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NASA NTRS1992

Overview of Venus geology: Preliminary description of terrain units for Venus global geological mapping

Venus terrain units can be categorized on the basis of morphology, reflectivity, backscatter, roughness, and emissivity. Morphology can be inferred from Magellan left-looking nominal incidence angle image mosaics, right-looking coverage, and more limited left-looking stereo. The typical resolution is about 300 m down to about 120 m near periapsis in the cycle one nominal coverage. The scale of geologic mapping governs definition of mappable terrain units. Initial global mapping is being compiled at a scale of 1:50 million. At this scale, the smallest individual features that can be mapped are about 125 km. The categories of terrain types are plains, complex ridge terrain, features with morphology suggesting volcanic or volcano-tectonic origin, features interpreted to be tectonic in origin, crater units, and surficial units such as splotches and streaks. Brief descriptions of terrain units are provided.

Saunders, R. Stephen↗

NASA NTRS1992

Winter warming from large volcanic eruptions

An examination of the Northern Hemisphere winter surface temperature patterns after the 12 largest volcanic eruptions from 1883-1992 shows warming over Eurasia and North America and cooling over the Middle East which are significant at the 95 percent level. This pattern is found in the first winter after tropical eruptions, in the first or second winter after midlatitude eruptions, and in the second winter after high latitude eruptions. The effects are independent of the hemisphere of the volcanoes. An enhanced zonal wind driven by heating of the tropical stratosphere by the volcanic aerosols is responsible for the regions of warming, while the cooling is caused by blocking of incoming sunlight.

Robock, Alan↗

NASA NTRS1993

Venus Interior Structure Mission (VISM): Establishing a Seismic Network on Venus

Magellan radar data show the surface of Venus to contain a wide range of geologic features (large volcanoes, extensive rift valleys, etc.). Although networks of interconnecting zones of deformation are identified, a system of spreading ridges and subduction zones like those that dominate the tectonic style of the Earth do not appear to be present. In addition, the absence of a mantle low-viscosity zone suggests a strong link between mantle dynamics and the surface. As a natural follow-on to the Magellan mission, establishing a network of seismometers on Venus will provide detailed quantitative information on the large scale interior structure of the planet. When analyzed in conjunction with image, gravity, and topography information, these data will aid in constraining mechanisms that drive surface deformation.

Stofan, E. R.↗

NASA NTRS1993

Effect of subglacial volcanism on changes in the West Antarctic Ice Sheet

Rapid changes in the West Antarctic Ice Sheet (WAIS) may affect future global sea-level changes. Alley and Whillans note that 'the water responsible for separating the glacier from its bed is produced by frictional dissipation and geothermal heat,' but assume that changes in geothermal flux would ordinarily be expected to have slower effects than glaciological parameters. I suggest that episodic subglacial volcanism and geothermal heating may have significantly greater effects on the WAIS than is generally appreciated. The WAIS flows through the active, largely asiesmic West Antarctic rift system (WS), which defines the sub-sea-level bed of the glacier. Various lines of evidence summarized in Behrendt et al. (1991) indicate high heat flow and shallow asthenosphere beneath the extended, weak lithosphere underlying the WS and the WAIS. Behrendt and Cooper suggest a possible synergistic relation between Cenozoic tectonism, episodic mountain uplift and volcanism in the West Antarctic rift system, and the waxing and waning of the Antarctic ice sheet beginning about earliest Oligocene time. A few active volcanoes and late-Cenozoic volcanic rocks are exposed throughout the WS along both flanks, and geophysical data suggest their presence beneath the WAIS. No part of the rift system can be considered inactive. I propose that subglacial volcanic eruptions and ice flow across areas of locally (episodically?) high heat flow--including volcanically active areas--should be considered possibly to have a forcing effect on the thermal regime resulting in increased melting at the base of the ice streams.

Behrendt, John C.↗

NASA NTRS1992

MSATT Workshop on Chemical Weathering on Mars

The topics covered with respect to chemical weathering on Mars include the following: Mars soil, mineralogy, spectroscopic analysis, clays, silicates, oxidation, iron oxides, water, chemical reactions, geochemistry, minerals, Mars atmosphere, atmospheric chemistry, salts, planetary evolution, volcanology, Mars volcanoes, regolith, surface reactions, Mars soil analogs, carbonates, meteorites, and reactivity.

Burns, Roger↗

NASA NTRS1992

Location of nanophase Fe-oxides in palagonitic soils: Implication for Martian pigments

Palagonitic materials from Mauna Kea, Hawaii, were identified as Mars analogs based on their spectral and magnetic properties. These materials probably resulted from hydrothermal alteration during eruption of the volcano and/or from weathering under ambient conditions. The reflectance spectra of the Mars surface obtained by Earth-based telescopes and the reflectance spectra of analogs obtained in the laboratory show features due to electronic transitions of Fe(III) in oxide particles that range in size from nanometer (nanophase) to micrometer sized or larger. The presence of Fe(III) suggests oxidizing conditions during the alteration process in Mars that may have occurred in the past or during a slow ongoing process. Two naturally altered basaltic samples from Hawaii (HWMK12 and HWMK13) and a laboratory-altered (PH-13-DCGT2) basaltic glass similar in elemental composition to the above two samples was examined. All three samples exhibited spectral characteristics similar to martian bright-region spectra. Chemical and mineralogical changes occurring at the surface of these basalts were studied in order to understand the basis for their Mars-like properties. The spectral properties of the three samples were examined after the removal of Fe oxides by chemical extractants.

Golden, D. C.↗

NASA NTRS1992

Chemical transport during formation and alteration of Martian impact and volcanic deposits

Much of the surface of Mars, including volcanic and cratered terrains, probably experienced alteration and degassing processes. These processes may have depleted or enriched many important elements in surface materials, including bedrock, dust, and soils. The composition of the martian soil may represent the best estimate, for some elements, of the average composition of the martian crust, similar to the composition of loess created by glacial action on the Earth. The martian soil may represent the only convenient, globally or regionally averaged sample of the martian crust. In order to understand the composition of the source material for the soil, however, we need to understand the contributions of volcanic vs. impact sources for this material and the chemical fractionations involved in its production. The processes to be addressed include degassing of volcanic deposits, as observed in the Valley of Ten Thousand Smokes at Katmai, Alaska, and degassing of meltbearing impact ejecta as inferred for suevite ejecta sheets at the Ries Crater, and alteration or palagonitization of volcanic deposits, as documented for volcanos in British Columbia and many other volcanic terrains, and impact crater deposits. The process of palagonitization has been the subject of several studies with reference to Mars, and palagonite is a good analogue for the spectroscopic properties of the martian dust. The role of impact in cratering has not been as well studied, although other researchers have established that both degassing and alteration are common features of impact crater deposits. Other relevant sources of experimental data include the extensive literature on the corrosion of nuclear waste glass and leaching of shocked materials.

Newsom, H. E.↗

NASA NTRS1992

Chemical modeling constraints on Martian surface mineralogies formed in an early, warm, wet climate, and speculations on the occurrence of phosphate minerals in the Martian regolith

This is one in a series of reports summarizing our chemical modeling studies of water-rock-gas interactions at the martian surface through time. The purpose of these studies is to place constraints on possible mineralogies formed at the martian surface and to model the geochemical implications of martian surficial processes proposed by previous researchers. Plumlee and Ridley summarize geochemical processes that may have occurred as a result of inferred volcano- and impact-driven hydrothermal activity on Mars. DeBraal et al. model the geochemical aspects of water-rock interactions and water evaporation near 0 C, as a prelude to future calculations that will model sub-0 C brine-rock-clathrate interactions under the current martian climate. In this report, we discuss reaction path calculations that model chemical processes that may have occurred at the martian surface in a postulated early, warm, wet climate. We assume a temperature of 25 C in all our calculations. Processes we model here include (1) the reaction of rainwater under various ambient CO2 and O2 pressures with basaltic rocks at the martian surface, (2) the formation of acid rain by volcanic gases such as HCl and SO2, (3) the reactions of acid rain with basaltic surficial materials, and (4) evaporation of waters resulting from rainwater-basalt interactions.

Plumlee, Geoffrey S.↗

NASA NTRS1992

Small volcanic edifices and volcanism in the plains of Venus

The different types of eruption that have occurred over time in the Venusian plains are considered. The most extensive volcanic units consist of flood lavas, the largest of which have volumes of the order of thousands of cubic kilometers. They are inferred to have erupted at high effusion rates, and they exhibit a range of radar backscatter characteristics indicating different surface textures and ages. Small edifices on the plains occur mainly in clusters associated with fracture belts. The majority are shield volcanos that may be up to a few tens of kilometers across but are generally 10 km or less in diameter. Volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 cu cm. These are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava. For many domes, the flanks were unstable during and after eruption and experienced gravity sliding that produced steep scalloped outer margins.

Guest, John E.↗

NASA NTRS1992

Anatomy of a Venusian hot spot - Geology, gravity, and mantle dynamics of Eistla Regio

Results of a study of the western and central portions of the Venusian hot spot Eistla Regio are presented. Magellan radar images were mapped to elucidate the general geologic history of the region. Radial fracture systems both on the rises and volcanoes indicate that uplift and associated faulting accompanied volcanic construction. Prominent fracture zones strike WNW to NW, parallel to the long axis of the highlands. The largest of these, Guor Linea, exhibits a progressive deformation history that may include minor clockwise rotation in addition to bulk NNE-SSW extension. Pioneer Venus line-of-sight accelerations were inverted for vertical gravity which, when combined with topography, were used to solve for mass anomalies on the crust-mantle boundary and in the upper levels of the mantle convective system.

Grimm, Robert E.↗

NASA NTRS1992

Winter warming from large volcanic eruptions

An examination of the Northern Hemisphere winter surface temperature patterns after the 12 largest volcanic eruptions from 1883-1992 shows warming over Eurasia and North America and cooling over the Middle East which are significant at the 95-percent level. This pattern is found in the first winter after tropical eruptions, in the first or second winter after midlatitude eruptions, and in the second winter after high latitude eruptions. The effects are independent of the hemisphere of the volcanoes. An enhanced zonal wind driven by heating of the tropical stratosphere by the volcanic aerosols is responsible for the regions of warming, while the cooling is caused by blocking of incoming sunlight.

Robock, Alan↗

NASA NTRS1992

The physical volcanology of Mars

The physical volcanology of Mars is reviewed, with particular attention given to the diversity of volcanic landforms, the implied styles of eruption associated with the construction of these landforms, the inferred internal structure of the volcanoes, and the influence that the eruptions have had on the Martian environment (both local and global in scale). Volcanism in the central highlands appears to have been explosive in character, while most of the constructional activity in the northern plains was effusive. Highlands volcanism appears to be relatively old compared to that in the northern hemisphere. There is evidence for the existence of large magma chambers and very high effusion rate eruptions on Mars. Tectonic deformation associated with volcanic constructs is primarily a consequence of loading and magma transport, while deformation in the volcanic plains reflects stresses associated with Tharsis and major impact basins.

Mouginis-Mark, Peter J.↗

NASA NTRS1993

Lava flow topographic measurements for radar data interpretation

Topographic profiles at 25- and 5-cm horizontal resolution for three sites along a lava flow on Kilauea Volcano are presented, and these data are used to illustrate techniques for surface roughness analysis. Height and slope distributions and the height autocorrelation function are evaluated as a function of varying lowpass filter wavelength for the 25-cm data. Rms slopes are found to increase rapidly with decreasing topographic scale and are typically much higher than those found by modeling of Magellan altimeter data for Venus. A more robust description of the surface roughness appears to be the ratio of rms height to surface height correlation length. For all three sites this parameter falls within the range of values typically found from model fits to Magellan altimeter waveforms. The 5-cm profile data are used to estimate the effect of small-scale roughness on quasi-specular scattering.

Campbell, Bruce A.↗

NASA NTRS1993

Modified HNO3 seasonality in volcanic layers of a polar ice core - Snow-pack effect or photochemical perturbation?

Changes in atmospheric HNO3 chemistry following the Laki (1783), Tambora (1815), and Katmai (1912) volcanic eruptions are presently investigated in view of a central Greenland ice core's chemical composition. Attention is given to the concentration of several cations and anions, using ion chromatography. Following the eruptions, the ratio of winter to summer depositions of NO3(-) was significantly higher than during nonvolcanic periods. While this may be due to ice pack effects, it is proposed that large concentrations of the stratospheric H2SO4 particles ejected by the volcanoes favored HNO3 removal during Arctic winter.

Laj, Paolo↗

NASA NTRS1993

Stratospheric chlorine injection by volcanic eruptions - HCl scavenging and implications for ozone

Because the output of volatile chlorine during a major volcanic event can greatly exceed the annual anthropogenic emissions of chlorine to the atmosphere, the fate of volcanic chlorine must be known. Although numerous observations have shown that volcanoes do not significantly contribute to the stratospheric chlorine burden, no quantitative explanation has been published. Hydrogen chloride (HCl) scavenging processes during the early phases of a volcanic eruption are discussed. A plume dynamics and thermodynamics model is used to show that HCl removal in condensed supercooled water can reduce HCl vapor concentrations by up to four orders of magnitude, preventing substantial stratospheric chlorine injection.

Tabazadeh, A.↗

NASA NTRS1993

The new face of Venus

The Magellan data on the Venus landscape which revealed volcanoes, shining mountains, relaxing plateaus, and craters on the surface of about 500 C, with an atmospheric pressure 90 times that of earth, are discussed. Venus is considered to be a planet that is both incredibly similar and dissimilar to earth. Venus might not exhibit plate tectonism but it might be dominated by catastrophes. The greenhouse around Venus has operated for at least the last 500 million years. The Magellan data revealed channels extending for thousands of kilometers, beautiful outflows surrounding impact craters, and odd volcanic constructs like the steep-sided domes.

Stofan, Ellen R.↗

NASA NTRS1993

The grandest tour - Voyager

The Voyager mission based on two spacecraft, Voyager 1 and Voyager 2, launched on September 5, 1977 and August 20 1977, respectively, is reviewed. The mission was designed to take advantage of a rare geometric arrangement of Jupiter, Saturn, Uranus, and Neptune that occurs only once every 176 years. Particular attention is given to data on Jupiter's ring and moons, the discovery of active volcanoes on Io, Jovian magnetism, Saturn's atmosphere, Titan and other moons, Uranus and Miranda, Uranian moons and rings, Triton's atmosphere.

Murrill, Mary B.↗

NASA NTRS1993

Earth observations during Space Shuttle Mission STS-42 - Discovery's mission to planet earth

The noteworthy imagery acquired during Space Shuttle Mission STS-42 is documented. Attention is given to frozen Tibetan lakes, Merapi Volcano in Java, Mt. Pinatubo in the Philippines, the coastline east of Tokyo Japan, land use in southern India, and the Indus River Delta. Observations of Kamchatka Peninsula, Lake Baikal, Moscow, Katmai National Park and Mt. Augustine, Alaska, the Alaskan coast by the Bering Sea, snow-covered New York, the Rhone River valley, the Strait of Gibraltar, and Mt. Ararat, Turkey, are also reported.

Lulla, Kamlesh P.↗