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 crat...

Abstract. Mars Pathfinder obtained multispectral, elemental, magnetic, and physical measurements of soil and dust at the Sagan Memorial Station during the course of its 83 sol mission. We describe initial results from these measurements, concentrating on multispectral and elemental data, and use these data, along with previous Viking, SNC meteorite, and telescopic results, to help constrain the origin and evolution of Martian soil and dust. We find that soils and dust can be divided into at least eight distinct spectral units, based on parameterization of Imager for Mars Pathfinder (IMP) 400 to 1000 nm multispectral images. The most distinctive spectral parameters for soils and dust are the reflectivity in the red, the red/blue reflectivity ratio, the near-IR spectral slope, and the strength of the 800 to 1000 nm absorption feature. Most of the Pathfinder spectra are consistent with the presence of poorly crystalline or nanophase ferric oxide(s), sometimes mixed with small but varying degrees of well-crystalline ferric and ferrous phases. Darker soil units appear to be coarser-grained. compacted, and/or mixed with a larger amount of dark ferrous materials relative to bright soils. Nanophase goethite, akaganeite, schwertmannite, and maghemite are leading candidates for the origin of the absorption centered near 900 nm in IMP spectra.

This full-scale model with fully operational lander cameras and surface-sampler subsystem was installed adjacent to a large sand box representing the area in reach of the surface sampler. The Science Test Lander was used during the mission to develop and verify surface-sampler commands and to perform some experiments for the Physical Properties Investigation. CircularS-band radio antenna of lander is 0.76 m wide. Locations of spacecraft parts, cameras, and surface-sampler subsystem are shown in figures 1 and 3. PHYSICAL PROPERTIES OF THE SURFACE MATERIALS AT CONTENTS i\bstract ---------------------------Introduction--------------------------Viking missions ----------------------Supplemental sections-------------------i\cknowledgments---------------------Mission description and conventions -------------Viking lander -----------------------Spacecraft parts ---------------------Surface-sampler subsystem ----------------Mission operations --------------------Other Viking experiments and investigations -------Coordinate systems and measurements ----------Time --------·_ ------------------Sample-field definition ------------------The landing sites ----·-------------------Orbiter views -----------------------Lander panoramas --------------------Sample fields -----------------------Landing ---'-------------------------I>escent -----------------------~ --Touchdown ------------------------Lander 1 ----------------------Lander 2 ----------------------I>iscussion ---------------------- exhaust erosion ------------------35 Surface-sampler data ---------------------44 i\ctivities -------------------------46 Sample trenches ---------------------48 I>rift material --------------------48 Blocky material -------------------52 Crusty to cloddy material --------------56 I>iscussion ----------------------58 Surface-bearing tests -------------------63 I>rift material --------------------63 Blocky material -------------------66 Crusty to cloddy material --------------68 · Backhoe touchdowns -------------------70 Surface impacts ----------------------77 Slope stability-------------------------81 Trench walls -----------------------81 XRFS 1 trench -------------------82 Physical Properties 1 trench ------------. 82 I>eep Hole 1 ---------------------84 Inferred cohesions------------------84 Natural slopes-----------------------84 Conical piles and tailings -----------------88 Summary -------------------------88 The quest for rocks ----------------------88 Sample acquisitions --------------------90 Purges and rock piles -------------------91 Comminution -----------------------92 Rock pushing -----------------------99 Summary -------------------------101 Temperatures -------------------------101 Surface-sampler diurnal temperatures-----------102 Lander 1 ----------------------102 Lander 2 ----------------------103 Interim period ----------------------104I>iscussion----------------------. :. . . --104 Page Visual observations of changes ----------------105 Conical piles------------------------106 Materials in the footpads ----------------...

Abstract. Rocks at the Mars Pathfinder site are probably locally derived. Textures on rock surfaces may indicate volcanic, sedimentary, or impact-generated rocks, but aeolian abration and dust coatings prevent unambiguous interpretation. Multispectral imaging has resolved four spectral classes of rocks: gray and red, which occur on different surfaces of the same rocks; pink, which is probably soil crusts; and maroon, which occurs as large boulders, mostly in the far field. Rocks are assigned to two spectral trends based on the position of peak reflectance: the primary spectral trend contains gray, red, and pink rocks; maroon rocks constitute the secondary spectral trend. The spatial pattern of spectral variations observed is oriented along the prevailing wind direction. The primary spectral trend arises from thin ferric coatings of aeolian dust on darker rocks. The secondary spectral trend is apparently due to coating by a different mineral, probably maghemite or ferrihydrite. A chronology based on rock spectra suggests that rounded maroon boulders constitute the oldest petrologic unit (a flood deposit), succeeded by smaller cobbles possibly deposited by impact, and followed by aeolian erosion and deposition. Nearly linear chemical trends in alpha proton X-ray spectrometer rock compositions are interpreted as mixing lines between rock and adhering dust, a conclusion supported by a correlation between sulfur abundance and red/blue spectral ratio. Extrapolations of regression lines to zero sulfur give the composition of a presumed igneous rock. The chemistry and normative mineralogy of the sulfurfree rock resemble common terrestrial volcanic rocks, and its classification corresponds to andesite. Igneous rocks of this composition my occur with clastic sedimentary rocks or impact melts and breccias. However, the spectral mottling expected on conglomerates or breccias is not observed in any APXS-analyzed rocks. Interpretation of the rocks as andesites is complicated by absence of a "1 gm" pyroxene absorption band. Plausible explanations include impact glass, band masking by magnetite, or presence of calcium-and iron-rich pyroxenes and olivine which push the absorption band minimum past the imager's spectral range. The inferred andesitic composition is most sinfilar to terrestrial anorogenic icelandites, formed by fractionation of tholeiitic basaltic magmas. Early melting of a relatively primitive Martian mantle could produce an appropriate parent magma, supporting the ancient age of Patlff•nder rocks inferred from their incorporation in Hesperian flood deposits. Although rocks of andesitic composition at the Patlff•nder site may represent samples of ancient Martian crust, inferences drawn about a necessary role for water or plate tectonics in their petrogenesis are probably unwarranted.

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Sketch map of Tharsis region of Mars, showing geographic relations between volcanoes and lava flows. Area approximately the same as in figure I-1.

Chemical analyses returned by Mars Pathfinder indicate that some rocks may be high in silica, implying differentiated parent materials. Rounded pebbles and cobbles and a possible conglomerate suggest fluvial processes that imply liquid water in equilibrium with the atmosphere and thus a warmer and wetter past. The moment of inertia indicates a central metallic core of 1300 to 2000 kilometers in radius. Composite airborne dust particles appear magnetized by freeze-dried maghemite stain or cement that may have been leached from crustal materials by an active hydrologic cycle. Remote-sensing data at a scale of generally greater than ϳ1 kilometer and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catastrophic floods that are relatively dust-free.Mars Pathfinder (named the Sagan Memorial Station) landed on the surface of Mars on 4 July 1997 (Figs. 1 and 2), deployed a small rover (named Sojourner) (Fig. 3), and collected data from three scientific instruments [named Imager for Mars Pathfinder (IMP), ␣-proton x-ray spectrometer (APXS), and atmospheric structure investigation/meteorology package (ASI/MET)] and technology experiments (1). In the first month of surface operations the mission returned about 1.2 gigabits of data, which include 9669 lander and 384 rover images and about 4 million temperature, pressure, and wind measurements. The rover traversed a total of about 52 m in 114 commanded movements, performed 10 chemical analyses of rocks and soil, carried out soil mechanics and technology experiments, and explored over 100 m 2 of the martian surface.Pathfinder used a rover, carrying a chemical analysis instrument, to characterize the rocks and soils in a landing area over hundreds of square meters on Mars, which provides a calibration point or "ground truth" for orbital remote-sensing observations (1, 2). The combination of spectral imaging of the landing area by the IMP, chemical analyses by the APXS aboard the rover, and close-up imaging of colors, textures, and morphologies with the rover cameras offers the potential for identifying rocks (petrology and mineralogy). Before the Pathfinder mission, knowledge of the kinds of rocks present on Mars was based mostly on the martian meteorites (all mafic igneous rocks) and inferences from Viking data (3, 4). In addition, small valley networks in heavily cratered terrain on Mars have been used to argue that the early martian environment may have been warmer and wetter (with a thicker atmosphere), at which time liquid water may have been stable (5).The Ares Vallis landing site (Fig. 4) was selected because it appeared acceptably safe and offered the prospect of analyzing a variety of rock types expected to be deposited by catastrophic floods, which enable addressing first-order scientific questions such as differentiation of the crust, the development of weathering products, and the nature of the early martian environment and its subsequent evolution (2). In the selection of the Pathfinder landing site...

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