The structure of the upper layer of a comet is a product of its surface activity. The Rosetta Lander Imaging System (ROLIS) on board Philae acquired close-range images of the Agilkia site during its ...descent onto comet 67P/Churyumov-Gerasimenko. These images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters. At the highest resolution of 1 centimeter per pixel, the surface appears granular, with no apparent deposits of unresolved sand-sized particles. The thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters. The presence of aeolian-like features resembling wind tails hints at regolith mobilization and erosion processes. Modeling suggests that abrasion driven by airfall-induced particle "splashing" is responsible for the observed formations.
Gullies on Mars resemble water-carved channels on Earth, but they are mostly at elevations where liquid water is not expected under current climate conditions. It has been suggested that sublimation ...of carbon dioxide ice alone could have formed Martian gullies. We used a general circulation model to show that the highest-elevation Martian gullies coincide with the boundary of terrain that experienced pressures above the triple point of water when Mars' rotational axis tilt reached 35°. Those conditions have occurred repeatedly over the past several million years, most recently ~630,000 years ago. Surface water ice, if present at these locations, could have melted when temperatures rose >273 kelvin. We propose a dual gully formation scenario that is driven by melting of water ice followed by carbon dioxide ice sublimation.
New maps of kilometer‐scale topographic roughness and concavity of the Moon reveal a very distinctive roughness signature of the proximal ejecta deposits of the Orientale basin (the Hevelius ...Formation). No other lunar impact basin, even the just‐preceding Imbrium basin, is characterized by this type of signature although most have similar types of ejecta units and secondary crater structures. The preservation of this distinctive signature, and its lack in basins formed prior to Orientale, is interpreted to be the result of seismically induced smoothing caused by this latest major basin‐forming event. Intense seismic waves accompanying the Orientale basin‐forming event preceded the emplacement of its ejecta in time and operated to shake and smooth steep and rough topography associated with earlier basin deposits such as Imbrium. Orientale ejecta emplaced immediately following the passage of the seismic waves formed the distinctive roughness signature that has been preserved for almost 4 billion years.
Key Points
Maps of topographic roughness and concavity are derived from LOLA data
Orientale ejecta have unique roughness/concavity signature
Orientale impact caused global smoothing due to seismic shaking
High‐resolution altimetry and imaging have revealed the presence of a meters‐thick sedimentary layer at middle to high northern and southern latitudes presently covering at least 23% of the planet. ...The layer is interpreted to be water‐ice‐rich, and to undergone degradation recently. Its activity very likely coincided with the last major obliquity excursion a few hundred thousand years ago. The majority of the layer at higher latitudes, however, persisted for a much longer time in the Late Amazonian. Stratigraphic analysis suggests a complex history of successive episodes of deposition and removal. Repeated deposition and removal of the mantles are interpreted to be responsible for the unusual statistical properties of kilometer‐scale topography in the transitional mid‐latitude zones.
Recent ice ages on Mars Head, James W; Mustard, John F; Kreslavsky, Mikhail A ...
Nature,
12/2003, Letnik:
426, Številka:
6968
Journal Article
Recenzirano
A key pacemaker of ice ages on the Earth is climatic forcing due to variations in planetary orbital parameters. Recent Mars exploration has revealed dusty, water-ice-rich mantling deposits that are ...layered, metres thick and latitude dependent, occurring in both hemispheres from mid-latitudes to the poles. Here we show evidence that these deposits formed during a geologically recent ice age that occurred from about 2.1 to 0.4 Myr ago. The deposits were emplaced symmetrically down to latitudes of approximately 30 degrees--equivalent to Saudi Arabia and the southern United States on the Earth--in response to the changing stability of water ice and dust during variations in obliquity (the angle between Mars' pole of rotation and the ecliptic plane) reaching 30-35 degrees. Mars is at present in an 'interglacial' period, and the ice-rich deposits are undergoing reworking, degradation and retreat in response to the current instability of near-surface ice. Unlike the Earth, martian ice ages are characterized by warmer polar climates and enhanced equatorward transport of atmospheric water and dust to produce widespread smooth deposits down to mid-latitudes.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Permafrost patterned ground forms on the surface of nearly all landscapes between 50°N and 70°N on Mars. This landform appears in satellite imagery as an interconnected network of polygonal shapes. ...Previous studies used geomorphologic observations to characterize patterned ground terrains on Mars. These classification systems prove useful but suffer from somewhat subjective methods. We find objective analysis of the polygons comprising patterned ground terrains on the surface of Mars feasible using High Resolution Imaging Science Experiment imagery acquired from orbit. We perform a two‐dimensional Fourier transform on 124 images of patterned surfaces to characterize the spatial scale pertinent to scenes and analyze the distribution of these properties on the surface of Mars between 50°N and 70°N. We find two distinct sets of polygons: large polygons which form below 60°N and small polygons primarily above 60°N with exception for sites within Acidalia Planitia. Our findings show similar trends to those found in previous studies but also fundamental differences in the populations of polygons above and below 60°N. The polygons within 60°N–70°N share roughly the same polygonal scale, implying that seasonal temperature change is not the only factor forcing polygon development as previous models found. In some cases, our method indicates the presence of multiple scales of polygons, although it cannot quantify the larger scale. This method in conjunction with observational analysis improves our ability to characterize surfaces and examine patterned ground terrains on Mars.
Key Points
Fourier transforms of Martian patterned ground yield reliable characterizations
Fourier transforms of patterned ground give results similar to previous surveys
The characteristic spatial scale is a proxy for polygon diameter
Martian high‐latitude zones are covered with a smooth, layered ice‐rich mantle containing characteristic polygonal patterns. The mantle is especially uniform and homogeneous in the northern plains. ...We performed a survey of decameter‐scale mantle textures and circular features, including impact craters, in the northern lowlands down to 50°N latitude. Strongly altered and mantled craters of impact and non‐impact origin form a population of subtle circular features below the mantle. Pits of non‐impact origin are numerous in some regions at lower latitudes. Impact craters superposed on the mantle are small and very sparse. The inferred mean crater retention age of the mantle is ∼0.1 Ma. The spatial distribution of young craters suggests an age difference between the highest latitudes (younger) and some lower‐latitude regions (older). Latitudinal trends in polygon textures agree with impact crater evidence for a latitudinal age progression of mantle properties.
Secondary impact craters, features created by projectiles ejected from a primary impact, contain important information about the primary cratering event and the nature and distribution of its ejecta. ...The Orientale impact basin (D ~ 930 km) is the youngest and the least degraded large impact basin on the Moon and has the most recognizable secondary impact craters. We identified and mapped 2,728 secondary craters in the investigated area of ~1.66 × 107 km2, covering an area from the rim of Orientale to six radii. Secondary crater diameters range from ~2 to 27 km, and the median diameter decreases as distance increases. Secondary craters are concentrated predominantly in the northwest and southwest. The ejecta deposit pattern inferred from secondary crater distribution suggests that the Orientale basin was formed by an oblique impact in which the downrange direction was 240°–265° in azimuth, and the incidence angle was steeper than 20°. The cumulative size‐frequency distribution of mapped secondary craters steepens as diameter increases and is very well approximated with a Weibull distribution with an exponent 1.32. A widely used crater scaling relationship predicts that the fragments that produced the secondary craters were predominantly in ~0.5–2‐km diameter range over the investigated area; the diameter of the largest fragment, however, decreases with increasing distance from Orientale. On the basis of the diameter of the largest secondary crater of Orientale, and other craters and basins, the largest secondary crater of the South Pole‐Aitken basin is estimated to be ~40 km in diameter. We explore the implications of these findings for the evolution of the megaregolith and future sample return missions.
Key Points
We mapped 2,728 Orientale basin secondary craters (diameter ≈2–27 km) within 6 radii from the Orientale rim crest
The spatial distribution of secondaries indicates that the Orientale impactor approached from the 60° to 85° azimuth direction with an incidence angle >20°
The largest Orientale basin ejecta fragment size decreases with increasing distance and fragments diameters in 0.5–2 km are dominant in the investigated area
The acquisition of new global elevation data from the Lunar Orbiter Laser Altimeter, carried on the Lunar Reconnaissance Orbiter, permits quantification of the surface roughness properties of the ...Moon at unprecedented scales and resolution. We map lunar surface roughness using a range of parameters: median absolute slope, both directional (along‐track) and bidirectional (in two dimensions); median differential slope; and Hurst exponent, over baselines ranging from ∼17 m to ∼2.7 km. We find that the lunar highlands and the mare plains show vastly different roughness properties, with subtler variations within mare and highlands. Most of the surface exhibits fractal‐like behavior, with a single or two different Hurst exponents over the given baseline range; when a transition exists, it typically occurs near the 1 km baseline, indicating a significant characteristic spatial scale for competing surface processes. The Hurst exponent is high within the lunar highlands, with a median value of 0.95, and lower in the maria (with a median value of 0.76). The median differential slope is a powerful tool for discriminating between roughness units and is useful in characterizing, among other things, the ejecta surrounding large basins, particularly Orientale, as well as the ray systems surrounding young, Copernican‐age craters. In addition, it allows a quantitative exploration on mare surfaces of the evolution of surface roughness with age.
We examine a narrow latitudinal band (60°N–70°N) on Mars to place constraints on the seasonally averaged velocity Q of boulder movements over patterned ground. These latitudes comprise a region of ...the northern lowlands where patterned ground covers nearly every surface. Here boulders meters in diameter are consistently found to be concentrated at or near the cracks that define the polygonal networks, indicating a mobilization process. Because impact craters are the source for many boulders, we can use craters and their degradation to estimate the time scales for boulder movement. We study and catalog 1018 degraded impact craters (100 m < D < 1 km) in 55 High Resolution Imaging Science Experiment (HiRISE) images. We find that crater degradation occurs on a time scale <∼1 Ma, which is too recent for melting‐related mechanisms in a past, warmer epoch. Clustering of boulders occurs at a time scale of a few Ma or shorter, which means that boulders on 5–20 m diameter polygons move at seasonal velocities Q ∼1–10 μm/yr or faster.
Key Points
Boulders on patterned ground terrain cluster requiring boulder movement
We place limits on the timescales of crater degradation and boulder clustering
Crater degradation and boulder distribution evolution are linked
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