Solar System Environments Research Articles

Mid-infrared imaging spectroscopic measurements of C2H4 frost simulating the outer solar system environments

In the dense and cold atmosphere of Titan, the presence of C2H4 haze has been confirmed by the observations of spacecraft. In the present study, original cryogenic experimental equipment was developed to simulate the low-temperature solid formation of C2H4 in combination with in-situ infrared spectroscopic measurements. As a result, out-of-plane bending vibration ν7 of solid-phase C2H4 located at ~ 10.5 μm was successfully detected with high sensitivity, and two-dimensional spectrographs of C2H4 at low temperatures were obtained. The obtained spectra of C2H4 can be fitted to the double Lorentzian function with various heights, central wavelengths, and full widths at half the maximum (FWHM) of the two-component Lorentzian functions. They were classified into three types using the fitting parameters. However, their spectral shapes are different from the amorphous, metastable, and crystalline forms obtained by the previous laboratory experiment in terms of the distance of two peak wavelengths and FWHM. The results may link to understanding the spectral band properties of C2H4 condensation in the haze component of Titan.Graphical

A systematic mid-infrared spectroscopic study of thermally processed SO2 ices.

The use of mid-infrared spectroscopy to characterise the chemistry of icy interstellar and Solar System environments will be exploited in the near future to better understand the chemical processes and molecular inventories in various astronomical environments. This is, in part, due to observational work made possible by the recently launched James Webb Space Telescope as well as forthcoming missions to the outer Solar System that will observe in the mid-infrared spectroscopic region (e.g., the Jupiter Icy Moons Explorer and the Europa Clipper missions). However, such spectroscopic characterisations are crucially reliant upon the generation of laboratory data for comparative purposes. In this paper, we present an extensive mid-infrared characterisation of SO2 ice condensed at several cryogenic temperatures between 20 and 100 K and thermally annealed to sublimation in an ultrahigh-vacuum system. Our results are anticipated to be useful in confirming the detection (and possibly thermal history) of SO2 on various Solar System bodies, such as Ceres and the icy Galilean moons of Jupiter, as well as in interstellar icy grain mantles.

Mechanochemical generation of perchlorate

Perchlorate (ClO4−) is widespread in the solar system having been detected on Earth, on Mars, in chondrite meteorites and in lunar samples. On Mars, perchlorates expand the potential for habitable conditions by lowering the freezing point of liquid water in the formation of brines. In future manned space exploration their presence poses a hazard to human health, however, it also represents opportunities as a source of oxygen and fuel. Despite their prevalence, the mechanism(s) of perchlorate formation in different solar system environments are poorly understood. Here we demonstrate that perchlorate can be generated through the mechanical activation of silicates in the presence of chloride.

Laboratory experiments on the radiation astrochemistry of water ice phases

Water (H2O) ice is a ubiquitous component of the universe, having been detected in a variety of interstellar and Solar System environments where radiation plays an important role in its physico-chemical transformations. Although the radiation chemistry of H2O astrophysical ice analogues has been well studied, direct and systematic comparisons of different solid phases are scarce and are typically limited to just two phases. In this article, we describe the results of an in-depth study of the 2 keV electron irradiation of amorphous solid water (ASW), restrained amorphous ice (RAI) and the cubic (Ic) and hexagonal (Ih) crystalline phases at 20 K so as to further uncover any potential dependence of the radiation physics and chemistry on the solid phase of the ice. Mid-infrared spectroscopic analysis of the four investigated H2O ice phases revealed that electron irradiation of the RAI, Ic, and Ih phases resulted in their amorphization (with the latter undergoing the process more slowly) while ASW underwent compaction. The abundance of hydrogen peroxide (H2O2) produced as a result of the irradiation was also found to vary between phases, with yields being highest in irradiated ASW. This observation is the cumulative result of several factors including the increased porosity and quantity of lattice defects in ASW, as well as its less extensive hydrogen-bonding network. Our results have astrophysical implications, particularly with regards to H2O-rich icy interstellar and Solar System bodies exposed to both radiation fields and temperature gradients.Graphical abstract

Radiolytic Destruction of Uracil in Interstellar and Solar System Ices.

Uracil is one of the four RNA nucleobases and a component of meteoritic organics. If delivered to the early Earth, uracil could have been involved in the origins of the first RNA-based life, and so this molecule could be a biomarker on other worlds. Therefore, it is important to understand uracil's survival to ionizing radiation in extraterrestrial environments. Here we present a study of the radiolytic destruction kinetics of uracil and mixtures of uracil diluted in H2O or CO2 ice. All samples were irradiated by protons with an energy of 0.9 MeV, and experiments were performed at 20 and 150 K to determine destruction rate constants at temperatures relevant to interstellar and Solar System environments. We show that uracil is destroyed much faster when H2O ice or CO2 ice is present than when these two ices are absent. Moreover, destruction is faster for CO2-dominated ices than for H2O-dominated ones and, to a lesser extent, at 150 K compared with 20 K. Extrapolation of our laboratory results to astronomical timescales shows that uracil will be preserved in ices with half-lives of up to ∼107 years on cold planetary bodies such as comets or Pluto. An important implication of our results is that for extraterrestrial environments, the application of laboratory data measured for the radiation-induced destruction of pure (neat) uracil samples can greatly underestimate the molecule's rate of destruction and significantly overestimate its lifetime, which can lead to errors of over 1000%.

Classification of the Biogenicity of Complex Organic Mixtures for the Detection of Extraterrestrial Life.

Searching for life in the Universe depends on unambiguously distinguishing biological features from background signals, which could take the form of chemical, morphological, or spectral signatures. The discovery and direct measurement of organic compounds unambiguously indicative of extraterrestrial (ET) life is a major goal of Solar System exploration. Biology processes matter and energy differently from abiological systems, and materials produced by biological systems may become enriched in planetary environments where biology is operative. However, ET biology might be composed of different components than terrestrial life. As ET sample return is difficult, in situ methods for identifying biology will be useful. Mass spectrometry (MS) is a potentially versatile life detection technique, which will be used to analyze numerous Solar System environments in the near future. We show here that simple algorithmic analysis of MS data from abiotic synthesis (natural and synthetic), microbial cells, and thermally processed biological materials (lab-grown organisms and petroleum) easily identifies relational organic compound distributions that distinguish pristine and aged biological and abiological materials, which likely can be attributed to the types of compounds these processes produce, as well as how they are formed and decompose. To our knowledge this is the first comprehensive demonstration of the utility of this analytical technique for the detection of biology. This method is independent of the detection of particular masses or molecular species samples may contain. This suggests a general method to agnostically detect evidence of biology using MS given a sufficiently strong signal in which the majority of the material in a sample has either a biological or abiological origin. Such metrics are also likely to be useful for studies of possible emergent living phenomena, and paleobiological samples.

Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: formation and destruction of solid carbonic acid upon 1 keV electron irradiation

Context. Carbonic acid (H2CO3) is a weak acid relevant to astrobiology which, to date, remains undetected in space. Experimental work has shown that the β-polymorph of H2CO3 forms under space relevant conditions through energetic (UV photon, electron, and cosmic ray) processing of CO2- and H2O-rich ices. Although its α-polymorph ice has been recently reassigned to the monomethyl ester of carbonic acid, a different form of H2CO3 ice may exist and is synthesized without irradiation through surface reactions involving CO molecules and OH radicals, that is to say γ-H2CO3. Aims. We aim to provide a systematic set of vacuum ultraviolet (VUV) photoabsorption spectroscopic data of pure carbonic acid that formed and was destroyed under conditions relevant to space in support of its future identification on the surface of icy objects in the Solar System by the upcoming Jupiter ICy moons Explorer mission and on interstellar dust by the James Webb Space Telescope spacecraft. Methods. We present VUV photoabsorption spectra of pure and mixed CO2 and H2O ices exposed to 1 keV electrons at 20 and 80 K to simulate different interstellar and Solar System environments. Ices were then annealed to obtain a layer of pure H2CO3 which was further exposed to 1 keV electrons at 20 and 80 K to monitor its destruction pathway. Fourier-transform infrared (FT-IR) spectroscopy was used as a secondary probe providing complementary information on the physicochemical changes within an ice. Results. Our laboratory work shows that the formation of solid H2CO3, CO, and O3 upon the energetic processing of CO2:H2O ice mixtures is temperature-dependent in the range between 20 and 80 K. The amorphous to crystalline phase transition of H2CO3 ice is investigated for the first time in the VUV spectral range by annealing the ice at 200 and 225 K. We have detected two photoabsorption bands at 139 and 200 nm, and we assigned them to β-H2CO3 and γ-H2CO3, respectively. We present VUV spectra of the electron irradiation of annealed H2CO3 ice at different temperatures leading to its decomposition into CO2, H2O, and CO ice. Laboratory results are compared to Cassini UltraViolet Imaging Spectrograph observations of the 70−90 K ice surface of Saturn’s satellites Enceladus, Dione, and Rhea.

Spitzer’s Solar System studies of comets, centaurs and Kuiper belt objects

In its 16 years of scientific measurements, the Spitzer Space Telescope performed ground-breaking and key infrared measurements of Solar System objects near and far. Targets ranged from the smallest planetesimals to the giant planets; Spitzer helped us to reshape our understanding of these objects while also laying the groundwork for future infrared space-based observations like those to be undertaken by the James Webb Space Telescope in the 2020s. In this Review Article, we describe how Spitzer advanced our knowledge of Solar System formation and evolution through observations of small outer Solar System planetesimals—that is, comets, centaurs and Kuiper belt objects (KBOs). Relics from the early formation era of our Solar System, these objects hold important information about the processes that created them.We group Spitzer’s key contributions into three broad classes: characterization of new Solar System objects (comets D/ISON 2012 S1, C/2016 R2 and 1I/‘Oumuamua); large population surveys of known objects (comets, centaurs and KBOs); and compositional studies through spectral measurements of body surfaces and emitted materials. In the Spitzer Space Telescope’s 16 years of operation, it observed many Solar System objects and environments. In this first Review Article of a pair, Spitzer’s insights into comets, centaurs and Kuiper belt objects—all remnants of the Solar System’s formation—are summarized.

Spitzer’s Solar System studies of asteroids, planets and the zodiacal cloud

In its 16 years of scientific measurements, the Spitzer Space Telescope performed a number of groundbreaking and key infrared measurements of Solar System objects near and far. In this second of two Review Articles, we describe results from Spitzer observations of asteroids, dust rings and planets that provide new insight into the formation and evolution of our Solar System. The key Spitzer results presented here can be grouped into three broad classes: characterizing the physical properties of asteroids, notably including a large survey of near-Earth objects; detection and characterization of several dust/debris disks in the Solar System; and comprehensive characterization of ice giant (Uranus and Neptune) atmospheres. Many of these observations provide critical foundations for future infrared space-based observations. In the Spitzer Space Telescope’s 16 years of operation, it observed many Solar System objects and environments. In this second Review Article of a pair, Spitzer’s insight into asteroids, dust clouds and rings and the ice giant planets are summarized.

Plasma Waves in Space: The Importance of Properly Accounting for the Measuring Device

AbstractElectric fields are generally measured or calculated using two intuitive assumptions: (1) the electric field equals the voltage divided by the antenna length when the antenna is electromagnetically short (2) the antenna responds best to electric field along its length. Both assumptions are often incorrect for electrostatic fields because they scale as the Debye length or as the electron gyroradius, which may be smaller than the antenna length. Taking into account this little‐known fact enables us to complete or correct several recent papers on plasma spontaneous fluctuations in various solar system environments.

Crystal structure of propionitrile (CH3CH2CN) determined using synchrotron powder X-ray diffraction.

The structure and thermal expansion of the astronomical molecule propionitrile have been determined from 100 to 150 K using synchrotron powder X-ray diffraction. This temperature range correlates with the conditions of Titan's lower stratosphere, and near surface, where propionitrile is thought to accumulate and condense into pure and mixed-nitrile phases. Propionitrile was determined to crystallize in space group, Pnma (No. 62), with unit cell a = 7.56183 (16) Å, b = 6.59134 (14) Å, c = 7.23629 (14), volume = 360.675 (13) Å3 at 100 K. The thermal expansion was found to be highly anisotropic with an eightfold increase in expansion between the c and b axes. These data will prove crucial in the computational modelling of propionitrile-ice systems in outer Solar System environments, allowing us to simulate and assign vibrational peaks in the infrared spectra for future use in planetary astronomy.

Gravitational wave solutions to linearized Jordan Brans Dicke theory on a cosmological background

Approximate vacuum solutions of Jordan-Brans-Dicke theory for perturbed scalar field and perturbed Robertson-Walker metric, are found. Solutions for the scale factor and the scalar field in unperturbed JBD theory are dependent on the $\omega$ parameter which determines how the scalar field is coupled to geometry of space-time. After adding a metric perturbation to Robertson-Walker metric and a perturbation to the scalar field, we solved the linearized JBD equations and found the scale factor and the scalar field as $a\propto t$ and $\phi\propto t^{-2}$ with $\omega=$-3/2. The results are necessary conditions for ordinary and scalar gravitational waves to exist in the vacuum case. Despite omega is a large positive number for current solar system environment observations, this value of omega makes JBD theory conformally invariant and fits recent supernovae type Ia data. We also looked for the value of omega for the case which has nonzero spatial curvature parameter.

(Invited) Electrochemical Measurement Approaches for Planetary Exploration

2018 marks the 10-year anniversary of the landing of the NASA Phoenix spacecraft on the ice-rich northern plains of Mars. The Phoenix Lander carried the Wet Chemistry Laboratory (WCL), a component instrument of the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) and was used to perform a comprehensive electrochemical characterization of martian soil in an aqueous solution. Designed to characterize the habitability of the martian surface, WCL contained an array of ion selective electrodes, a conductivity cell, a platinum electrode to measure redox potential, electrodes to perform perform cyclic voltammetry, chronopotentiometry and anodic stripping voltammetry, as well as a set of chemical reagents used to perform a sulfate titration and sample acidification. The measurements made with WCL resulted in a number of major discoveries, including the detection of percent levels of perchlorate on the martian surface, and represent the first, and to date, the only time electrochemical methods have been used for planetary exploration beyond Earth. NASA is now embarking on a new era of astrobiology exploration as plans are being formulated for missions to Ocean Worlds of the outer solar system, including Europa, an ice covered Jovian moon. Based on lessons learned during the development of WCL and its operation on the surface of Mars, we will discuss new design approaches and strategies that will enable the implementation of autonomous microfluidic electrochemical sensor arrays during upcoming missions to these yet to be explored solar system environments.

Simple prebiotic synthesis of high diversity dynamic combinatorial polyester libraries

It is widely believed that the origin of life depended on environmentally driven complexification of abiotically produced organic compounds. Polymerization is one type of such complexification, and it may be important that many diverse polymer sequences be produced for the sake of selection. Not all compound classes are easily polymerized under the environmental conditions present on primitive planets, and it is possible that life’s origin was aided by other monomers besides those used in contemporary biochemistry. Here we show that alpha-hydroxy acids, which are plausibly abundant prebiotic monomers, can be oligomerized to generate vast, likely sequence-complete libraries, which are also stable for significant amounts of time. This occurs over a variety of reaction conditions (temperature, concentration, salinity, and presence of congeners) compatible with geochemical settings on the primitive Earth and other solar system environments. The high-sequence heterogeneity achievable with these compounds may be useful for scaffolding the origin of life.

Dichotomous Hypothesis of the Solar System Origin: Substantiation and Consequences for Astrobiology

The aim of this study consists in the advancement of the author’s dichotomous hypothesis of hot origin of the solar system that is alternate to the well-known hypotheses of cold accretion. The hypothesis proposes formation of solar planets and satellites through dichotomous division of the superheated protoplanetary mass ejected from the youngest Sun due to strong non-equilibrium competition between gravitational contraction and heat expansion. Like the cold accretion hypotheses, the dichotomous hypothesis also explains the key regularities of the solar system structure (angular momentum distribution, position of the asteroid belt, back rotation of Venus and Uranus, etc.). Besides, it offers explanation of new data on extra solar planetary systems that is difficult to understand basing on the gradual cold accretion process (in particular, the misaligned protoplanetary discs in the binary protostar IRS 43). Some predictions following of the given explanation can be examined during future observations. The proposed hot origin of planets and satellites provides some new opportunities and directions for search of life in the solar system, including comets and hydrothermal environments on Mars and Europa.

Diversity of the initial rocky planetary building materials at the edge of the solar system

AbstractAsteroids and comets are surviving members of the vast planetesimal population that was distributed across the early solar system. They appear to be a diverse set of bodies but we present evidence from comet samples that the body‐to‐body diversity of the initial rocky component mix in planetesimals may have declined with distance from the Sun. Laboratory measurements of the minor element Mn in olivine collected from Comet Wild 2 suggests that the micron‐sized rocky crystalline contents of this comet formed in numerous inner solar system environments. The results are consistent with a scenario where silicates such as olivine form at incandescent temperatures in multiple environments and then mix as they are transported to distant cold regions where silicates could accrete with ice and organics to form comets. Accreting far from silicate formation regions, many ice‐rich planetesimals are likely to have started with similar complex mixtures of diverse rocky components formed in various high‐temperature environments. This contrasts with asteroidal meteorite parent bodies whose silicates retain regional properties that give different chondrite classes their distinctive properties.

N2H+and N15NH+toward the prestellar core 16293E in L1689N

Understanding the processes that could lead to enrichment of molecules in $^{15}$N atoms is of particular interest in order to shed light on the relatively large variations observed in the $^{14}$N/$^{15}$N ratio in various solar system environments. Currently, the sample of molecular clouds where $^{14}$N/$^{15}$N ratios have been measured is small and has to be enlarged in order to allow statistically significant studies. In particular, the N$_2$H$^+$ molecule currently shows the largest spread of $^{14}$N/$^{15}$N ratios in high-mass star forming regions. However, the $^{14}$N/$^{15}$N ratio in N$_2$H$^+$ was obtained in only two low-mass star forming regions (L1544 and B1b). The current work extends this sample to a third dark cloud. We targeted the 16293E prestellar core, where the N$^{15}$NH$^+$ $J$=1-0 line was detected. Using a model previously developed for the physical structure of the source, we solved the molecular excitation with a non-local radiative transfer code. For that purpose, we computed specific collisional rate coefficients for the N$^{15}$NH$^+$-H$_2$ collisional system. As a first step of the analysis, the N$_2$H$^+$ abundance profile was constrained by reproducing the N$_2$H$^+$ $J$=1-0 and 3-2 maps. A scaling factor was then applied to this profile to match the N$^{15}$NH$^+$ $J$=1-0 spectrum. We derive a column density ratio N$_2$H$^+$ / N$^{15}$NH$^+$ = $330^{+170}_{-100}$. The current estimate $\sim$330 agrees with the value typical of the elemental isotopic ratio in the local ISM. It is however lower than in some other cores, where values as high as 1300 have been reported.

High-resolution mapping of lunar polar hydrogen with a low-resource orbital mission

Lunar permanently shaded regions (PSRs) are unique solar-system environments. Their low temperatures (<100K) facilitate cold trapping of volatile materials over timescales comparable to the lifetime of the solar system. While much has been learned about these regions from orbital spacecraft, important missing information includes the spatial and depth-dependent distribution of bulk hydrogen concentrations in and around PSRs. We present two complementary mission scenarios where orbital neutron spectroscopy will provide significantly improved understanding of lunar polar bulk hydrogen concentrations. In the first mission concept, a six-month orbital mission will measure bulk hydrogen concentrations with sensitivity better than 50ppm and a spatial resolution of order 20km over the entire lunar South Polar region (poleward of 80ºS). Spatial reconstruction analyses of the returned data will improve the final spatial resolution to better than 10km. The presence and burial depth (<25cm) of subsurface deposits will be quantified with latitude-dependent sensitivities ranging from 50 to 350ppm. The second concept envisions a few, very low altitude (~5km) flyovers of one or more PSRs to quantify the hydrogen concentrations and spatial heterogeneities with a hydrogen sensitivity less than 200 and spatial scale size of 5km. Both concepts can be combined in a single mission where full-coverage polar measurements are made with a hydrogen spatial resolution of 20km, and higher spatial resolution measurements are made for a few strategically selected PSRs at the end of the mission.

Observations of the Icy Universe

Freeze-out of the gas-phase elements onto cold grains in dense interstellar and circumstellar media builds up ice mantles consisting of molecules that are mostly formed in situ (H2O, NH3, CO2, CO, CH3OH, and more). This review summarizes the detected infrared spectroscopic ice features and compares the abundances across Galactic, extragalactic, and Solar System environments. A tremendous amount of information is contained in the ice band profiles. Laboratory experiments play a critical role in the analysis of the observations. Strong evidence is found for distinct ice formation stages, separated by CO freeze-out at high densities. The ice bands have proven to be excellent probes of the thermal history of their environment. The evidence for the long-held idea that processing of ices by energetic photons and cosmic rays produces complex molecules is weak. Recent state-of-the-art observations show promise for much progress in this area with planned infrared facilities.

In situ apparatus for the study of clathrate hydrates relevant to solar system bodies using synchrotron X-ray diffraction and Raman spectroscopy

Context. Clathrate hydrates are believed to play a significant role in various solar system environments, e.g. comets, and the surfaces and interiors of icy satellites. However, the structural factors governing their formation and dissociation are poorly understood.