In situ sampling missions to detect biosignatures on ocean worlds requires thorough sample preparation to manage the expected chemical complexity of such environments. Proposed instruments must be capable of automatic liquid sample handling to ensure sensitive and accurate detections of biosignatures, regardless of the initial chemical composition. Herein, we outline the design, build, and test of the integrated Biosignature Preparation for Ocean Worlds (BioPOW) system capable of purifying amino acids from icy samples. This four step modular instrument 1) melts ice samples, 2) purifies amino acids via cation exchange chromatography, 3) concentrates via vacuum drying, and 4) derivatizes amino acids to volatilize and enable detection with downstream analytical instruments. Initial experiments validated the thermal performance of the system by melting ice in the sample cup (1 mL sample, 3°C–28°C, <5 min, 1.4 kJ) and heating the derivatization tank past the concentration temperature (20°C–60°C, 12 min, 3.6 kJ) to the derivatization temperature (60°C–90°C, 25 min, 7.5 kJ). Later experiments investigated important factors for automatic cation exchange using a design of experiments approach, and found that initial salt concentration, sample and eluate flow rates, and water wash volumes all play significant roles in reducing conductivity (1.1 x–6.7 x) while maintaining phenylalanine yields between 31% and 94%. The modules were then integrated into a 12 cm × 20 cm × 20 cm fieldable platform for analysis, and the maturation of this design for future spaceflight is discussed.

The surface strength of small rubble-pile asteroids, which are aggregates of unconsolidated material under microgravity, is poorly constrained but critical to understanding surface evolution and geologic history of the asteroid. Here we use images of an impact ejecta deposit and downslope avalanche adjacent to a 70-m-diameter impact crater on the rubble-pile asteroid (101955) Bennu to constrain the asteroid’s surface properties. We infer that the ejecta deposited near the crater must have been mobilized with velocities less than Bennu’s escape velocity (20 cm s –1 ); such low velocities can be explained only if the effective strength of the local surface is exceedingly low, nominally ≤2 Pa. This value is four orders of magnitude below strength values commonly used for asteroid surfaces, but it is consistent with recent estimates of internal strength of rubble-pile asteroids and with the surface strength of another rubble-pile asteroid, Ryugu. We find a downslope avalanche indicating a surface composed of material readily mobilized by impacts and that has probably been renewed multiple times since Bennu’s initial assembly. Compared with stronger surfaces, very weak surfaces imply (1) more retention of material because of the low ejecta velocities and (2) lower crater-based age estimates—although the heterogeneous structure of rubble piles complicates interpretation.

The surface of the rubble‐pile asteroid (101955) Bennu has been characterized in detail by the OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission. By examining global and local digital terrain models, we observed that Bennu possesses terraces, that is, a series of roughly latitude‐parallel, step‐like slope breaks. These partially circumscribe the poles and extend east‐west over several longitudinal quadrants at mid‐ to high (≥30°) latitudes. The terraces are subtle in amplitude, with heights ranging from 1 to 5 m. They often exhibit back‐wasting that results in V‐shaped scarps that open downslope in some locations. When boulders >5–10 m are absent at or near a terrace, the steeper portion (the drop) of the terrace lacks rocks, whereas the flatter portion (the bench) of the terrace has accumulations of rocks at its crest. When boulders >5–10 m are present, their steep downslope faces often make up the drop from the terrace crest, and they retain debris upslope, thereby enhancing the terrace structure. A geotechnical stability analysis indicates that Bennu's surface is likely unstable and that surface cohesion is <0.6 Pa. Bennu's terraces strongly resemble scarps generated in laboratory and numerical simulations of a cohesionless granular bed as the slope of the bed increases quasi‐statically. We conclude that terraces are probably actively forming on Bennu as its surface slowly fails owing to creep induced by spin acceleration.

Global geologic maps are useful tools for efficient interpretation of a planetary body, and they provide global context for the diversity and evolution of the surface. We used data acquired by the OSIRIS-REx spacecraft to create the first global geologic map of the near-Earth asteroid (101955) Bennu. As this is the first geologic map of a small, non-spherical, rubble-pile asteroid, we discuss the distinctive mapping challenges and best practices that may be useful for future exploration of similar asteroids, such as those to be visited with the Hera and Janus missions. By mapping on two centimeter-scale global image mosaics (2D projected space) and a centimeter-scale global shape model (3D space), we generated three input maps respectively describing Bennu's shape features, geologic features, and surface texture. Based on these input maps, we defined two geologic units: the Smooth Unit and the Rugged Unit. The units are differentiated primarily on the basis of surface texture, concentrations of boulders, and the distributions of lineaments, mass movement features, and craters. They are bounded by several scarps. The Rugged Unit contains abundant boulders and signs of recent mass movement. It also has fewer small (<20 m), putatively fresh craters than the Smooth Unit, suggesting that such craters have been erased in the former. Based on these geologic indicators, we infer that the Rugged Unit has the younger surface of the two. Differential crater sizefrequency distributions and the distribution of the freshest craters suggest that both unit surfaces formed -10-65 million years ago, when Bennu was located in the Main Asteroid Belt, and the Smooth Unit has not been significantly resurfaced in the past 2 million years. Meanwhile, the Rugged Unit has experienced resurfacing within the past -500,000 years during Bennu's lifetime as a near-Earth asteroid. The geologic units are consistent with global diversity in slope, surface roughness, normal albedo, and thermal emission spectral characteristics. The site on Bennu where the OSIRIS-REx mission collected a regolith sample is located in the Smooth Unit, in a small crater nested within a larger one. So although the Smooth Unit is an older surface than the Rugged Unit, the impact-crater setting indicates that the material sampled was recently exposed. Several similarities are apparent between Bennu and asteroid (162173) Ryugu from a global geologic perspective, including two geologic units distinguishable by variations in the number density of boulders, as well as in other datasets such as brightness.

The impactor-to-crater size scaling relationships that enable estimates of planetary surface ages rely on an accurate formulation of impactor–target physics. An armouring regime, specific to rubble-pile surfaces, has been proposed to occur when an impactor is comparable in diameter to a target surface particle (for example, a boulder). Armouring is proposed to reduce crater diameter, or prevent crater formation in the asteroid surface, at small crater diameters. Here, using measurements of 1,560 craters on the rubble-pile asteroid (101955) Bennu, we show that the boulder population controls a transition from crater formation to armouring at crater diameters ~2–3 m, below which crater formation in the bulk surface is increasingly rare. By combining estimates of impactor flux with the armouring scaling relationship, we find that Bennu’s crater retention age (surface age derived from crater abundance) spans from 1.6–2.2 Myr for craters less than a few meters to ~10–65 Myr for craters >100 m in diameter, reducing the maximum surface age by a factor of >15 relative to previous estimates. The range of crater retention ages, together with latitudinal variations in large-crater spatial density, indicate that ongoing resurfacing processes render the surface many times younger than the bulk asteroid.

Persephone is a NASA concept mission study that addresses key questions raised by New Horizons' encounters with Kuiper Belt objects (KBOs), with arguably the most important being "Does Pluto have a subsurface ocean?". More broadly, Persephone would answer four significant science questions: (1) What are the internal structures of Pluto and Charon? (2) How have the surfaces and atmospheres in the Pluto system evolved? (3) How has the KBO population evolved? (4) What are the particles and magnetic field environments of the Kuiper Belt? To answer these questions, Persephone has a comprehensive payload, and would both orbit within the Pluto system and encounter other KBOs. The nominal mission is 30.7 years long, with launch in 2031 on a Space Launch System (SLS) Block 2 rocket with a Centaur kick stage, followed by a 27.6 year cruise powered by existing radioisotope electric propulsion (REP) and a Jupiter gravity assist to reach Pluto in 2058. En route to Pluto, Persephone would have one 50- to 100-km-class KBO encounter before starting a 3.1 Earth-year orbital campaign of the Pluto system. The mission also includes the potential for an 8-year extended mission, which would enable the exploration of another KBO in the 100- to 150-km-size class. The mission payload includes 11 instruments: Panchromatic and Color High-Resolution Imager; Low-Light Camera; Ultra-Violet Spectrometer; Near-Infrared (IR) Spectrometer; Thermal IR Camera; Radio Frequency Spectrometer; Mass Spectrometer; Altimeter; Sounding Radar; Magnetometer; and Plasma Spectrometer. The nominal cost of this mission is 3.0B, making it a large strategic science mission.

Images of asteroids (162173) Ryugu and (101955) Bennu acquired by the Hayabusa2 and OSIRIS-REx missions, respectively, reveal rocky worlds covered in rubble. These two asteroids do not have hydrostatic shapes, indicating that they possess some internal friction and/or cohesion even if they lack tensile strength. Understanding the deviation of the surfaces of these bodies from those of idealized shapes helps constrain the mechanical properties of their interiors. Here, we focus on the feedback between YORP-induced spin-up (in which asymmetric reflection and re-emission of solar radiation from the surface systematically change the rotation rate), long-wavelength topography (which provides a structure to control the orientation), and surface roughness on Ryugu and Bennu. By performing spherical harmonic analyses of the shapes of these two asteroids, we find that although they are superficially similar, they exhibit different long-wavelength topography that implies different internal structures and rotational histories. Bennu's shape and rotation rate require a modest amount of internal strength through some combination of at least 17° of internal friction or a few Pa of cohesion, whereas Ryugu could be nearly strengthless. Bennu's longitudinal ridges make it susceptible to YORP spin-up, consistent with the observed increase in rotation rate that is not seen on Ryugu. These longitudinal ridges also suggest a heterogeneous density structure for Bennu, consistent with gravity data.

GEOLOGIC MAP. Erica R. Jawin1, T. J. McCoy1, K. J. Walsh2, H. C. Connolly Jr.3,4, R.-L. Ballouz4, A. J. Ryan4, M. Pajola5, O. S. Barnouin6, H. H. Kaplan7, V. E. Hamilton2, J. P. Emery8, D. N. DellaGiustina4, D. J. Scheeres9, M. G. Daly10, C. A. Bennett4, D. R. Golish4, M. Perry6, R. T. Daly6, E. B. Bierhaus11, M. C. Nolan4, H. L. Enos4, and D. S. Lauretta4, Smithsonian National Museum of Natural History, Washington, DC, USA (jawine@si.edu), 2Southwest Research Institute, Boulder, CO, USA, 3Department of Geology, Rowan University, Glassboro, NJ, USA, 4Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA, 5INAF-Astronomical Observatory of Padova, Padova, Italy, 6Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, 7Goddard Space Flight Research Center, Greenbelt, MD, USA, 8Northern Arizona University, Flagstaff, AZ, USA, 9University of Colorado, Boulder, CO, USA, 10Centre for Research in Earth and Space Science, York University, Toronto, Canada, 11Lockheed Martin Space, Littleton, CO, USA.

NASA’s OSIRIS-REx mission to asteroid (101955) Bennu relied on the production of real-time shape models for both spacecraft navigation and scientific analysis. The primary method of constructing shape models during the early phases of the mission was image-based stereophotoclinometry (SPC). The SPC shape models were used for operational planning, navigation, sample site selection, and initial scientific investigations. To this end, detailed analyses of the quality of each shape model and a thorough documentation of all sources of error were vital to ensure proper considerations of the limitations of each model. In this paper, we present methods used during the OSIRIS-REx mission to validate the SPC shape models and construct the associated quality reports. Although developed for the OSIRIS-REx mission, these validation techniques can be applied to SPC-derived shape models of other planetary bodies.

Here we highlight recent advances in our knowledge about Saturn's ring system and bring forward the outstanding science issues that could be addressed by studying the ring systems of the ice giants.We focus on interactions between planetary rings and other elements in the system, including the moons, host planet, and its magnetosphere, and conclude that ring science investigations, in accordance with magnetospheric and atmospheric science disciplines, are essential in advancing our knowledge of solar system evolution, the origin and evolution of the moons and Ocean Worlds, as well as contemporary phenomena observed in the ice giant systems.We request that the study of ice giant ring systems to be considered a top priority for all future ice giant explorations.

Bierhaus2, O. S. Barnouin1, M. G. Daly3, J. A. Seabrook3, J. H. Roberts1, C. M. Ernst1, M. E. Perry1, H. Nair1, R. C. Espiritu1, E. E. Palmer4, R. W. Gaskell4, J. R. Weirich4, H. C. M. Susorney5, C. L. Johnson5,6, K. J. Walsh7, M. C. Nolan8, E. R. Jawin9, P. Michel10, D. Trang11, and D. S. Lauretta8. 1Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA (terik.daly@jhuapl.edu); 2Lockheed Martin Space, Littleton, CO, USA; 3Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada; 4Planetary Science Institute, Tucson, AZ, USA; 5School of Earth Sciences, University of Bristol, Bristol, UK; 6Dept. of Earth, Ocean & Atmospheric Sciences, University of British Columbia, Vancouver, Canada; 7Southwest Research Institute, Boulder, CO, USA; 8Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA; 9Smithsonian Institution National Museum of Natural History, Washington, D.C., USA; 10Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France; 11University of Hawai’i at Mānoa, Honolulu, HI, USA.

A planetary surface’s resistance to change is generally described as its “strength” (units of stress). The surface strength of small, rubble-pile asteroids, which consist of fragments of larger bodies that were collisionally disrupted, is poorly constrained due to their wide departure from terrestrial analogs. Here, we report the observation of an ejecta deposit surrounding an impact crater that limits the maximum surface strength of the near-Earth rubble-pile asteroid (101955) Bennu. The presence of this deposit implies that ejecta were mobilized with velocities less than the escape velocity of Bennu, 20 cm/s. Because ejecta velocities increase with surface strength, the ejecta deposit can only be explained if the effective strength of the surface material near the crater is exceedingly low, ≤100 Pa. This is three orders of magnitude below values commonly used for asteroid surfaces, but is supported by previous observations of an artificial impact crater on a similar asteroid, Ryugu. Our findings indicate a mobile surface that has likely been renewed multiple times since Bennu’s initial assembly and have far-reaching implications for interpreting observations of Bennu and other rubble piles.

Astrobiology is an exciting field of science focused on understanding the origins, evolution, distribution, and future of life in the universe. NASA focuses much of its research and technology developments on astrobiology, and the Johns Hopkins University Applied Physics Laboratory (APL) is a major contributor through research, technology, and missions. Astrobiology efforts at APL range from constraining when life first emerged on Earth and researching biosignature (i.e., signals of past or present life) preservation, to developing instruments and missions aiming to detect biosignatures and characterize the capability of an extreme planetary environment to harbor and support life. Beginning with APL's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO), which searches for past wet and potentially habitable regions on Mars, APL has continued to develop cutting-edge techniques and instruments to search for biosignatures, remotely and in situ. Additionally, APL is leading and serving as a key partner in several exciting NASA missions that will occur in the coming decades with habitability and biosignature detection goals. In this article, we summarize current efforts and look forward, over the coming 25 years, to the potential astrobiology exploration and discoveries that await.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

R.T. Daly1, E.B. Bierhaus2, O.S. Barnouin1, M.G. Daly3, J. Seabrook3, J.H. Roberts1, C.M. Ernst1, M.E. Perry1, E.E. Palmer4, R.W. Gaskell4, J.R. Weirich4, H.C.M. Susorney5, C.L. Johnson4,5, K.J. Walsh6, M.C. Nolan7, E.R. Jawin8, P. Michel9, D. Trang10, and D.S. Lauretta7. 1Johns Hopkins Applied Physics Laboratory, Laurel, MD (terik.daly@jhuapl.edu); 2Lockheed Martin Space, Littleton, CO; 3Centre for Research in Earth and Space Science, York Univ., Toronto, Ontario, Canada; 4Planetary Science Institute, Tucson, AZ; 5Dept. of Earth, Ocean & Atmospheric Sciences, Univ. of British Columbia, Vancouver, Canada; 6Southwest Research Institute, Boulder, CO; 7Lunar and Planetary Laboratory, Univ. of Arizona, Tucson, AZ; 8Smithsonian National Museum of Natural History, Washington, D.C.; 9Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France; 10University of Hawa’ii, Honolulu, HI.

We investigate the shape of near-Earth asteroid (101955) Bennu by constructing a high-resolution (20 cm) global digital terrain model from laser altimeter data. By modeling the northern and southern hemispheres separately, we find that longitudinal ridges previously identified in the north extend into the south but are obscured there by surface material. In the south, more numerous large boulders effectively retain surface materials and imply a higher average strength at depth to support them. The north has fewer large boulders and more evidence of boulder dynamics (toppling and downslope movement) and surface flow. These factors result in Bennu's southern hemisphere being rounder and smoother, whereas its northern hemisphere has higher slopes and a less regular shape. We infer an originally asymmetric distribution of large boulders followed by a partial disruption, leading to wedge formation in Bennu's history.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Enceladus' plume is the dominant source of neutrals and plasma in Saturn's magnetosphere. The plasma results from the ionization of icy particles and water vapor, which are vented into Saturn's inner magnetosphere through fissures in Enceladus' southern polar region. These fissures are subjected to tidal stresses that can vary as Enceladus moves in a slightly eccentric orbit around Saturn. Plume activity and brightness have also been shown to vary with the moon's orbital position, reaching a maximum when Enceladus is farthest away from Saturn in its orbit (the Enceladus orbital apoapsis). In this paper we will show that temporal variations in the thermal electron density distribution correlate with the position of Enceladus in its orbit around Saturn, with the strongest density enhancements in the vicinity of Enceladus when the moon is in the post‐apoapsis sector of its orbit.

Bennu is an similar to 500-m-diameter rubble-pile asteroid that is the target of detailed study by the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission. Here we use data from the OSIRIS-REx Laser Altimeter to assess depth-to-diameter ratios (d/D) of 108 impact craters larger than 10 m in diameter. The d/D of craters on Bennu ranges from 0.02 to 0.19. The mean is 0.10 +/- 0.03. The smallest craters show the broadest range in d/D, consistent with d/D measurements on other asteroids. A few craters have central mounds, which is interpreted as evidence that a more competent substrate lies a few meters beneath them. The range of d/D narrows as crater size increases, with craters larger than 80 m tending toward smaller d/D. At large scales, increases in target strength with depth, combined with target curvature, may affect crater morphometry. Plain Language Summary Between 2018 and 2020, National Aeronautics and Space Administration (NASA)'s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft orbited a small asteroid called Bennu in preparation to collect a sample for return to Earth. Bennu is a "rubble-pile" asteroid, meaning an aggregate of rock fragments that have coalesced together in space. OSIRIS-REx observations showed that Bennu has many craters on its surface, which formed when other, smaller objects collided with it in the past. Crater depths and widths (diameters), in addition to relating to the size and speed of the impacting object, also reflect the physical characteristics of the impacted surface. Accordingly, we measured the depths and diameters of many of Bennu's craters to better understand the surface and interior properties of this rubble-pile asteroid and how it compares to other asteroids. The smaller craters on Bennu have a variety of depths, even among similarly sized craters. The largest are so wide that they appear to be affected by the curvature of Bennu's surface and by the presence of stronger material at depth. We observe mounds inside some of the smaller craters, supporting the idea that a more competent substrate underlies the surface material.