The Comet Interceptor Mission.

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA's F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum ΔV capability of 600 ms-1. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes - B1, provided by the Japanese space agency, JAXA, and B2 - that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission's science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

SISPO: Space Imaging Simulator for Proximity Operations.

This paper describes the architecture and demonstrates the capabilities of a newly developed, physically-based imaging simulator environment called SISPO, developed for small solar system body fly-by and terrestrial planet surface mission simulations. The image simulator utilises the open-source 3-D visualisation system Blender and its Cycles rendering engine, which supports physically based rendering capabilities and procedural micropolygon displacement texture generation. The simulator concentrates on realistic surface rendering and has supplementary models to produce realistic dust- and gas-environment optical models for comets and active asteroids. The framework also includes tools to simulate the most common image aberrations, such as tangential and sagittal astigmatism, internal and external comatic aberration, and simple geometric distortions. The model framework's primary objective is to support small-body space mission design by allowing better simulations for characterisation of imaging instrument performance, assisting mission planning, and developing computer-vision algorithms. SISPO allows the simulation of trajectories, light parameters and camera's intrinsic parameters.

A new model for silicification of cyanobacteria in Proterozoic tidal flats.

Microbial fossils preserved by early diagenetic chert provide a window into the Proterozoic biosphere, but seawater chemistry, microbial processes, and the interactions between microbes and the environment that contributed to this preservation are not well constrained. Here, we use fossilization experiments to explore the processes that preserve marine cyanobacterial biofilms by the precipitation of amorphous silica in a seawater medium that is analogous to Proterozoic seawater. These experiments demonstrate that the exceptional silicification of benthic marine cyanobacteria analogous to the oldest diagnostic cyanobacterial fossils requires interactions among extracellular polymeric substances (EPS), photosynthetically induced pH changes, magnesium cations (Mg2+ ), and >70 ppm silica.

Light-driven anaerobic microbial oxidation of manganese.

Oxygenic photosynthesis supplies organic carbon to the modern biosphere, but it is uncertain when this metabolism originated. It has previously been proposed1,2 that photosynthetic reaction centres capable of splitting water arose by about 3 billion years ago on the basis of the inferred presence of manganese oxides in Archaean sedimentary rocks. However, this assumes that manganese oxides can be produced only in the presence of molecular oxygen3, reactive oxygen species4,5 or by high-potential photosynthetic reaction centres6,7. Here we show that communities of anoxygenic photosynthetic microorganisms biomineralize manganese oxides in the absence of molecular oxygen and high-potential photosynthetic reaction centres. Microbial oxidation of Mn(II) under strictly anaerobic conditions during the Archaean eon would have produced geochemical signals identical to those used to date the evolution of oxygenic photosynthesis before the Great Oxidation Event1,2. This light-dependent process may also produce manganese oxides in the photic zones of modern anoxic water bodies and sediments.

Open-source sensor for measuring oxygen partial pressures below 100 microbars.

The ability to measure partial pressures of oxygen below 100 microbars and nanomolar dissolved oxygen concentrations in in situ laboratory systems benefits many fields including microbiology, geobiology, oceanography, chemistry, and materials science. Here, we present an easily constructible open-source design for a networked luminescence lifetime measurement system for in situ measurements in arbitrary laboratory containers. The system is well suited for measuring oxygen partial pressures in the 0-100 µbar range, with the maximum potentially usable upper range limit at around 10 mbar, depending on experimental conditions. The sensor has a limited drift and its detectability limit for oxygen is at 0.02 µbar for short timescale measurements. Each sensor can connect to a Wi-Fi network and send the logged data either over the Internet or to a local server, enabling a large number of parallel unattended experiments. Designs are also provided for attaching the sensor to various commercially available containers used in laboratories. The design files are released under an open source license, which enables other laboratories to build, customize, and use these sensors.

Evaluation of Electron-Phonon Coupling and Spectral Densities of Pigment-Protein Complexes by Line-Narrowed Optical Spectroscopy.

Spectral hole burning (SHB) and difference fluorescence line narrowing (ΔFLN) are routinely used for investigations of electron-phonon coupling in photosynthetic pigment-protein complexes as well as in other amorphous systems at cryogenic temperatures. Nevertheless, the Huang-Rhys factors S, an integral measure of electron-phonon coupling strength, and the phonon spectral densities obtained by SHB and ΔFLN over the past years have differed significantly in the case of certain photosynthetic pigment-protein complexes. In this work, the specific properties of both types of line-narrowing spectroscopic techniques that may lead to these discrepancies are critically analyzed by a combined experimental and computational approach, using the CP29 antenna complex of green plants as a suitable model system. We confirm that only ΔFLN at low fluence, by providing access to the homogeneously broadened spectrum, is able to deliver correct S values, while SHB may significantly under- or overestimate them, depending on the burn fluence. We also discuss possible other sources of discrepancies in the literature data, e.g., in the case of LHC II aggregates and correct numerical errors found in some previous records.

Unified analysis of ensemble and single-complex optical spectral data from light-harvesting complex-2 chromoproteins for gaining deeper insight into bacterial photosynthesis.

Bacterial light-harvesting pigment-protein complexes are very efficient at converting photons into excitons and transferring them to reaction centers, where the energy is stored in a chemical form. Optical properties of the complexes are known to change significantly in time and also vary from one complex to another; therefore, a detailed understanding of the variations on the level of single complexes and how they accumulate into effects that can be seen on the macroscopic scale is required. While experimental and theoretical methods exist to study the spectral properties of light-harvesting complexes on both individual complex and bulk ensemble levels, they have been developed largely independently of each other. To fill this gap, we simultaneously analyze experimental low-temperature single-complex and bulk ensemble optical spectra of the light-harvesting complex-2 (LH2) chromoproteins from the photosynthetic bacterium Rhodopseudomonas acidophila in order to find a unique theoretical model consistent with both experimental situations. The model, which satisfies most of the observations, combines strong exciton-phonon coupling with significant disorder, characteristic of the proteins. We establish a detailed disorder model that, in addition to containing a C_{2}-symmetrical modulation of the site energies, distinguishes between static intercomplex and slow conformational intracomplex disorders. The model evaluations also verify that, despite best efforts, the single-LH2-complex measurements performed so far may be biased toward complexes with higher Huang-Rhys factors.

Subtle spectral effects accompanying the assembly of bacteriochlorophylls into cyclic light harvesting complexes revealed by high-resolution fluorescence spectroscopy.

We have observed that an assembly of the bacteriochloropyll a molecules into B850 and B875 groups of cyclic bacterial light-harvesting complexes LH2 and LH1, respectively, results an almost total loss of the intra-molecular vibronic structure in the fluorescence spectrum, and simultaneously, an essential enhancement of its phonon sideband due to electron-phonon coupling. While the suppression of the vibronic coupling in delocalized (excitonic) molecular systems is predictable, as also confirmed by our model calculations, a boost of the electron-phonon coupling is rather unexpected. The latter phenomenon is explained by exciton self-trapping, promoted by mixing the molecular exciton states with charge transfer states between the adjacent chromophores in the tightly packed B850 and B875 arrangements. Similar, although less dramatic trends were noted for the light-harvesting complexes containing chlorophyll pigments.

Excitons in intact cells of photosynthetic bacteria.

Live cells and regular crystals seem fundamentally incompatible. Still, effects characteristic to ideal crystals, such as coherent sharing of excitation, have been recently used in many studies to explain the behavior of several photosynthetic complexes, especially the inner workings of the light-harvesting apparatus of the oldest known photosynthetic organisms, the purple bacteria. To this date, there has been no concrete evidence that the same effects are instrumental in real living cells, leaving a possibility that this is an artifact of unnatural study conditions, not a real effect relevant to the biological operation of bacteria. Hereby, we demonstrate survival of collective coherent excitations (excitons) in intact cells of photosynthetic purple bacteria. This is done by using excitation anisotropy spectroscopy for tracking the temperature-dependent evolution of exciton bands in light-harvesting systems of increasing structural complexity. The temperature was gradually raised from 4.5 K to ambient temperature, and the complexity of the systems ranged from detergent-isolated complexes to complete bacterial cells. The results provide conclusive evidence that excitons are indeed one of the key elements contributing to the energetic and dynamic properties of photosynthetic organisms.

Cataract diagnosis by measurement of backscattered light.

We present a portable optical cataract assessment technology which measures with a circular photodetector the fraction of light scattered backwards by the human eye lens when illuminated by a laser diode. As our signal arises directly from the fundamental pathology-increased scattering in the lens-it directly assesses cataract extent and progression. Initial clinical results in undilated human eyes show device reading correlations in agreement with clinical examination and Scheimpflug photography.

Davydov splitting of excitons in cyclic bacteriochlorophyll a nanoaggregates of bacterial light-harvesting complexes between 4.5 and 263 K.

The nature of electronic excitations created by photon absorption in the cyclic B850 aggregates of 18 bacteriochlorophyll molecules of LH2 antenna complexes of photosynthetic bacteria is studied over a broad temperature range using absorption, fluorescence, and fluorescence anisotropy spectra. The latter technique has been proved to be suitable for revealing the hidden structure of excitons in inhomogeneously broadened spectra of cyclic aggregates. A theoretical model that accounts for differences of absorbing excitons in undeformed and emitting exciton polarons in deformed antenna lattices is also developed. Only a slight decrease of the exciton bandwidth and exciton coupling energy with temperature is observed. Survival of excitons in the whole temperature span from cryogenic to nearly ambient temperatures strongly suggests that collective, coherent electronic excitations might play a role in the functional light-harvesting process taking place at physiological temperatures.