Research

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Open AccessArticle How Do Modern Extreme Hydrothermal Environments Inform the Identification of Martian Habitability? The Case of the El Tatio Geyser Field

by Roberto Barbieri and Barbara Cavalazzi

Challenges 2014, 5(2), 430-443; doi:10.3390/challe5020430

Received: 26 August 2014 / Revised: 27 October 2014 / Accepted: 3 November 2014 / Published: 13 November 2014

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Abstract

Despite the success in knowledge gained by the Mars missions in the last two decades, the search for traces of life on Mars is still in progress. The reconstruction of (paleo-) environments on Mars have seen a dramatic increase, in particular with regard

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Despite the success in knowledge gained by the Mars missions in the last two decades, the search for traces of life on Mars is still in progress. The reconstruction of (paleo-) environments on Mars have seen a dramatic increase, in particular with regard to the potentially habitable conditions, and it is now possible to recognize a significant role to subaerial hydrothermal processes. For this reason, and because the conditions of the primordial Earth—when these extreme environments had to be common—probably resembled Mars during its most suitable time to host life, research on terrestrial extreme hydrothermal habitats may assist in understanding how to recognize life on Mars. A number of geological and environmental reasons, and logistics opportunities, make the geothermal field of El Tatio, in the Chilean Andes an ideal location to study. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

Open AccessArticle The Radiation Environment of Exoplanet Atmospheres

by Jeffrey L. Linsky

Challenges 2014, 5(2), 351-373; doi:10.3390/challe5020351

Received: 15 August 2014 / Revised: 2 October 2014 / Accepted: 10 October 2014 / Published: 29 October 2014

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Abstract

Exoplanets are born and evolve in the radiation and particle environment created by their host star. The host star’s optical and infrared radiation heats the exoplanet’s lower atmosphere and surface, while the ultraviolet, extreme ultraviolet and X-radiation control the photochemistry and mass loss

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Exoplanets are born and evolve in the radiation and particle environment created by their host star. The host star’s optical and infrared radiation heats the exoplanet’s lower atmosphere and surface, while the ultraviolet, extreme ultraviolet and X-radiation control the photochemistry and mass loss from the exoplanet’s upper atmosphere. Stellar radiation, especially at the shorter wavelengths, changes dramatically as a host star evolves leading to changes in the planet’s atmosphere and habitability. This paper reviews the present state of our knowledge concerning the time-dependent radiation emitted by stars with convective zones, that is stars with spectral types F, G, K, and M, which comprise nearly all of the host stars of detected exoplanets. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

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Open AccessArticle Towards a Mathematical Description of Biodiversity Evolution

by Jorge E. Horvath

Challenges 2014, 5(2), 324-333; doi:10.3390/challe5020324

Received: 24 June 2014 / Revised: 21 August 2014 / Accepted: 12 September 2014 / Published: 23 September 2014

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Abstract

We outline in this work a mathematical description of biodiversity evolution based on a second-order differential equation (also known as the “inertial/Galilean view”). After discussing the motivations and explicit forms of the simplest “forces”, we are lead to an equation analogue to a

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We outline in this work a mathematical description of biodiversity evolution based on a second-order differential equation (also known as the “inertial/Galilean view”). After discussing the motivations and explicit forms of the simplest “forces”, we are lead to an equation analogue to a harmonic oscillator. The known solutions for the homogeneous problem are then tentatively related to the biodiversity curves of Sepkoski and Alroy et al., suggesting mostly an inertial behavior of the time evolution of the number of genera and a quadratic behavior in some long-term evolution after extinction events. We present the Green function for the dynamical system and apply it to the description of the recovery curve after the Permo-Triassic extinction, as recently analyzed by Burgess, Bowring and Shen. Even though the agreement is not satisfactory, we point out direct connections between observed drop times after massive extinctions and mathematical constants and discuss why the failure ensues, suggesting a more complex form of the second-order mathematical description. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

Open AccessArticle On the Habitability of Aquaplanets

by Rolando Cardenas, Noel Perez, Jesus Martinez-Frias and Osmel Martin

Challenges 2014, 5(2), 284-293; doi:10.3390/challe5020284

Received: 29 May 2014 / Revised: 18 August 2014 / Accepted: 20 August 2014 / Published: 28 August 2014

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Abstract

An Aquatic Habitability Index is proposed, based on Quantitative Habitability Theory, and considering a very general model for life. It is a primary habitability index, measuring habitability for phytoplankton in the first place. The index is applied to some case studies, such as

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An Aquatic Habitability Index is proposed, based on Quantitative Habitability Theory, and considering a very general model for life. It is a primary habitability index, measuring habitability for phytoplankton in the first place. The index is applied to some case studies, such as the habitability changes in Earth due to environmental perturbations caused by asteroid impacts. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

Open AccessArticle What Does It Take to Establish that a World Is Uninhabited Prior to Exploitation? A Question of Ethics as well as Science

by Erik Persson

Challenges 2014, 5(2), 224-238; doi:10.3390/challe5020224

Received: 25 June 2014 / Revised: 30 July 2014 / Accepted: 4 August 2014 / Published: 12 August 2014

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Abstract

If we find life on another world, it will be an extremely important discovery and we will have to take great care not to do anything that might endanger that life. If the life we find is sentient we will have moral obligations

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If we find life on another world, it will be an extremely important discovery and we will have to take great care not to do anything that might endanger that life. If the life we find is sentient we will have moral obligations to that life. Whether it is sentient or not, we have a duty to ourselves to preserve it as a study object, and also because it would be commonly seen as valuable in its own right. In addition to this we would also have a duty to our fellow humans and other earthly life forms not to expose them to danger by advertently or inadvertently exposing them to potentially harmful space organisms. When space exploration turns into exploitation it will therefore be important to be able to show that a world that is up for exploitation is uninhabited before the exploitation starts. Showing that a world is uninhabited is, however, a different kind of task than showing that it is inhabited. The latter task can be accomplished through one positive finding but it is not clear how to go about the former task. In this paper I suggest that it is a gradual process asymptotically approaching certainty rather than a discovery in the traditional sense of the word. It has to be handled in two steps. The first is to connect degree of certainty with research setup. The second is to decide how certain we need to be. The first step is about the number, diversity and quality of observations. The second step is a decision we have to make based on the purpose of the investigation. The purpose and therefore the degree of certainty needed to establish that a world is uninhabited will be different for a world that is up for exploitation than for a world that is not. In the latter case it is only a matter of epistemic values. In the former case also ethical values have to be considered. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

Open AccessArticle Planetary Atmosphere and Surfaces Chamber (PASC): A Platform to Address Various Challenges in Astrobiology

by Eva Mateo-Marti

Challenges 2014, 5(2), 213-223; doi:10.3390/challe5020213

Received: 30 June 2014 / Revised: 29 July 2014 / Accepted: 30 July 2014 / Published: 8 August 2014

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Abstract

The study of planetary environments of astrobiological interest has become a major challenge. Because of the obvious technical and economical limitations on in situ planetary exploration, laboratory simulations are one of the most feasible research options to make advances both in planetary science

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The study of planetary environments of astrobiological interest has become a major challenge. Because of the obvious technical and economical limitations on in situ planetary exploration, laboratory simulations are one of the most feasible research options to make advances both in planetary science and in developing a consistent description of the origin of life. With this objective in mind, we applied vacuum technology to the design of versatile vacuum chambers devoted to the simulation of planetary atmospheres’ conditions. These vacuum chambers are able to simulate atmospheres and surface temperatures representative of the majority of planetary objects, and they are especially appropriate for studying the physical, chemical and biological changes induced in a particular sample by in situ irradiation or physical parameters in a controlled environment. Vacuum chambers are a promising potential tool in several scientific and technological fields, such as engineering, chemistry, geology and biology. They also offer the possibility of discriminating between the effects of individual physical parameters and selected combinations thereof. The implementation of our vacuum chambers in combination with analytical techniques was specifically developed to make feasible the in situ physico-chemical characterization of samples. Many wide-ranging applications in astrobiology are detailed herein to provide an understanding of the potential and flexibility of these experimental systems. Instruments and engineering technology for space applications could take advantage of our environment-simulation chambers for sensor calibration. Our systems also provide the opportunity to gain a greater understanding of the chemical reactivity of molecules on surfaces under different environments, thereby leading to a greater understanding of interface processes in prebiotic chemical reactions and facilitating studies of UV photostability and photochemistry on surfaces. Furthermore, the stability and presence of certain minerals on planetary surfaces and the potential habitability of microorganisms under various planetary environmental conditions can be studied using our apparatus. Therefore, these simulation chambers can address multiple different challenging and multidisciplinary astrobiological studies. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

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Open AccessArticle Enantioselective Crystallization of Sodium Chlorate in the Presence of Racemic Hydrophobic Amino Acids and Static Magnetic Fields

by María-Paz Zorzano, Susana Osuna-Esteban, Marta Ruiz-Bermejo, Cesar Menor-Salván and Sabino Veintemillas-Verdaguer

Challenges 2014, 5(1), 175-192; doi:10.3390/challe5010175

Received: 27 March 2014 / Revised: 7 May 2014 / Accepted: 13 May 2014 / Published: 5 June 2014

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Abstract

We study the bias induced by a weak (200 mT) external magnetic field on the preferred handedness of sodium chlorate crystals obtained by slow evaporation at ambient conditions of its saturated saline solution with 20 ppm of added racemic (dl) hydrophobic amino acids.

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We study the bias induced by a weak (200 mT) external magnetic field on the preferred handedness of sodium chlorate crystals obtained by slow evaporation at ambient conditions of its saturated saline solution with 20 ppm of added racemic (dl) hydrophobic amino acids. By applying the Fisher test to pairs of experiments with opposing magnetic field orientation we conclude, with a confidence level of 99.7%, that at the water-air interface of this saline solution there is an enantioselective magnetic interaction that acts upon racemic mixtures of hydrophobic chiral amino acids. This interaction has been observed with the three tested racemic hydrophobic amino acids: dl-Phe, dl-Try and dl-Trp, at ambient conditions and in spite of the ubiquitous chiral organic contamination. This enantioselective magnetic dependence is not observed when there is only one handedness of added chiral amino-acid, if the added amino acid is not chiral or if there is no additive. This effect has been confirmed with a double blind test. This novel experimental observation may have implications for our view of plausible initial prebiotic scenarios and of the roles of the geomagnetic field in homochirality in the biosphere. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

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Open AccessArticle Assessing the Possibility of Biological Complexity on Other Worlds, with an Estimate of the Occurrence of Complex Life in the Milky Way Galaxy

by Louis N. Irwin, Abel Méndez, Alberto G. Fairén and Dirk Schulze-Makuch

Challenges 2014, 5(1), 159-174; doi:10.3390/challe5010159

Received: 19 March 2014 / Revised: 19 May 2014 / Accepted: 20 May 2014 / Published: 28 May 2014

Cited by 9 | PDF Full-text (297 KB) | HTML Full-text | XML Full-text

Abstract

Rational speculation about biological evolution on other worlds is one of the outstanding challenges in astrobiology. With the growing confirmation that multiplanetary systems abound in the universe, the prospect that life occurs redundantly throughout the cosmos is gaining widespread support. Given the enormous

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Rational speculation about biological evolution on other worlds is one of the outstanding challenges in astrobiology. With the growing confirmation that multiplanetary systems abound in the universe, the prospect that life occurs redundantly throughout the cosmos is gaining widespread support. Given the enormous number of possible abodes for life likely to be discovered on an ongoing basis, the prospect that life could have evolved into complex, macro-organismic communities in at least some cases merits consideration. Toward that end, we here propose a Biological Complexity Index (BCI), designed to provide a quantitative estimate of the relative probability that complex, macro-organismic life forms could have emerged on other worlds. The BCI ranks planets and moons by basic, first-order characteristics detectable with available technology. By our calculation only 11 (~1.7%) of the extrasolar planets known to date have a BCI above that of Europa; but by extrapolation, the total of such planets could exceed 100 million in our galaxy alone. This is the first quantitative assessment of the plausibility of complex life throughout the universe based on empirical data. It supports the view that the evolution of complex life on other worlds is rare in frequency but large in absolute number. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

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Open AccessArticle CO Self-Shielding as a Mechanism to Make ¹⁶O-Enriched Solids in the Solar Nebula

by Joseph A. Nuth, III, Natasha M. Johnson and Hugh G. M. Hill

Challenges 2014, 5(1), 152-158; doi:10.3390/challe5010152

Received: 1 April 2014 / Revised: 8 May 2014 / Accepted: 13 May 2014 / Published: 21 May 2014

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Abstract

Photochemical self-shielding of CO has been proposed as a mechanism to produce solids observed in the modern, ¹⁶O-depleted solar system. This is distinct from the relatively ¹⁶O-enriched composition of the solar nebula, as demonstrated by the oxygen isotopic composition of the

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Photochemical self-shielding of CO has been proposed as a mechanism to produce solids observed in the modern, ¹⁶O-depleted solar system. This is distinct from the relatively ¹⁶O-enriched composition of the solar nebula, as demonstrated by the oxygen isotopic composition of the contemporary sun. While supporting the idea that self-shielding can produce local enhancements in ¹⁶O-depleted solids, we argue that complementary enhancements of ¹⁶O-enriched solids can also be produced via C¹⁶O-based, Fischer-Tropsch type (FTT) catalytic processes that could produce much of the carbonaceous feedstock incorporated into accreting planetesimals. Local enhancements could explain observed ¹⁶O enrichment in calcium-aluminum-rich inclusions (CAIs), such as those from the meteorite, Isheyevo (CH/CHb), as well as in chondrules from the meteorite, Acfer 214 (CH3). CO self-shielding results in an overall increase in the ¹⁷O and ¹⁸O content of nebular solids only to the extent that there is a net loss of C¹⁶O from the solar nebula. In contrast, if C¹⁶O reacts in the nebula to produce organics and water then the net effect of the self-shielding process will be negligible for the average oxygen isotopic content of nebular solids and other mechanisms must be sought to produce the observed dichotomy between oxygen in the Sun and that in meteorites and the terrestrial planets. This illustrates that the formation and metamorphism of rocks and organics need to be considered in tandem rather than as isolated reaction networks. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

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Open AccessReview The Detection and Characterization of Extrasolar Planets

by Ken Rice

Challenges 2014, 5(2), 296-323; doi:10.3390/challe5020296

Received: 1 July 2014 / Revised: 3 September 2014 / Accepted: 3 September 2014 / Published: 19 September 2014

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Abstract

We have now confirmed the existence of > 1800 planets orbiting stars other thanthe Sun; known as extrasolar planets or exoplanets. The different methods for detectingsuch planets are sensitive to different regions of parameter space, and so, we are discoveringa wide diversity of

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We have now confirmed the existence of > 1800 planets orbiting stars other thanthe Sun; known as extrasolar planets or exoplanets. The different methods for detectingsuch planets are sensitive to different regions of parameter space, and so, we are discoveringa wide diversity of exoplanets and exoplanetary systems. Characterizing such planets isdifficult, but we are starting to be able to determine something of their internal compositionand are beginning to be able to probe their atmospheres, the first step towards the detectionof bio-signatures and, hence, determining if a planet could be habitable or not. Here, Iwill review how we detect exoplanets, how we characterize exoplanetary systems and theexoplanets themselves, where we stand with respect to potentially habitable planets and howwe are progressing towards being able to actually determine if a planet could host life or not. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

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Open AccessReview Biogenicity and Syngeneity of Organic Matter in Ancient Sedimentary Rocks: Recent Advances in the Search for Evidence of Past Life

by Dorothy Z. Oehler and Sherry L. Cady

Challenges 2014, 5(2), 260-283; doi:10.3390/challe5020260

Received: 12 May 2014 / Revised: 20 August 2014 / Accepted: 21 August 2014 / Published: 27 August 2014

Cited by 7 | PDF Full-text (325 KB) | HTML Full-text | XML Full-text

Abstract

The past decade has seen an explosion of new technologies for assessment of biogenicity and syngeneity of carbonaceous material within sedimentary rocks. Advances have been made in techniques for analysis of in situ organic matter as well as for extracted bulk samples of

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The past decade has seen an explosion of new technologies for assessment of biogenicity and syngeneity of carbonaceous material within sedimentary rocks. Advances have been made in techniques for analysis of in situ organic matter as well as for extracted bulk samples of soluble and insoluble (kerogen) organic fractions. The in situ techniques allow analysis of micrometer-to-sub-micrometer-scale organic residues within their host rocks and include Raman and fluorescence spectroscopy/imagery, confocal laser scanning microscopy, and forms of secondary ion/laser-based mass spectrometry, analytical transmission electron microscopy, and X-ray absorption microscopy/spectroscopy. Analyses can be made for chemical, molecular, and isotopic composition coupled with assessment of spatial relationships to surrounding minerals, veins, and fractures. The bulk analyses include improved methods for minimizing contamination and recognizing syngenetic constituents of soluble organic fractions as well as enhanced spectroscopic and pyrolytic techniques for unlocking syngenetic molecular signatures in kerogen. Together, these technologies provide vital tools for the study of some of the oldest and problematic carbonaceous residues and for advancing our understanding of the earliest stages of biological evolution on Earth and the search for evidence of life beyond Earth. We discuss each of these new technologies, emphasizing their advantages and disadvantages, applications, and likely future directions. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

Open AccessReview Prebiotic Phosphorylation Reactions on the Early Earth

by Maheen Gull

Challenges 2014, 5(2), 193-212; doi:10.3390/challe5020193

Received: 28 May 2014 / Revised: 19 June 2014 / Accepted: 9 July 2014 / Published: 18 July 2014

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Abstract

Phosphorus (P) is an essential element for life. It occurs in living beings in the form of phosphate, which is ubiquitous in biochemistry, chiefly in the form of C-O-P (carbon, oxygen and phosphorus), C-P, or P-O-P linkages to form life. Within prebiotic chemistry,

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Phosphorus (P) is an essential element for life. It occurs in living beings in the form of phosphate, which is ubiquitous in biochemistry, chiefly in the form of C-O-P (carbon, oxygen and phosphorus), C-P, or P-O-P linkages to form life. Within prebiotic chemistry, several key questions concerning phosphorus chemistry have developed: what were the most likely sources of P on the early Earth? How did it become incorporated into the biological world to form the P compounds that life employs today? Can meteorites be responsible for the delivery of P? What were the most likely solvents on the early Earth and out of those which are favorable for phosphorylation? Or, alternatively, were P compounds most likely produced in relatively dry environments? What were the most suitable temperature conditions for phosphorylation? A route to efficient formation of biological P compounds is still a question that challenges astrobiologists. This article discusses these important issues related to the origin of biological P compounds. Full article

(This article belongs to the Special Issue Challenges in Astrobiology)

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