We show evidence of very recent (≤25–40 Myr) geologic activity on the eastern flank of Olympus Mons volcano that includes a suite of fluvial (channel networks), volcanic (emplacement of lava flows and dikes), and tectonic (wrinkle ridges and troughs) processes. The combination and youth of these features confirms the importance of geological activity continuing to the present on Mars.
Densities of impact craters on tessera, which is complex ridged terrain of tectonic origin, and on the remainder of the planet, which is mostly volcanic plains, were studied using Magellan images for about 96% of the surface of Venus. The density of large (D>16 km) impact craters on tessera is higher by a factor of about 1.4 than on the remainder of the planet. This means that the tessera crater retention age is larger than the age of the plains. This is in agreement with the well known fact that tessera is embayed by the surrounding volcanic plains. The density of small (D<16 km) impact craters on tessera is lower than on the remainder of the planet, evidently an observational bias caused by a difficulty in recognizing the small craters on rough tessera terrain. The absence of recognizable tectonic deformation in most of the large on‐tessera craters means that during the period of crater emplacement most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformation.
Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.
This paper considers the morphology of several rocks and rock fragments within and near the Rock Garden at the Mars Pathfinder landing site. We have analyzed stereo images taken both by the lander IMP camera and by the rover forward cameras. The rocks were found to differ in their roundness/angularity and in their densities of millimeter‐to‐centimter‐sized pits and flute‐like features on the rock facets. No correlation between rock roundness and the degree of rock pitting was found, indicating that rounding and pitting formed by different processes. Pits are either gas bubbles (implying the rocks are volcanic), or resulted from eolian abrasion/deflation (putting no constraints on the nature of the rocks), or both. If pits are of eolian origin, variations in the density of pits on different rocks imply that either the rock surfaces have been exposed for different times or the susceptibility of different rocks to pitting varies significantly from rock to rock, implying differences in composition and/or lithology. The densities of pits and flutes on the rocks show anti‐correlation. This may be connected to their formation but probably is mostly related to issues of observation and definition of what is a pit and what is a flute. Flute‐like features may be a result of eolian abrasion, but in this case, the spatial closeness of fluted rocks and those having delicately pitted textures requires explanation. Other mechanisms producing flute‐like features, such as collisions in the deposit‐forming flows and the breaking of rock in impact cratering, should not be ignored. Our observations agree with the conclusion of Golombek et al. [this issue] that deflational exhumation in this area was minor. The observed, relatively large, rocks typically do not show a concentration of small rock fragments at their feet. This implies diurnal thermal cycles are very ineffective at destructing rocks. The rock, Half Dome, may be a conglomerate (as suggested by Smith et al. [1997a] for this and several other rocks), but this hypothesis remains to be proven. Impact craters of a few meters to a few tens meters in diameter may have played a role in the local geology.
Using Magellan SAR images and the Schaber et al. [1998] crater data base we examined impact craters in the area north of 35°N and determined the geologic units on which they are superposed. The crater density of the regional plains with wrinkle ridges (Pwr) was found to be very close to the global average and thus the mean surface age of the plains is close to the mean surface age of the planet (T). About 80 to 97% of the craters superposed on a composite unit that includes materials of Tessera terrain (Tt), Densely fractured plains (Pdf), Fractured and ridged plains (Pfr), and Fracture Belts (FB), also postdate the regional plains. Thus, the time interval between the formation of these older units and emplacement of the regional plains (ΔT) should be geologically short, from a few percent to about 20% of T, or approximately 40 to 150 m.y. This means that in the area under study, volcanic and tectonic activity in the beginning of the morphologically recognizable part of the geologic history of Venus (about the last 750 m.y.) was much more active than in the subsequent time.
