The Gaia optical astrometric mission has measured the precise positions ofmillions of objects in the sky, including extragalactic sources also observedby Very Long Baseline Interferometry (VLBI). In the recent Gaia EDR3 release,an effect of negative parallax with a magnitude of approximately -17 $\mu$aswas reported, presumably due to technical reasons related to the relativisticdelay model. A recent analysis of a 30-year set of geodetic VLBI data revealeda similar negative parallax with an amplitude of $-15.8 \pm 0.5$ $ \mu$as.Since both astrometric techniques, optical and radio, provide consistentestimates of this negative parallax, it is necessary to investigate thepotential origin of this effect. We developed the extended group relativisticdelay model to incorporate the additional parallactic effect for radio sourcesat distances less than 1 Mpc and found that the apparent annual signal mightappear due the non-orthogonality of the fundamental axes, which are defined bythe positions of the reference radio sources themselves. Unlike theconventional parallactic ellipse, the apparent annual effect in this caseappears as a circular motion for all objects independently of their eclipticlatitude. The measured amplitude of this circular effect is within a range of10-15 $\mu$as that is consistent with the ICRF3 stability of the fundamentalaxis. This annual circular effect could also arise if a G\"odel-typecosmological metric were applied, suggesting that, in the future, thisphenomenon could be used to indicate global cosmic rotation.
Abstract A broadband, multi‐channel Very Low Frequency (VLF) radio receiver (BBR), developed as a sensitive analog, vector wave receiver for whistler‐mode signals in the VLF range, was successfully flown on the Air Force Demonstration Science Experiment (DSX) Mission to Mid‐Earth Orbit (Johnston et al., 2023, https://doi.org/10.1029/2022JA030771 ). The BBR is a radiation resistant, 5 × 2 channel receiver, integrated into the Wave Induced Precipitation of Electron Radiation (WIPER) instrument package on DSX. The BBR accepts electric wave signal inputs from (a) an 81.6 m tip‐to‐tip dipole VLF antenna on the DSX Y‐boom, (b) a 16.3 m tip‐to‐tip dipole antenna on the DSX Z‐boom, and (c) signals from a Tri‐Axial Search Coil (TASC), an three‐orthogonal axes magnetic wave search coil magnetometer mounted on the DSX + Z boom. The electric and magnetic VLF signals are processed in the BBR by two independent, radiation hardened five channel receivers: (a) a receiver of heritage design with commercial off‐the‐shelf components (COTS), and (b) a micro‐receiver incorporating custom, radiation resistant, micro‐electronics. The bandwidth of all five channels in both the heritage and micro designs covers from 10 Hz to 50 kHz. A software “receiver”, SRx, running in the on‐board flight computer, the ECS, manages the BBR's data flow and data delivery to the ground. The SRx additionally computes supporting science data products such as Fourier transforms, multi‐band filters and cross correlations among the BBR's electric and magnetic field channels to facilitate production of VLF wave normals.
Abstract We report on the detection of protons and the potential detection of H 2 + between Saturn's F and G rings based on Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer (IMS) time‐of‐flight (TOF) composition measurements acquired during Saturn Orbit Insertion (SOI) outbound pass. The range in dipole L shell is 2.3 < L < 2.8. Initial results based on TOF data were presented in E. C. Sittler et al. (2017). Here we present the latest results of our analysis. During the SOI outbound pass between the F and G rings the CAPS IMS was in a mode of reduced post‐acceleration voltage at −6 kV instead of the usual −14.6 kV. This reduced voltage still allows the analysis of protons since 6 keV protons are minimally scattered by the instrument's ultrathin carbon foils when compared to heavier ions O + and O 2 + . Background noise from penetrating radiation and ghost peaks produced by foil‐scattered O + ions within the instrument were considered in our analysis. The analysis allowed an determination of the proton density, temperature and flow velocity, accounting for spacecraft potential by assuming a convected Maxwellian for the proton velocity distribution function. We find average proton density n P = 3.4 ± 1.2 #/cm 3 , proton temperature T P = 1.74 ± 0.12 eV, proton corotation flow speed V P = 24 ± 1.5 km/s in spacecraft reference frame and spacecraft potential Φ SC = −0.8 ± 1.5 V. These results are compared with previous theoretical estimates of H + and H 2 + ions within Saturn's inner magnetosphere.
Abstract One of the key scientific goals of China's first Mars mission Tianwen‐1 is to search for ground ice. This study focuses on investigating potential water ice reservoirs in the vicinity of the landing site of the Zhurong rover to provide geological context and references for data interpretation. Our study area is centered on Utopia Planitia (UP), where Shallow Radar onboard the Mars Reconnaissance Orbiter (SHARAD) previously detected subsurface echoes that could be interpreted as ice deposits. Based on the SHARAD data, we have estimated the thickness, dielectric properties, and possible material composition of the surface deposition layer. The inferred water ice volume content ranges from approximately 55%–85%, which is consistent with deposits found on the western edge of UP. Based on morphological features and radar data products, we interpret the detected sediment layer as the latitude‐dependent mantle (LDM). We have conducted a comprehensive analysis of the distribution and morphology of various periglacial landforms, including Decameter‐scale Rimmed Depressions (DRDs), polygonal landforms, and scalloped depressions on the surface of UP. The implications for the level of degradation are discussed. The radar results provide evidence that DRDs have formed as a result of the degradation of the LDM layer. Additionally, our statistical analysis of concentric crater humps (CCH) linked to subsurface pure glacial ice suggests the possible presence of an icy layer that may be as thick as a kilometer beneath the LDM unit.
Abstract Martian dust is mobilized throughout the year by local, regional, and global dust storms, influencing atmospheric opacity and inhibiting observations of the surface. Using Mars Hand Lens Imager (MAHLI) images and methods of Schmidt et al. (2018), https://doi.org/10.1029/2018je005553 , areal dust coverages on 368 near‐horizontal, undisturbed rock surfaces were estimated along six Mars Years of the Mars Science Laboratory (MSL) Curiosity rover's geologic traverse from mission sols 46–3,409, corresponding to Mars Year 31 (MY#; Clancy et al., 2000, https://doi.org/10.1029/1999JE001089 ), areocentric solar longitude (Ls) 175.9° through MY36, Ls 177.6°. Targets were evaluated for potential geospatial and seasonal (Ls) correlations. Dust coverages increased at each spring equinox (Ls = 0°) with the highest coverage (76.6%) recorded at the top of Vera Rubin Ridge (VRR) prior to the Mars Year 34 (MY34) global dust storm. Dust coverages annually decreased as prevailing wind strengths in the Gale crater increased during southern summer. Following the ascent of VRR, New Year maximums have decreased by approximately 15% annually (MY35 60.7%, and MY36 52.8%), suggesting that dust is less abundant at higher elevations on Mount Sharp, and/or that dust suspension or removal is enhanced at higher elevations by stronger winds at higher elevations. Two regions with relatively low dust coverages (<20%) were found in proximity to active aeolian sand dunes and are interpreted to result from saltating sand particles striking and lofting dust particles. This research represents the single longest recording of surface dust deposits to date for landed missions.
Abstract Jezero crater contains a sedimentary fan deposit previously interpreted as a delta, which can be studied to better understand the aqueous history of Mars. After a year of traversing and sampling the crater floor, the Mars 2020 Perseverance rover encountered the Enchanted Lake area at the base of the sedimentary fan. Data were collected at Enchanted Lake between sols ∼420–426 and again between sols 556–629 when the rover returned for sampling. The goal of this paper is to describe and characterize the facies within this outcrop to help constrain the paleoenvironment of Enchanted Lake in the context of the overall fan system. Facies are defined based on observed sedimentary structures, bedding geometries, and grain size. The lack of extensive cross‐stratification combined with an abundance of soft sediment deformation, planar bedding, and normal grading leads us to interpret Enchanted Lake as a turbidite succession formed in a generally unconfined prodelta environment. Such depositional environments have high preservation potential for organic matter and potential biosignatures. Samples from these rocks have been collected by Perseverance for a planned Mars Sample Return mission. Our interpretation of Enchanted Lake is also consistent with other sedimentary deposits in the Jezero crater, such as the Kodiak butte, which is interpreted as deltaic in origin. Placing Enchanted Lake in context with Kodiak provides some constraints on the relative timing of these deposits within the Jezero fan system.