Radar evidence of subglacial liquid water on Mars Orosei, R; Lauro, S E; Pettinelli, E ...
Science (American Association for the Advancement of Science),
08/2018, Letnik:
361, Številka:
6401
Journal Article
Recenzirano
Odprti dostop
The presence of liquid water at the base of the martian polar caps has long been suspected but not observed. We surveyed the Planum Australe region using the MARSIS (Mars Advanced Radar for ...Subsurface and Ionosphere Sounding) instrument, a low-frequency radar on the Mars Express spacecraft. Radar profiles collected between May 2012 and December 2015 contain evidence of liquid water trapped below the ice of the South Polar Layered Deposits. Anomalously bright subsurface reflections are evident within a well-defined, 20-kilometer-wide zone centered at 193°E, 81°S, which is surrounded by much less reflective areas. Quantitative analysis of the radar signals shows that this bright feature has high relative dielectric permittivity (>15), matching that of water-bearing materials. We interpret this feature as a stable body of liquid water on Mars.
Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies. Numerous studies have invoked its importance in driving ...landscape evolution, yet morphological features produced by thermal fracture processes have never been definitively observed on an airless body, or any surface where other weathering mechanisms may be ruled out. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission provides an opportunity to search for evidence of thermal breakdown and assess its significance on asteroid surfaces. Here we show boulder morphologies observed on Bennu that are consistent with terrestrial observations and models of fatigue-driven exfoliation and demonstrate how crack propagation via thermal stress can lead to their development. The rate and expression of this process will vary with asteroid composition and location, influencing how different bodies evolve and their apparent relative surface ages from space weathering and cratering records.
“LICIACube – the Light Italian Cubesat for Imaging of Asteroids” is managed by the Italian Space Agency (ASI) and will be part of the NASA DART mission, with the aim of i) documenting the DART ...impact’s effects on the secondary member of the (65803) Didymos binary asteroid system, ii) characterizing the shape of the target, and iii) performing dedicated scientific investigations on it. DART probe will be launched at the end of 2021 and LICIACube will be hosted as piggyback during the interplanetary cruise, then released 10 days before the impact, and autonomously guided along its fly-by trajectory. The LICIACube payload is composed by LEIA, a narrow FoV camera, and LUKE, a wide FoV imager with an RGB Bayer pattern filter, that will collect and transmit to Earth several unique images of the effects of the DART impact on the asteroid, such as the formation and the development of the plume potentially determined by the impact.
LICIACube will be the first deep space mission developed and autonomously managed by an Italian team: the design, integration and test of the CubeSat have been assigned by ASI to the aerospace company Argotec, while the LICIACube Ground Segment has a complex architecture based on the Argotec Mission Control Center, antennas of the NASA Deep Space Network and data archiving and processing, managed at the ASI Space Science Data Center. The LICIACube team includes a wide Italian scientific community, involved in the definition of all the aspects of the mission: trajectory design; mission definition (and real-time orbit determination during operations); impact, plume and imaging simulation and modelling, in preparation of a suitable framework for the analysis and interpretation of in-situ data. The major technological mission challenge, i.e. the autonomous targeting and imaging of such a small body during a fast fly-by, to be accomplished with the limited resources of a CubeSat, is affordable thanks to a strong synergy of all the mentioned teams in support of the engineering tasks.
-LICIACube is the first purely Italian spacecraft operating in deep space.-LICIACube is managed by the Italian Space Agency and will be part of the NASA DART mission.-It will analyze the output of the first kinetic impact test at a realistic scale.-The payload is composed by LEIA, a narrow FoV camera, and LUKE, a wide FoV imager with an RGB Bayer pattern filter.-Aims: documenting the DART impact on Dimorphos; characterizing the target shape; performing scientific investigations.
Context. The Rosetta mission of the European Space Agency has been orbiting the comet 67P/Churyumov-Gerasimenko (67P) since August 2014 and is now in its escort phase. A large complement of ...scientific experiments designed to complete the most detailed study of a comet ever attempted are onboard Rosetta. Aims. We present results for the photometric and spectrophotometric properties of the nucleus of 67P derived from the OSIRIS imaging system, which consists of a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). The observations presented here were performed during July and the beginning of August 2014, during the approach phase, when OSIRIS was mapping the surface of the comet with several filters at different phase angles (1.3°–54°). The resolution reached up to 2.1 m/px. Methods. The OSIRIS images were processed with the OSIRIS standard pipeline, then converted into I/F radiance factors and corrected for the illumination conditions at each pixel using the Lommel-Seeliger disk law. Color cubes of the surface were produced by stacking registered and illumination-corrected images. Furthermore, photometric analysis was performed both on disk-averaged photometry in several filters and on disk-resolved images acquired with the NAC orange filter, centered at 649 nm, using Hapke modeling. Results. The disk-averaged phase function of the nucleus of 67P shows a strong opposition surge with a G parameter value of −0.13 ± 0.01 in the HG system formalism and an absolute magnitude Hv(1,1,0) = 15.74 ± 0.02 mag. The integrated spectrophotometry in 20 filters covering the 250−1000 nm wavelength range shows a red spectral behavior, without clear absorption bands except for a potential absorption centered at ~290 nm that is possibly due to SO2 ice. The nucleus shows strong phase reddening, with disk-averaged spectral slopes increasing from 11%/(100 nm) to 16%/(100 nm) in the 1.3°−54° phase angle range. The geometric albedo of the comet is 6.5 ± 0.2% at 649 nm, with local variations of up to ~16% in the Hapi region. From the disk-resolved images we computed the spectral slope together with local spectrophotometry and identified three distinct groups of regions (blue, moderately red, and red). The Hapi region is the brightest, the bluest in term of spectral slope, and the most active surface on the comet. Local spectrophotometry shows an enhancement of the flux in the 700−750 nm that is associated with coma emissions.
Context. The complex shape of comet 67P and its oblique rotation axis cause pronounced seasonal effects. Irradiation and hence activity vary strongly. Aims. We investigate the insolation of the ...cometary surface in order to predict the sublimation of water ice. The strongly varying erosion levels are correlated with the topography and morphology of the present cometary surface and its evolution. Methods. The insolation as a function of heliocentric distance and diurnal (spin dependent) variation is calculated using >105 facets of a detailed digital terrain model. Shading, but also illumination and thermal radiation by facets in the field of view of a specific facet are iteratively taken into account. We use a two-layer model of a thin porous dust cover above an icy surface to calculate the water sublimation, presuming steady state and a uniform surface. Our second model, which includes the history of warming and cooling due to thermal inertia, is restricted to a much simpler shape model but allows us to test various distributions of active areas. Results. Sublimation from a dirty ice surface yields maximum erosion. A thin dust cover of 50 μm yields similar rates at perihelion. Only about 6% of the surface needs to be active to match the observed water production rates at perihelion. A dust layer of 1 mm thickness suppresses the activity by a factor of 4 to 5. Erosion on the south side can reach more than 10 m per orbit at active spots. The energy input to the concave neck area (Hapi) during northern summer is enhanced by about 50% owing to self-illumination. Here surface temperatures reach maximum values along the foot of the Hathor wall. Integrated over the whole orbit this area receives the least energy input. Based on the detailed shape model, the simulations identify “hot spots” in depressions and larger pits in good correlation with observed dust activity. Three-quarters of the total sublimation is produced while the sub-solar latitude is south, resulting in a distinct dichotomy in activity and morphology. Conclusions. The northern areas display a much rougher morphology than what is seen on Imhotep, an area at the equator that will be fully illuminated when 67P is closer to the Sun. Self-illumination in concave regions enhance the energy input and hence erosion. This explains the early activity observed at Hapi. Cliffs are more prone to erosion than horizontal, often dust covered, areas, which leads to surface planation. Local activity can only persist if the forming cliff walls are eroding. Comet 67P has two lobes and also two distinct sides. Transport of material from the south to the north is probable. The morphology of the Imhotep plain should be typical for the terrains of the yet unseen southern hemisphere.
Carbonaceous asteroids, such as (101955) Bennu, preserve material from the early Solar System, including volatile compounds and organic molecules. We report spacecraft imaging and spectral data ...collected during and after retrieval of a sample from Bennu’s surface. The sampling event mobilized rocks and dust into a debris plume, excavating a 9-meter-long elliptical crater. This exposed material is darker, spectrally redder, and more abundant in fine particulates than the original surface. The bulk density of the displaced subsurface material was 500 to 700 kilograms per cubic meter, which is about half that of the whole asteroid. Particulates that landed on instrument optics spectrally resemble aqueously altered carbonaceous meteorites. The spacecraft stored 250 ± 101 grams of material, which will be delivered to Earth in 2023.
Grabbing a sample of asteroid Bennu
The near-Earth carbonaceous asteroid Bennu was the target of the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) sample return mission. After rendezvousing with the asteroid, the spacecraft spent 2 years studying its surface and selecting a suitable site. Lauretta
et al
. describe the sample collection process and its effects on Bennu. The asteroid provided almost no resistance to contact, and the gas released by the spacecraft blew a crater several meters wide, exposing redder rocks and dust. So much material was gathered that the collection chamber overflowed. Approximately 250 grams was successfully stowed, well above the mission goal of 60 grams. The samples are expected to arrive on Earth in September 2023. —KTS
Samples of the near-Earth asteroid Bennu were collected by the OSIRIS-REx mission and should arrive on Earth in 2023.
An asteroid's history is determined in large part by its strength against collisions with other objects
(impact strength). Laboratory experiments on centimetre-scale meteorites
have been extrapolated ...and buttressed with numerical simulations to derive the impact strength at the asteroid scale
. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples
and images of the asteroid (25143) Itokawa
. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu's metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75 ± 0.75 million years.
Context. We present an investigation of the surface properties of areas on the nucleus of comet 67P/Churyumov-Gerasimenko. Aims. We aim to show that transport of material from one part of the ...cometary nucleus to another is a significant mechanism that influences the appearance of the nucleus and the surface thermal properties. Methods. We used data from the OSIRIS imaging system onboard the Rosetta spacecraft to identify surface features on the nucleus that can be produced by various transport mechanisms. We used simple calculations based on previous works to establish the plausibility of dust transport from one part of the nucleus to another. Results. We show by observation and modeling that “airfall” as a consequence of non-escaping large particles emitted from the neck region of the nucleus is a plausible explanation for the smooth thin deposits in the northern hemisphere of the nucleus. The consequences are also discussed. We also present observations of aeolian ripples and ventifacts. We show by numerical modeling that a type of saltation is plausible even under the rarified gas densities seen at the surface of the nucleus. However, interparticle cohesive forces present difficulties for this model, and an alternative mechanism for the initiation of reptation and creep may result from the airfall mechanism. The requirements on gas density and other parameters of this alternative make it a more attractive explanation for the observations. The uncertainties and implications are discussed.
Aims. The OSIRIS camera onboard the Rosetta spacecraft has been acquiring images of the comet 67P/Churyumov-Gerasimenko (67P)’s nucleus at spatial resolutions down to ~0.17 m/px ever since Aug. 2014. ...These images have yielded unprecedented insight into the morphological diversity of the comet’s surface. This paper presents an overview of the regional morphology of comet 67P. Methods. We used the images that were acquired at orbits ~20–30 km from the center of the comet to distinguish different regions on the surface and introduce the basic regional nomenclature adopted by all papers in this Rosetta special feature that address the comet’s morphology and surface processes. We used anaglyphs to detect subtle regional and topographical boundaries and images from close orbit (~10 km from the comet’s center) to investigate the fine texture of the surface. Results. Nineteen regions have currently been defined on the nucleus based on morphological and/or structural boundaries, and they can be grouped into distinctive region types. Consolidated, fractured regions are the most common region type. Some of these regions enclose smooth units that appear to settle in gravitational sinks or topographically low areas. Both comet lobes have a significant portion of their surface covered by a dusty coating that appears to be recently placed and shows signs of mobilization by aeolian-like processes. The dusty coatings cover most of the regions on the surface but are notably absent from a couple of irregular large depressions that show sharp contacts with their surroundings and talus-like deposits in their interiors, which suggests that short-term explosive activity may play a significant role in shaping the comet’s surface in addition to long-term sublimation loss. Finally, the presence of layered brittle units showing signs of mechanical failure predominantly in one of the comet’s lobes can indicate a compositional heterogeneity between the two lobes.
Context . A number of bodies in the Solar System are characterized by dark surfaces, from carbonaceous asteroids to the enigmatic surface of Phobos and Deimos. Our understanding of the spectroscopic ...behavior of low-albedo surfaces remains incomplete. To improve the interpretation of remote sensing data, laboratory studies continue to serve as a pivotal tool for unveiling the physical state and composition of such surfaces. Aims . Several processes can be simulated in the laboratory, however, the preparation and analysis of a complex mixing of analog material is one of the most fundamental among them, while also being one of the most complex when multiple components are used. In this work, we aim to study how dark material mixed with basaltic material at different grain sizes can affect the spectroscopic features from the near- to mid- infrared (1.25–25 µm). Methods . Our sample set includes four series of basaltic mix (feldspar and pyroxene) at different grain sizes from <50 µm to 1000 µm, mixed with amorphous carbon at increasing weight percentages ranging from 1% to 50%. We analyzed several features on the spectrum of each mineral mixture. In particular, we investigated the behavior of the: (i) near-infrared slope; (ii) 2.7 µm OH-stretching band; (iii) Christiansen features; and (iv) Reststrahlen band and Transparency feature. Results . The measurements presented in this work, which take into account a large wavelength range for the first time, point toward a critical effect of dark material, but with a different outcomes for each grain size. Some of the most interesting results involve the slope trend of modification with dark material and the variant behavior of the Reststrahlen band and Transparency feature. Conclusions . This dataset will offer a key support in the interpretation of data collected on dark surfaces by past and future space missions. This knowledge will be also important in the context of linking analyses of returned samples with remote sensing data collected on planetary surfaces.
PDF
