‘Trojan’ asteroids in far reaches of solar system more common than previously thought

Aug. 29, 2013 — UBC astronomers have discovered the first Trojan asteroid sharing the orbit of Uranus, and believe 2011 QF99 is part of a larger-than-expected population of transient objects temporarily trapped by the gravitational pull of the Solar System’s giant planets.Trojans are asteroids that share the orbit of a planet, occupying stable positions known as Lagrangian points. Astronomers considered their presence at Uranus unlikely because the gravitational pull of larger neighbouring planets would destabilize and expel any Uranian Trojans over the age of the Solar System.To determine how the 60 kilometre-wide ball of rock and ice ended up sharing an orbit with Uranus the astronomers created a simulation of the Solar System and its co-orbital objects, including Trojans.”Surprisingly, our model predicts that at any given time three per cent of scattered objects between Jupiter and Neptune should be co-orbitals of Uranus or Neptune,” says Mike Alexandersen, lead author of the study to be published tomorrow in the journal Science. This percentage had never before been computed, and is much higher than previous estimates.Several temporary Trojans and co-orbitals have been discovered in the Solar System during the past decade. QF99 is one of those temporary objects, only recently (within the last few hundred thousand years) ensnared by Uranus and set to escape the planet’s gravitational pull in about a million years.”This tells us something about the current evolution of the Solar System,” says Alexandersen. “By studying the process by which Trojans become temporarily captured, one can better understand how objects migrate into the planetary region of the Solar System.”UBC astronomers Brett Gladman, Sarah Greenstreet and colleagues at the National Research Council of Canada and Observatoire de Besancon in France were part of the research team.

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New explanation for slow earthquakes on San Andreas

June 3, 2013 — New Zealand’s geologic hazards agency reported this week an ongoing, “silent” earthquake that began in January is still going strong. Though it is releasing the energy equivalent of a 7.0 earthquake, New Zealanders can’t feel it because its energy is being released over a long period of time, therefore slow, rather than a few short seconds.These so-called “slow slip events” are common at subduction zone faults — where an oceanic plate meets a continental plate and dives beneath it. They also occur on continents along strike-slip faults like California’s San Andreas, where two plates move horizontally in opposite directions. Occurring close to the surface, in the upper 3-5 kilometers (km) of the fault, this slow, silent movement is referred to as “creep events.”In a study published this week in Nature Geoscience, scientists from Woods Hole Oceanographic Institution (WHOI), McGill University, and GNS Science New Zealand provide a new model for understanding the geological source of silent earthquakes, or “creep events” along California’s San Andreas fault. The new study shows creep events originate closer to the surface, a much shallower source along the fault.”The observation that faults creep in different ways at different places and times in the earthquake cycle has been around for 40 years without a mechanical model that can explain this variability,” says WHOI geologist and co-author Jeff McGuire. “Creep is a basic feature of how faults work that we now understand better.”Fault creep occurs in shallow portions of the fault and is not considered a seismic event. There are two types of creep. In one form, creep occurs as a continuous stable sliding of unlocked portions of the fault, and can account for approximately 25 millimeters of motion along the fault per year. The other type is called a “creep event,” sudden slow movement, lasting only a few hours, and accommodating approximately 3 centimeters of slip per event. Creep events are separated by long intervals of slow continuous creep.”Normal earthquakes happen when the locked portions of the fault rupture under the strain of accumulated stress and the plates move or slip along the fault,” says the study’s lead author, WHOI postdoctoral scholar Matt Wei. …

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