Search Results - (Author, Cooperation:J. W. Head)

2011-10-01

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

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2012-03-23

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2012-03-23

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

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2012-12-12

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

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2015-05-09

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

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2014-10-04

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

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2011-10-01

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2012-06-23

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

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2013-07-05

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

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1365-246X

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

Explosive eruptions of mafic magmas produce lava fountains whose heights are a function of the exsolved volatile content of the magma, its erupted mass flux, and the geometry of the vent (which may be an elongate fissure or a localized, near-circular conduit). The geometry of the initial vent (and the eruptive behaviour) can be distinctly modified by lava drainback and accumulating ejecta. Hot pyroclasts landing near the vent may coalesce to form rootless flows, some of which may drain back into the vent to be recycled into the eruption products. Rootless flows may be at least partially confined by pre-existing topographic features, or by spatter or cinder ramparts being built up by the eruption itself, so that they accumulate into a lava pond over and around the vent. The erupting jet of magmatic gas and pyroclasts must force its way through such a pond and will entrain some of the pond lava as it does so. The energy expended in entraining and accelerating previously erupted materials will reduce the eruption velocity and the lava fountain height by an amount which can be calculated as a function of the eruption conditions and the lava pond depth (or lava drainback rate). The results of such calculations are presented, and are used to assess the influence of this process on attempts to infer magma volatile contents from field observations of lava fountain heights.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] GANYMEDE, Jupiter's largest moon, is the largest known icy object in the Solar System. Voyager images of its surface show a complex and unique geology with widespread evidence for tectonic activity1'2. For silicate bodies on which significant lateral motion of the lithosphère does not occur, global ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Gillespie et al. concur with our interpretation that certain lobate equatorial and mid-latitude features on Mars are due to debris-covered glaciers formed largely during past periods of increased spin-axis obliquity, when climate regimes favoured snow and ice accumulation and glacial ...

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1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Images from the Mars Express HRSC (High-Resolution Stereo Camera) of debris aprons at the base of massifs in eastern Hellas reveal numerous concentrically ridged lobate and pitted features and related evidence of extremely ice-rich glacier-like viscous flow and sublimation. Together with new ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] The ice-rich surface of the jovian satellite Europa is sparsely cratered, suggesting that this moon might be geologically active today. Moreover, models of the satellite's interior indicate that tidal interactions with Jupiter might produce enough heat to maintain a subsurface liquid water ...

Electronic Resource

1573-0794

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract The complex morphology and topography of Eastern Ishtar Terra have been interpreted as due to tectonic deformation. Models proposed to account for this deformation include: crustal flow through “asthenospheric flow and thermal-gravitational sliding”; rifting, gravity spreading, and fold belt formation; and horizontal convergence and crustal thickening. In this study we map the detailed structural and topographic fabric of this region in order to explore and test these hypotheses. Eastern Ishtar can be divided into four major provinces: Maxwell Montes/Western Fortuna Tessera, a high plateau and mountain belt dominated by long NNW trending ridges; Central Fortuna Tessera, a low region of orthogonally oriented short WNW trending ridges and long, NNE trending troughs; Eastern Fortuna Tessera, a broad, E-W trending topographic rise characterized by ENE trending troughs and a complex pattern of intersecting ridges; and Northern Fortuna Tessera, a region of steep, NE-facing topographic scarps and ridges that trend WNW. On the basis of structural and topographic relationships, the features within these provinces are found to be inconsistent with a formation through either downslope crustal flow or rifting. We find that the mapped features are most consistent with a formation through convergence, collison, and underthrusting of thickened crustal terranes. These terranes are suggested to have been created through processes of seafloor-type spreading and crustal collision. Based on relationships between the different terranes, several accretional events are proposed in which Eastern Ishtar is produced by the collision of crustal terranes beginning at Lakshmi Planum and extending to the east. This sequence is initiated with the formation of Maxwell Montes and Western Fortuna Tessera during east-west crustal convergence, underthrusting, and stacking. The next step involves the northeast to southwest convergence of a preexisting thick block of tessera in Central Fortuna, which produces shear deformation within Western Fortuna. This northeast to southwest convergence also produces Northern Fortuna Tessera through crustal imbrication, a process recognized along the entire northern boundary of Ishtar Terra. Finally, Laima Tessera converges with Fortuna from the southeast and collides with Eastern Fortuna Tessera producing shear within Eastern Fortuna and the linear convergence zones along the edges of Laima. High resolution images returned by the Magellan spacecraft will enable us to examine the features involved in the proposed production and suturing of crustal terranes.

Electronic Resource

1573-0794

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract Recent high resolution, high incidence angle Arecibo radar images of southern Ishtar Terra and flanking plains of Guinevere and Sedna on Venus reveal details of topographic features resolved by Pioneer Venus. The high incidence angles of Arecibo images favor the detection of surface roughness-related features, and complement recently obtained low incidence angle Venera 15/16 images in which changes in surface topographic slope are well portrayed. Four provinces have been defined on the basis of radar characteristics in Arecibo images and topography. Volcanism and tectonism are the dominant processes in the mapped area, which has an average age of about 0.5–1.0 billion years (Ivanov et al., 1986). These processes vary in relative significance in the mapped provinces and it is likely that geologic activity has occurred simultaneously in all four provinces. On the basis of stratigraphic evidence, however, a general sequence is proposed which represents the major activity in each area. The low predominantly volcanic plains of Guinevere and Sedna Planitiae are the relatively oldest terrain. A major region of complex tectonic deformation, the Southern Ishtar Transition Zone, postdates much of the low plains and delineates the steep-sloped flanks of Ishtar Terra. Lakshmi Planum is characterized by a distinctive volcanic style (large low edifices, calderas, flanking plains) and at least in part postdates the Southern Ishtar Transition Zone. Relatively recent plains-style volcanism occurs locally in Sedna Planitia and embays the Southern Ishtar Transition Zone. Compressional deformation appears to dominate the mountains of the Ishtar plateau, but the nature of the tectonic deformation in the Southern Ishtar Transition Zone is very complex and likely represents a combination of extension, compression and strikeslip deformation. Arecibo data reveal additional coronae in the lowlands, suggesting that corona formation is an even more widespread process than indicated by the Venera data.

Electronic Resource

1573-0794

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract New radar images (resolution 1.5–2.0 km) obtained from the Arecibo Observatory are used to assess the geology of a portion of the equatorial region of Venus (1

Electronic Resource

1573-0794

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract The details of stratigraphic units and structures making up six coronae and their regional surroundings on Venus were examined using full resolution Magellan images and stereoscopic coverage. Altimetry and stereoscopic coverage were essential in establishing the local stratigraphic relationships and the timing of corona-related topography. The degree of preservation of signatures of earlier corona-related activities and the scale of later corona-related activities vary significantly from corona to corona. We compared the geologic sequence in each corona to regional and global stratigraphic units, placing the coronae in the broader context of the geologic history of Venus. The results of this study were compared with earlier analyses bringing the total number of corona considered to about 15% of the total corona population. We found that corona started forming soon after tessera formation and largely spanned a significant part of the subsequent geologic history of Venus, over about 200–400 million years. Topographic annulae were initiated in early post-tessera time but were largely completely formed by the time of emplacement of regional plains with wrinkle ridges. Some coronae ceased activity by this time, while others continued until closer to the present, although showing evidence of waning activity. Coronae-associated volcanism dominated many coronae during this later stage. Convincing evidence of pre-regional plains corona- related volcanism was not found in the population examined here. We conclude that coronae formed in a two stage process; the first stage (tectonic phase) involved the annular warping of early extensive stratigraphic units of volcanic origin and the second (volcanic phase) involved coronae-related lava flow activity and local fracturing. For the vast majority of coronae, the first tectonic phase was largely complete prior to the emplacement of the regional plains (Pwr, plains with wrinkle ridges). The vast majority of corona-related volcanic activity (emplacement of Pl, lobate flows) occurred subsequent to the emplacement of regional plains. We found no evidence of coronae initiation in substantially later periods of the observed history of Venus.

Electronic Resource

1573-0794

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract Characteristics of rock populations on the surfaces of Mars and Venus can be derived from analyses of rock morphology and morphometry data. We present measurements of rock sizes and sphericities made from Viking lander images using an interactive digital image display system. The rocks considered are in the gravel size range (16–256 mm in diameter). Mean sphericities, form ratios, and roundness factors are found to be very similar for both Viking lander sites. Size distributions, however, demonstrate differences between the sites; there are significantly more cobble size fragments at VL-2 than at VL-1. A model calling for aphanitic basalts emplaced as ejecta or lava flows at the Viking sites is supported by the rock shape, size, and roundness data. Morphologic features pertaining to the modification history of a rock are considered for Mars and Venus. A multi-parameter clustering algorithm is utilized to objectively categorize martian and venusian rocks in terms of various criteria. Erosional markings such as flutes are demonstrated to be most important in separating VL-1 rock morphologic groups, while rock form (i.e., shape) represents the primary separator of subpopulations at VL-2 and the Venera landing sites. Fillets are common around VL-1 and Venera 10 fragments. Obstacle scours occur frequently only at VL-1. Cavities in rocks are ubiquitous at all lander sites except Venera 9. Eolian processes, possibly assisted by local solution weathering, are a strong candidate for the origin of cavities and flutes in martian rocks.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Two YEARS after Apollo 17, S. Ross Taylor published Lunar Science: A Post-Apollo View, a book designed to bring some order to our knowledge of the Moon's evolutionary history resulting from the abundance of information derived from the Apollo and Luna missions. In this book, published eight years ...

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