Prebiotic Chemistry of Pluto

The evolution of organic material on Asteroid 162173 Ryugu and its delivery to Earth

The recent return of samples from asteroid 162173 Ryugu provides a first insight into early Solar System prebiotic evolution from known planetary bodies. Ryugu's samples are CI chondrite-like, rich in water and organic material, and primarily composed of phyllosilicate. This phyllosilicate surrounds micron to submicron macromolecular organic particles known as insoluble organic matter. Using advanced microscopy techniques on Hayabusa-2 samples, we find that aqueous alteration on Ryugu produced organic particles richer in aromatics compared to less altered carbonaceous chondrites. This challenges the view that aromatic-rich organic matter formed pre-accretion. Additionally, widespread diffuse organic material occurs in phyllosilicate more aliphatic-, carboxylic-rich, and aromatic-poor than the discrete organic particles, likely preserving the soluble organic material. Some organic particles evolved to encapsulate phyllosilicate, indicating that aqueous alteration on Ryugu led to the containment of soluble organic matter within these particles. Earth therefore has been, and continues to be, delivered micron-sized polymeric organic objects containing biologically relevant molecules.

The COSPAR planetary protection policy for missions to Icy Worlds: A review of history, current scientific knowledge, and future directions

Recent discoveries related to the habitability and astrobiological relevance of the outer Solar System have expanded our understanding of where and how life may have originated. As a result, the Icy Worlds of the outer Solar System have become among the highest priority targets for future spacecraft missions dedicated to astrobiology-focused and/or direct life detection objectives. This, in turn, has led to a renewed interest in planetary protection concerns and policies for the exploration of these worlds and has been a topic of discussion within the COSPAR (Committee on Space Research) Panel on Planetary Protection. This paper summarizes the results of those discussions, reviewing the current knowledge and the history of planetary protection considerations for Icy Worlds as well as suggesting ways forward. Based on those discussions, we therefore suggest to (1) Establish a new definition for Icy Worlds for Planetary Protection that captures the outer Solar System moons and dwarf planets like Pluto, but excludes more primitive bodies such as comets, centaurs, and asteroids: Icy Worlds in our Solar System are defined as all bodies with an outermost layer that is believed to be greater than 50 % water ice by volume and have enough mass to assume a nearly round shape. (2) Establish indices for the lower limits of Earth life with regards to water activity (LLAw) and temperature (LLT) and apply them into all areas of the COSPAR Planetary Protection Policy. These values are currently set at 0.5 and -28 °C and were originally established for defining Mars Special Regions; (3) Establish LLT as a parameter to assign categorization for Icy Worlds missions. The suggested categorization will have a 1000-year period of biological exploration, to be applied to all Icy Worlds and not just Europa and Enceladus as is currently the case. (4) Have all missions consider the possibility of impact. Transient thermal anomalies caused by impact would be acceptable so long as there is less than 10-4 probability of a single microbe reaching deeper environments where temperature is >LLT in the period of biological exploration. (5) Restructure or remove Category II* from the policy as it becomes largely redundant with this new approach, (6) Establish that any sample return from an Icy World should be Category V restricted Earth return.

"Sea Water" Supplemented with Calcium Phosphate and Magnesium Sulfate in a Long-Term Miller-Type Experiment Yields Sugars, Nucleic Acids Bases, Nucleosides, Lipids, Amino Acids, and Oligopeptides

The standard approach to exploring prebiotic chemistry is to use a small number of highly purified reactants and to attempt to optimize the conditions required to produce a particular end product. However, purified reactants do not exist in nature. We have previously proposed that what drives prebiotic evolution are complex chemical ecologies. Therefore, we have begun to explore what happens if one substitutes "sea water", with its complex mix of minerals and salts, for distilled water in the classic Miller experiment. We have also adapted the apparatus to permit it to be regassed at regular intervals so as to maintain a relatively constant supply of methane, hydrogen, and ammonia. The "sea water" used in the experiments was created from Mediterranean Sea salt with the addition of calcium phosphate and magnesium sulfate. Tests included several types of mass spectrometry, an ATP-monitoring device capable of measuring femtomoles of ATP, and a high-sensitivity cAMP enzyme-linked immunoadsorption assay. As expected, amino acids appeared within a few days of the start of the experiment and accumulated thereafter. Sugars, including glucose and ribose, followed as did long-chain fatty acids (up to C20). At three-to-five weeks after starting the experiment, ATP was repeatedly detected. Thus, we have shown that it is possible to produce a "one-pot synthesis" of most of the key chemical prerequisites for living systems within weeks by mimicking more closely the complexity of real-world chemical ecologies.

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%.

Synthesis of CxNy-rich polycyclic oligomers from primeval monomers in aqueous media

A multichannel, non-thermolytic and efficient pathway is described toward the formation of functionalized carbon nitride-like oligomers, starting from readily available cyanamide and glyoxal (in ratios >2), in aqueous media under mild conditions. Such oligomers can be isolated as stable solids that result from structures involving cyanamide self-additions along with structures formally derived from the condensation of cyanamide, dicyandiamide or melamine with glyoxal, leading occasionally to oxygen-containing units. The oligomeric aggregates have masses up to 500 u, as inferred from mass spectra analyses, and their formation can be rationalized in terms of polyadditions of cyanamide (up to 10-mer) and glyoxal. The latter is not only a willing reaction partner, but also promotes facile condensation by enhancing the reactivity of nitrile fragments and inducing a significant lowering of the energy barriers. This mechanistic surmise is also supported by DFT calculations of the early condensation steps. As a result, melamine/triazine-type structures are obtained in aquatic environments under much milder conditions than those usually required by other synthetic procedures. Moreover, our results also help unveil the abiotic processes affording complex organic matter on celestial bodies and early earth.

Organic Components of Small Bodies in the Outer Solar System: Some Results of the New Horizons Mission

The close encounters of the Pluto–Charon system and the Kuiper Belt object Arrokoth (formerly 2014 MU₆₉) by NASA’s New Horizons spacecraft in 2015 and 2019, respectively, have given new perspectives on the most distant planetary bodies yet explored. These bodies are key indicators of the composition, chemistry, and dynamics of the outer regions of the Solar System’s nascent environment. Pluto and Charon reveal characteristics of the largest Kuiper Belt objects formed in the dynamically evolving solar nebula inward of ~30 AU, while the much smaller Arrokoth is a largely undisturbed relic of accretion at ~45 AU. The surfaces of Pluto and Charon are covered with volatile and refractory ices and organic components, and have been shaped by geological activity. On Pluto, N₂, CO and CH₄ are exchanged between the atmosphere and surface as gaseous and condensed phases on diurnal, seasonal and longer timescales, while Charon’s surface is primarily inert H₂O ice with an ammoniated component and a polar region colored with a macromolecular organic deposit. Arrokoth is revealed as a fused binary body in a relatively benign space environment where it originated and has remained for the age of the Solar System. Its surface is a mix of CH₃OH ice, a red-orange pigment of presumed complex organic material, and possibly other undetected components.

Organic Molecules: Is It Possible to Distinguish Aromatics from Aliphatics Collected by Space Missions in High Speed Impacts?

A prime site of astrobiological interest within the Solar System is the interior ocean of Enceladus. This ocean has already been shown to contain organic molecules and is thought to have the conditions necessary for more complex organic biomolecules to emerge and potentially even life itself. This sub-surface ocean has been accessed by Cassini, an unmanned spacecraft that interacted with the water plumes ejected naturally from Enceladus. The encounter speed with these plumes and their contents was 5 km s−1 and above. Encounters at such speeds allow analysis of vaporised material from submicron-sized particles within the plume, but sampling micron-sized particles remains an open question. The latter particles can impact metal targets exposed on the exterior of future spacecraft, producing impact craters lined with impactor residue, which can then be analysed. Although there is considerable literature on how mineral grains behave in such high-speed impacts, and also on the relationship between the crater residue and the original grain composition, far less is known regarding the behaviour of organic particles. Here we consider a deceptively simple yet fundamental scientific question: for impacts at speeds of around 5–6 kms−1 would the impactor residue alone be sufficient to enable us to recognise the signature conferred by organic particles? Furthermore, would it be possible to identify the organic molecules involved, or at least distinguish between aromatic and aliphatic chemical structures? For polystyrene (aromatic-rich) and poly (methyl methacrylate) (solely aliphatic) latex particles impinging at around 5 km s−1 onto metal targets, we found that sufficient residue is retained at the impact site to permit identification of a carbon-rich projectile, but not of the particular molecules involved, nor is it currently possible to discriminate between aromatic-rich and solely aliphatic particles. This suggests that an alternative analytical method to simple impacts on metal targets is required to enable successful collection of organic samples in a fly-by Enceladus mission.