Mass-radius relationships and constraints on the composition of Pluto,II

A new estimate of Pluto's mass within the range of possible masses considered in an earlier work has enabled us to refine our model of Pluto's interior.     

On the astrometric detection of neighboring planetary systems,II

Extensive testing suggests that astrometric techniques can be employed to detect and study virtually any planetary system that may exist within 40 light years (12.5 parsec) of the Sun. Following the conclusion of Paper I [G. Gatewood, Icarus27 (1976), 1–12], the astrometric group at the Allegheny Observatory began an intensive survey of 20 nearby stars to detect the nonlinear variations in their motion that planetary systems would induce. Several tests conducted to further our understanding of the limitations of this survey indicated that the photographic detector itself is responsible for the majority of the random error. A new photoelectric detector has been designed and a simplified prototype of it successfully tested. The new detector is expected to be able to utilize virtually all of the astrometric information transmitted through the Earth's atmosphere. This is sufficient to determine relative positions to within an accuracy of approximately 1 milliarcsec/hr. Such precisions exceed the design capabilities of the best existing astrometric telescopes, thus a feasibility study has been conducted for the design of an improved instrument. The study concludes that a new ground-based telescope and the new detector combined should be able to study stars as faint as the 17th magnitude with an annual accuracy of a few tenths of a milliarcsecond. However, to obtain the ultimate accuracy possible from current technology, we must place an astrometric system above the Earth's atmosphere. A space-borne instrument utilizing the new detector would in theory have sufficient accuracy to detect any Earth-like planet orbiting any of the several hundred stars nearest the Sun.     

Numerical simulation of the final stages of terrestrial planet formation

Three representative numerical simulations of the growth of the terrestrial planets by accretion of large protoplanets are presented. The mass and relative-velocity distributions of the bodies in these simulations are free to evolve simultaneously in response to close gravitational encounters and occasional collisions between bodies. The collisions between bodies, therefore, arise in a natural way and the assumption of expressions for the relative velocity distribution and the gravitational collision cross section is unnecessary. These simulations indicate that the growth of bodies with final masses approaching those of Venus and the Earth is possible, at least for the case of a two-dimensional system. Simulations assuming an initial uniform distribution of orbital eccentricities on the interval from 0 to e_max are found to produce final states containing too many bodies with masses which are too small when e_max < 0.10, while simulations with e_max > 0.20 result in too many catastrophic collisions between bodies thus preventing rapid accretion of planetary-size bodies. The e_max = 0.15 simulation ends with a state surprisingly similar to that of the present terrestrial planets and, therefore, provides a rough estimate of the range of radial sampling to be expected for the terrestrial planets.     

Newtonian cosmology with a varying gravitational constant

Newtonian cosmology is developed with the assumption that the gravitational constantG diminishes with time. The functional form adopted forG(t), a modification of a suggestion of Dirac, isG=A(k+t) ^–1, wheret is the age of the Universe and a small constantk is inserted to avoid a singularity in the two-body problem. IfR is the scale factor, normalized to unity at an epoch time , the differential equation is then . Here ₀ is the mean density at the epoch time. With the above form forG(t), the solution is reducible to quadratures.The scale factorR either increases indefinitely or has one and only one maximum. LetH ₀ be the present value of Hubble's constant /R and _0c the minimum density for a maximum ofR, i.e., for closure of the Universe. The conditions for a maximum lead to a boundary curve of _0c versusH ₀ and the numbers indicate strongly that thisG-variable Newtonian model corresponds to an open universe. An upward estimate of the age of the Universe from 10¹⁰ yr to five times such a value would still lead to the same conclusion.The present Newtonian cosmology appears to refute the statement, sometimes made, that the Dirac model forG necessarily leads to the conclusion that the age of the Universe is one-third the Hubble time. Appendix B treats this point, explaining that this incorrect conclusion arises from using all the assumptions in Dirac (1938). The present paper uses only Dirac's final result, viz,G(k+t)^–1, superposing it on the differential equation .     

Differential unroofing within the central metasedimentary Belt of the Grenville Orogen: constraints from 40Ar/39Ar thermochronology

An ⁴⁰Ar/³⁹Ar thermochronological investigation of upper greenschist to granulite facies gneiss, amphibolite and marble was conducted in the Central Metasedimentary Belt (CMB), Ontario, to constrain its cooling history. Incremental ⁴⁰Ar/³⁹Ar release spectra indicate that substantial differential unroofing occurred in the CMB between 1000 and 600 Ma. A consistent pattern of significantly older hornblende and phlogopite ⁴⁰Ar/³Ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation. Variations in hornblende ⁴⁰Ar/³⁹Ar age plateaus exceeding 200 Ma occur over distances less than 50 km with major age discontinuities occurring across the Robertson Lake shear zone and the Sharbot Lake mylonite zone which separate the Sharbot Lake terrane from the Elzevir and Frontenac terranes. Extensional displacements of up to 14 km are inferred between the Frontenac and Elzevir terranes of the CMB. No evidence for significant post argon-closure vertical displacement is indicated in the vicinity of the Perth Road mylonite within the Frontenac terrane. Variations of nearly 100 Ma in phlogopite ⁴⁰Ar/³⁹Ar plateau ages occur in undeformed marble on either side of the Bancroft Shear Zone. Phlogopites from sheared and mylonitized marble within the shear zone yield ⁴⁰Ar/³⁹Ar diffusional loss profiles, but have older geologically meaningless ages thought to reflect incorporation of excess argon. By 900 Ma, southeast directed extension was occurring throughout the CMB, possibly initiated along previous zones of compressional shearing. An easterly migration of active zones of extension is inferred, possibly related to an earlier, overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone. The duration of extensional shearing is not well constrained, but must have ceased before 600 Ma as required by the deposition of overlying undeformed Cambrian and/or Ordovician sedimentary rocks.Contribution No. 481 from the Mineralogical Laboratory, University of Michigan     

Age — heaping: contrasting patterns from china

Detailed examination of China's 1982 census data confirms that the standard of age reporting among the Han Chinese, and among many of the ethnic minorities, is very good. Furthermore the quality of age reporting is unaffected by large variations in levels of literacy which exist between men and women, between young and old, and between rural and urban areas. Much of the credit for the high quality of China's age reporting stems from the distinctive nature of Chinese traditional calendars. However, in the far west of China, in the Xinjiang Autonomous Region, age heaping and age exaggeration are as bad as they are in many other developing countries. The authors show that the poor quality of age reporting in Xinjiang is confined to three of the region's ethnic communities, the Uygur, Kirgiz and Tajik, who appear to neither know, nor care, how old they are.     

IGU commission on the world political map

The International Geographical Union Reports

IGU commission on the world political map     

Book reviews

REGIONAL GEOGRAPHY: Current Developments and Future Prospects edited by R. J. Johnston, J Hauer and G. A. Hoekveld. 14 × 22 cm, xiii and 216 pages. Routledge: London 1990 (ISBN 0 415 05247 5) $A89.95 (hard).

THE MAKING OF THE AMERICAN LANDSCAPE edited by M. P. Conzen. 19 × 25 cm, xvi and 433 pages. Unwin Hyman: Boston 1990 (ISBN 0 04 917010 4) $A59.95 (soft).

EASTERN EUROPE: An Historical Geography 1815–1945 by D. Turnock. 14 × 22 cm, ix and 357 pages. Routledge: London 1989 (ISBN 0 415 01269 4) $A115.00 (hard), distributed by the Law Book Co.

THE HUMAN GEOGRAPHY OF EASTERN EUROPE by D. Turnock. 14 × 22 cm, xii and 345 pages. Routledge: London 1989 (ISBN 0 415 00469 1) $A105.00 (hard), distributed by the Law Book Co.

A HISTORY OF THE PACIFIC ISLANDS by I. C. Campbell. 14 × 21 cm, 239 pages. University of Queensland Press: St Lucia 1990 (ISBN 0 7022 2291 7) $A34.95 (soft).

THE CORPORATE FIRM IN A CHANGING WORLD ECONOMY: Case Studies in the Geography of Enterprise edited by M. de Smidt and E. Wev‐er. 14 × 23 cm, xiv and 247 pages. Routledge: London 1990 (ISBN 0 415 03497 3) $A105.00 (hard).

THE CAPITALIST CITY: Global Restructuring and Community Politics by M. P. Smith and J. R. Feagin. 15 × 23 cm, 393 pages. Basil Black‐well: Oxford 1988 (ISBN 0 631 15182 6) £35 (hard).

CONFLICT AND CHANGE IN THE COUNTRYSIDE: Rural Society, Economy and Planning in the Developed World by G. M. Robinson. 15 × 23 cm, xxvi and 482 pages. Belhaven Press: London 1990 (ISBN 1 85293 043 8) £40.00 (hard); (ISBN 1 85293 044 6) £14.95 (soft).

ENVIRONMENTAL SCIENCE (Collins Reference Dictionary) by G. Jones, A. Robertson, J. Forbes and G. Hollier. 13 × 20 cm, vi and 473 pages. Collins: London 1990 (ISBN 0 00 434348 4) £5.95 (soft).

THE HUMAN IMPACT ON THE NATURAL ENVIRONMENT (Third Edition) by A. Goudie. 17 × 24 cm, xi and 388 pages. Blackwell: Oxford 1990 (ISBN 0 631 16164 3) $A39.95 (soft).

PATTERNS OF LIFE: Biogeography of a Changing World by H. W. Mielke. 19 × 24 cm, xiv and 370 pages. Unwin Hyman: London 1989 (ISBN 0 04 574032 1).