Age and origin of the lowlands of Mars

One of the many significant findings of the Mars Global Surveyor mission is the presence of hundreds of quasi-circular depressions (QCDs) observed from high-resolution MOLA topography data. Their presence has recently been interpreted to reflect a northern lowlands that archive some of the earliest recorded rocks on Mars, mostly below a veneer of Hesperian and Amazonian materials. Here we analyze these data, coupled with a recent synthesis of geologic, geophysical, geomorphic, topographic, and magnetic information. Such analysis allows us to suggest a potential plate tectonic phase during the recorded Early into Middle Noachian martian history that transitioned into a monoplate world with episodic magmatic-driven activity persisting to present. This working hypothesis is based on: (1) the observation that the basement of the northern plains is younger than the basement of the southern highlands, but older than the material exposures of the cratered highlands, suggesting different formational ages for each one of the three geologic-time units; (2) the observation that parts of the very ancient highland's crust are highly magnetized, thus suggesting that most if not all of the formation of the lowlands basement postdates the shut off of the martian dynamo, some 4 Gyr ago, and so allowing hundreds of millions of years for the shaping of the buried lowlands. Consequently, the role of endogenic processes in the earliest geological evolution of Mars (Early perhaps into Middle Noachian) requires reconsideration, since MOLA topographic and MGS magnetic data afford a temporal window sufficient for very early, non-primordial shaping of the northern lowlands' basement.     

Putative Meteoritic Craters in Río Cuarto (Central Argentina) Interpreted as Eolian Structures

Ten oblong aligned depressions in the Río Cuarto area (provincia de Córdoba, Argentina) were supposed to be the result of very-low-angle Holocene meteoroid impacts. However, we consider that authors that studied the structures did not demonstrated their extraterrestrial origin. We suggest that an eolian origin for the structures of Río Cuarto is more likely. Actually, these landforms integrate large systems of similar deflation/accumulation geoforms aligned according to predominant winds during different periods.     

The helicity constraint in turbulent dynamos with shear

The evolution of magnetic fields is studied using simulations of forced helical turbulence with strong imposed shear. After some initial exponential growth, the magnetic field develops a large-scale travelling wave pattern. The resulting field structure possesses magnetic helicity, which is conserved in a periodic box by the ideal magnetohydrodynamics equations and can hence only change on a resistive time-scale. This strongly constrains the growth time of the large-scale magnetic field, but less strongly constrains the length of the cycle period. Comparing this with the case without shear, the time-scale for large-scale field amplification is shortened by a factor Q , which depends on the relative importance of shear and helical turbulence, and which also controls the ratio of toroidal to poloidal field. The results of the simulations can be reproduced qualitatively and quantitatively with a mean-field α Ω-dynamo model with alpha-effect and turbulent magnetic diffusivity coefficients that are less strongly quenched than in the corresponding α ²-dynamo.     

The NASA Astrophysics Data System: Free Access to the Astronomical Literature On-line and through Email

The Astrophysics Data System (ADS) provides access to the astronomical literature through the World Wide Web. It is a NASA funded project and access to all the ADS services is free to everybody world-wide. The ADS Abstract Service allows the searching of four databases with abstracts in Astronomy, Instrumentation, Physics/Geophysics, and the LANL Preprints with a total of over 2.2 million references. The system also provides access to reference and citation information, links to on-line data, electronic journal articles, and other on-line information. The ADS Article Service contains the full articles for most of the astronomical literature back to volume 1. It contains the scanned pages of all the major journals (Astrophysical Journal, Astronomical Journal, Astronomy & Astrophysics, Monthly Notices of the Royal Astronomical Society, and Solar Physics), as well as most smaller journals back to volume 1.The ADS can be accessed through any web browser without signup or login. Alternatively an email interface is available that allows our users to execute queries via email and to retrieve scanned articles via email. This might be interesting for users on slow or unreliable links, since the email system will retry sending information automatically until the transfer is complete. There are now 9 mirror sites of the ADS available in different parts of the world to improve access. The ADS is available at: http://ads.harvard.edu This revised version was published online in July 2006 with corrections to the Cover Date.     

Statistical analysis of the temperature records for the Northern Territory of Australia

The Northern Territory of Australia has a unique situation of an extension larger than France and a population of 200,000, with only three meteorology stations open for more than 40 years, Darwin (DW), Alice Springs (AS) and Tennant Creek, and only two of them, DW and AS, providing data over 100 years, and from 500 to more than 1,000 km separating these stations and the stations in the neighbouring states of Australia. Homogenizations of data in between different measuring sites for the same location as well as the way to derive the missed data to complete at least 100 years from the neighbouring locations are analysed in details and the effects on the temperature trends are straightforwardly investigated. Using properly homogenised data over 130 years and a linear fitting, the warming maximum and minimum temperatures are +0.009 and +0.057 °C/10 years for Alice Springs and ?0.025 and 0.064 °C/10 years for Darwin. With the data available, the only option to produce warming trends is to overweight the cold years in the middle of the 1970s and the subsequent return to warmer temperatures. Starting from 1980, to compute trends, there is still a clear warming in Alice Springs, but also clear cooling in Tennant Creek, and a mixed behaviour with warming maximum temperatures and cooling minimum temperatures in Darwin.     

The Villalbeto de la Peña meteorite fall: II. Determination of atmospheric trajectory and orbit

Abstract— The L6 ordinary chondrite Villalbeto de la Peña fall occurred on January 4, 2004, at 16: 46: 45 ± 2 s UTC. The related daylight fireball was witnessed by thousands of people from Spain, Portugal, and southern France, and was also photographed and videotaped from different locations of León and Palencia provinces in Spain. From accurate astrometric calibrations of these records, we have determined the atmospheric trajectory of the meteoroid. The initial fireball velocity, calculated from measurements of 86 video frames, was 16.9 ± 0.4 km/s. The slope of the trajectory was 29.0 ± 0.6° to the horizontal, the recorded velocity during the main fragmentation at a height of 27.9 ± 0.4 km was 14.2 ± 0.2 km/s, and the fireball terminal height was 22.2 ± 0.2 km. The heliocentric orbit of the meteoroid resided in the ecliptic plane (i = 0.0 ± 0.2°), having a perihelion distance of 0.860 ± 0.007 AU and a semimajor axis of 2.3 ± 0.2 AU. Therefore, the meteorite progenitor body came from the Main Belt, like all previous determined meteorite orbits. The Villalbeto de la Peña fireball analysis has provided the ninth known orbit of a meteorite in the solar system.     

Orbital Elements of 2004 Perseid Meteoroids Perturbed by Jupiter

Jupiter and Saturn produce important gravitational impulses on meteoroids released by comet 109P/Swift-Tuttle. The meteoroids from this comet once released follow retrograde orbits that during their periodic approaches to these planets (within 1.6 and 0.9 A.U., respectively) are impulsed gaining orbital energy. This perturbation effect is translated into a net inward shift in the node of the perturbed meteoroids. Such geometry with Jupiter occurred in 2004 over a meteoroid trail ejected by this comet during the 1862 A.D. return of the comet to perihelion. In order to study the predicted outburst produced by one-revolution meteoroids, the Spanish Photographic Meteor Network (SPMN) performed an extensive campaign. As a part of this observational effort here are presented 10 accurate meteoroid orbits. We discuss their origin by comparing them with the theoretical orbital elements of the dust trails intercepting the Earth during the 2004 Perseid return.