Distribution Of Impact Craters Research Articles

Venus's shield terrain

Research Article| May 01, 2005 Venus's shield terrain Vicki L. Hansen Vicki L. Hansen 1Department of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Vicki L. Hansen 1Department of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812, USA Publisher: Geological Society of America Received: 02 Mar 2004 Revision Received: 11 Oct 2004 Accepted: 14 Oct 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2005) 117 (5-6): 808–822. https://doi.org/10.1130/B256060.1 Article history Received: 02 Mar 2004 Revision Received: 11 Oct 2004 Accepted: 14 Oct 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Vicki L. Hansen; Venus's shield terrain. GSA Bulletin 2005;; 117 (5-6): 808–822. doi: https://doi.org/10.1130/B256060.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Plains, planitiae, or lowlands—expanses of gentle, long-wavelength (∼1000 km) basins—cover ∼80% of Venus's surface. These regions are widely accepted as covered by volcanic flows, although the mechanism(s) responsible for resurfacing remains elusive; in addition, a volcanic origin for the lowland surface may be open to question. Lowland resurfacing is typically attributed to catastrophic emplacement (10–100 m.y.) of globally extensive, thick (1–3 km) flood-type lava. This model of resurfacing has been postulated on the basis of impact crater distribution, taken together with a lack of obvious volcanic flows or edifices, and a lack of viable alternative models. Ongoing geologic mapping of ∼15,000,000 km2 (0–25N/90–150E) using 225 m/pixel and 75 m/ pixel NASA Magellan SAR (synthetic aperture radar) data indicates that small edifices, called shields (1–15-km-diameter edifices, <<1 km high), play a major role in lowland resurfacing. Individual shields are radar-smooth or -rough, quasi-circular to circular features with or without a central pit. Shield shapes, which have been previously documented, range from shield, dome, or cone, to flat-topped or flat. Shield deposits typically coalesce, forming a thin, regionally extensive but discontinuous, mechanically strong layer, herein called shield paint. Shield paint conforms to delicate local topography, providing evidence of its thin character and indicating generally low viscosity during emplacement. Shield terrain (shields and shield paint) covers more than 10,000,000 km2 within the study area. Detailed mapping of five 2° × 2° regions using coregistered normal and inverted right- and left-illumination SAR imagery indicates shield densities of 3500–33,500 shields/106 km2; thus, the map area hosts more than 35,000–335,000 shields. Shield terrain generally postdates, but is also locally deformed by, fractures and wrinkle ridges, indicating tim-transgressive formation relative to local deformation and/ or reactivation. The regional scale crust was strong throughout shield-terrain formation. Shield terrain may extend across much of Venus's surface. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

미행성 지구충돌의 역사 II: 2002년 데이터베이스를 이용한 주기분석

Grieve(1991), Moon et al.(2001), Earth Impact Database(2002; DB02) 등 세 가지 충돌구 DB를 이용해서 미행성 지구충돌 사건의 주기성 여부를 조사했다. 특히 DB02는 과거 사용됐던 데이터베이스들에 비해서 연령 측정오차가 크게 향상되었으며, 이를 이용한 주기분석 발표는 이번이 처음이다. DB02(나이 <TEX>$t{\leq}250Myr$</TEX> 및 <TEX>$t{\leq}150Myr$</TEX>, 나이오차 <TEX>${\Delta}t{\leq}{\pm}10Myr$</TEX>)에 대 한 Fourier 분석 결과, <TEX>$D{\geq}5km$</TEX>와 <TEX>$D{\geq}20km$</TEX>인 표본의 경우, 주기성이 검출되지 않았다. 그러나 <TEX>$D{\geq}30km$</TEX>인 표본인 경우 16.1Myr와 34.7Myr에서 두 개의 피크가 나타났고, <TEX>$D{\geq}45km$</TEX>인 표본은 16.1Myr부근에서 강한 파워를 보였다. 이로부터 우리는 <TEX>$d{\geq}1.5km$</TEX>인 천체의 주기적인 지구 충돌 가능성을 고려할 수 있으며, 충돌구 나이의 정밀도는 물론 t와 D의 범위, 그리고 데이터베이스의 선택이 미행성의 지구충돌의 역사를 재구성하는데 중요한 변수로 작용한다는 사실을 확인했다. We examined the hypothesis that the crater formation rate exhibits periodicity, employing data sets of Grieve (1991), Moon et al. (2001), and the Earth Impact Database(2002; DB02). DB02 is known to supercede previous compilations in terms of its accuracy and precision of the ages; it is the first time that this database has been used for periodicity analysis. For data sets comprising impact structures with <TEX>$D{\geq}5km$</TEX>(and also those with <TEX>$D{\geq}20km$</TEX>), there is no convincing evidence for periodicities in the crater ages, according to our Fourier analysis. However, we detected two peaks at 16.1Myr and 34.7Myr for craters with <TEX>$D{\geq}30km$</TEX>; we confirm that the age distribution of impact craters with <TEX>$D{\geq}45km$</TEX> has dominant power at 16.1Myr. Thus, we may conjecture a probable periodic shower of Earth impactors with sizes <TEX>$d{\geq}1.5km$</TEX>. In addition, we found that the selection of data sets, the lower limits on the ages and diameters of impact craters, as well as the accuracy and precision of the ages, all constitute crucial factors in reconstructing the impact cratering history of the Earth.

William Wilson Mullins

William Wilson Mullins

Morphology, evolution and tectonics of Valles Marineris wallslopes (Mars)

Hillslopes up to 11 km in height can be found along the walls of the Valles Marineris troughs. The widest and deepest troughs are grabens, in which tectonics probably exerted the primary control on the wall morphology. Geographical variations in the wall morphology and profiles show that they result from complex, persistent tectonic influences, and that significant changes in erosional processes occurred during this evolution, from late Hesperian to late Amazonian. Preliminary calculations suggest that about 85–95% of the fault-controlled wall relief probably formed in an “ancient” stage prior to this transitional period. A study of the volatile content of the wall rocks, based upon the morphology and distribution of impact craters on the surrounding plateaus, shows that extreme erosional widening of the Central Valles Marineris troughs occurred during the “ancient” stage of high ground ice content. During the subsequent “recent” stage of tectonic and morphological evolution, the wall materials were partly desiccated.

Catastrophic Resurfacing and Episodic Subduction on Venus

There is clear evidence that a catastrophic resurfacing event occurred on Venus in the relatively recent past. A primary data source for this resurfacing event is the spatial distribution of impact craters. In this paper, we apply the pair-correlation technique to the observed crater distribution and find that the result is identical to that for a random distribution. In order to test the sensitivity of the technique, we also apply it to the spatial distribution of coronae on Venus. For the coronae, we find substantial deviations from a random distribution. One explanation for the catastrophic resurfacing is the episodic subduction hypothesis. We model episodic subduction using a thermal boundary-layer stability analysis. We find that episodic subduction events with intervals of 500 to 700 Myr can transport only 15–25% of the radiogenic heat produced within the planet. We suggest that the remainder of the heat must be lost to the surface during a period of vigorous tectonic activity, following the subduction event but prior to the subsequent stabilization of a global lithosphere.

Modeling of Ejecta Produced upon Hypervelocity Impacts

Each time a debris particle or a meteoroid strikes a satellite in orbit, a great amount of secondary particles is ejected in the neighborhood of the impact site. This phenomenon is important in particular for brittle materials, such as those used for solar arrays or thermal control paint. The secondary particles that do not impact other parts of the spacecraft are added to the primary debris population and hence increase the small debris particle flux. We describe an ejecta production model that gives the size and the velocity distribution of ejected particles as a function of primary impact parameters. The model has been used to explain the discrepancy between measurements and modeling of impact crater distribution on the solar arrays of the EuReCa spacecraft.

Statistics for modeling heavy tailed distributions in geology : Part II. Applications

In this paper, we present three diverse types of applications of extreme value statistics in geology, namely: earthquakes magnitudes, diamond values, and impact crater size distribution on terrestrial planets. Each of these applications has a different perspective toward tail modeling, yet many of these phenomena exhibit heavy or long tails which can be modeled by power laws. It is shown that the estimation of important tail characteristics, such as the extreme value index, is directly linked to the interpretation of the underlying geological process. Only the most extreme data are useful for studying such phenomena, so thresholds must be selected above which the data become power laws. In the case of earthquake magnitudes, we investigate the use of extreme value statistics in predicting large events on the global scale and for shallow intracontinental earthquakes in Asia. Large differences are found between estimates obtained from extreme value statistics and the usually applied standard statistical techniques. In the case of diamond deposits, we investigate the impact of the most precious stones in the global valuation of primary deposits. It is shown that in the case of Pareto-type behavior, the expected value of few extreme stones in the entire deposit has considerable influence on the global valuation. In the case of impact crater distributions, we study the difference between craters distributions on Earth and Mars and distributions occurring on other planets or satellites within the solar system. A striking result is that all planets display the same distributional tail except for Earth and Mars. In a concluding account, we demonstrate the apparent loghyperbolic variation in all of the above-mentioned examples.

Europa: Initial Galileo Geological Observations

Images of Europa from the Galileo spacecraft show a surface with a complex history involving tectonic deformation, impact cratering, and possible emplacement of ice-rich materials and perhaps liquids on the surface. Differences in impact crater distributions suggest that some areas have been resurfaced more recently than others; Europa could experience current cryovolcanic and tectonic activity. Global-scale patterns of tectonic features suggest deformation resulting from non-synchronous rotation of Europa around Jupiter. Some regions of the lithosphere have been fractured, with icy plates separated and rotated into new positions. The dimensions of these plates suggest that the depth to liquid or mobile ice was only a few kilometers at the time of disruption. Some surfaces have also been upwarped, possibly by diapirs, cryomagmatic intrusions, or convective upwelling. In some places, this deformation has led to the development of chaotic terrain in which surface material has collapsed and/or been eroded.

Duration of tessera deformation on Venus

The density and distribution of impact craters superposed on the highly deformed tessera terrain on Venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains. Eighty percent of tesserae craters are undeformed. No existing craters exhibit evidence of contractional deformation, suggesting that the early compressional stage of tessera deformation ended abruptly. The small number of craters fractured by late‐stage tessera extension constrains the duration of this phase to less than 20% of the average crater retention age of the tesserae, or approximately 30–60 Ma. These results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains.

Dating volcanism and rifting on Venus using impact crater densities

Although the distribution of impact craters on Venus is indistinguishable from a completely spatially random population on the basis of crater data alone, the addition of geologic information indicates that the craters are not random with respect to geology. Areas of low crater density correlate with concentrations of tectonic and volcanic features, as well as high proportions of faulted and embayed craters. High‐resolution mapping of large volcanoes, flood‐type lava flow fields, rifts, and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance. Because these features also appear young based on stratigraphic data, difference in age is the most likely reason for differences in crater density between geologically defined terrains. Crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters, as long as the mapping criteria are independent of impact crater locations. This result provides a tool to begin identifying and dating major geologic provinces, and to begin developing a geologic history of Venus. Using a simple data model, and calculating crater ages relative to an assumed global mean age of 300 Ma, we show that the ages of large volcanoes (72 ± 45 Ma), flow fields (128 ± 91 Ma), rifts (130 ± 145 Ma), and coronae (120 ± 115 Ma) are all substantially younger than the mean plains age, and probably represent ongoing volcanic and tectonic activity, rather than the end of a brief global resurfacing event with a mean age of 300 Ma.

Comment on “The global resurfacing of Venus” by R. G. Strom, G. G. Schaber, and D. D. Dawson

The distribution of impact craters on Venus has been the subject of a great deal of analysis since the return of Magellan data. Phillips el al. (1992) performed Monte Carlo two-dimensional (2-D) modeling of the areal distribution of craters, and the results of that exercise allowed a restricted, but still quite large, range of possible planetary resurfacing histories, including the possibility that the crater, were emplaced on a geologically inactive planet. However, the nonrandom distribution of embayed and deformed craters (Phillips el al., 1992), the hypsometric distribution of craters (Herrick and Phillips, 1994), the varied degradation states of craters (Izenberg et al., 1994), their nonrandom distribution with different geologic terrain types (Namiki and Solomon, 1994; Price et al, 1994), and three-dimensional resurfacing modeling (Bullock el al., 1993) all seem to argue against that particular possibility. In contrast, Strom el al. (1994) have collected a refined and more comprehensive data set of impact features, and they input these data into more sophisticated 2-D Monte Carlo modeling and statistical analyses of the areal distribution of craters, the hypsometric distribution of craters, and the number of embayed craters. They concluded that 'Venus experienced a global resurfacing event about 300 m.y. ago followed by a dramatic reduction of volcanism and tectonism. This global resurfacing event ended abruptly (less than 10 m.y.). The present crater population has accumulated since then and remains largely intact . . . only about 4%-6% of the planet has been volcanically resurfaced since the global event . . .' If these conclusions are well-founded, this work certainly represents a significant advancement in restricting tile number of plausible resurfacing histories for the planet. If Strom et al. (1994) are correct, it would also mean that all of the other aforementioned works are in error to various degrees, or at least represent overzealous interpretation of the data. However, we have identified apparent flaws in the observations, modeling, and interpretations presented by Strom el al. (1994) that lead us to question whether their conclusions are warranted. We limit our comments to three areas of their analysis: (1) observations pertaining to the number and area of disrupted and pristine craters and crater-related features, (2) modeling of the areal and elevation distribution of craters, and (3) interpretations of resurfacing models.

The tectonics of Venus

Solid Venus has several differences from solid Earth: a mild variation in topography, with marked departures of some kilometres confined to less than 10% of the surface; no interconnected system of ridges, such as would be associated with a spreading lithospheric boundary layer; a high correlation of gravity with topography; and a ratio of gravity to topography implying compensation depths in excess of 100 km for major features. This high admittance ratio implies a stiff upper mantle, as also indicated by the high depth-diameter ratio of craters. The details of Magellan radar imagery enable some inference of event sequences. The morphology generally indicates a more regional scale of change than on Earth. The principal chronometer is impact crater distribution. The number of impact craters identified is 914, which is equivalent to about 500 Ma’s infall of bodies big enough to penetrate the atmosphere. But the number of craters per unit area has but slightly more variations from the mean than random and only a mild negative correlation with topographic elevation. Only one-third of the craters evidence tectonic or volcanic modification. Hence the rate of resurfacing over the last few 100 Ma must be quite slight compared to Earth’s. The main debate is whether Venerean tectonic activity is in monotonic decline, or whether it is episodic on a 500 Ma time-scale. Evidencing some current tectonic and volcanic activity are a few limited regions of marked topographic and geoidal highs, exceeding 8 km and 80 m respectively, with steep slopes. Clearly, plate tectonics does not exist on Venus. The underlying cause of this different evolution appears to be the lack of water. This dryness makes the upper mantle stiff enough to regionalize the tectonics and inhibit recycling of crust. The lack of water also prevents erosion and thus the recycling of secondary differentiates. These deficiencies in recycling imply a strong net upward differentiation of heat sources and a thick crust, now allowed by new results on dry diabase rheology.

Effects of the Venusian Atmosphere on Incoming Meteoroids and the Impact Crater Population

The dense atmosphere on Venus prevents craters smaller than about 2 km in diameter from forming and also causes formation of several crater fields and multiple-floored craters (collectively referred to as multiple impacts). A model has been constructed that simulates the behavior of a meteoroid in a dense planetary atmosphere. This model was then combined with an assumed flux of incoming meteoroids in an effort to reproduce the size-frequency distribution of impact craters and several aspects of the population of crater fields and multiple-floored craters on Venus. The modeling indicates that it is plausible that the observed rollover in the size-frequency curve for Venus is due entirely to atmospheric effects on incoming meteoroids. However, there must be substantial variation in the density and behavior of incoming meteoroids in the atmosphere. Lower-density meteoroids must be less likely to survive atmospheric passage than simple density differences can account for. Consequently, it is likely that the percentage of craters formed by high-density meteoroids is very high at small crater diameters, and this percentage decreases substantially with increasing crater diameter. Overall, high-density meteoroids created a disproportionately large percentage of the impact craters on Venus. Also, our results indicate that a process such as meteoroid flattening or atmospheric explosion of meteoroids must be invoked to prevent craters smaller than the observed minimum diameter (2 km) from forming. In terms of using the size-frequency distribution to age-date the surface, the model indicates that the observed population has at least 75% of the craters over 32 km in diameter that would be expected on an atmosphereless Venus; thus, this part of the curve is most suitable for comparison with the calibrated curves for the Moon. Separation of meteoroid fragments by aerodynamic drag alone is not adequate to explain either the number of multiple impacts or the dispersion of individual craters in multiple impacts. A large transverse velocity must be imparted on the fragments at breakup. This requires that meteoroids that form multiple impacts must have enough strength to resist breakup until sufficient pressure is built up on the leading edge of the meteoroid. It is likely that most multiple impacts were formed by asteroidal, and not cometary, meteoroids.

Geology and distribution of impact craters on Venus: What are they telling us?

Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a data base describing the 842 craters on 89% of Venus' surface mapped through orbit 2578. (The craters range in diameter from 1.5 to 280 km.) We have studied the distribution, size‐density, morphology, geology, and associated surface properties of these craters both in the aggregate and, for some craters, in greater detail. We find that (1) the spatial distribution of craters is highly uniform; (2) the size‐density distribution of large craters (diameters ≥35 km) is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0.5 Ga (based on the estimated population of Venus‐crossing asteroids); (3) unlike the case on other planets, the density of small craters (diameters ≤35 km) declines rapidly with decreasing diameters because of atmospheric filtering; (4) the spectrum of crater modification differs greatly from that on other planets: 62% of all craters are pristine, only 4% are embayed by lavas, and the remainder are affected by tectonism, but none are severely and progressively depleted (as extrapolated from the size‐density distribution of larger craters); (5) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (6) diffuse radar‐bright or ‐dark features surround some craters, and 367 similar diffuse “splotches” with no central crater are observed; and (7) other crater features unique to Venus include radar‐bright or ‐dark parabolic arcs opening westward and extensive outflows originating in crater ejecta. The first three of these observations are entirely unexpected. We interpret them as indicating that the planet's cratering record was erased by a global resurfacing event or events, the latest ending about 0.5 Ga, after which volcanic activity declined (but did not cease entirely). Since the last resurfacing event, a maximum of 10% of the planet has been resurfaced and only about 4% of the craters have been obliterated. Convective thermal evolution models support this interpretation (Arkani‐Hamed and Toksoz, 1984). Observations 3–7 confirm quantitatively the expectation that the dense atmosphere of Venus has strongly affected the production of craters. Large impactors have been relatively unaffected, intermediate‐sized ones have been fragmented and have produced overlapping or multiple craters, a narrow size range has produced shock‐induced “splotches” but no craters, and the smallest bodies have had no observable effect on the surface. The number of craters eliminated by the “atmospheric filter” is enormous, about 98% of the craters between 2 and 35 km in diameter that Magellan might have observed on a hypothetical airless Venus. Unique crater‐related features such as parabolas and outflow deposits demonstrate the roles of Venus' high atmospheric density and temperature in modifying the crater formation process. Finally, heavily fractured craters and lava‐embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis, and Phoebe regiones. These craters thus provide physical evidence for recent volcanic and tectonic activity at a low level.

Can cometary bombardment disrupt synchronous rotation of planetary satellites?

The impact crater distribution observed on the moons of Jupiter and Saturn is more symmetric than that expected from cometary bombardment, which should preferentially scar the leading hemisphere of a synchronously rotating moon. The possibility that cometary impacts may have “spun up” these satellites, resulting in an interchange of leading and trailing hemispheres, is investigated. It is found that such a “spinup” is unlikely for the Galilean satellites of Jupiter and for Saturn's inner midsized moons.

Terrestrial mass extinctions, cometary impacts and the Sun's motion perpendicular to the galactic plane

Episodes of mass extinctions on the Earth are now strongly suspected to be cyclical1. We report here that our analysis of the data of Raup and Sepkoski1 suggests that the dominant cyclicity in major marine mass extinctions during at least the past 250 Myr is 30 ± 1 Myr, with the standard deviation of an individual episode being ±9 Myr. We find this terrestrial cycle to be strongly correlated with the time needed for the Solar System to oscillate vertically about the plane of the Galaxy, which is 33 ± 3 Myr according to the best current astronomical evidence. It is argued that galactic triggering or forcing of terrestrial biological crises may arise as a result of collisions (or close encounters) of the Solar System with intermediate-sized to large-sized interstellar clouds of gas and dust, which are sufficiently concentrated towards the galactic plane to produce the observed cyclicity and its scatter. Among other consequences, a nearby interstellar cloud would gravitationally perturb the Solar System's family of comets and thereby increase the flux of comets and comet-derived bodies near the Earth, leading to large-body impacts. We find a dominant cyclicity of 31 ± 1 Myr in the observed age distribution of impact craters on Earth, the phase of this cycle agreeing with that shown by the major biological crises. Our galactic hypothesis can thus simultaneously account for the mean interval between major terrestrial crises and for the 50% scatter of the time intervals about their mean value.

Use of equivalency in estimating the historical size distribution of impact craters

A previous analysis of a stochastic model of lunar-type impact cratering is extended to utilize geological age data by defining a more general statistic Ω i(t) to be the number of equivalent whole craters of original diameter d i and age ≤ t in an observational area A; each crater is taken to be equivalent to the fraction of its rim (or area) that is in A and not occluded by later craters. By integration of a new gain-loss differential equation, a generalization of the previous basic equation is obtained that relates the expected value ω i(t) = E[Ω i(t)] to the process functions specifying the size distribution and flux of craters (primary or secondary) as they form. The results are specialized to the plausible case in which the cratered body can be subdivided into geological provinces of increasing ages t 1, t 2, …, t i … and the size probability distribution can be approximated as constant within each of the periods t i+1 - t i . It is shown that use of the Ω i permits, in principle, a reconstruction of the historical values of the process functions and correctly compensates for the effect of overlap by removing the false bias favoring large craters that results from the usual method of crater counting. Possible generalizations of the gain-loss equation are indicated.

Some Geologic Problems of Mars

Data on the body of Mars are as yet insufficient to allow computation of meaningful models of the internal structure or density stratification within the planet. The flattening of the solid surface appears to be greater than the flattening of the equipotential surface by about a factor of two. The resultant possibility that the equatorial regions are high compared to the polar areas has many geological and biological implications. Most of the inner planets—Mercury, Venus, Earth, Moon, and Mars—have bulk compositions which differ from one another as well as from that of the stony meteorites. The thermal histories of the planets have therefore differed because of different original compositions as well as different sizes and distances from the Sun. More data on the surface topography are required in order to make geological interpretations which are not largely subjective. The dark areas could be either highlands or lowlands relative to surrounding bright areas. Although sharp relief of several thousand feet is probably absent or nearly so, substantial mountain ranges, plateaus, or basins could exist if the slopes on their flanks are gradual. Liquid water cannot exist on the surface of Mars, and aqueous erosion features, if they once existed, have become subdued by aeolian activity. Winds on Mars may move more and coarser material than winds on Earth. Deep accumulations of aeolian deposits have formed, at least locally. Mariner IV pictures show an impact-crater distribution much like the highland areas of the Moon. Many craters with original depths of less than about 8000–10,000 feet have become obliterated on Mars by windblown dust.