is another indicator of pressure gradients. Osmotic pressure gradients may also exist in planetary oceans that could be conducive for the support of life (Schulze-Makuch and Irwin, Submitted for publication). Those gradients are much harder to con5rm and would probably need to be measured directly by a probe. Electromagnetism is another energy source that occurs wherever electromagnetic 5elds are induced or traversed (Schulze-Makuch and Irwin, 2001). In the Jovian system a strong magnetic 5eld is created by Jupiter’s magnetospheric plasma that corotates with the planet at a velocity of 118 km=s (Beatty and Chaikin, 1990). Energetic ion and electron intensities throughout the Jovian magnetosphere have been measured by Galileo using an energetic particle detector. Saturn generates a less massive but still large magnetosphere that will be mapped in detail by the Cassini orbiter. More benign electromagnetic 5elds and their Puctuations can be measured directly using a magnetometer.
9. Liquid medium as solvent Finally, living processes appear to require a liquid medium for concentrating without immobilizing interacting constituents within a bounded internal environment. This assumption is usually taken to mean an aqueous medium, though organic compounds and water mixtures with ammonia and other miscible molecules can exist in liquid form at temperatures well below the freezing point of water. That a Puid medium is necessary for harboring life is speculative, as the possibility that life could exist in dense atmospheres has also been suggested (Sagan and Salpeter, 1976). There is for example both experimental and observational evidence for organic synthesis in Jupiter’s atmosphere (Sagan et al., 1967; Raulin and Bossard, 1985; Guillemin, 2000). However, it is diHcult to envision how the boundary conditions necessary for compartmentalizing the Pow of energy and restricting the population of interacting molecules could be established under such conditions. Thus, while the possibility cannot be excluded that life could form and persist in dense gaseous environments, it does not seem very likely (Irwin and Schulze-Makuch, 2001). Liquid water is known for certain only on Earth, but may exist as subsurface water on Mars in underground aquifers, or on Europa and Ganymede where subsurface oceans are inferred from electromagnetic measurements from the Galileo orbiter and from the presence of hydrated salt minerals on the surface. Water–ammonia–organic compound mixtures are another possibility on cold planetary bodies because these mixtures can be in a liquid state at much lower temperatures than water. Theoretical models indicate the presence of subsurface stores that are liquid at extremely cold temperatures on Titan (Coustenis and Lorenz, 1999) and possibly some of the satellites of Uranus and Neptune. Liquid water at or close to the surface can easily be detected by radar and the absorption spectrum of water, but not when it is present in the deep subsurface or shielded by a thick layer of ice. 10. Implications for future exploration Neither biosignatures nor geosignatures have been found on any planetary body beyond Earth to date, though the search should continue as resolution improves. Mars is the only other planetary body where life as we know it could plausibly be discovered by direct sampling in the foreseeable future. Thus, missions to Mars should remain a priority. In the meantime and for the coming decades, search for habitats suitable for life beyond Mars should focus on geoindicators such as those listed above. The current emphasis on visualization of surface features by the Mars Global Surveyor and Mars Odyssey Orbiter that arrived October, 2001, the Galileo Orbiter currently surveying the Jovian system, and the
D. Schulze-Makuch et al. / Planetary and Space Science 50 (2002) 675–683
681
Table 3 Presence of geoindicators for life on the planets and major satellites of our solar system based on current knowledge
Major planetary body
Signi5cant atmosphere
Internal di9erentiation
Polymeric chemistry
Energy source
Liquid solvent
Mercury Venus Earth Moon
No Yes Yes No
Yes Yes Yes Yes
No Yes? Yes No
L L L L
None None? H2 O None
Mars Jupiter Io Ganymede Europa Callistro
Yes Yes No Yesa Yesa Yesa ?
Yes Yes Yes Yes Yes Yes
Yes? Yes Yes Yes Yes Yes?
LCH LCH K PM LH M G C H K G P ∗∗ M C H K G P ∗∗ M C H K G P ∗∗ M
H2 O None H2 SO4 ? H2 O H2 O H2 O?
Saturn Tethys Dione Rhea Enceladus Iapetus Titan
Yes No No No No No Yes
Yes No No No No? No Yes
Yes Yes? Yes? Yes? Yes Yes Yes
CH K PM M M M CH K M CM CH M
None None? None? None? H2 O None? CH2 , CH4 ; H2 ONH4 mix?
Uranus Titania Ariel Miranda Umbriel Oberon
Yes No No No No No
Yes Yes? No No No No
Yes? Yes? No? No? No? No?
CH K PM CH C? H ? C? H ? C? H ? C? H ?
None H2 O None None None None
Neptune Triton
Yes Yesa
Yes Yes?
Yes? Yes
CH K PM C H G P ∗∗
None H2 O
Pluto Comets and Asteroids
No No
Yes? Some
Yes? Some
CG L for some
None None
H CH P CH K GPM G
a Note:
These moons have no signi5cant gaseous atmosphere, but an ice shield that may perform a similar function as an atmosphere, P ∗∗ pressure as energy source not based on atmospheric pressure gradients but osmotic pressure gradients in a possible high-salinity subsurface ocean. Legend: L=light energy, C =chemical cycling, H =heat energy, K =kinetic energy (motion), G=gravitational energy (tides), P=pressure energy, M = electromagnetic energy.
Cassini-Huygens Mission due to arrive at Saturn in 2004 are compatible with this strategy. These missions all have the capability to detect energy gradients, organic chemicals, and near-subsurface as well as surface water. The Huygens probe to Titan in January, 2005, will add detailed knowledge of that body’s atmosphere, weather, and surface chemistry. Because of the apparent similarity of its atmosphere to that of the early Earth, and its abundance of organic constituents, Titan should remain a high-priority target for exploration. In fact, geoindicators discussed here point to Titan as a suitable environment of life (Table 3), and thus Titan should be considered to be upgraded to a category II (favorable) plausibility of life index (Irwin and Schulze-Makuch, 2001). Organic constituents appear also to be present on Triton and possibly Iapetus. Enceladus,
Triton, and Titania show evidence of resurfacing that would indicate the generation of internal energy. Enceladus and Triton, along with Io, may be geothermally active. Higher resolution images and infrared data from Io would provide a more detailed picture of the complex thermal gradients on that body. All these examples merit increased attention as future robotic exploration of the outer solar system is planned. The current emphasis on Europa, is appropriate, including plans for a Europa orbiter. At the same time, growing evidence for liquid oceans on Ganymede and Callisto should boost priority for acquiring data on those satellites to a level equivalent to that of Europa, given the larger size of Ganymede and Callisto. Development of new remote-based ice-penetrating probes for melting through the surface ice sheet to explore directly the
682
D. Schulze-Makuch et al. / Planetary and Space Science 50 (2002) 675–683
subsurface ocean should be considered a necessity for gaining the type of detailed information that would aid in evaluating the suitability of those environments for life. 11. Conclusions The search for extraterrestrial life everywhere but on our closest planetary neighbor (Mars), is limited for the foreseeable future by our inability to obtain physical samples. Therefore, information that can only be obtained by remote sensing and robotic probes will for now provide the only clues concerning the existence of life elsewhere. The set of search parameters we have proposed emphasize the importance of detecting the presence of physical and chemical gradients of all kinds, because of their potential for generating free energy. Evidence for polymeric chemistry in association with chemical cycling, the presence of an atmosphere or ice shield, suHcient mass for endogenic heating, and the availability of a liquid that may act as a solvent to enhance chemical reactions are other geoindicators that would enhance the prospects of life. Also, any unusual topographical features or surface patterns that cannot be easily explained by well understood geological and geochemical processes, should be regarded as evidence for the possibility of environmental changes induced by living systems. Acknowledgements We would like to acknowledge the frequent, stimulating discussions with other members of the GeoBiological Groundwater Research Group (URL: http:==www.geo.utep. edu/pub/dirksm/geobiowater/geobiowater.html) that promoted our interest and helped to shape the paper into its current form. NASA grant NAG5-9542 supported a portion of the work presented here.
References Anbar, A.D., 2001. Iron isotope biosignatures: promise and progress. EOS, Trans. Am. Geophys. Union 82 (15), 173, 178–179. Asher, W.E., Winebrenner, D.P., 2000. Sensitivity and selectivity of prokaryote detection by optical circular polarization di9erential scattering. EOS, Trans. Am. Geophys. Union 81 (48), F810. Beatty, J.K., Chaikin, A., 1990. The New Solar System, 3rd Edition. Sky Publishing Corporation, Cambridge, MA. Cairns-Smith, A.G., 1982. On the nature of primary genetic materials. In: Cairns-Smith (Ed.), Genetic Takeover. Cambridge University Press, Cambridge, UK, pp. 136 –163. Chyba, C.F., McDonald, G.D., 1995. The origin of life in the solar system: current issues. Ann. Rev. Earth Planet. Sci. 23, 215–249. Connerney, J.E.P., Acuna, M.H., Wasilewski, P.J., Ness, N.F., Reme, H., Mazelle, C., Vignes, D., Lin, R.P., Mitchell, D.L., Cloutier, P.A., 1999. Magnetic lineations in the ancient crust of Mars. Science 284 (5415), 794–798.
Coustenis, A., Lorenz, R.D., 1999. Titan. In: Weissman, P.R., Johnson, A.T.V. (Eds.), Encyclopedia of the Solar System, 1st Edition. Academic Press, New York, pp. 377–404. Gervin, J.C., Kerber, A.G., Witt, R.G., Lu, Y.C., Sekhon, R., 1985. Comparison of level I land cover accuracy for MSS and AVHRR data. Int. J. Remote Sensing 6, 47–57. Gordon, H.R., Clark, D.K., Mueller, J.L., Hovis, W.A., 1980. Phytoplankton pigments from the Nimbus-7 coastal zone color scanner — comparison with surface measurements. Science 210, 63–66. GriHth, C.A., Owen, T., Wagener, R., 1991. Titan’s surface and troposphere, investigated with ground-based near-infrared observations. Icarus 93, 362–378. Guillemin, J.-C., 2000. Organic photochemistry in the atmosphere of Jupiter and Saturn — The role played by H2 S, PH3 and NH3 . Origins Life Evol. Biosphere 30 (2– 4), 236. Hoppa, G.V., Tufts, B.R., Greenberg, R., Geissler, P.E., 1999. Formation of cycloidal features on Europa. Science 285, 1899–1902. Hovis, W.A., co-authors, 1980. Nimbus-7 CZCS coastal zone color scanner — system description and early imagery. Science 210, 60 – 63. Irwin, L.N., Schulze-Makuch, D., 2001. Assessing the plausibility of life on other worlds. Astrobiology 1 (2), 143–160. Khurana, K.K., Kivelson, M.G., Stevenson, D.J., Schubert, G., Russell, C.T., Walker, R.J., Polanskey, C., 1998. Induced magnetic 5elds as evidence for subsurface oceans in Europa and Callistro. Nature 395, 777–780. Kivelson, M.G., Khurana, K.K., Russell, C.T., Volwerk, M., Walker, R.J., Zimmer, C., 2000. Galileo magnetometer measurements: a stronger case for a subsurface ocean at Europa. Science 289, 1340–1343. Lorenz, R.D., 1993. The surface of Titan in the context of the ESA Huygens probe. ESA J. 17, 275–292. Lorenz, R.D., Lunine, J.I., 1997. Titan’s surface reviewed: the nature of bright and dark terrain. Planet. Space Sci. 45 (8), 981–992. Lovejoy, J.E., 2000. The Gaia hypothesis. In: Margulis, L., Matthews, C., Haselton, A. (Eds.), Environmental Evolution. MIT Press, Cambridge, MA, pp. 1–28. Lwo9, A., 1962. Biological Order. MIT Press, Cambridge, MA. Margulis, L., 1998. Symbiotic planet. Amherst, Mass., Sciencewriters, Brockman, Inc. McKay, D.S., Everett Jr., K.G., Thomas-Keprta, K.L., Vali, H., Romanek, C.S., Clemett, S.J., Chillier, X.D.F., Maechling, C.R., Zare, R.N., 1996. Search for past life on Mars: possible relic biogenic activity in Martian Meteorite ALH84001. Science 273, 924–930. Moreno, A., Fernandez, A., Etxeberria, A., 1990. Cybernetics, autopoiesis and de5nition of life. In: Trappl (Ed.), Cybernetics and Systems’90. World Scienti5c, Singapore, pp. 357–364. Morowitz, H.J., 1968. Energy Flow in Biology. Academic Press, New York. Prockter, L.M., Pappalardo, R.T., 2000. Folds on Europa: implications for crustal cycling and accommodation of extension. Science 289, 941–943. Raulin, F., Bossard, A., 1985. Organic synthesis in gas phase and chemical evolution in planetary atmospheres. Adv. Space Res. 4, 75–82. Russell, M.J., Ingham, J.K., Zedef, V., Maktav, D., Sunar, F., Hall, A.J., Fallick, A.E., 1999. Search for signs of ancient life on Mars: expectations from hydromagnesite microbialites, Salda Lake, Turkey. J. Geolog. Soc. London 156, 869–888. Sabins, F.F., 1997. Remote Sensing: Principles and Interpretation, 3rd Edition. W.H. Freeman and Company, New York, USA, 494p. Sagan, C., Salpeter, E.E., 1976. Astrophys. J. Suppl. Ser. 32, 737. Sagan, C., 1994. Pale Blue Dot. Random House, New York. Sagan, C.E., Lippincott, E.R., Dayho9, M.O., Eck, R.V., 1967. Organic molecules and the coloration of Jupiter. Nature 213, 273–274. Sagan, C., Thompson, W.R., Carlsson, R., Gurnett, D., Hord, C., 1993. A search for life on Earth from the Galileo spacecraft. Nature 365, 715–721. Schidlowski, M., 1988. A 3800-million-year isotopic record of life from carbon in sedimentary rocks. Nature 333 (6171), 313–318.
D. Schulze-Makuch et al. / Planetary and Space Science 50 (2002) 675–683 Schroedinger, E., 1944. What is life?: The Physical Aspect of the Living Cell. Cambridge University Press, Cambridge, UK. Schulze-Makuch, D., Guan, H., Irwin, L.N., Vega, E., 2001. Rede5ning life: an ecological, thermodynamic and bioinformatic approach. Fundamentals of Life. Elsevier, Amsterdam, pp. 41–51. Schulze-Makuch, D., Irwin, L.N. Energy cycling and hypothetical organisms in Europa’s ocean. Astrobiology, Submitted for publication. Schulze-Makuch, D., Irwin, L.N., 2001. Alternative energy sources could support life on Europa. EOS, Trans. Am. Geophys. Union 82 (13), 150. Sleep, N.H., 1994. Martian plate tectonics. J. Geophys. Res. 99, 5639. Spencer, J.R., Tamppari, L.K., Martin, T.Z., Travis, L.D., 1999. Temperatures on Europa from Galileo photopolarimeter–radiometer. Science 284, 1514–1516. Spencer, J.R., Martin, T.Z., Goguen, J., Tamppari, L.K., Barnard, L., Travis, L.D., 2001. Galileo PPR observations of the Galilean satellites.
683
Jupiter: Planets, Satellites & Magnetosphere, Abstract volume, Boulder, Colorado, pp. 105 –106. Sullivan, R., Greeley, R., Homan, K., Klemaszewski, J., Belton, M.J.S., Carr, M.H., Chapman, C.R., Tufts, R., Head III, J.W., Pappalardo, R., Moore, J., Thomas, P., the Galileo Imaging Team, ., 1998. Episodic plate separation and fracture in5ll on the surface of Europa. Nature 391, 371–373. Szent-GyBorgyi, A., 1972. The Living State, with Observations on Cancer. Academic Press, New York. Tapponnier, P., Molnier, P., 1977. Active faulting and tectonics in China. J. Geophys. Res. 82, 2905–2930. Tucker, C.J., Townshend, J.R.G., Go9, T.E., 1985. African land-cover classi5cation using satellite data. Science 227, 369–375. Ward, P.D., Brownlee, D., 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus, New York. Winebrenner, D.P., 2001. Scientist at the Applied Physics Laboratory, University of Washington, personal communication.
