An objective Bayesian analysis of life's early start and our late arrival.

Life emerged on Earth within the first quintile of its habitable window, but a technological civilization did not blossom until its last. Efforts to infer the rate of abiogenesis, based on its early emergence, are frustrated by the selection effect that if the evolution of intelligence is a slow process, then life's early start may simply be a prerequisite to our existence, rather than useful evidence for optimism. In this work, we interpret the chronology of these two events in a Bayesian framework, extending upon previous work by considering that the evolutionary timescale is itself an unknown that needs to be jointly inferred, rather than fiducially set. We further adopt an objective Bayesian approach, such that our results would be agreed upon even by those using wildly different priors for the rates of abiogenesis and evolution-common points of contention for this problem. It is then shown that the earliest microfossil evidence for life indicates that the rate of abiogenesis is at least 2.8 times more likely to be a typically rapid process, rather than a slow one. This modest limiting Bayes factor rises to 8.7 if we accept the more disputed evidence of 13C-depleted zircon deposits [E. A. Bell, P. Boehnke, T. M. Harrison, W. L. Mao, Proc. Natl. Acad. Sci. U.S.A. 112, 14518-14521 (2015)]. For intelligence evolution, it is found that a rare-intelligence scenario is slightly favored at 3:2 betting odds. Thus, if we reran Earth's clock, one should statistically favor life to frequently reemerge, but intelligence may not be as inevitable.

A Catalog of Spectra, Albedos, and Colors of Solar System Bodies for Exoplanet Comparison.

We present a catalog of spectra and geometric albedos, representative of the different types of solar system bodies, from 0.45 to 2.5 µm. We analyzed published calibrated, uncalibrated spectra, and albedos for solar system objects and derived a set of reference spectra and reference albedos for 19 objects that are representative of the diversity of bodies in our solar system. We also identified previously published data that appear contaminated. Our catalog provides a baseline for comparison of exoplanet observations to 19 bodies in our own solar system, which can assist in the prioritization of exoplanets for time intensive follow-up with next-generation extremely large telescopes and space-based direct observation missions. Using high- and low-resolution spectra of these solar system objects, we also derive colors for these bodies and explore how a color-color diagram could be used to initially distinguish between rocky, icy, and gaseous exoplanets. We explore how the colors of solar system analog bodies would change when orbiting different host stars. This catalog of solar system reference spectra and albedos is available for download through the Carl Sagan Institute.

Summary and implications of reported amino acid concentrations in the Murchison meteorite

A study of literature reports of the concentrations of amino acids in extracts from the Murchison meteorite shows that many of the concentration ratios are constant. There are two possible interpretations of these ratios. One is that they are controlled by the pathways through which the amino acids formed, from which it follows that the amino acids are distributed in the same proportions throughout the meteorite. The other interpretation is that the ratios result from the analytical procedures used to extract the amino acids from the meteorite. These methods rely heavily on high-temperature (100 degrees C) aqueous extraction and subsequent high-temperature acid hydrolysis. A correlation was observed in the present study between the relative concentrations of several amino acids in the meteorite extracts and their relative aqueous solubilities at 100 degrees C (alanine, valine, leucine, isoleucine, norleucine, aspartic acid, glutamic acid and glycine). The extract solutions are dilute, and far from the saturation limits, but these correlations suggest that the sampling procedure affects directly the reported concentrations for these amino acids. Ratios of the concentration of serine to those of glycine are also constant but cannot be accounted for solely by relative solubilities, and, as suggested elsewhere, serine as well as phenylalanine and methionine may be terrestrial contaminants. Data for beta-alanine, alpha-aminobutyric acid, proline, sarcosine, alloisoleucine, beta-aminoisobutyric acid, beta-aminobutyric acid, and threonine also show constant abundances relative to glycine, but lack of solubility data at extraction conditions prohibits evaluating the extent of possible sampling bias for these amino acids. If the extraction process does not bias the results, and all extractable amino acids are removed from meteorite samples, then the properties of amino acids which control both their solubilities and their concentrations in the meteorite need to be established. The possibility of sampling bias needs to be tested experimentally before concluding that extraction is complete, and that the constant relative abundances indicate that the relative concentrations of amino acids are homogeneous in the meteorite.

Risk and value analysis of SETI.

This paper attempts to apply a traditional risk and value analysis to the Search for Extraterrestrial Intelligence--SETI. In view of the difficulties of assessing the probability of success, a comparison is made between SETI and a previous search for extraterrestrial life, the biological component of Project Viking. Our application of simple Utility Theory, given some reasonable assumptions, suggests that SETI is at least as worthwhile as the biological experiment on Viking.