Molecular nitrogen (N2) plays a profound role in supporting processes on the surface and in the atmosphere of Pluto, yet the origin of Pluto's N2 remains a mystery. However, this may begin to change ...as the 14N/15N ratio of N2 was recently estimated based on a non-detection of HC15N in Pluto's atmosphere, while accounting for 14N/15N fractionation between HCN and N2 using a photochemical model. Here, I show that, if this latter step of translating isotope ratios is adequately understood, then the derived 14N/15N ratio represents the first distinguishing constraint on the origin of Pluto's N2. One notable finding of the present study is that isotopic fractionation between atmospheric N2 and N2-rich ices on the surface of Pluto does not appear to be significant. I infer a lower limit of ∼197 for the 14N/15N ratio of the dominant (solid) reservoir of N2 on Pluto; i.e., mostly contained in Sputnik Planitia. From this lower limit, an endmember ammonia source of Pluto's N2 can be ruled out. I perform N isotope mixing calculations that enable quantitative understanding of the relationships between contributions by primordial N2, NH3, and nitrogen originally sourced in organic materials (Norg) to Pluto's observed N2 inventory. These calculations also address how uncertainties in the isotopic composition of Norg and the history of atmospheric escape affect the allowed ranges of primordial N2, NH3, and Norg contributions. While present uncertainties are substantial, I find that a contribution by primordial N2, Norg, or both is implied, and the sum of their contributions should be at least ∼45%. Hence, it is likely that Pluto formed from building blocks that were cold enough to trap N2 (e.g., <30 K), or Pluto has a thermally processed and dynamic interior that supports generation of N2 from Norg (at temperatures above ∼350 °C) and N2 transport to the surface. Furthermore, the lower limit on 14N/15N suggests that NH3 has been a less significant contributor to the origin of N2 on Pluto than on Titan, which is indicative of a key difference in the origin and evolution of these worlds. Recommendations are given for future work that can continue to advance and contextualize understanding of the origin of Pluto's N2. A new mission that can determine the origin of N2 on Neptune's moon Triton should be a priority. Such a mission would offer an unprecedented opportunity for comparison of volatile origins on large Kuiper belt objects (past or present) with distinct histories.
•Determining the origin of N2 is fundamental to understanding the origin and evolution of Pluto.•The ratio of 14N/15N isotopes in N2 can serve as an important constraint on the source(s) of N2.•The present lower limit on the 14N/15N ratio requires significant contributions from a primordial N2 or an organic N source.•Pluto should have formed from cold building blocks, or Pluto's core would have needed to be hot and geologically active.•A relatively high 14N/15N ratio in Pluto's N2 suggests a diminished role of NH3 as a N2 source on Pluto compared with Titan.
The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a 'fossil' record of the chemical composition of the initial protoplanetary disk. Metal-rich ...stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA's Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.
Phosphorus is often present in meteorites as the mineral schreibersite, in which P is in a reduced oxidation state as a phosphide. Phosphides such as schreibersite have been proposed to be important ...to the development of life on the earth and may serve as indicators of metamorphic grade on meteorite parent bodies. Here we investigate how synthetic schreibersite (as the iron end-member, Fe3P) oxidizes into calcium phosphates through reaction with silicates under high temperature conditions, at specific oxygen fugacities, and in the absence of water. We find that schreibersite readily oxidizes to phosphates at temperatures of 750–850 °C over a few weeks depending on the oxygen fugacity of the environment. The rate of this process is best matched by diffusion-limited kinetics. Therefore, the metamorphic heating timescale required to equilibrate phosphorus in meteoritic samples with small schreibersite grains (∼1 μm), such as in the type 3 ordinary chondrites (3.0–3.3), was short (10–100 days).
Determining the source(s) of hydrogen, carbon, and nitrogen accreted by Earth is important for understanding the origins of water and life and for constraining dynamical processes that operated ...during planet formation. Chondritic meteorites are asteroidal fragments that retain records of the first few million years of solar system history. The deuterium/hydrogen (D/H) values of water in carbonaceous chondrites are distinct from those in comets and Saturn's moon Enceladus, implying that they formed in a different region of the solar system, contrary to predictions of recent dynamical models. The D/H values of water in carbonaceous chondrites also argue against an influx of water ice from the outer solar system, which has been invoked to explain the nonsolar oxygen isotopic composition of the inner solar system. The bulk hydrogen and nitrogen isotopic compositions of CI chondrites suggest that they were the principal source of Earth's volatiles.
► We present new kinetics data on the radiolytic destruction of amino acids at 15, 100, and 140K. ► The amino acids glycine, alanine, and phenylalanine were studied. ► Destruction of amino acids was ...measured in situ with infrared spectroscopy. ► The half-lives of amino acids were estimated for various extraterrestrial environments.
We present new kinetics data on the radiolytic destruction of amino acids measured in situ with infrared spectroscopy. Samples were irradiated at 15, 100, and 140K with 0.8-MeV protons, and amino-acid decay was followed at each temperature with and without H2O present. Observed radiation products included CO2 and amines, consistent with amino-acid decarboxylation. The half-lives of glycine, alanine, and phenylalanine were estimated for various extraterrestrial environments. Infrared spectral changes demonstrated the conversion from the non-zwitterion structure NH2CH2(R)COOH at 15K to the zwitterion structure +NH3CH2(R)COO− at 140K for each amino acid studied.
The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the ...lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO2, H2O, F, S and Cl) contents of the most primitive basalts in the Moon-the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.
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