Mechanical and electrostatic experiments with dust particles collected in the inner coma of comet 67P by COSIMA onboard Rosetta.

The in situ cometary dust particle instrument COSIMA (COmetary Secondary Ion Mass Analyser) onboard ESA's Rosetta mission has collected about 31 000 dust particles in the inner coma of comet 67P/Churyumov-Gerasimenko since August 2014. The particles are identified by optical microscope imaging and analysed by time-of-flight secondary ion mass spectrometry. After dust particle collection by low speed impact on metal targets, the collected particle morphology points towards four families of cometary dust particles. COSIMA is an in situ laboratory that operates remotely controlled next to the comet nucleus. The particles can be further manipulated within the instrument by mechanical and electrostatic means after their collection by impact. The particles are stored above 0°C in the instrument and the experiments are carried out on the refractory, ice-free matter of the captured cometary dust particles. An interesting particle morphology class, the compact particles, is not fragmented on impact. One of these particles was mechanically pressed and thereby crushed into large fragments. The particles are good electrical insulators and transform into rubble pile agglomerates by the application of an energetic indium ion beam during the secondary ion mass spectrometry analysis.This article is part of the themed issue 'Cometary science after Rosetta'.

High-molecular-weight organic matter in the particles of comet 67P/Churyumov-Gerasimenko.

The presence of solid carbonaceous matter in cometary dust was established by the detection of elements such as carbon, hydrogen, oxygen and nitrogen in particles from comet 1P/Halley. Such matter is generally thought to have originated in the interstellar medium, but it might have formed in the solar nebula-the cloud of gas and dust that was left over after the Sun formed. This solid carbonaceous material cannot be observed from Earth, so it has eluded unambiguous characterization. Many gaseous organic molecules, however, have been observed; they come mostly from the sublimation of ices at the surface or in the subsurface of cometary nuclei. These ices could have been formed from material inherited from the interstellar medium that suffered little processing in the solar nebula. Here we report the in situ detection of solid organic matter in the dust particles emitted by comet 67P/Churyumov-Gerasimenko; the carbon in this organic material is bound in very large macromolecular compounds, analogous to the insoluble organic matter found in the carbonaceous chondrite meteorites. The organic matter in meteorites might have formed in the interstellar medium and/or the solar nebula, but was almost certainly modified in the meteorites' parent bodies. We conclude that the observed cometary carbonaceous solid matter could have the same origin as the meteoritic insoluble organic matter, but suffered less modification before and/or after being incorporated into the comet.

Pressurized Martian-Like Pure CO2 Atmosphere Supports Strong Growth of Cyanobacteria, and Causes Significant Changes in their Metabolism.

Surviving of crews during future missions to Mars will depend on reliable and adequate supplies of essential life support materials, i.e. oxygen, food, clean water, and fuel. The most economical and sustainable (and in long term, the only viable) way to provide these supplies on Martian bases is via bio-regenerative systems, by using local resources to drive oxygenic photosynthesis. Selected cyanobacteria, grown in adequately protective containment could serve as pioneer species to produce life sustaining substrates for higher organisms. The very high (95.3 %) CO2 content in Martian atmosphere would provide an abundant carbon source for photo-assimilation, but nitrogen would be a strongly limiting substrate for bio-assimilation in this environment, and would need to be supplemented by nitrogen fertilizing. The very high supply of carbon, with rate-limiting supply of nitrogen strongly affects the growth and the metabolic pathways of the photosynthetic organisms. Here we show that modified, Martian-like atmospheric composition (nearly 100 % CO2) under various low pressure conditions (starting from 50 mbar to maintain liquid water, up to 200 mbars) supports strong cellular growth. Under high CO2 / low N2 ratio the filamentous cyanobacteria produce significant amount of H2 during light due to differentiation of high amount of heterocysts.

Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years.

Comets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer. The presence of water-ice grains in periodic comets is now well established. Modelling of infrared spectra obtained about ten kilometres from the nucleus of comet Hartley 2 suggests that larger dust particles are being physically decoupled from fine-grained water-ice particles that may be aggregates, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion. Micrometre-sized interplanetary dust particles collected in the Earth's stratosphere and certain micrometeorites are assumed to be of cometary origin. Here we report that grains collected from the Jupiter-family comet 67P/Churyumov-Gerasimenko come from a dusty crust that quenches the material outflow activity at the comet surface. The larger grains (exceeding 50 micrometres across) are fluffy (with porosity over 50 per cent), and many shattered when collected on the target plate, suggesting that they are agglomerates of entities in the size range of interplanetary dust particles. Their surfaces are generally rich in sodium, which explains the high sodium abundance in cometary meteoroids. The particles collected to date therefore probably represent parent material of interplanetary dust particles. This argues against comet dust being composed of a silicate core mantled by organic refractory material and then by a mixture of water-dominated ices. At its previous recurrence (orbital period 6.5 years), the comet's dust production doubled when it was between 2.7 and 2.5 astronomical units from the Sun, indicating that this was when the nucleus shed its mantle. Once the mantle is shed, unprocessed material starts to supply the developing coma, radically changing its dust component, which then also contains icy grains, as detected during encounters with other comets closer to the Sun.

Microbial life in a liquid asphalt desert.

Pitch Lake in Trinidad and Tobago is a natural asphalt reservoir nourished by pitch seepage, a form of petroleum that consists of mostly asphaltines, from the surrounding oil-rich region. During upward seepage, pitch mixes with mud and gases under high pressure, and the lighter portion evaporates or is volatilized, which produces a liquid asphalt residue characterized by low water activity, recalcitrant carbon substrates, and noxious chemical compounds. An active microbial community of archaea and bacteria, many of them novel strains (particularly from the new Tar ARC groups), totaling a biomass of up to 10(7) cells per gram, was found to inhabit the liquid hydrocarbon matrix of Pitch Lake. Geochemical and molecular taxonomic approaches revealed diverse, novel, and deeply branching microbial lineages with the potential to mediate anaerobic hydrocarbon degradation processes in different parts of the asphalt column. In addition, we found markers for archaeal methane metabolism and specific gene sequences affiliated with facultative and obligate anaerobic sulfur- and nitrite-oxidizing bacteria. The microbial diversity at Pitch Lake was found to be unique when compared to microbial communities analyzed at other hydrocarbon-rich environments, which included Rancho Le Brea, a natural asphalt environment in California, USA, and an oil well and a mud volcano in Trinidad and Tobago, among other sites. These results open a window into the microbial ecology and biogeochemistry of recalcitrant hydrocarbon matrices and establish the site as a terrestrial analogue for modeling the biotic potential of hydrocarbon lakes such as those found on Saturn's largest moon Titan.

Phenotypes and functional effects caused by various viral RNA silencing suppressors in transgenic Nicotiana benthamiana and N. tabacum.

RNA silencing suppressor genes derived from six virus genera were transformed into Nicotiana benthamiana and N. tabacum plants. These suppressors were P1 of Rice yellow mottle virus (RYMV), P1 of Cocksfoot mottle virus, P19 of Tomato bushy stunt virus, P25 of Potato virus X, HcPro of Potato virus Y (strain N), 2b of Cucumber mosaic virus (strain Kin), and AC2 of African cassava mosaic virus (ACMV). HcPro caused the most severe phenotypes in both Nicotiana spp. AC2 also produced severe effects in N. tabacum but a much milder phenotype in N. benthamiana, although both HcPro and AC2 affected the leaf tissues of the two Nicotiana spp. in similar ways, causing hyperplasia and hypoplasia, respectively. P1-RYMV caused high lethality in the N. benthamiana plants but only mild effects in the N. tabacum plants. Phenotypic alterations produced by the other transgenes were minor in both species. Interestingly, the suppressors had very different effects on crucifer-infecting Tobamovirus (crTMV) infections. AC2 enhanced both spread and brightness of the crTMV-green fluorescent protein (GFP) lesions, whereas 2b and both P1 suppressors enhanced spread but not brightness of these lesions. P19 promoted spread of the infection into new foci within the infiltrated leaf, whereas HcPro and P25 suppressed the spread of crTMV-GFP lesions.

Homochirality in an early peptide world.

A recently proposed model of non-autocatalytic reactions in dipeptide formation that leads to spontaneous symmetry breaking and homochirality was examined. The model is governed by activation, polymerization, epimerization, and depolymerization of amino acids. Symmetry breaking was determined to result primarily from the different rates of reactions that involve homodimers and heterodimers, i.e., stereoselective reactions, and the fact that epimerization can only occur on the N-terminal residue and not on the C-terminal residue. This corresponds to an auto-inductive cyclic process that works only in one direction. It is argued that epimerization mimics autocatalytic behavior as well as mutual antagonism, both of which are known to be crucial for the production of full homochirality.

Biological feedbacks as cause and demise of the Neoproterozoic icehouse: astrobiological prospects for faster evolution and importance of cold conditions.

Several severe glaciations occurred during the Neoproterozoic eon, and especially near its end in the Cryogenian period (630-850 Ma). While the glacial periods themselves were probably related to the continental positions being appropriate for glaciation, the general coldness of the Neoproterozoic and Cryogenian as a whole lacks specific explanation. The Cryogenian was immediately followed by the Ediacaran biota and Cambrian Metazoan, thus understanding the climate-biosphere interactions around the Cryogenian period is central to understanding the development of complex multicellular life in general. Here we present a feedback mechanism between growth of eukaryotic algal phytoplankton and climate which explains how the Earth system gradually entered the Cryogenian icehouse from the warm Mesoproterozoic greenhouse. The more abrupt termination of the Cryogenian is explained by the increase in gaseous carbon release caused by the more complex planktonic and benthic foodwebs and enhanced by a diversification of metazoan zooplankton and benthic animals. The increased ecosystem complexity caused a decrease in organic carbon burial rate, breaking the algal-climatic feedback loop of the earlier Neoproterozoic eon. Prior to the Neoproterozoic eon, eukaryotic evolution took place in a slow timescale regulated by interior cooling of the Earth and solar brightening. Evolution could have proceeded faster had these geophysical processes been faster. Thus, complex life could theoretically also be found around stars that are more massive than the Sun and have main sequence life shorter than 10 Ga. We also suggest that snow and glaciers are, in a statistical sense, important markers for conditions that may possibly promote the development of complex life on extrasolar planets.

Suitability of different photosynthetic organisms for an extraterrestrial biological life support system.

In the present era of intensive space and planetary research, efficient life support systems (LSSs) are needed to maintain suitable living conditions when humans move into space, i.e. away from the Earth's atmosphere. Thus far, such suitable conditions on various space flights and on the space stations (Mir and the International Space Station) have been maintained solely via physical and chemical means (transport of O2, H2O and food from the Earth, cleaning and recycling of air and water). However, for long-duration missions to distant destinations, such as exploratory missions to Mars, biological life support systems (BLSSs) may be needed to convert local CO2 and H2O to O2, and to food. As on earth, this conversion process would need to be based on photosynthesis. Use of higher plants and microalgae as BLSS organisms has been intensively studied. Here we review the growth requirements of these two types of photosynthetic organisms, with particular attention to their suitability for use in harsh Martian conditions, i.e. low temperatures, low atmospheric pressure, high CO2 concentration, high UV radiation and dryness.