Interpretation of radar sounder reflections to infer the structure and composition of the martian polar caps depends on whether bright returns correspond to single packed dust layers or a more finely ...layered structure. Reflections from multiple layers can create strong resonant scattering (interference) effects that impact analyses of radargram reflectors and inference of dielectric contrast. We identify resonant behavior for an areally extensive reflector in the north polar layered deposits from Shallow Radar data processed in two frequency bands. Echo strength varies by ∼2 dB between subband reflections across a region ∼400 km in extent, with the stronger echo shifting abruptly from the high‐ to low‐frequency band outside the central region of Gemina Lingula. This behavior can arise from resonant scattering between two layers of dust (0.3–0.6 m thick) separated by 0.5–3 m of ice. Such layering requires there be little postdepositional aeolian activity to preserve layer thickness and spacing.
Plain Language Summary
The north polar cap of Mars is made of layers of ice and dust that are a record of the changing climate over the past few million years. These north polar layered deposits (NPLD) have been imaged with radar sounder data from the Shallow Radar (SHARAD) instrument. The radar data reveal reflectors caused by changes in the electrical properties of the layered ice and dust. However, as the radar beam is reflected off multiple layers it can interfere with itself, changing the appearance of the reflector. We identified resonance by splitting SHARAD data into two bands at different frequencies and looking for differences in the reflectors. We used two models to simulate interference and suggest plausible thicknesses for the ice and dust layers. Our simulations suggest a portion of the NPLD formed in less than 200 thousand years, at a time when the region was much less windy than it is currently.
Key Points
We identify resonant radar scattering behavior in radargrams of the martian north polar layered deposits using Shallow Radar multiband data
Resonant behavior can affect interpretation of radargrams and can arise from two thin dust layers separated by less than a few meters of ice
We characterize a reflector that formed during quiescent periods of sustained dust accumulation with minimal aeolian activity
Abstract Planetary defense efforts rely on estimates of the mechanical properties of asteroids, which are difficult to constrain accurately from Earth. The mechanical properties of asteroid material ...are also important in the interpretation of the Double Asteroid Redirection Test (DART) impact. Here we perform a detailed morphological analysis of the surface boulders on Dimorphos using images, the primary data set available from the DART mission. We estimate the bulk angle of internal friction of the boulders to be 32.7 ± 2. 5° from our measurements of the roundness of the 34 best-resolved boulders ranging in size from 1.67–6.64 m. The elongated nature of the boulders around the DART impact site implies that they were likely formed through impact processing. Finally, we find striking similarities in the morphology of the boulders on Dimorphos with those on other rubble pile asteroids (Itokawa, Ryugu and Bennu). This leads to very similar internal friction angles across the four bodies and suggests that a common formation mechanism has shaped the boulders. Our results provide key inputs for understanding the DART impact and for improving our knowledge about the physical properties, the formation and the evolution of both near-Earth rubble-pile and binary asteroids.
The north and south polar layered deposits (PLD) on Mars are composed of stacks of layered ice and dust, but the SPLD is approximately twice as bright as the NPLD in 20‐MHz radar echoes. We use ...Shallow Radar (SHARAD) data in ∼4‐MHz bands centered on 17.5 MHz (“L”) and 22.5 MHz (“H”) to determine whether radar reflectivity variations are due to scattering effects related to closely spaced, near‐surface dielectric layering. We mapped the ratio of the surface echo power at the two frequencies (H/L) for both PLDs. The NPLD has large areas where H and L echo power differ, consistent with destructive interference in the H band within the uppermost ∼20 m. The SPLD is dominated by H ∼ L (unity), except for isolated regions in and near the residual CO2 cap and Australe Lingula. The H/L variations can be partly explained by near‐surface structure, where large variations in H/L match locations with numerous near‐surface reflecting interfaces, and locations where H ∼ L may contain few such reflectors. There is no obvious connection between H/L and surface morphology, but the distribution of non‐unity H/L resembles the extent of a widespread, recent accumulated package (WRAP) at both poles. The spatial association between H/L and WRAP and interference indicated by H/L suggests that large regions in the NPLD—and isolated areas in the SPLD—are characterized by shallow layer(s) of consistent thickness/separation potentially deposited within the past few tens of kyr as Mars emerged from the last obliquity‐driven ice age.
Plain Language Summary
The north and south poles of Mars reflect radar energy differently, where the south polar region is more reflective than the north. We used radar data from the Shallow Radar instrument to investigate how the near‐surface ice and dust layering could affect reflectivity. We found that the north polar region is more densely layered near the surface, leading to radar signal interference that changes the reflectivity. The south polar region, in contrast, has fewer near‐surface reflectors in many locations. Both poles show evidence of interference in similar locations as a widespread young deposit of ice and dust layers that formed in the past ∼300,000 years. Since our data (which describe to the top several meters of the surface) suggest interference occurs in regions similar to this young deposit, one explanation is that these regions of strong radar interference accumulated over the past ∼20,000 years.
Key Points
North and south polar layered deposits have different radar surface reflectivities due in part to layering in the uppermost ∼20 m
Interference effects in radar scattering occur more often in the densely layered north polar deposits
Differences in reflectivity and near‐surface structure could indicate the extent of recent ice/dust accumulation in the past ∼20 kyr
The Lunar Science for Landed Missions workshop was convened at the National Aeronautics and Space Administration Ames Research Center on 10–12 January, 2018. Interest in the workshop was broad, with ...110 people participating in person and 70 people joining online. In addition, the workshop website (https://lunar‐landing.arc.nasa.gov) includes video recordings of many of the presentations. This workshop defined a set of targets that near‐term landed missions could visit for scientific exploration. The scope of such missions was aimed primarily, but not exclusively, at commercial exploration companies with interests in pursuing ventures on the surface of the Moon. Contributed and invited talks were presented that detailed many high priority landing site options across the surface of the Moon that would meet scientific goals in a wide variety of areas, including impact cratering processes and dating, volatiles, volcanism, magnetism, geophysics, and astrophysics. Representatives from the Japan Aerospace Exploration Agency and the European Space Agency also presented about international plans for lunar exploration and science. This report summarizes the set of landing sites and/or investigations that were presented at the workshop that would address high priority science and exploration questions. In addition to landing site discussions, technology developments were also specified that were considered as enhancing to the types of investigations presented. It is evident that the Moon is rich in scientific exploration targets that will inform us on the origin and evolution of the Earth‐Moon system and the history of the inner Solar System, and also has enormous potential for enabling human exploration and for the development of a vibrant lunar commercial sector.
Plain Language Summary
Where should we explore next on the Moon? This report summarizes potential future landing sites on the surface of the Moon, as presented at the Lunar Science for Landed Missions Workshop in January 2018 at NASA Ames.
Key Points
This report outlines lunar sites that landed missions could visit for scientific exploration
High‐priority lunar mission targets will aid future public‐private partnerships on the Moon
A series of mission enhancing technology priorities are identified
When the OSIRIS-REx spacecraft pressed its sample collection mechanism into the surface of Bennu, it provided a direct test of the poorly understood near-subsurface physical properties of rubble-pile ...asteroids, which consist of rock fragments at rest in microgravity. Here, we find that the forces measured by the spacecraft are best modeled as a granular bed with near-zero cohesion that is half as dense as the bulk asteroid. The low gravity of a small rubble-pile asteroid such as Bennu effectively weakens its near subsurface by not compressing the upper layers, thereby minimizing the influence of interparticle cohesion on surface geology. The underdensity and weak near subsurface should be global properties of Bennu and not localized to the contact point.
Mars has experienced widespread glaciation across the mid-latitudes during the Late Amazonian. Deglaciation has altered these mid-latitude regions, and the characteristics of deglaciation can be ...useful in determining the variability in environmental response to climate change. The paraglacial period describes the period over which deglaciation occurred, and is characterized by a suite of features that form due to ice loss. Glaciated craters in the martian mid-latitudes were documented for evidence of paraglacial activity to determine how local crater setting affects deglaciation. Five paraglacial features were identified: spatulate depressions, washboard terrain, gullies, polygons, and broad pits, and their occurrence in each glaciated crater was recorded. 71% of glaciated craters (~450 craters) contained some evidence of paraglacial activity. Relatively more southern hemisphere glaciated craters contained paraglacial features (89%) than northern hemisphere glaciated craters (42%). The spatial density of paraglacial features varies with location. Different combinations of paraglacial features were found in each crater, although some features were preferentially associated, including washboard terrain and gullies, and broad pits and polygons, suggesting dependent formation mechanisms. More types of paraglacial features (up to all five features) were found in small craters (~5–10 km) at a range of elevations, and at modest latitudes (~35–45°), which corresponds to a large region of the southern highlands. Few or no paraglacial features were found in craters containing glacial fill exceeding ~70% of their predicted depth. The thickness of crater fill appears to be the dominant control on paraglacial response, which is affected by crater diameter, and partly by latitude. The variation in thickness of crater fill is attributed to variable accumulation and ablation rates during peak glacial periods during the Late Amazonian. The similarity in paraglacial feature morphology across the mid-latitudes of both hemispheres suggests that deglaciation and paraglaciation operate via similar mechanisms, although climatic conditions and geologic setting at each crater will determine the specific pathway for paraglacial activity to occur. These observations can be used to predict where future paraglacial activity will occur, and where it may be inhibited.
•Martian glaciated craters contain abundant evidence of Late Amazonian deglaciation•Paraglacial modification indicates geologic activity postdating glaciation•Southern hemisphere craters experienced more paraglacial activity than northern•Craters experience different degrees of deglaciation based on preserved CCF depth
Abstract
The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission recently returned a sample of rocks and dust collected from asteroid Bennu. ...We analyzed the highest-resolution thermal data obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES) to gain insight into the thermal and physical properties of the sampling site, including rocks that may have been sampled, and the immediately surrounding Hokioi Crater. After correcting the pointing of the OTES data sets, we find that OTES fortuitously observed two dark rocks moments before they were contacted by the spacecraft. We derived thermal inertias of 100–150 (±50) J m
−2
K
−1
s
−1/2
for these two rocks—exceptionally low even compared with other previously analyzed dark rocks on Bennu (180–250 J m
−2
K
−1
s
−1/2
). Our simulations indicate that monolayer coatings of sand- to pebble-sized particles, as observed on one of these rocks, could significantly reduce the apparent thermal inertia and largely mask the properties of the substrate. However, the other low-thermal-inertia rock that was contacted is not obviously covered in particles. Moreover, this rock appears to have been partially crushed, and thus potentially sampled, by the spacecraft. We conclude that this rock may be highly fractured and that it should be sought in the returned sample to better understand its origin in Bennu’s parent body and the relationship between its thermal and physical properties.
The sample of asteroid (101955) Bennu was collected from the Nightingale sample site by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer spacecraft and ...arrived on Earth on 24 September 2023. To better understand Bennu's parent body, we identified boulders over 2 m in diameter around the Nightingale region and analyzed normal albedo, morphology, and surface roughness. We found that boulders can be separated into two groups based on albedo, and four groups using morphology including angularity, texture, and the presence or absence of clasts, layers, and bright spots: Type A is rounded, rugged, and clastic, with the highest root‐mean square deviation roughness; Type B is sub‐angular with intermediate roughness and polygonal surface fractures; Type C is angular, has distinct fractures, and the lowest roughness; and Type D is sub‐angular with intermediate roughness and bright spots. Unsupervised clustering algorithms showed that our Type A‐D classification represents the diversity in the morphology and albedo data. Using documented contacts between boulder groups, we conclude that boulders on Bennu originated on a single, heterogeneous parent body that experienced vertical mixing via impacts prior to or during its disruption. The boulder morphologies on Bennu bear striking resemblance to those on asteroid Ryugu, potentially suggesting a shared origin. Finally, from analyses of sample collection images, we predict that the sample will be heterogeneous in morphology, brightness, and degree of aqueous alteration and dominated by darker Type A and B material. These predictions are supported by initial analyses of the Ryugu sample.
Plain Language Summary
Asteroid Bennu is composed of fragments of an ancient, disrupted parent body. The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer spacecraft investigated Bennu and collected a sample, which arrived on Earth on 24 September 2023. We investigated the brightness, appearance, and roughness of boulders near where the sample was collected to better understand the parent body and the diversity of Bennu's surface materials. We found that boulders can be broken into four groups: Type A are dark, rough, and have clasts; Type B are smoother and are similarly bright and rough to Type A; Type C boulders are brighter and very smooth; and Type D boulders have distinct bright spots and similar brightness as Type C. Certain boulders contain more than one morphology, suggesting they formed near each other on the parent body, and impacts onto the parent body mixed rocks from different depths and cemented them into the boulders we observe on Bennu. Bennu boulders resemble those on Ryugu, which may mean they are related. We predict that dark Type A and B particles will be most abundant in the returned sample.
Key Points
Boulders in the Nightingale sample region can be divided into four groups that vary in albedo, morphology, and surface roughness
Contacts between groups indicate a single heterogeneous parent body modified by impacts and aqueous alteration prior to disruption
We predict the returned sample will be diverse in reflectance and morphology that reflects the heterogeneity of meter‐scale boulders
•The paraglacial period describes transient post-glacial processes on Earth.•Martian paraglacial modification is observed in a mid-latitude crater interior.•Key features are: spatulate depressions, ...gullies, washboard terrain, and polygons.•The Mars paraglacial period has been active for at least the last few million years.•Cold, arid conditions extend the martian paraglacial period relative to Earth.
On Earth a transitional phase between glacial and interglacial periods is referred to as the paraglacial period. This period immediately postdates glacial retreat and is characterized by ice removal, glacial unloading, and the exposure of steep slopes and large sediment stores. These responses led to the development of a suite of morphologic units (e.g., talus cones, gullies, sackungen, and polygons) which, when observed together, are indicative of the paraglacial period. A similar period of transitional climate and deglaciation is identified on Mars in the Late Amazonian, characterized by the association of features in a glaciated 10.6 km diameter mid-latitude crater. This crater contains concentric crater fill (CCF) formed by debris-covered glaciers, as well as a suite of stratigraphically younger geomorphic units (e.g., spatulate depressions, washboard terrain, gullies, and polygonal terrain) that are all indicative of the local environmental response to deglaciation. These features are interpreted to represent a geologically recent martian paraglacial period within this crater. The morphology and relative stratigraphic relationships among these paraglacial features are described in order to assess the processes operating during deglaciation and to document the recent history of glaciation on Mars: spatulate depressions formed by the differential sublimation of pure glacial ice near the base of the crater wall; subsequently, due to the loss of basal support and steepened slopes, remnant ice on the crater wall began to flow downhill, and formed transverse crevasses that created washboard terrain. Continuous thermal cycling of sediment-mantled ice on crater walls created fractures that formed polygonal terrain. During this time and after, gullies formed by the transport of sediment downslope from crater rim alcoves. Analyses of modeled obliquity variations suggest that the paraglacial period could have operated within the last ∼5 Myr and may still be ongoing, suggesting that the current martian paraglacial period is much longer in duration than typical paraglacial periods on Earth. Understanding the nature and sequence of paraglacial activity can help to identify variations in climate in recent Mars history.
Abstract
The Aristarchus plateau hosts a diversity of volcanic features, including the largest pyroclastic deposit on the Moon, the largest sinuous rille on the Moon, and intrusive and extrusive ...examples of evolved, Th-rich silicic lithologies. We provide an overview of previous remote-sensing measurements of the Aristarchus plateau and provide new analyses of Diviner Lunar Radiometer thermal IR data, Lunar Prospector Gamma Ray Spectrometer Th data, Chang’e-5 Microwave Radiometer data, and hyperspectral and multispectral visible/near-infrared images and spectra from the Chandrayaan-1 Moon Mineralogy Mapper and the Kaguya Multispectral Imager. The rich diversity of volcanic features on the Aristarchus plateau presents an opportunity for a sustained science and exploration program. We suggest a series of missions to the Aristarchus crater floor or ejecta, the Cobra Head, and Herodotus Mons to investigate the link between pyroclastic, effusive basaltic, and silicic volcanism in the region. Such missions would enable analyses of silicic rocks that are rare in the Apollo sample collection and demonstrate in situ resource utilization of FeO- and H
2
O-bearing pyroclastic materials.
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