Search

We obtained New Horizons LORRI images to measure the cosmic optical background (COB) intensity integrated over $0.4\lesssim\lambda\lesssim0.9{~\rm\mu m}.$ The survey comprises 16 high Galactic-latitude fields selected to minimize scattered diffuse Galactic light (DGL) from the Milky Way galaxy, as well as scattered light from bright stars. This work supersedes an earlier analysis based on observations of one of the present fields. Isolating the COB contribution to the raw total sky levels measured in the fields requires subtracting the remaining scattered light from bright stars and galaxies, intensity from faint stars within the fields fainter than the photometric detection-limit, and the DGL foreground. DGL is estimated from Planck HFI $350 {~\rm\mu m}$ and $550 {~\rm\mu m}$ intensities, using a new self-calibrated indicator based on the 16 fields augmented with eight additional DGL calibration fields obtained as part of the survey. The survey yields a highly significant detection ($6.8\sigma$) of the COB at ${\rm 11.16\pm 1.65~(1.47~sys,~0.75~ran) ~nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $\mu$m. The estimated integrated intensity from background galaxies, ${\rm 8.17\pm 1.18 ~nW ~m^{-2} ~sr^{-1}},$ can account for the great majority of this signal. The rest of the COB signal, ${\rm 2.99\pm2.03~ (1.75~sys,~1.03~ran) ~nW ~m^{-2} ~sr^{-1}},$ is formally classified as anomalous intensity but is not significantly different from zero. The simplest interpretation is that the COB is completely due to galaxies., Comment: Accepted for publication in the Astrophysical Journal; 30 pages, 14 figures. v3 has updated version of fig 14 to show total error value for Symons et al. 2023 result

Modeling observations of the archetypal debris disk around $\beta$ Pic, obtained in 2023 January with the MIRI MRS on board JWST, reveals significant differences compared with that obtained with the IRS on board Spitzer. The bright 5 - 15 $\mu$m continuum excess modeled using a $\sim$600 K black body has disappeared. The previously prominent 18 and 23 $\mu$m crystalline forsterite emission features, arising from cold dust ($\sim$100 K) in the Rayleigh limit, have disappeared and been replaced by very weak features arising from the hotter 500 K dust population. Finally, the shape of the 10 $\mu$m silicate feature has changed, consistent with a shift in the temperature of the warm dust population from $\sim$300 K to $\sim$500 K and an increase in the crystalline fraction of the warm, silicate dust. Stellar radiation pressure may have blown both the hot and the cold crystalline dust particles observed in the Spitzer spectra out of the planetary system during the intervening 20 years between the Spitzer and JWST observations. These results indicate that the $\beta$ Pic system has a dynamic circumstellar environment, and that periods of enhanced collisions can create large clouds of dust that sweep through the planetary system., Comment: 15 pages, 8 figures, ApJ in press

We present JWST MIRI Medium Resolution Spectrograph (MRS) observations of the $\beta$ Pictoris system. We detect an infrared excess from the central unresolved point source from 5 to 7.5 $\mu$m which is indicative of dust within the inner $\sim$7 au of the system. We perform PSF subtraction on the MRS data cubes and detect a spatially resolved dust population emitting at 5 $\mu$m. This spatially resolved hot dust population is best explained if the dust grains are in the small grain limit (2$\pi$a$\ll$$\lambda$). The combination of unresolved and resolved dust at 5 $\mu$m could suggest that dust grains are being produced in the inner few au of the system and are then radiatively driven outwards, where the particles could accrete onto the known planets in the system $\beta$ Pic b and c. We also report the detection of an emission line at 6.986 $\mu$m that we attribute to be [Ar II]. We find that the [Ar II] emission is spatially resolved with JWST and appears to be aligned with the dust disk. Through PSF subtraction techniques, we detect $\beta$ Pic b at the 5$\sigma$ level in our MRS data cubes and present the first mid-IR spectrum of the planet from 5 to 7 $\mu$m. The planet's spectrum is consistent with having absorption from water vapor between 5 and 6.5 $\mu$m. We perform atmosphere model grid fitting on spectra and photometry of $\beta$ Pic b and find that the planet's atmosphere likely has a sub-stellar C/O ratio., Comment: Accepted for Publication in ApJ

One of the key research themes identified by the Astro2020 decadal survey is Worlds and Suns in Context. The Advanced X-ray Imaging Satellite (AXIS) is a proposed NASA APEX mission that will become the prime high-energy instrument for studying star-planet connections from birth to death. This work explores the major advances in this broad domain of research that will be enabled by the AXIS mission, through X-ray observations of stars in clusters spanning a broad range of ages, flaring M-dwarf stars known to host exoplanets, and young stars exhibiting accretion interactions with their protoplanetary disks. In addition, we explore the ability of AXIS to use planetary nebulae, white dwarfs, and the Solar System to constrain important physical processes from the microscopic (e.g., charge exchange) to the macroscopic (e.g., stellar wind interactions with the surrounding interstellar medium)., Comment: This White Paper is part of a series commissioned for the AXIS Probe Concept Mission

We have examined inter-night variability of K2-discovered Dippers that are not close to being viewed edge-on, as determined from previously-reported ALMA images, using the SpeX spectrograph and the NASA Infrared Telescope facility (IRTF). The three objects observed were EPIC 203850058, EPIC 205151387, and EPIC 204638512 (2MASS J16042165-2130284). Using the ratio of the fluxes between two successive nights, we find that for EPIC 204638512 and EPIC 205151387, we find that the properties of the dust differ from that seen in the diffuse interstellar medium and denser molecular clouds. However, the grain properties needed to explain the extinction does resemble those used to model the disks of many young stellar objects. The wavelength-dependent extinction models of both EPIC 204638512 and EPIC 205151387 includes grains at least 500 microns in size, but lacks grains smaller than 0.25 microns. The change in extinction during the dips, and the timescale for these variations to occur, imply obscuration by the surface layers of the inner disks. The recent discovery of a highly mis-inclined inner disk in EPIC 204638512 is suggests that the variations in this disk system may point to due to rapid changes in obscuration by the surface layers of its inner disk, and that other face-on Dippers might have similar geometries. The He I line at 1.083 microns in EPIC 205151387 and EPIC 20463851 were seen to change from night to night, suggesting that we are seeing He I gas mixed in with the surface dust., Comment: 13 pages, 6 figures, 2 tables

Atom-interferometric quantum sensors could revolutionize navigation, civil engineering, and Earth observation. However, operation in real-world environments is challenging due to external interference, platform noise, and constraints on size, weight, and power. Here we experimentally demonstrate that tailored light pulses designed using robust control techniques mitigate significant error sources in an atom-interferometric accelerometer. To mimic the effect of unpredictable lateral platform motion, we apply laser-intensity noise that varies up to 20$\%$ from pulse-to-pulse. Our robust control solution maintains performant sensing, while the utility of conventional pulses collapses. By measuring local gravity, we show that our robust pulses preserve interferometer scale factor and improve measurement precision by 10$\times$ in the presence of this noise. We further validate these enhancements by measuring applied accelerations over a 200 $\mu g$ range up to 21$\times$ more precisely at the highest applied noise level. Our demonstration provides a pathway to improved atom-interferometric inertial sensing in real-world settings., Comment: 15 pages, 7 figures

The Vera C. Rubin Observatory is expected to start the Legacy Survey of Space and Time (LSST) in early to mid-2025. This multi-band wide-field synoptic survey will transform our view of the solar system, with the discovery and monitoring of over 5 million small bodies.The final survey strategy chosen for LSST has direct implications on the discoverability and characterization of solar system minor planets and passing interstellar objects. Creating an inventory of the solar system is one of the four main LSST science drivers. The LSST observing cadence is a complex optimization problem that must balance the priorities and needs of all the key LSST science areas. To design the best LSST survey strategy, a series of operation simulations using the Rubin Observatory scheduler have been generated to explore the various options for tuning observing parameters and prioritizations. We explore the impact of the various simulated LSST observing strategies on studying the solar system's small body reservoirs. We examine what are the best observing scenarios and review what are the important considerations for maximizing LSST solar system science. In general, most of the LSST cadence simulations produce +/-5% or less variations in our chosen key metrics, but a subset of the simulations significantly hinder science returns with much larger losses in the discovery and light curve metrics., Comment: Accepted to ApJS, 103 pages (including references), 43 figures, 9 Tables. Videos will be available in the online journal formatted and published version of the paper [v2.0 submission corrects the author list metadata from the arxiv initial submission and updates the abstract]

We present a retrospective analysis of Earth-based mid-infrared observations of Jupiter capturing the aftermath of the impacts by Comet D/Shoemaker-Levy 9 (henceforth SL9) in July 1994 and the Wesley impactor in July 2009. While the atmospheric effects of both impacts have been reported previously, we were motivated to re-examine both events using consistent methods to enable robust, quantitative comparisons. We analyzed spectrophotometry and spectroscopy capturing both impacts using two independent analyses: 1) a least-squares search over a grid of candidate mineral species to determine the composition of impact residue and 2) a radiative transfer analysis to derive atmospheric information. We observe that the SL9 impact sites are enhanced in stratospheric CH4 emissions at 7.9 um, due to shock heating and adiabatic compression from plume re-entry, and from 8.5 - 11.5 um due to stratospheric NH3 emission and non-gaseous cometary material. We derive NH3 concentrations of 5.7 ppmv at 30 mbar. In new findings, we find that the SL9 impact sites also exhibit a non-gaseous emission feature at 18 - 19 um. The non-gaseous emission at 8.5 - 11.5 and 18 - 19 um emission is best reproduced by predominantly amorphous olivine and obsidian at similar abundances. The Wesley impact site exhibits enhanced emissions from 8.8 - 11.5 and 18 - 19 um. We found this could be reproduced by predominantly amorphous olivine and stratospheric NH3 at concentrations of 150 ppbv at 30 mbar. Stratospheric NH3 abundances are a factor of 40 higher in the SL9 impacts compared to the Wesley impact, which confirms the former reached deeper, NH3-richer altitudes of the atmosphere. The absence of silicas in the Wesley impact would place an upper limit of 10 km/s on the incident velocity and 9 degree on the entry angle of the impactor such that temperatures were insufficient to convert silicates.

Precise radial velocity (PRV) surveys are important for the search of Earth analogs around nearby bright stars. Such planets induce a small stellar reflex motion with RV amplitude of $\sim$10 cm/s. Detecting such a small RV signal poses important challenges to instrumentation, data analysis, and the precision of astrophysical models to mitigate stellar jitter. In this work, we investigate an important component in the PRV error budget - the spectral contamination from the Earth's atmosphere (tellurics). We characterize the effects of telluric absorption on the RV precision and quantify its contribution to the RV budget over time and across a wavelength range of 350 nm - 2.5$\mu$m. We investigate the effectiveness in mitigating tellurics using simulated spectra of a solar twin star with telluric contamination over a year's worth of observations, and we extracted the RVs using two commonly adopted algorithms: dividing out a telluric model before performing cross-correlation or Forward Modeling the observed spectrum incorporating a telluric model. We assume various degrees of cleanness in removing the tellurics, including mimicking the lack of accurate knowledge of the telluric lines by using a mismatched line profile to model the "observed" tellurics. We conclude that the RV errors caused by telluric absorption can be suppressed to close to or even below the photon-limited precision in the optical region, especially in the blue, around 1-10 cm/s. At red through near-infrared wavelengths, however, the residuals of tellurics can induce an RV error on the m/s level even under the most favorable assumptions for telluric removal, leading to significant systematic noise in the RV time series and periodograms. If the red-optical or near-infrared becomes critical in the mitigation of stellar activity, systematic errors from tellurics can be eliminated with a space mission such as EarthFinder., Comment: 29 pages, 11 figures. AJ under review. Comments welcome

While beta Pic is known to host silicates in ring-like structures, whether the properties of these silicate dust vary with stellocentric distance remains an open question. We re-analyze the beta Pictoris debris disk spectrum from the Spitzer Infrared Spectrograph (IRS) and a new IRTF/SpeX spectrum to investigate trends in Fe/Mg ratio, shape, and crystallinity in grains as a function of wavelength, a proxy for stellocentric distance. By analyzing a re-calibrated and re-extracted spectrum, we identify a new 18 micron forsterite emission feature and recover a 23 micron forsterite emission feature with a substantially larger line-to-continuum ratio than previously reported. We find that these prominent spectral features are primarily produced by small submicron-sized grains, which are continuously generated and replenished from planetesimal collisions in the disk and can elucidate their parent bodies' composition. We discover three trends about these small grains: as stellocentric distance increases, (1) small silicate grains become more crystalline (less amorphous), (2) they become more irregular in shape, and (3) for crystalline silicate grains, the Fe/Mg ratio decreases. Applying these trends to beta Pic's planetary architecture, we find that the dust population exterior to the orbits of beta Pic b and c differs substantially in crystallinity and shape. We also find a tentative 3-5 micron dust excess due to spatially unresolved hot dust emission close to the star. From our findings, we infer that the surfaces of large planetesimals are more Fe-rich and collisionally-processed closer to the star but more Fe-poor and primordial farther from the star., Comment: 19 pages, 12 figures, Accepted for Publication in ApJ

We used New Horizons LORRI images to measure the optical-band ($0.4\lesssim\lambda\lesssim0.9{\rm\mu m}$) sky brightness within a high galactic-latitude field selected to have reduced diffuse scattered light from the Milky Way galaxy (DGL), as inferred from the IRIS all-sky $100~\mu$m map. We also selected the field to significantly reduce the scattered light from bright stars (SSL) outside the LORRI field. Suppression of DGL and SSL reduced the large uncertainties in the background flux levels present in our earlier New Horizons COB results. The raw total sky level, measured when New Horizons was 51.3 AU from the Sun, is $24.22\pm0.80{\rm ~nW ~m^{-2} ~sr^{-1}}.$ Isolating the COB contribution to the raw total required subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection-limit within the field, and the hydrogen plus ionized-helium two-photon continua. This yielded a highly significant detection of the COB at ${\rm 16.37\pm 1.47 ~nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $\mu$m. This result is in strong tension with the hypothesis that the COB only comprises the integrated light of external galaxies (IGL) presently known from deep HST counts. Subtraction of the estimated IGL flux from the total COB level leaves a flux component of unknown origin at ${\rm 8.06\pm1.92 ~nW ~m^{-2} ~sr^{-1}}.$ Its amplitude is equal to the IGL., Comment: Accepted for publication in the Astrophysical Journal Letters

The majority of exoplanets found to date have been discovered via the transit method, and transmission spectroscopy represents the primary method of studying these distant worlds. Currently, in-depth atmospheric characterization of transiting exoplanets entails the use of spectrographs on large telescopes, requiring significant observing time to study each planet. Previous studies have demonstrated trends for solar system worlds using color-color photometry of reflectance spectra, as well as trends within transmission spectra for hot Jupiters. Building on these concepts, we have investigated the use of transmission color photometric analysis for efficient, coarse categorization of exoplanets and for assessing the nature of these worlds, with a focus on resolving the bulk composition degeneracy to aid in discriminating super-Earths and sub-Neptunes. We present our methodology and first results, including spectrum models, model comparison frameworks, and wave band selection criteria. We present our results for different transmission "color" metrics, filter selection methods, and numbers of filters. Assuming noise-free spectra of isothermal atmospheres in chemical equilibrium, with our pipeline, we are able to constrain atmospheric mean molecular weight in order to distinguish between super-Earth and sub-Neptune atmospheres with >90$\%$ overall accuracy using as few as two specific low-resolution filter combinations. We also found that increasing the number of filters does not substantially impact this performance. This method could allow for broad characterization of large numbers of planets much more efficiently than current methods permit, enabling population and system-level studies. Additionally, data collected via this method could inform follow-up observing time by large telescopes for more detailed studies of worlds of interest.

We investigate what can be learned about a population of distant KBOs by studying the statistical properties of their light curves. Whereas others have successfully inferred the properties of individual, highly variable KBOs, we show that the fraction of KBOs with low amplitudes also provides fundamental information about a population. Each light curve is primarily the result of two factors: shape and orientation. We consider contact binaries and ellipsoidal shapes, with and without flattening. After developing the mathematical framework, we apply it to the existing body of KBO light curve data. Principal conclusions are as follows. (1) When using absolute magnitude H as a proxy for size, it is more accurate to use the maximum of the light curve rather than the mean. (2) Previous investigators have noted that smaller KBOs have higher-amplitude light curves, and have interpreted this as evidence that they are systematically more irregular in shape than larger KBOs; we show that a population of flattened bodies with uniform proportions could also explain this result. (3) Our analysis indicates that prior assessments of the fraction of contact binaries in the Kuiper Belt may be artificially low. (4) The pole orientations of some KBOs can be inferred from observed changes in their light curves; however, these KBOs constitute a biased sample, whose pole orientations are not representative of the population overall. (5) Although surface topography, albedo patterns, limb darkening, and other surface properties can affect individual light curves, they do not have a strong influence on the statistics overall. (6) Photometry from the OSSOS survey is incompatible with previous results and its statistical properties defy easy interpretation. We also discuss the promise of this approach for the analysis of future, much larger data sets such as the one anticipated from the Rubin Observatory.

We present analyses of Spitzer observations of 29P/Schwassmann-Wachmann 1 using 16 $\mu$m IRS "blue" peak-up (PU) and 24 $\mu$m and 70 $\mu$m MIPS images obtained on UT 2003 November 23 and 24 that characterize the Centaur's large-grain (10-100 $\mu$m) dust coma during a time of non-outbursting "quiescent" activity. Estimates of $\epsilon f \rho$ for each band (16 $\mu$m (2600 $\pm$ 43 cm), 24 $\mu$m (5800 $\pm$ 63 cm), and 70 $\mu$m (1800 $\pm$ 900 cm)) follow the trend between nucleus size vs. $\epsilon f \rho$ that was observed for the WISE/NEOWISE comet ensemble. A coma model was used to derive a dust production rate in the range of 50-100 kg/s. For the first time, a color temperature map of SW1's coma was constructed using the 16 $\mu$m and 24 $\mu$m imaging data. With peaks at $\sim$ 140K, this map implies that coma water ice grains should be slowly sublimating and producing water gas in the coma. We analyzed the persistent 24 $\mu$m "wing" (a curved southwestern coma) feature at 352,000 km (90$''$) from the nucleus attributed by Stansberry et al. (2004) to nucleus rotation and instead propose that it is largely created by solar radiation pressure and gravity acting on micron sized grains. We performed coma removal to the 16 $\mu$m PU image in order to refine the nucleus' emitted thermal flux. A new application of the Near Earth Asteroid Thermal Model (NEATM; Harris 1998) at five wavelengths (5.730 $\mu$m, 7.873 $\mu$m, 15.80 $\mu$m, 23.68 $\mu$m, and 71.42 $\mu$m) was then used to refine SW1's effective radius measurement to $R = 32.3 \pm 3.1$ km and infrared beaming parameter to $\eta = 1.1 \pm 0.2$, respectively.

We observed Episodically Active Asteroid (6478) Gault in 2020 with multiple telescopes in Asia and North America and have found that it is no longer active after its recent outbursts at the end of 2018 and start of 2019. The inactivity during this apparation allowed us to measure the absolute magnitude of Gault of H_r = 14.63 +/- 0.02, G_r = 0.21 +/- 0.02 from our secular phasecurve observations. In addition, we were able to constrain Gault's rotation period using time-series photometric lightcurves taken over 17 hours on multiple days in 2020 August, September and October. The photometric lightcurves have a repeating $\lesssim$0.05 magnitude feature suggesting that (6478) Gault has a rotation period of ~2.5 hours and may have a semi-spherical or top-like shape, much like Near-Earth Asteroids Ryugu and Bennu. The rotation period of ~2.5 hours is near to the expected critical rotation period for an asteroid with the physical properties of (6478) Gault suggesting that its activity observed over multiple epochs is due to surface mass shedding from its fast rotation spun up by the Yarkovsky-O'Keefe-Radzievskii-Paddack effect., Comment: 23 pages, 18 figures; Accepted by ApJ Letters

Experiments in Atomic, Molecular, and Optical (AMO) physics require precise and accurate control of digital, analog, and radio frequency (RF) signals. We present a control hardware based on a field programmable gate array (FPGA) core which drives various modules via a simple interface bus. The system supports an operating frequency of 10 MHz and a memory depth of 8 M (2$^{23}$) instructions, both easily scalable. Successive experimental sequences can be stacked with no dead time and synchronized with external events at any instructions. Two or more units can be cascaded and synchronized to a common clock, a feature useful to operate large experimental setups in a modular way.

We present visible and mid-infrared imagery and photometry of temporary Jovian co-orbital comet P/2019 LD$_2$ taken with HST/WFC3, Spitzer/IRAC, the GROWTH telescope network, visible spectroscopy from Keck/LRIS and archival ZTF observations taken between 2019 April and 2020 August. Our observations indicate that the nucleus of LD$_2$ has a radius between 0.2-1.8 km assuming a 0.08 albedo and a coma dominated by $\sim$100$\mu$ m-scale dust ejected at $\sim$1 m/s speeds with a $\sim$1'' jet pointing in the SW direction. LD$_2$ experienced a total dust mass loss of $\sim$10$^8$ kg at a loss rate of $\sim$6 kg/s with Af$\rho$/cross-section varying between $\sim$85 cm/125 km$^2$ and $\sim$200 cm/310 km$^2$ from 2019 April 9 to 2019 Nov 8. If the increase in Af$\rho$/cross-section remained constant, it implies LD$_2$'s activity began $\sim$2018 November when within 4.8 au of the Sun, implying the onset of H$_2$O sublimation. We measure CO/CO$_2$ gas production of $\lesssim$10$^{27}$ mol/s /$\lesssim$10$^{26}$ mol/s from our 4.5 $\mu$m Spitzer observations, $g$-$r$ = 0.59$\pm$0.03, $r$-$i$ = 0.18$\pm$0.05, $i$-$z$ = 0.01$\pm$0.07 from GROWTH observations, H$_2$O gas production of $\lesssim$80 kg/s scaling from our estimated $C_2$ production of $Q_{C_2}\lesssim$7.5$\times10^{24}$ mol/s from Keck/LRIS spectroscopy. We determine that the long-term orbit of LD$_2$ is similar to Jupiter family comets having close encounters with Jupiter within $\sim$0.5 Hill radius in the last $\sim$3 y, within 0.8 Hill radius in $\sim$9 y. Additionally, 78.8$\%$ of our orbital clones are ejected from the Solar System within $1 \times 10^{6}$ years having a dynamical half-life of 3.4 $\times 10^5$ years., Comment: 28 pages, 8 figures, 2 Tables, accepted for publication in AJ

We used existing data from the New Horizons LORRI camera to measure the optical-band ($0.4\lesssim\lambda\lesssim0.9{\rm\mu m}$) sky brightness within seven high galactic latitude fields. The average raw level measured while New Horizons was 42 to 45 AU from the Sun is $33.2\pm0.5{\rm ~nW ~m^{-2} ~sr^{-1}}.$ This is $\sim10\times$ darker than the darkest sky accessible to the {\it Hubble Space Telescope}, highlighting the utility of New Horizons for detecting the cosmic optical background (COB). Isolating the COB contribution to the raw total requires subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection-limit within the fields, and diffuse Milky Way light scattered by infrared cirrus. We remove newly identified residual zodiacal light from the IRIS $100\mu$m all sky maps to generate two different estimates for the diffuse galactic light (DGL). Using these yields a highly significant detection of the COB in the range ${\rm 15.9\pm 4.2\ (1.8~stat., 3.7~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 18.7\pm 3.8\ (1.8~stat., 3.3 ~sys.)~ nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $\mu$m. Subtraction of the integrated light of galaxies (IGL) fainter than the photometric detection-limit from the total COB level leaves a diffuse flux component of unknown origin in the range ${\rm 8.8\pm4.9\ (1.8 ~stat., 4.5 ~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 11.9\pm4.6\ (1.8 ~stat., 4.2 ~sys.) ~nW ~m^{-2} ~sr^{-1}}$. Explaining it with undetected galaxies requires the galaxy-count faint-end slope to steepen markedly at $V>24$ or that existing surveys are missing half the galaxies with $V< 30.$, Comment: 32 pages, 20 figures. Accepted for publication in ApJ

NASA is working to define the architecture needed for a Mars exploration campaign. Initial analysis assumptions allow for pre-deployment of essential cargo and equipment to support a crew landing, including the pre-positioning of a Mars Ascent Vehicle (MAV). This MAV is likely to represent the largest single payload that must be landed on the Mars surface. Its size would be influenced by the amount of mass that state-of-the-art Entry, Descent, and Landing (EDL) systems would be capable of placing on Mars. One possible method of increasing the usable size of the MAV without exceeding available EDL capabilities is to land the MAV without ascent propellant on board. Following such a method may necessitate a strategy to acquire sufficient ascent propellant to allow a crew to safely depart the Martian surface. This paper describes a conceptual return propellant strategy that uses a liquid transportation skid, or pallet, to be used in conjunction with a rover mobility system to transfer water across the Martian surface from a source point to an in-situ resource utilization (ISRU) plant that would use this water as a feedstock to generate oxygen and methane to enable launch of the MAV. Design considerations, concept of operations, and rover energetics will be discussed in this paper.

We present time-resolved visible spectrophotometry of minimoon 2020 CD$_3$, the second asteroid known to become temporarily captured by the Earth-Moon system's gravitational field. The spectrophotometry was taken with Keck I/LRIS between wavelengths 434 nm and 912 nm in $B$, $g$, $V$, $R$, $I$ and RG850 filters as it was leaving the Earth-Moon system on 2020 March 23 UTC. The spectrophotometry of 2020 CD$_3$ most closely resembles the spectra of V-type asteroids and some Lunar rock samples with a reddish slope of ~18$\%$/100 nm between 434 nm and 761 nm corresponding to colors of $g$-$r$ = 0.62$\pm$0.08, $r$-$i$ = 0.21 $\pm$ 0.06 and an absorption band at ~900 nm corresponding to $i$-$z$ = -0.54$\pm$0.10. Combining our measured 31.9$\pm$0.1 absolute magnitude with an albedo of 0.35 typical for V-type asteroids, we determine 2020 CD$_3$'s diameter to be ~0.9$\pm$0.1 m making it the first minimoon and one of the smallest asteroids to be spectrally studied. We use our time-series photometry to detect periodic lightcurve variations with a $<$10$^{-4}$ false alarm probability corresponding to a lightcurve period of ~573 s and a lightcurve amplitude of ~1 mag implying 2020 CD$_3$ possesses a $b/a$ axial ratio of ~2.5. In addition, we extend the observational arc of 2020 CD$_3$ to 37 days between 2020 February 15 UTC and 2020 March 23 UTC. From the improved orbital solution for 2020 CD$_3$, we estimate its likely duration of its capture to be ~2 y, and we measure the non-gravitation perturbation on its orbit due to radiation pressure with an area-to-mass ratio of 6.9$\pm$2.4$\times$10$^{-4}$ m$^2$/kg implying a density of 2.3$\pm$0.8 g/cm$^3$, broadly compatible with the densities of other meter-scale asteroids and Lunar rock. We searched for pre-discovery detections of 2020 CD$_3$ in the ZTF archive as far back as 2018 October, but were unable to locate any positive detections., Comment: 26 pages, 8 figures, 3 Tables, accepted for publication in ApJL

We consider the history of New Horizons target (486958) Arrokoth in the context of its sublimative evolution. Shortly after the Sun's protoplanetary disk (PPD) cleared, the newly intense sunlight sparked a sublimative period in Arrokoth's early history that lasted for ~10-100 Myr. Although this sublimation was too weak to significantly alter Arrokoth's spin state, it could drive mass transport around the surface significant enough to erase topographic features on length scales of ~10-100 m. This includes craters up to ~50-500 m in diameter, which suggests that the majority of Arrokoth's craters may not be primordial (dating from the merger of Arrokoth's lobes), but rather could date from after the end of this sublimative period. Thereafter, Arrokoth entered a Quiescent Period (which lasts to the present day), in which volatile production rates are at least 13 orders of magnitude less than the ~10^24 molecules/s detection limit of the New Horizons spacecraft (Lisse et al. 2020). This is insufficient to drive either mass transport or sublimative torques. These results suggest that the observed surface of Arrokoth is not primordial, but rather dates from the Quiescent Period. By contrast, the inability of sublimative torques to meaningfully alter Arrokoth's rotation state suggests that its shape is indeed primordial, and its observed rotation is representative of its spin state after formation., Comment: 19 pages, 2 figures, 1 table

In the next decades, the astrobiological community will debate whether the first observations of oxygen in an exoplanet$'$s atmosphere signifies life, so it is critical to establish procedures now for collection and interpretation of such data. We present a step-by-step observational strategy for using oxygen as a robust biosignature, to prioritize exoplanet targets and design future observations. It is premised on avoiding planets lacking subaerial weathering of continents, which would imply geochemical cycles drastically different from Earth$'$s, precluding use of oxygen as a biosignature. The strategy starts with the most readily obtained data: semi-major axis and stellar luminosity to ensure residence in the habitable zone; stellar XUV flux, to ensure an exoplanet can retain a secondary (outgassed) atmosphere. Next, high-precision mass and radius information should be combined with high-precision stellar abundance data, to constrain the exoplanet$'$s water content; those incompatible with less than 0.1 wt % H$_{2}$O can be deprioritized. Then, reflectance photometry or low-resolution transmission spectroscopy should confirm an optically thin atmosphere. Subsequent long-duration, high-resolution transmission spectroscopy should search for oxygen and ensure that water vapor and CO$_{2}$ are present only at low (10$^{2}$-10$^{4}$ ppm levels). Assuming oxygen is found, attribution to life requires the difficult acquisition of a detailed, multispectral light curve of the exoplanet to ensure both surface land and water. Exoplanets failing some of these steps might be habitable, even have observable biogenic oxygen, but should be deprioritized because oxygen could not be attributed unambiguously to life. We show how this is the case for the Solar System, the 55 Cnc System, and the TRAPPIST-1 System, in which only the Earth and TRAPPIST-1e successfully pass through our procedure., Comment: 27 Pages, 1 Figure, 0 Tables (accepted 09 June 2020, in press)

The search for life on exoplanets is one of the grand scientific challenges of our time. The strategy to date has been to find (e.g., through transit surveys like Kepler) Earth-like exoplanets in their stars habitable zone, then use transmission spectroscopy to measure biosignature gases, especially oxygen, in the planets atmospheres (e.g., using JWST, the James Webb Space Telescope). Already there are more such planets than can be observed by JWST, and missions like the Transiting Exoplanet Survey Satellite and others will find more. A better understanding of the geochemical cycles relevant to biosignature gases is needed, to prioritize targets for costly follow-up observations and to help design future missions. We define a Detectability Index to quantify the likelihood that a biosignature gas could be assigned a biological vs. non-biological origin. We apply this index to the case of oxygen gas, O2, on Earth-like planets with varying water contents. We demonstrate that on Earth-like exoplanets with 0.2 weight percent (wt%) water (i.e., no exposed continents) a reduced flux of bioessential phosphorus limits the export of photosynthetically produced atmospheric O2 to levels indistinguishable from geophysical production by photolysis of water plus hydrogen escape. Higher water contents >1wt% that lead to high-pressure ice mantles further slow phosphorus cycling. Paradoxically, the maximum water content allowing use of O2 as a biosignature, 0.2wt%, is consistent with no water based on mass and radius. Thus, the utility of an O2 biosignature likely requires the direct detection of both water and land on a planet.

We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by SOFIA with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 Aug 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 $\pm$ 0.0005 hours. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies., Comment: Submitted to Astronomical Journal (revised); 40 pages, 13 figures, 9 tables

We present high resolution, deep imaging of interstellar comet 2I/Borisov taken with the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2019 December 8 UTC and 2020 January 27 UTC (HST GO 16040, PI Bolin) before and after its perihelion passage in combination with HST/WFC3 images taken on 2019 October 12 UTC and 2019 November 16 UTC (HST GO/DD 16009, PI Jewitt) before its outburst and fragmentation of March 2020, thus observing the comet in a relatively undisrupted state. We locate 1-2\arcsec~long (2,000 - 3,000 km projected length) jet-like structures near the optocenter of 2I that appear to change position angles from epoch to epoch. With the assumption that the jet is located near the rotational pole supported by its stationary appearance on $\sim$10-100 h time frames in HST images, we determine that 2I's pole points near $\alpha$ = 322$\pm$10$^\circ$, $\delta$ = 37$\pm$10$^\circ$ ($\lambda$ = 341$^\circ$, $\beta$ = 48$^\circ$) and may be in a simple rotation state. Additionally, we find evidence for possible periodicity in the HST time-series lightcurve on the time scale of $\sim$5.3 h with a small amplitude of $\sim$0.05 mag implying a lower limit on its $b/a$ ratio of $\sim$1.5 unlike the large $\sim$2 mag lightcurve observed for 1I/`Oumuamua. However, these small lightcurve variations may not be the result of the rotation of 2I's nucleus due to its dust-dominated light-scattering cross-section. Therefore, uniquely constraining the pre-Solar System encounter, pre-outburst rotation state of 2I may not be possible even with the resolution and sensitivity provided by HST observations., Comment: 14 pages, 6 figures and 3 tables, accepted for publication in MNRAS on 23 July 2020

We present visible and near-infrared photometric and spectroscopic observations of interstellar object 2I/Borisov taken from 2019 September 10 to 2019 November 29 using the GROWTH, the APO ARC 3.5 m and the NASA/IRTF 3.0 m combined with post and pre-discovery observations of 2I obtained by ZTF from 2019 March 17 to 2019 May 5. Comparison with imaging of distant Solar System comets shows an object very similar to mildly active Solar System comets with an out-gassing rate of $\sim$10$^{27}$ mol/sec. The photometry, taken in filters spanning the visible and NIR range shows a gradual brightening trend of $\sim0.03$ mags/day since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The lightcurve from recent and pre-discovery data reveals a brightness trend suggesting the recent onset of significant H$_2$O sublimation with the comet being active with super volatiles such as CO at heliocentric distances $>$6 au consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 04, we estimate a diameter of 2I's nucleus of $\lesssim$1.4 km. We use the size estimates of 1I/'Oumuamua and 2I/Borisov to roughly estimate the slope of the ISO size-distribution resulting in a slope of $\sim$3.4$\pm$1.2, similar to Solar System comets and bodies produced from collisional equilibrium., Comment: 22 pages, 7 figures, 1 table. Accepted for publication in AJ on 12 May 2020