44 research outputs found
Advancing Tests of Relativistic Gravity via Laser Ranging to Phobos
Phobos Laser Ranging (PLR) is a concept for a space mission designed to
advance tests of relativistic gravity in the solar system. PLR's primary
objective is to measure the curvature of space around the Sun, represented by
the Eddington parameter , with an accuracy of two parts in ,
thereby improving today's best result by two orders of magnitude. Other mission
goals include measurements of the time-rate-of-change of the gravitational
constant, and of the gravitational inverse square law at 1.5 AU
distances--with up to two orders-of-magnitude improvement for each. The science
parameters will be estimated using laser ranging measurements of the distance
between an Earth station and an active laser transponder on Phobos capable of
reaching mm-level range resolution. A transponder on Phobos sending 0.25 mJ, 10
ps pulses at 1 kHz, and receiving asynchronous 1 kHz pulses from earth via a 12
cm aperture will permit links that even at maximum range will exceed a photon
per second. A total measurement precision of 50 ps demands a few hundred
photons to average to 1 mm (3.3 ps) range precision. Existing satellite laser
ranging (SLR) facilities--with appropriate augmentation--may be able to
participate in PLR. Since Phobos' orbital period is about 8 hours, each
observatory is guaranteed visibility of the Phobos instrument every Earth day.
Given the current technology readiness level, PLR could be started in 2011 for
launch in 2016 for 3 years of science operations. We discuss the PLR's science
objectives, instrument, and mission design. We also present the details of
science simulations performed to support the mission's primary objectives.Comment: 25 pages, 10 figures, 9 table
Young off-axis volcanism along the ultraslow-spreading Southwest Indian Ridge
Author Posting. © The Authors, 2010. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 3 (2010): 286-292, doi:10.1038/ngeo824.Mid-ocean ridge crustal accretion occurs continuously at all spreading rates
through a combination of magmatic and tectonic processes. Fast to slow spreading
ridges are largely built by adding magma to narrowly focused neovolcanic zones. In
contrast, ultraslow spreading ridge construction significantly relies on tectonic
accretion, which is characterized by thin volcanic crust, emplacement of mantle
peridotite directly to the seafloor, and unique seafloor fabrics with variable
segmentation patterns. While advances in remote imaging have enhanced our
observational understanding of crustal accretion at all spreading rates, temporal
information is required in order to quantitatively understand mid-ocean ridge
construction. However, temporal information does not exist for ultraslow spreading
environments. Here, we utilize U-series eruption ages to investigate crustal
accretion at an ultraslow spreading ridge for the first time. Unexpectedly young
eruption ages throughout the Southwest Indian ridge rift valley indicate that
neovolcanic activity is not confined to the spreading axis, and that magmatic crustal
accretion occurs over a wider zone than at faster spreading ridges. These
observations not only suggest that crustal accretion at ultraslow spreading ridges is
distinct from faster spreading ridges, but also that the magma transport
mechanisms may differ as a function of spreading rate.This work was supported by
the following NSF grants: NSF-OCE 0137325; NSF-OCE 060383800; and NSF-OCE
062705300
Impact of tree species on nutrient and light availability: evidence from a permanent plot study of old-field succession
Materials in movement: gold and stone in process in the Upton Lovell G2a burial
Excavated over two centuries ago, the Upton Lovell G2a ‘Wessex Culture’ burial has held a prominent place in research on Bronze Age Britain. In particular, was it the grave of a ‘shaman’ or a metalworker? We take a new approach to the grave goods, employing microwear analysis and scanning electron microscopy to map a history of interactions between people and materials, identifying evidence for the presence of Bronze Age gold on five artefacts, four for the first time. Advancing a new materialist approach, we identify a goldworking toolkit, linking gold, stone and copper objects within a chaîne opératoire, concluding that modern categorisations of these materials miss much of their complexity