ALBERT

Description: Establishing the abundance and physical properties of regolith and boulders on asteroids is crucial for understanding the formation and degradation mechanisms at work on their surfaces. Using images and thermal data from NASA's Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft, we show that asteroid (101955) Bennu's surface is globally rough, dense with boulders, and low in albedo. The number of boulders is surprising given Bennu's moderate thermal inertia, suggesting that simple models linking thermal inertia to particle size do not adequately capture the complexity relating these properties. At the same time, we find evidence for a wide range of particle sizes with distinct albedo characteristics. Our findings imply that ages of Bennu's surface particles span from the disruption of the asteroid's parent body (boulders) to recent in situ production (micrometre-scale particles).

Description: Active asteroids are those that show evidence of ongoing mass loss. We report repeated instances of particle ejection from the surface of (101955) Bennu, demonstrating that it is an active asteroid. The ejection events were imaged by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and SecurityRegolith Explorer) spacecraft. For the three largest observed events, we estimated the ejected particle velocities and sizes, event times, source regions, and energies. We also determined the trajectories and photometric properties of several gravitationally bound particles that orbited temporarily in the Bennu environment. We consider multiple hypotheses for the mechanisms that lead to particle ejection for the largest events, including rotational disruption, electrostatic lofting, ice sublimation, phyllosilicate dehydration, meteoroid impacts, thermal stress fracturing, and secondary impacts.

Description: Visible/near-infrared (VNIR) reflectance spectra of both Mars [1] and the Moon [2] include hydration bands that vary across the planet and are not well explained in some cases. Poorly crystalline phases have been found at ~30-70 wt.% by CheMin in Gale crater, Mars in all samples measured to date [3]. Here we report on VNIR reflectance spectra of a large collection of amorphous and poorly crystalline materials. These include opal, allophane, imogolite, iron hydroxides/ oxyhydroxides (FeOx), and several synthetic materials containing Si, Al and/or Fe. All of these contain hydration bands due to water and OH that can be used to identify these materials remotely on planetary bodies.

Description: Mars has a sedimentary history that spans billions of years. Orbital images have allowed for the identification of vast regional sedimentary deposits that can be traced over 100s of kilometers and are 100s of meters thick including localized alluvial, deltaic, and lacustrine deposits. Detections of secondary minerals in these deposits from orbital spectroscopy suggest the aqueous history of early Mars varied as a function of space and time. Orbital observations, however, provide a simplified and incomplete picture of Mars sedimentary history because measurements for inferring sediment transport and deposition, such as lithology, grain size, and internal structures, and measurements for inferring sediment source and aqueous alteration, such as outcrop-scale mineralogic and geochemical composition and diagenetic features, cannot be identified from orbit. Rover observations have significantly enhanced our view of ancient and modern sedimentary environments on Mars, resulting in detailed reconstructions of paleo-environments and habitability.

Description: Microbial contamination is of particular interest to geological curation as many microorganisms can change mineral composition and produce compounds used as biosignatures used for the detection of life. Microbial cells can change the mineral composition of rocks through organic acid production and direct enzymatic oxidation/reduction of transition metals. Enzymatic oxidation of iron and manganese can occur at a rate several orders of magnitude faster than under abiotic conditions and produce highly reactive nanoparticle- sized oxides that can react and sorb other metals and organic compounds. Many fungi can also produce organic acids that dissolve and chelate mineral matrices chemically reducing and dissolving rock surfaces. Finally, several common soil-associated bacteria and fungi produce secondary metabolites that contain unusual amino acid analogs and non-ribosomal peptides containing both L- and D- chirality used in characterizing carbonaceous chondrites and the detection of extraterrestrial life.

Description: Comets are time capsules from the birth of our Solar System that record pre-solar history, the initial stages of planet formation, and the sources of prebiotic organics and volatiles for the origin of life. These capsules can only be opened in laboratories on Earth. CAESAR (Comet Astrobiology Exploration Sample Return)s sample analysis objectives are to understand the nature of Solar System starting materials and how these components came together to form planets and give rise to life. Examination of these comet nucleus surface samples in laboratories around the world will also provide ground truth to remote observations of the innumerable icy bodies of the Solar System.

Description: Current sheets (CSs) play a crucial role in the storage and conversion of magnetic energy in planetary magnetotails. Using highresolution magnetic field data from MAVEN spacecraft, we report the existence of super thin current sheets (STCSs) in the Martian magnetotail. The typical halfthickness of the STCSs is ~5 km, and it is much less than the gyroradius of thermal protons (p). The STCSs are embedded into a thicker sheet with L p forming a multiscale current configuration. The formation of STCS does not depend on ion composition, but it is controlled by the small value of the normal component of the magnetic field at the neutral plane (BN). A number of the observed multiscale CSs are located in the parametric map close to the tearingunstable domain, and thus, the inner STCS can provide an additional free energy to excite ion tearing mode in the Martian magnetotail.

Description: The habitable zone (HZ) is commonly defined as the range of distances from a host star within which liquid water, a key requirement for life, may exist on a planet's surface. Substantially more CO2 than present in Earth's modern atmosphere is required to maintain clement temperatures for most of the HZ, with several bars required at the outer edge. However, most complex aerobic life on Earth is limited by CO2 concentrations of just fractions of a bar. At the same time, most exoplanets in the traditional HZ reside in proximity to M dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to promote greater abundances of gases that can be toxic in the atmospheres of orbiting planets, such as carbon monoxide (CO). Here we show that the HZ for complex aerobic life is likely limited relative to that for microbial life. We use a 1D radiative-convective climate and photochemical models to circumscribe a Habitable Zone for Complex Life (HZCL) based on known toxicity limits for a range of organisms as a proof of concept. We find that for CO2 tolerances of 0.01, 0.1, and 1 bar, the HZCL is only 21%, 32%, and 50% as wide as the conventional HZ for a Sun-like star, and that CO concentrations may limit some complex life throughout the entire HZ of the coolest M dwarfs. These results cast new light on the likely distribution of complex life in the universe and have important ramifications for the search for exoplanet biosignatures and technosignatures.

Description: The Compact Reconnaissance Imaging Spectral Mapper (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) obtains pole-to-pole observations (i.e., full MRO orbits) of vertical profiles for visible/near-IR spectra (=0.44.0 m), which are ideally suited to identifying the composition and particle sizes of Mars ice and dust aerosols over 50100 km altitudes in the Mars mesosphere. Within the coverage limitations of the CRISM limb data set, a distinct compositional dichotomy is found in Mars mesospheric ice aerosols. CO2 ice clouds appear during the aphelion period of Mars orbit (Solar Longitudes, Ls0160) at low latitudes (20S10N) over specific longitude regions (Meridiani, Valles Marineris) and at typical altitudes of 5575 km. Apart from faint water ice hazes below 55 km, mesospheric H2O ice clouds are primarily restricted to the perihelion orbital range (Ls160 350) at northern and southern mid-to-low latitudes with less apparent longitudinal dependences. Mars mesospheric CO2 clouds are presented in CRISM spectra with a surprisingly large range of particle sizes (cross section weighted radii, Reff=0.3 to 2.2 m). The smaller particle sizes (Reff 1 m) appear concentrated near the spatial (latitude and altitude) boundaries of their global occurrences. CRISM spectra of mesospheric CO2 clouds also show evidence of iridescence, indicating very narrow particle size distributions (effective variance, Veff0.03) and so very abrupt CO2 cloud nucleation. Furthermore, these clouds are sometimes accompanied by altitude coincident peaks in 1.27 m O2 dayglow, which indicates very dry, cold regions of formation. Mesospheric water ice clouds generally exhibit small particle sizes (Reff=0.10.3 m), although larger particle sizes (Reff=0.40.7 m) appear infrequently. On average, water ice cloud particle sizes decrease with altitude over 5080 km in the perihelion mesosphere. Water ice mass appears similar in clouds over a large range of observed cloud particle sizes, with particle number densities increasing to 10 cm3 for Reff=0.2 m. Near coincident Mars Climate Sounder (MCS) temperature and aerosol profile measurements for a subset of CRISM mesospheric aerosol measurements indicate near saturation (H2O and CO2) conditions for ice clouds and distinct mesospheric temperature increases associated with mesospheric dust loading. Dayside (3 pm) mesospheric CO2 clouds with larger particle sizes (Reff 0.5 m) scatter surface infrared emission in MCS limb infrared radiances, as well as solar irradiance in the MCS solar band channel. Scattering of surface infrared emission is most strikingly presented in nighttime (3 am) MCS observations at 5560 km altitudes, indicating extensive mesospheric nighttime CO2 clouds with considerably larger particle sizes (Reff7 m). Mesospheric CO2 ice clouds present cirrus-like waveforms over extensive latitude and longitude regions (1010), as revealed in coincident Mars Color Imager (MARCI) nadir imaging. Solar tides, gravity waves, and the large orbital variation of the extended thermal structure of the Mars atmosphere influence all of these behaviors. Mesospheric dust aerosols appear infrequently over the non-global (planet encircling) dust storm era of the CRISM limb data set (20092016), and exhibit smaller particle sizes (Reff=0.20.7 m) relative to dust in the lower atmosphere. One isolated case of an aphelion (Ls=96) mesospheric dust layer with large dust particle sizes (Reff 2 m) over Syria Planum may reflect high altitude, non-local transport of dust over elevated regions.

Description: Pluto was discovered in 1930 at Lowell Observatory in the belated resumption of a wide-field photographic search originally be-gun at Percival Lowells direction prior to his death in 1916. Photometry in the 1950s established the rotation period of 6.4 hours and a color redder than the Sun, but the mass, density, size and albedo were unknown. Near-infrared photometry in 1976 indicated the presence of CH4 frost, suggestive of a relatively high surface albedo and a diameter comparable to the Moon. The large satellite Charon was discovered in 1978, followed by an epoch of mutual transits and occultations of Pluto and Charon from 1985 to 1990, as viewed from Earth. These events resulted in reliable sizes and masses of the two bodies, as well as the orbit of Charon. The mutual events also demonstrated that Pluto and Charon are in locked synchronous rotation and revolution, a configuration unique among the planets. The atmosphere of Pluto was discovered in 1988 from a stellar occultation observed from the Kuiper Airborne Observatory and ground stations, with indications of a haze layer (or a temperature inversion) in the lower atmosphere. Sub-sequent stellar occultations showed that the extent of the atmosphere is variable on a timescale of a few years. The spectroscopic detection of N2 and CO ice in 1993 demonstrated that the atmosphere must be primarily composed of N2, with CH4 and CO as minor components; the spectroscopic detection of gaseous CH4 was reported in 1994.