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PubMed2002

Biogenesis and early life on Earth and Europa: favored by an alkaline ocean?

Recent discoveries about Europa--the probable existence of a sizeable ocean below its ice crust; the detection of hydrated sodium carbonates, among other salts; and the calculation of a net loss of sodium from the subsurface--suggest the existence of an alkaline ocean. Alkaline oceans (nicknamed "soda oceans" in analogy to terrestrial soda lakes) have been hypothesized also for early Earth and Mars on the basis of mass balance considerations involving total amounts of acids available for weathering and the composition of the early crust. Such an environment could be favorable to biogenesis since it may have provided for very low Ca2+ concentrations mandatory for the biochemical function of proteins. A rapid loss of CO2 from Europa's atmosphere may have led to freezing oceans. Alkaline brine bubbles embedded in ice in freezing and impact-thawing oceans could have provided a suitable environment for protocell formation and the large number of trials needed for biogenesis. Understanding these processes could be central to assessing the probability of life on Europa.

Alkalies↗
PubMed2003

Physical simulation for low-energy astrobiology environmental scenarios.

Speculations about the extent of life of independent origin and the potential for sustaining Earth-based life in subsurface environments on both Europa and Mars are of current and relevant interest. Theoretical modeling based on chemical energetics has demonstrated potential options for viable biochemical metabolism (metabolic pathways) in these types of environments. Also, similar environments on Earth show microbial activity. However, actual physical simulation testing of specific environments is required to confidently determine the interplay of various physical and chemical parameters on the viability of relevant metabolic pathways. This testing is required to determine the potential to sustain life in these environments on a specific scenario by scenario basis. This study examines the justification, design, and fabrication of, as well as the culture selection and screening for, a psychrophilic/halophilic/anaerobic digester. This digester is specifically designed to conform to physical testing needs of research relating to potential extent physical environments on Europa and other planetary bodies in the Solar System. The study is a long-term effort and is currently in an early phase, with only screening-level data at this time. Full study results will likely take an additional 2 years. However, researchers in electromagnetic biosignature and in situ instrument development should be aware of the study at this time, as they are invited to participate in planning for future applications of the digester facility.

Acetates↗
PubMed2003

Spectral behavior of hydrated sulfate salts: implications for Europa mission spectrometer design.

Remote sensing of the surface of Europa with near-infrared instruments has suggested the presence of hydrated materials, including sulfate salts. Attention has been focused on these salts for the information they might yield regarding the evolution of a putative interior ocean, and the evaluation of its astrobiological potential. These materials exhibit distinct infrared absorption features due to bound water. The interactions of this water with the host molecules lead to fine structure that can be used to discriminate among these materials on the basis of their spectral behavior. This fine structure is even more pronounced at the low temperatures prevalent on icy satellites. Examination of hydrated sulfate salt spectra measured under cryogenic temperature conditions provides realistic constraints for future remote-sensing missions to Europa. In particular, it suggests that a spectrometer system capable of 2-5 nm spectral resolution or better, with a spatial resolution approaching 100 m, would be able to differentiate among proposed hydrated surface materials, if present, and constrain their distributions across the surface. Such information would provide valuable insights into the evolutionary history of Europa.

Astronomical Phenomena↗
PubMed2003

The search for life on Europa: limiting environmental factors, potential habitats, and Earth analogues.

The putative ocean of Europa has focused considerable attention on the potential habitats for life on Europa. By generally clement Earth standards, these Europan habitats are likely to be extreme environments. The objectives of this paper were to examine: (1) the limits for biological activity on Earth with respect to temperature, salinity, acidity, desiccation, radiation, pressure, and time; (2) potential habitats for life on Europa; and (3) Earth analogues and their limitations for Europa. Based on empirical evidence, the limits for biological activity on Earth are: (1) the temperature range is from 253 to 394 K; (2) the salinity range is a(H2O) = 0.6-1.0; (3) the desiccation range is from 60% to 100% relative humidity; (4) the acidity range is from pH 0 to 13; (5) microbes such as Deinococcus are roughly 4,000 times more resistant to ionizing radiation than humans; (6) the range for hydrostatic pressure is from 0 to 1,100 bars; and (7) the maximum time for organisms to survive in the dormant state may be as long as 250 million years. The potential habitats for life on Europa are the ice layer, the brine ocean, and the seafloor environment. The dual stresses of lethal radiation and low temperatures on or near the icy surface of Europa preclude the possibility of biological activity anywhere near the surface. Only at the base of the ice layer could one expect to find the suitable temperatures and liquid water that are necessary for life. An ice layer turnover time of 10 million years is probably rapid enough for preserving in the surface ice layers dormant life forms originating from the ocean. Model simulations demonstrate that hypothetical oceans could exist on Europa that are too cold for biological activity (T < 253 K). These simulations also demonstrate that salinities are high, which would restrict life to extreme halophiles. An acidic ocean (if present) could also potentially limit life. Pressure, per se, is unlikely to directly limit life on Europa. But indirectly, pressure plays an important role in controlling the chemical environments for life. Deep ocean basins such as the Mariana Trench are good analogues for the cold, high-pressure ocean of Europa. Many of the best terrestrial analogues for potential Europan habitats are in the Arctic and Antarctica. The six factors likely to be most important in defining the environments for life on Europa and the focus for future work are liquid water, energy, nutrients, low temperatures, salinity, and high pressures.

Cold Temperature↗
PubMed2003

Strategy for modeling putative multilevel ecosystems on Europa.

A general strategy for modeling ecosystems on other worlds is described. Two alternative biospheres beneath the ice surface of Europa are modeled, based on analogous ecosystems on Earth in potentially comparable habitats, with reallocation of biomass quantities consistent with different sources of energy and chemical constituents. The first ecosystem models a benthic biosphere supported by chemoautotrophic producers. The second models two concentrations of biota at the top and bottom of the subsurface water column supported by energy harvested from transmembrane ionic gradients. Calculations indicate the plausibility of both ecosystems, including small macroorganisms at the highest trophic levels, with ionotrophy supporting a larger biomass than chemoautotrophy.

Biomass↗
PubMed2003

The production of oxidants in Europa's surface.

The oxidants produced by radiolysis and photolysis in the icy surface of Europa may be necessary to sustain carbon-based biochemistry in Europa's putative subsurface ocean. Because the subduction of oxidants to the ocean presents considerable thermodynamic challenges, we examine the formation of oxygen and related species in Europa's surface ice with the goal of characterizing the chemical state of the irradiated material. Relevant spectral observations of Europa and the laboratory data on the production of oxygen and related species are first summarized. Since the laboratory data are incomplete, we examine the rate equations for formation of oxygen and its chemical precursors by radiolysis and photolysis. Measurements and simple rate equations are suggested that can be used to characterize the production of oxidants in Europa's surface material and the chemical environment produced by radiolysis. Possible precursor molecules and the role of radical trapping are examined. The possibility of oxygen reactions on grain surfaces in Europa's regolith is discussed, and the earlier estimates of the supply of O(2) to the atmosphere are increased.

Carbon↗
PubMed2003

Locating potential biosignatures on Europa from surface geology observations.

We evaluated the astrobiological potential of the major classes of geologic units on Europa with respect to possible biosignatures preservation on the basis of surface geology observations. These observations are independent of any formational model and therefore provide an objective, though preliminary, evaluation. The assessment criteria include high mobility of material, surface concentration of non-ice components, relative youth, textural roughness, and environmental stability. Our review determined that, as feature classes, low-albedo smooth plains, smooth bands, and chaos hold the highest potential, primarily because of their relative young age, the emplacement of low-viscosity material, and indications of material exchange with the subsurface. Some lineaments and impact craters may be promising sites for closer study despite the comparatively lower astrobiological potential of their classes. This assessment will be expanded by multidisciplinary examination of the potential for habitability of specific features.

Exobiology↗
PubMed2003

Tírez lake as a terrestrial analog of Europa.

Tírez Lake (La Mancha, central Spain) is proposed as a terrestrial analogue of Europa's ocean. The proposal is based on the comparison of the hydrogeochemistry of Tírez Lake with the geochemical features of the alteration mineralogy of meteoritic precursors and with Galileo's Near Infrared Mapping Spectrometer data on Europa's surface. To validate the astrobiological potential of Tírez Lake as an analog of Europa, different hydrogeochemical, mineral, and microbial analyses were performed. Experimental and theoretical modeling helped to understand the crystallization pathways that may occur in Europa's crust. Calculations about the oxidation state of the hypothetical Europan ocean were estimated to support the sulfate-rich neutral liquid model as the origin of Europa's observed hydrated minerals and to facilitate their comparison with Tírez's hydrogeochemistry. Hydrogeochemical and mineralogical analyses showed that Tírez waters corresponded to Mg-Na-SO(4)-Cl brines with epsomite, hexahydrite, and halite as end members. A preliminary microbial ecology characterization identified two different microbial domains: a photosynthetically sustained community represented by planktonic/benthonic forms and microbial mat communities, and a subsurficial anaerobic realm in which chemolithotrophy predominates. Fluorescence in situ hybridization has been used to characterize the prokaryotic diversity of the system. The subsurficial community seemed to be dominated by sulfate-reducing bacteria and methanogens. Frozen Tírez brines were analyzed by Fourier-transform infrared techniques providing spectra similar to those reported previously using pure components and to the Galileo spectral data. Calorimetric measurements of Tírez brines showed pathways and phase metastability for magnesium sulfate and sodium chloride crystallization that may aid in understanding the processes involved in the formation of Europa's icy crust. The use of fluorescence hybridization techniques for microbial detection and characterization in hypersaline environments makes this methodology strongly advisable for future Europa astrobiological missions.

Chemical Phenomena↗
PubMed2006

Clathrate hydrates of oxidants in the ice shell of Europa.

Europa's icy surface is radiolytically modified by high-energy electrons and ions, and photolytically modified by solar ultraviolet photons. Observations from the Galileo Near Infrared Mapping Spectrometer, ground-based telescopes, the International Ultraviolet Explorer, and the Hubble Space Telescope, along with laboratory experiment results, indicate that the production of oxidants, such as H2O2, O2, CO2, and SO2, is a consequence of the surface radiolytic chemistry. Once created, some of the products may be entrained deeper into the ice shell through impact gardening or other resurfacing processes. The temperature and pressure environments of regions within the europan hydrosphere are expected to permit the formation of mixed clathrate compounds. The formation of carbon dioxide and sulfur dioxide clathrates has been examined in some detail. Here we add to this analysis by considering oxidants produced radiolytically on the surface of Europa. Our results indicate that the bulk ice shell could have a approximately 1.7-7.6% by number contamination of oxidants resulting from radiolysis at the surface. Oxidant-hosting clathrates would consequently make up approximately 12-53% of the ice shell by number relative to ice, if oxidants were entrained throughout. We examine, in brief, the consequences of such contamination on bulk ice shell thickness and find that clathrate formation could lead to substantially thinner ice shells on Europa than otherwise expected. Finally, we propose that double occupancy of clathrate cages by O2 molecules could serve as an explanation for the observation of condensed-phase O2 on Europa. Clathrate-sealed, gas-filled bubbles in the near surface ice could also provide an effective trapping mechanism, though they cannot explain the 5771 A (O2)2 absorption.

Ice↗
PubMed2006

Infrared spectra and radiation stability of H2O2 ices relevant to Europa.

In this paper we present spectra of H2O2-containing ices in the near- and mid-infrared (IR) regions. Spectral changes on warming are shown, as is a comparison of near-IR bands of H2O and H2O2-containing ices. An estimate of the A-value (absolute intensity) for the largest near- IR feature of H2O2 is given. Radiation-decay half-lives are reported for 19 K and 80 K, and are related to the surface radiation doses on Europa. The radiation data show that H2O2 destruction is slower at 80 K than 19 K, and are consistent with the claim that icy material in the outermost micrometer of Europa's surface has been heavily processed by radiation.

Hydrogen Peroxide↗
PubMed1989

Correlation of postoperative bone scintigraphy with healing of vascularized fibula transfer: a clinical study.

This study examines the usefulness and reliability of bone scintigraphy in correlation with radiological and clinical evidence of bone healing in 15 patients who underwent microvascular transfer of the fibula. All patients were followed for a minimum of 18 months postoperatively. Technetium-99 methylene diphosphonate bone scans and the most recent radiographs were blindly rereviewed. Bone scintigraphic results were characterized as (1) clearly positive (i.e., excellent visualization of the fibula), (2) clearly negative (i.e., no evidence of tracer uptake in the fibula), or (3) indeterminate (i.e., artifact present as a result of metallic or soft tissue interference). Bone radiographs were classified into three typical patterns: (1) complete bony union and graft hypertrophy, (2) incomplete union (either distal or proximal) requiring a second procedure), and (3) nonunion, with increased proximal and distal lucency (with or without pathological fracture) and loss of graft definition. Eleven patients had positive scintigraphic scans postoperatively. In 8 no subsequent procedure was necessary; 2 patients required additional bone grafts to augment the osseous reconstruction; viable fibulas were seen at reoperation. One patient with a positive scan showed decreased graft definition at four months followed by autograft fracture. Three patients had indeterminate scans, 2 of whom evidenced uncomplicated clinical and radiological union. One patient had a clearly negative scan and ultimately tibia-fibula synostosis was required to attain stability. Bone scintigraphy appears to correlate with survival, but not necessarily union, of a vascularized fibula autograft. Additional monitoring techniques should be used in combination with a one-time bone scan to both monitor the patency of the microanastomoses and to prioritize the orthopedic management of the patient.

Arteries↗
PubMed1991

Treatment of chronic traumatic bone wounds. Microvascular free tissue transfer: a 13-year experience in 96 patients.

During a 13-year period at the Massachusetts General Hospital, Boston, Massachusetts, 97 microvascular free tissue transfers have been performed for soft-tissue reconstruction in 96 patients following bone debridement for chronic traumatic bone wounds. These 96 patients comprise a continuation study of 18 original patients reported in 1982. During a 13-year follow-up period (mean, 77.1 months), 95.8% of these 96 patients have enjoyed complete wound closure with a lack of drainage after the debridement and free tissue transfer. Most of the patients (89.6%) encountered in this study are ambulatory without assist and 5.2% of patients have undergone amputation. Twenty-three per cent of patients required subsequent segmental bone defect reconstruction in the lower extremity after infection eradication. The pathophysiology of chronic traumatic bony wounds is different from that of chronic hematogenous osteomyelitis and thus a high incidence of long-term successful management can be seen through complete wound debridement and adequate soft-tissue coverage.

Adolescent↗
PubMed1996

Blade plate reconstruction of metaphyseal nonunion of the tibia.

Sixteen patients with nonunion of the metaphyseal region of the proximal or distal tibia after fracture or reconstructive procedures were surgically treated using a fixed angle blade plate and autogenous bone grafting. Eight of these nonunions were complicated by prior sepsis. The average time of followup assessment was 23 months. Fourteen of these 16 nonunions went on to uncomplicated union after the index procedure. Failure of the tibial fracture to unite in 2 patients required further surgical intervention with a ring external fixator. The authors recommend blade plate reconstruction for metaphyseal nonunion of the proximal and distal tibia, especially when complicated by angular deformity, small osteopenic metaphyseal fragments, and stiff adjacent joints.

Adult↗
PubMed1997

Recalcitrant nonunion.

One surgeon treated 13 patients with 14 long home fractures that remained ununited for 10 or more years (average, 16 years) and after an average of three prior surgeries. The clavicle was involved in two cases, the humerus in five (one proximal, three diaphyseal, and one distal intraarticular), the femoral diaphysis in three, and the tibial diaphysis in four. The patients were observed for an average of 54 months. All of the fractures healed, and every patient in the series regained functional use of the involved limb without reports of pain, instability, or persistent swelling related to the site of nonunion. Three patients had persistent leg length discrepancies, and five had substantial residual stiffness of one or both adjacent joints. This experience has shown that despite the longevity of the nonunions, healing can be achieved using the basic concepts of the creation of a stable skeletal fixation in the presence of a well vascularized environments with the addition of autogenous bone graft. By the same token, the duration of the nonunion will lead to soft tissue maladaptation and contracture that at times compromise successful restoration of limb length or adjacent articular mobility.

Adult↗
PubMed1999

Salvage of contaminated fractures of the distal humerus with thin wire external fixation.

Fractures and osteotomies of the distal humerus that are contaminated or infected represent a difficult management problem. Stable anatomic fixation with plates and screws, the acknowledged key to a good result in the treatment of bicondylar fractures, may be unwise. A thin wire circular (Ilizarov) external fixator was used as salvage treatment in such complex situations in five patients. The fixator allowed functional mobilization of the elbow while allowing achievement of the primary goal of eradicating the infection or colonization. Two patients required a second operation for fixation of a fibrous union of the lateral condyle. One patient with a vascularized fibular graft later required triple plate fixation for malalignment at the distal host and graft junction. Four of five patients ultimately achieved complete union. The fracture remained ununited in one patient who has declined additional intervention. All five patients achieved at least 85 degrees ulnohumeral motion, two after a secondary elbow capsulectomy performed after healing was achieved. This experience suggested that the Ilizarov construct, although not a panacea, represents a reliable method of skeletal stabilization that allows functional mobilization while elimination of infection or colonization is ensured. If necessary, stiffness and incomplete healing can be addressed with an increased margin of safety at subsequent operations.

Adolescent↗
PubMed1999

Infected nonunion of the tibia.

The treatment of infected nonunited fractures of the tibia using the techniques of Ilizarov was compared with autogenous cancellous bone graft application under a well vascularized soft tissue envelope. There were 10 patients in the Ilizarov group and 17 in the bone graft group. Soft tissue coverage with a free vascularized or a rotational muscle flap was used more frequently among the patients having bone graft (71%) than the Ilizarov group (30%). All 27 patients had bony defects (average, 3.7 cm; range, 1-18 cm). At an average followup of 6 years, 26 patients had a functional limb, and one patient (Ilizarov group) ultimately required a below knee amputation. Three patients in each group required a second plate and bone graft procedure to gain union. Infection persisted in four patients (all in the Ilizarov group). If a well vascularized soft tissue envelope is present (particularly after flap coverage), bone grafting procedures are safe and efficacious. The Ilizarov technique may be best suited for the treatment of very proximal or distal metaphyseal nonunions and nonunions associated with large leg length discrepancies.

Adult↗