A bimodal distribution of haze in Pluto’s atmosphere

Pluto, Titan, and Triton make up a unique class of solar system bodies, with icy surfaces and chemically reducing atmospheres rich in organic photochemistry and haze formation. Hazes play important roles in these atmospheres, with physical and chemical processes highly dependent on particle sizes, but the haze size distribution in reducing atmospheres is currently poorly understood. Here we report observational evidence that Pluto’s haze particles are bimodally distributed, which successfully reproduces the full phase scattering observations from New Horizons. Combined with previous simulations of Titan’s haze, this result suggests that haze particles in reducing atmospheres undergo rapid shape change near pressure levels ~0.5 Pa and favors a photochemical rather than a dynamical origin for the formation of Titan’s detached haze. It also demonstrates that both oxidizing and reducing atmospheres can produce multi-modal hazes, and encourages reanalysis of observations of hazes on Titan and Triton.

Menura: a code for simulating the interaction between a turbulent solar wind and solar system bodies

Despite the close relationship between planetary science and plasma physics, few advanced numerical tools allow to bridge the two topics. The code Menura proposes a breakthrough towards the self-consistent modelling of these overlapping field, in a novel 2-step approach allowing for the global simulation of the interaction between a fully turbulent solar wind and various bodies of the solar system. This article introduces the new code and its 2-step global algorithm, illustrated by a first example: the interaction between a turbulent solar wind and a comet.

Coherent backscatter enhancement in bistatic Ku-/X-band radar observations of dry snow

The coherent backscatter opposition effect (CBOE) enhances the backscatter intensity of electromagnetic waves by up to a factor of two in a very narrow cone around the direct return direction when multiple scattering occurs in a weakly absorbing, disordered medium. So far, this effect has not been investigated in terrestrial snow in the microwave spectrum. It has also received little attention in scattering models. We present the first characterization of the CBOE in dry snow using ground-based and space-borne bistatic radar systems. For a seasonal snow pack in Ku-band (17.2 GHz), we found backscatter enhancement of 50–60 % (+1.8–2.0 dB) at zero bistatic angle and a peak half-width-at-half-maximum (HWHM) of 0.25°. In X-band (9.65 GHz), we found backscatter enhancement of at least 35 % (+1.3 dB) and an estimated HWHM of 0.12° in the accumulation areas of glaciers in the Jungfrau-Aletsch region, Switzerland. Sampling of the peak shape at different bistatic angles allows estimating the scattering and absorption mean free paths, ΛT and ΛA. In the VV polarization, we obtained ΛT = 0.4 ± 0.1 m and ΛA = 19 ± 12 m at Ku-band, and ΛT = 2.1 ± 0.4 m, ΛA = 21.8 ± 2.7 m at X-band. The HH polarization yielded similar results. The observed backscatter enhancement is thus significant enough to require consideration in backscatter models describing monostatic and bistatic radar experiments. Enhanced backscattering beyond the Earth, on the surface of solar system bodies, has been interpreted as being caused by the presence of water ice. In agreement with this interpretation, our results confirm the presence of the CBOE at X- and Ku-band frequencies in terrestrial snow.

The Infrared Complex Refractive Index of Amorphous Ammonia Ice at 40 K (1.43–22.73 μm) and Its Relevance to Outer Solar System Bodies

A near-IR absorption band at 2.2 μm linked to ammonia-containing ice has been detected on icy bodies throughout the solar system and appears in the extensive volume of data for Pluto and Charon returned by New Horizons. This band is an important clue for understanding the abundance of ammonia and ammoniated compounds on the surface of outer solar system bodies and requires new laboratory data for its full analysis. To satisfy this data need, the complex refractive index of amorphous ammonia ice was calculated from experimental infrared transmission spectra with ice deposition and measurements conducted at 40 K, a characteristic surface temperature for outer solar system bodies. The measured imaginary part of the complex refractive index and associated band strength calculations are generally larger than prior published values for amorphous ammonia ice at 30 K. The complex refractive index for amorphous ammonia at 40 K computed in the mid-infrared region (2.5–22.73 μm) will also be valuable for interpreting observations of both solar system and astrophysical sources anticipated with the Near InfraRed Spectrograph and Mid-Infrared Instrument on the James Webb Space Telescope.

New Evidence for Wet Accretion of Inner Solar System Planetesimals from Meteorites Chelyabinsk and Benenitra

We investigated the hydrogen isotopic compositions and water contents of pyroxenes in two recent ordinary chondrite falls, namely, Chelyabinsk (2013 fall) and Benenitra (2018 fall), and compared them to three ordinary chondrite Antarctic finds, namely, Graves Nunataks GRA 06179, Larkman Nunatak LAR 12241, and Dominion Range DOM 10035. The pyroxene minerals in Benenitra and Chelyabinsk are hydrated (∼0.018–0.087 wt.% H2O) and show D-poor isotopic signatures (δDSMOW from −444‰ to −49‰). On the contrary, the ordinary chondrite finds exhibit evidence of terrestrial contamination with elevated water contents (∼0.039–0.174 wt.%) and δDSMOW values (from −199‰ to −14‰). We evaluated several small parent-body processes that are likely to alter the measured compositions in Benenitra and Chelyabinsk and inferred that water loss in S-type planetesimals is minimal during thermal metamorphism. Benenitra and Chelyabinsk hydrogen compositions reflect a mixed component of D-poor nebular hydrogen and water from the D-rich mesostases. A total of 45%–95% of water in the minerals characterized by low δDSMOW values was contributed by nebular hydrogen. S-type asteroids dominantly composed of nominally anhydrous minerals can hold 254–518 ppm of water. Addition of a nebular water component to nominally dry inner solar system bodies during accretion suggests a reduced need of volatile delivery to the terrestrial planets during late accretion.

Lunar-like silicate material forms the Earth quasi-satellite (469219) 2016 HO3 Kamoʻoalewa

Little is known about Earth quasi-satellites, a class of near-Earth small solar system bodies that orbit the sun but remain close to the Earth, because they are faint and difficult to observe. Here we use the Large Binocular Telescope (LBT) and the Lowell Discovery Telescope (LDT) to conduct a comprehensive physical characterization of quasi-satellite (469219) Kamoʻoalewa and assess its affinity with other groups of near-Earth objects. We find that (469219) Kamoʻoalewa rotates with a period of 28.3 (+1.8/−1.3) minutes and displays a reddened reflectance spectrum from 0.4–2.2 microns. This spectrum is indicative of a silicate-based composition, but with reddening beyond what is typically seen amongst asteroids in the inner solar system. We compare the spectrum to those of several material analogs and conclude that the best match is with lunar-like silicates. This interpretation implies extensive space weathering and raises the prospect that Kamo’oalewa could comprise lunar material.

Planetary Mass Spectrometry for Agnostic Life Detection in the Solar System

For the past fifty years of space exploration, mass spectrometry has provided unique chemical and physical insights on the characteristics of other planetary bodies in the Solar System. A variety of mass spectrometer types, including magnetic sector, quadrupole, time-of-flight, and ion trap, have and will continue to deepen our understanding of the formation and evolution of exploration targets like the surfaces and atmospheres of planets and their moons. An important impetus for the continuing exploration of Mars, Europa, Enceladus, Titan, and Venus involves assessing the habitability of solar system bodies and, ultimately, the search for life—a monumental effort that can be advanced by mass spectrometry. Modern flight-capable mass spectrometers, in combination with various sample processing, separation, and ionization techniques enable sensitive detection of chemical biosignatures. While our canonical knowledge of biosignatures is rooted in Terran-based examples, agnostic approaches in astrobiology can cast a wider net, to search for signs of life that may not be based on Terran-like biochemistry. Here, we delve into the search for extraterrestrial chemical and morphological biosignatures and examine several possible approaches to agnostic life detection using mass spectrometry. We discuss how future missions can help ensure that our search strategies are inclusive of unfamiliar life forms.