MARSIS radar sounder evidence of buried basins in the northern lowlands of Mars

A hemispheric dichotomy on Mars is marked by the sharp contrast between the sparsely cratered northern lowland plains and the heavily cratered southern highlands. Mechanisms proposed to remove ancient crust or form younger lowland crust include one or more giant impacts, subcrustal transport by mantle convection, the generation of thinner crust by plate tectonics, and mantle overturn following solidification of an early magma ocean. The age of the northern lowland crust is a significant constraint on these models. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on the European Space Agency's Mars Express spacecraft is providing new constraints on the martian subsurface. Here we show evidence of buried impact basins ranging in diameter from about 130 km to 470 km found over approximately 14 per cent of the northern lowlands. The number of detected buried basins >200 km in diameter indicates that the lowland crust is ancient, dating back to the Early Noachian epoch. This crater density is a lower limit because of the likelihood that not all buried basins in the area surveyed by MARSIS have been detected. An Early Noachian age for the lowland crust has been previously suggested on the basis of a large number of quasi-circular topographic depressions interpreted to be evidence of buried basins. Only a few of these depressions in the area surveyed by MARSIS, however, correlate with the detected subsurface echoes. On the basis of the MARSIS data, we conclude that the northern lowland crust is at least as old as the oldest exposed highland crust. This suggests that the crustal dichotomy formed early in the geologic evolution of Mars.     

Navigation: bat orientation using Earth's magnetic field

Bats famously orientate at night by echolocation, but this works over only a short range, and little is known about how they navigate over longer distances. Here we show that the homing behaviour of Eptesicus fuscus, known as the big brown bat, can be altered by artificially shifting the Earth's magnetic field, indicating that these bats rely on a magnetic compass to return to their home roost. This finding adds to the impressive array of sensory abilities possessed by this animal for navigation in the dark.     

Introns and the origin of nucleus-cytosol compartmentalization

The origin of the eukaryotic nucleus marked a seminal evolutionary transition. We propose that the nuclear envelope's incipient function was to allow mRNA splicing, which is slow, to go to completion so that translation, which is fast, would occur only on mRNA with intact reading frames. The rapid, fortuitous spread of introns following the origin of mitochondria is adduced as the selective pressure that forged nucleus-cytosol compartmentalization.     

Links between annual, Milankovitch and continuum temperature variability

Climate variability exists at all timescales-and climatic processes are intimately coupled, so that understanding variability at any one timescale requires some understanding of the whole. Records of the Earth's surface temperature illustrate this interdependence, having a continuum of variability following a power-law scaling. But although specific modes of interannual variability are relatively well understood, the general controls on continuum variability are uncertain and usually described as purely stochastic processes. Here we show that power-law relationships of surface temperature variability scale with annual and Milankovitch-period (23,000- and 41,000-year) cycles. The annual cycle corresponds to scaling at monthly to decadal periods, while millennial and longer periods are tied to the Milankovitch cycles. Thus the annual, Milankovitch and continuum temperature variability together represent the response to deterministic insolation forcing. The identification of a deterministic control on the continuum provides insight into the mechanisms governing interannual and longer-period climate variability.     

A computationally efficient approximation to the nearest neighbor interchange metric

The nearest neighbor interchange (nni) metric is a distance measure providing a quantitative measure of dissimilarity between two unrooted binary trees with labeled leaves. The metric has a transparent definition in terms of a simple transformation of binary trees, but its use in nontrivial problems is usually prevented by the absence of a computationally efficient algorithm. Since recent attempts to discover such an algorithm continue to be unsuccessful, we address the complementary problem of designing an approximation to the nni metric. Such an approximation should be well-defined, efficient to compute, comprehensible to users, relevant to applications, and a close fit to the nni metric; the challenge, of course, is to compromise these objectives in such a way that the final design is acceptable to users with practical and theoretical orientations. We describe an approximation algorithm that appears to satisfy adequately these objectives. The algorithm requires O(n) space to compute dissimilarity between binary trees withn labeled leaves; it requires O(n logn) time for rooted trees and O(n ² logn) time for unrooted trees. To help the user interpret the dissimilarity measures based on this algorithm, we describe empirical distributions of dissimilarities between pairs of randomly selected trees for both rooted and unrooted cases.The Natural Sciences and Engineering Research Council of Canada partially supported this work with Grant A-4142.     

Unclassed matrix shading and optimal ordering in hierarchical cluster analysis

A method is presented for the graphic display of proximity matrices as a complement to the common data analysis techniques of hierarchical clustering. The procedure involves the use of computer generated shaded matrices based on unclassed choropleth mapping in conjunction with a strategy for matrix reorganization. The latter incorporates a combination of techniques for seriation and the ordering of binary trees.Partial support for this research was provided by NIJ Grant #82-IJ-CX-0019 and NSF Grant #SES82-06067. The authors wish to acknowledge the assistance of Professors L.J. Hubert, R.G. Golledge, and W.R. Tobler.     

In memoriam: William Wallace Newby (1902-1977)

W. W. Newby, professor of biology at the University of Utah from 1927 to 1971, was renowned as a great teacher and a clear and forceful writer and for his meticulous research in invertebrate embryology. Ancillary skills were counseling (especially of premedical students), illustrating, wood crafting, and paper preserving. Some of his writings pertained to the history of research in the biological sciences at the University of Utah. His specialty areas in teaching were embryology and genetics, and he served as chairman of the Department of Genetics and Cytology from the year of its creation in 1948 until 1962. The most outstanding example of his research pertained to the early embryology of the echiuroid worm Urechis caupo .         

Algal composition of microbiotic crusts from the Central Desert of Baja California, Mexico

A total of 66 algal species representing 32 genera were recovered from soils of 10 sites in the Catavi?a region of the Central Desert of Baja California, Mexico. The most common species encountered were the cyanophytes Nostoc commune and Schizothrix calcicola , the chlorophyte Myrmecia astigmatica , and the diatoms Hantzschia amphioxys, Hantzschia amphyoxys f. capitata, Luticola cohnii, Luticola mutica , and Pinnularia borealis var. scalarts . Nine species not found in any previous studies of North American desert soils were present in our study sites, including 3 taxa new to science: Cylindrocystis brebissonii var. deserti , var. nov.; Elakatothrix obtusata , sp. nov.; and Fasciculochloris mexicana , sp. nov. Attempts to correlate species composition with soil chemical and physical parameters were unsuccessful apart from a pH effect on cyanobacterial distribution. Overall composition of the soil algal community in the Catavi?a region is distinct from other desert sites we have studies, although some cosmopolitan desert soil taxa were present.