in synchronous rotation, as is the Moon about the Earth, they keep the same face towards Saturn and the same faces leading and trailing in orbital motion. E. M. Shoemaker of the U.S. Geological Survey has estimated that the cratering rates, produced primarily by impact of cometary nucleii, differ from the leading hemispheres to the trailing hemispheres of these moons by factors of about 10 to 1. Based on this model, it is likely, then, that the trailing hemispheres have received far fewer impacts and have been "impact gardened" to an order of magnitude less than the leading hemispheres. It is probable, therefore, that the patterns have survived only in the trailing hemisphere and, at one time, laced the entire globes of Dione and Rhea. Detailed inspection of high-resolution images along the extensions of these bright markings suggest that they are formed on fractures. It is probable that fluids escaping along these fractures produced frost deposits on the surface. Images of Dione show that, in its trailing hemisphere, the bright swaths cross over the top of the impact craters, indicating that the loss of internal fluids along the fracture system had to occur well after cessation of heavy bombardment. This was the first evidence for some endogenic or internal activity of these small moons.

Prior to the Voyager encounters with the Saturnian system, it was known that lapetus was a very bizarre world in that the leading hemisphere was extremely dark and that the trailing hemisphere was very bright. The dark side has an albedo of only a few percent, about that of carbon black or soot. The albedo of the trailing hemisphere, however, is similar to the rest of Saturn's icy moons with a reflectivity of about 50 percent, or about that of dirty snow. One of the models suggested that the leading hemisphere of lapetus might be dark because dark material was falling in from space and being deposited preferentially on the leading hemisphere. The purported source of this material was postulated to be Phoebe. Ground-based telescopic observations of Phoebe suggested that it had colors like carbonaceous chondrites and dark asteroids and, therefore, also might have a very dark surface.

Although Voyager 1 confirmed that Phoebe is extremely dark, Earth-based telescopic data, which was confirmed by Voyager observations, indicated that the color of Phoebe's surface and lapetus's dark leading hemisphere are very different. The Voyager images of lapetus showed that the boundary between the leading and trailing hemispheres was probably too complex to be produced simply by infalling material. Voyager 1 images showed a ring of dark material about 125

miles in diameter extending from the dark hemisphere into the bright hemisphere. The features resembles a moat produced when large craters with central prominences were flooded on the Moon. Voyager 2 images showed that not only is the boundary between the leading and trailing hemispheres angular, but part of that boundary extends deep into the trailing hemisphere. Additionally, Voyager 2 images showed floors of craters covered by the dark material which are located in the center of the trailing hemisphere. The evidence now indicates that some internal process is responsible for the extrusion of dark materials onto the surface of Iapetus which form the dark regions, flooding crater floors, and large basins. The conclusions now are tentative and highly speculative and will have to await a future mission to this strange moon for detailed understanding of its geologic evolution.

Mimas, Enceladus, and Hyperion are the smallest of the classical Saturnian satellites and have diameters between about 150 and 200 miles. By mass, they each are about ten-millionths the mass of the Earth. Prior to the encounters of Voyagers 1 and 2, the suspicion was that these bodies were far too small to have any substantial internal geologic activity. The Voyager images showed these three small objects to have geologic histories that range in diversity over the spectrum of planets we have so far seen in the solar system.

Mimas, an approximately spherical body, is likely an object of cold accretion. An enourmous impact crater found on its leading hemispher is nearly one-half the diameter of the small satellite. Most likely Mimas was subjected to even larger impacts early in its history. Under such an impact, the body would be blasted apart. Because the fragments would have low relative velocities (typically only a few hundred feet per second, comparable to Mimas' escape velocity) compared to Mimas' orbital spread of 15 miles per second, the material would continue orbiting in a tight wreath and slowly reaccrete, thus reforming the moon. Evidently little other geologic activity, except recratering of the surface early in its history, has gone on. Hyperion, in contrast, resembles the new or minor satellites. It is a very irregularly shaped moon, evidently a piece of a larger object that, like Mimas, was intensely battered, but, in this case one large fragment evidently survived. Enceladus, in great contrast to Mimas and Hyperion, was the real surprise of the Voyager 2 encounter. Scientists were suspicious that it might be unusual in that it has an extremely high albedo reflecting nearly 100 percent of the light it

recieves from the Sun. Additionally, Enceladus' orbit coincides with the peak intensity of the E ring, a diffuse ring of material well outside the main ring system. Normally, satellites clean out or sweep out material along their orbits; the evidence indicated that Enceladus is a source rather than a sink of material. Finally, C. Yoder of the Jet Propulsion Laboratory realized that there is an orbital resonance between Dione and Enceladus similar to that by which Europa heats lo, the volcanically active moon of Jupiter. Dione causes Enceladus to move through Saturn's gravitational field in such a way that the surface is flexed. Voyager 2 images of Enceladus show that, in fact, it does have an extremely complex geologic history. Terrains vary in their crater populations from cases where craters are nearly shoulder to shoulder to those where craters are absent, at least to the limit of resolution of the Voyager images. A wide array of other terrains that are intermediate in crater density between these two extreme crater densities also can be identified. Complex "ropy" ridges are found on the margins of some of the crater-free plains. Evidently Enceladus has undergone geologic activity, perhaps episodically, throughout its history. Preliminary estimates by A. Cook and R. Terrile of the Voyager Imaging Science Team suggested that tidal energy may well be inadequate to heat a body composed of pure-water ice. One possible. explanation is that the interiors of the icy moons may contain more volatile species. Methane and ammonia are likely candidates and are known to be abundant in the atmosphere of Titan. Either one of these materials, if present in substantial quantities within the moons, would lower their melting points by perhaps 180° F. to ranges not far above their surface temperatures. Under such conditions, tidal heating, perhaps assisted by some early radiogenic heating, could easily keep such a tiny icy world geologically active.

World Energy ResourcesThe Need to Know Their Quantity and Location

For more than 100 years the U.S. Geological Survey has had the responsibility for assessing mineral resources to provide a basis for informed decisions and policies in both the public and

private sector of the United States. The need for reliable data on the extent, location, and quality of these mineral resources has continued to grow with the broadening of our concerns and commerce. The United States requires mineral supplies from countries around the world, as do all technically advanced societies. Not only is a wide variety of minerals in demand, but, the energy minerals, very large quantities are needed.

Fortunately, the United States is very well endowed with mineral wealth and especailly with the energy minerals. We are, and have been throughout the period of the Industrial Revolution, one of the world's principal producers of petroleum, coal, and more recently uranium. Our capacity to produce and discover remains high, but so does our demand. For instance, the demand for petroleum over the past decade has been about three times the rate of domestic reserves additions. The Nation presently is embarked on a major exploratory effort to discover and produce more petroleum. At the same time, we are taking steps to divert petroleum demand to other energy sources, such as coal, and to reduce consumption. However, it is clear that continued cooperation, on as broad a base as possible, with other supplier nations is essential to our economic well being and hence national security.

In decades past, a stable price and supply rendered petroleum readily available to all nations with such reliability that national security was not affected. Now, however, extraordinary prices, political upheavals in the Middle East, changing economic principles in supplier countries, and domestic resource depletion require that we broaden our base of knowledge about the distribution and potential future availability of energy minerals throughout the world. This knowledge is essential to formulating international relations and to maintaining our national security.

From the perspective of the U.S. Government, there is no need for direct assistance for exploration activities conducted by the private sector at this time; however, the U.S. Government does need a broad knowledge of the worldwide regional resource availability as an aid to the conduct of international relations. A case in point is the sudden emergence of Mexico as a major producer of petroleum. In retrospect, the signs of that emergence clearly were evident 10 years ago. But in the absence of a program capable of recognizing the clues and analyzing their significance, the U.S. Government had no base of geologic resource understanding over a period of

several years to enable it to respond politically and diplomatically to the changing resource realities.

The principal focus of the U.S. Geological Survey's World Energy Resources Program is on petroleum, but a modest effort to build a base for the worldwide investigation of other energy mineral resources has been initiated. The intent of the program is to provide an understanding of world energy mineral resources for the purposes of policy planning and analysis, including domestic resource assessment, that will be useful to the President, Congress, and other Federal agencies such as the Departments of State, Energy, and Commerce. The initial program objective has been to develop a geologic synthesis of the major petroleum producing regions of the world and, in cooperation with petroleum engineers in other agencies, to assess the present and future producibility of those resources. Secondly, we have initiated studies in frontier areas of the world that offer great promise for future production or are significant areas of international concern (Antarctica, for example). And finally, resource studies have begun in areas of modest resource potential but ones that show geologic promise of at least supplying some measure of local energy mineral needs. The program is coordinated with and depends critically on research activities in the domestic energy resource area. Relations with other Survey international programs also are maintained for best use of available manpower and data sources.

The principal products of the program will be Survey Circulars reporting on our assessment of resource potential in a given country or basin, coupled with a brief discussion of the geology leading to the assessment. Separate publications will include a more detailed presentation of the resource geology which will provide a baseline of information for ongoing analysis. To gain full advantage of the resource investigations, the assessment must be continuing, and, for each area studied, the program will maintain a surveillance of exploration activity as a check on the assessment process. At the completion of the first year of operation, assessments have been completed and are available as Open-File Reports for the following countries or parts of countries: ArabianIranian Basin (subdivided by country); West Siberia, Volga Urals, and Middle Caspian Basin, U.S.S.R.; Venezuela; Indonesia; Malaysia-Brunei; world offshore basins; southeast Mexico, Belize, and northern Guatemala; Trinidad; and northeast Mexico.

Hard Minerals From the
Deep Sea: The Role of
Spreading Ridges

For more than three decades the sedimentary
rocks of the world's continental shelves have
been a prolific source of oil and gas. In recent
years, due both to advances in marine mineral
recovery technology and the increasing cost of
locating and producing onshore supplies, there
has been a marked growth of interest in the sea
floor as a prospective source of nonfuel minerals
as well. These hard minerals, such as zinc, cop-
per, and silver and so called because they are
formed and concentrated in hard crystalline rocks
as distinguished from the softer sedimentary
deposits that are the source and habitat of oil,
gas, and coal, are presently the focus of extensive
commerical and scientific investigation.

Of particular interest to scientists has been the evidence of mineral formation along the great 45,000-mile-long system of oceanic spreadingcenter-ridges that circles the world and is the site where new crust is being continously formed by the upwelling of molten rock from the underlying mantle (fig. 1). As the new crust is formed, it moves away from the spreading center on both sides of the ridge at varying rates up to 6.3 inches per year. Recent investigations have disclosed the occurrence of mineral-rich submarine hot springs that are a source of potentially valuable minerals along the spreading-center-ridge system. Some areas of the Western United States and Alaska contain sections of oceanic crust that have been transported via plate tectonic movement and incorporated into the continental crust. Studying the current mineralization processes on oceanic spreading ridges will thus enhance our ability to identify these onshore areas and target them for mineral explorations.

In 1979, a detainled photographic and geophysical survey of the Pacific sea floor off Mexico discovered a number of hot springs forming concentrations of zinc, copper, and silver, in ore-grade sulfide-mineral deposits on very young glassy volcanic rocks. The initial work, which used unmanned vehicles, was immediately followed by manned submersible investigation of selected submarine springs, and mineral deposits and hot water samples were recovered. The hydrothermal waters were much hotter (700°-750°F.) than previously observed or suspected. Since the initial discovery, this

hydrothermal site at 21° north latitude on the East Pacific Rise has become the focus of investigations of the formation of metallic mineral deposits on the deep-sea floor. Since 1974, Geological Survey scientists have been involved in mapping the geology, crustal generation, and hydrothermal processes of the sea floor at this

site.

Submarine hydrothermal activity on the deepsea floor is known from other sites along the world-encircling oceanic spreading-center-ridge system. Lower temperature hydrothermal vents have been observed during submersible work along the Galapagos Rift System in the equatorial Pacific. Sulfide minerals have been dredged from Guaymas Basin in the Gulf of California, and hot water vents similar to those at 21° north latitude off Mexico have been photographed in the South Pacific on the East Pacific Rise west of Chile (fig. 2). All these occurrences are on oceanic spreading-center ridges where the rate of separation of the plates is greater than 2 inches per year. One-half the length of the spreading-ridge system has separation rates that exceed 2 inches per year. All detailed geophysical, photographic, or submersible studies of small segments of ridge crest having separation rates exceeding 2 inches per year have found active hydrothermal systems or mineral deposits left by these submarine

springs. Thus, it is likely that many more areas of actively forming mineral deposits will be found on the deep-ocean floor. The similarity of bottomdwelling animals at these widely separated hydrothermal sites is additional support for suspecting that hydrothermal systems will be found along most of the world's spreading ridge system.

Resource Potential and Application to Contential Deposits

The submersible studies of the Galapagos Rift and the East Pacific Rise off Mexico provide the most comprehensive picture of hard mineral deposits formed at spreading centers. The sulfide deposits form small shallow mounds occasionally topped by one or more conical sulfide spires or chimneys. The mounds rest on fine-grained marine sediment (generally siliceous ooze) or directly on pillow lavas of the sea floor. Mounds observed on the East Pacific Rise site are 50 to 100 feet across and 6 feet high. The sulfide chimneys are 3 to 15 feet high.

The mounds and chimneys mark the location of hydrothermal vents that discharge metal-rich fluid at temperatures up to 700°F. onto the sea floor. The chimneys are constructed from sulfide