The following highlights summarize research papers in Geophysical Research Letters (GL), Journal of Geophysical Research--Space Physics (JA), Journal of Geophysical Research--Solid Earth (JB), Journal of Geophysical Research--Oceans (JC), Journal of Geophysical Research--Atmospheres (JD), and Water Resources Research (WR). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.
1. Water properties needed to carve Martian channels
Subsurface aquifers similar to those on Earth could have caused the flood channels seen in a Martian landscape known as Cerberus Fossae. Michael Manga analyzed the requirements needed to form the deep troughs observed on Mars, including the surface permeability, water flow speed, and the discharge rate necessary to carve the formations. Manga suggests that existing calculations used to estimate subsurface properties on Earth may help explain how water contained in underground aquifers below Mars' frozen crust could release rapidly and form the channels. The model assumed that the channel-forming floods were initiated by water stored in volcanic aquifers a few kilometers [miles] below Mars' cryosphere [permanently frozen area] that escaped periodically to the surface through cracks in the ground. The simulation predicts that the channel formation would have required multiple floods, but would not need additional weather effects.
Title: Martian floods at Cerberus Fossae can by produced by groundwater discharge
Michael Manga, University of California-Berkeley, Berkeley, California.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018958, 2004
2. Estimating the depth of Mercury's crust
An analysis of Mercury's surface features has allowed researchers to estimate the depth of the planet's crust. Nimmo and Watters speculate that the shape of fault scarps observed by the Mariner spacecraft can be used to measure the thickness of the brittle layer where the faulting occurs. The thickness of the brittle layer indicates that Mercury's heat flux was relatively high at the time of faulting and that its lithosphere [solid area] was relatively weak. Their study indicates that the planet's crust must be less than 140 kilometers [90 miles] thick; otherwise, the high heat flux would have melted the crust. Such a depth is nearly 30 percent of Mercury's total interior volume, compared to approximately one percent for Earth's crust. Title: Depth of faulting on Mercury: Implications for heat flux and crustal and effective elastic thickness
Francis Nimmo, University of California--Los Angeles, Los Angeles, California; T. R. Watters, Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D.C.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018847, 2004
3. Borehole analysis can simulate earthquake dynamics
A recently published paper analyzing the seismic reaction after injecting a borehole with fluids may provide researchers with an improved method to estimate earthquake probabilities. Parotidis et al. modeled the seismic reactions after fluid was injected into a borehole, which is done regularly for oil extraction or intentional rock fracturing, and report that the same spread of pore pressure that causes rocks to fail in an earthquake also occurs in a borehole. Their study helps explain the ground's response to the pressure buildup from fluids in seismically active regions and defines a way that pressure spreads in an identifiable motion through a rock before it fractures. The mechanism, termed the "back front of induced seismicity," was matched to three case studies in quake-prone areas and nearly matched the actual fracture dynamics. The authors conclude that new information may allow them to infer the spread and fracture point of a quake.
Title: Back front of seismicity induced after termination of borehole fluid injection
Miltos Parotidis, S. A. Shapiro, E. Rothert, Free University of Berlin, Berlin, Germany.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018987, 2004
4. International Space Station sprites images provide new details
Images of sprites taken from the International Space Station provide new estimates for the phenomena and suggest for the first time that the unusual events may be coupled to the atmosphere, ionosphere and magnetosphere. Blanc et al. report the first sprite images taken from space from directly above, which can aid current observations that estimate the intensity, frequency and characteristics of the lightning-like features. Sprites are upward luminous flashes associated with electrical field charges in thunderstorms, but they occur at higher altitudes than normal lightning. The camera images taken from the space station show that most sprites are much brighter when seen from directly above and can easily be differentiated from regular lightning by their spectral response. The information can be used to better identify sprites using satellite observations and can help researchers determine the atmospheric dynamics in the near-Earth space environment that may affect sprite formation.
Title: Nadir observations of sprites from the International Space Station
Elisabeth Blanc, T. Farges, R. Roche, D. Brebion, Atomic Energy Commissiion, Bruyeres le Chatel, France; T. Hua, Astrophysical Laboratory of Marseille, Marseille, France; A. Labarthe, National Center for Space Studies, Toulouse, France; V. Melnikov, Rocket Space Corporation ENERGIA, Korolev, Russia.
Source: Journal of Geophysical Research--Space Physics (JA) paper 10.1029/2003JA009972, 2004
5. First direct observation of interstellar hydrogen shadow
The Cassini spacecraft has made the first measurements of a large shadow in the interstellar hydrogen population and recorded the first direct observations of interstellar pickup ions outside of Jupiter's orbit. McComas et al. analyzed data from the mission's ion mass spectrometer and confirm that neutral interstellar helium atoms travel far into the solar system, forming a focusing cone even at these large distances. The study also provides the first direct evidence of a hydrogen shadow shaped by the Sun's motion through the interstellar medium. The authors report that most hydrogen particles become charged as they try to pass close by the Sun and are swept away in the solar wind, leaving a large depletion, or interstellar shadow, in the neutral hydrogen downstream in the interstellar flow. Their report also shows how particles that flow into the heliosphere [the region in which the solar wind is identifiable] are engaged by the Sun's gravitational field and solar radiation pressure, which, depending on the neutral species, collectively bend the trajectories toward or away from the Sun.
Title: Interstellar hydrogen shadow: Observations of interstellar pickup ions beyond Jupiter
David J. McComas, N. A. Schwadron, F. J. Crary, H. A. Elliott, D. T. Young, Southwest Research Institute, San Antonio, Texas; J. T. Gosling, M. F. Thomsen, Los Alamos National Laboratory, Los Alamos, New Mexico; E. Sittler, NASA Goddard Space Flight Center, Greenbelt, Maryland; J.-J. Berthelier, National Research Center, St. Maur, France; K. Szego, KFKI Research Institute for Particles and Nuclear Physics, Budapest, Hungary; A. J. Coates, University College London, Surrey, UK. Source: Journal of Geophysical Research--Space Physics (JA) paper 10.1029/2003JA010217, 2004
6. The underground heating effect on cities
The first comprehensive study to analyze how urban areas affect subsurface temperatures can help researchers use boreholes and monitoring wells near cities to better study climate change. Ferguson and Woodbury studied the temperature field below Winnipeg, Manitoba, Canada, and found that downward heat flow from urban sources reached more than 130 meters [430 feet] beneath the city and likely raised groundwater temperatures in parts of a regional aquifer by nearly five degrees Celsius [nine degrees Fahrenheit]. Their results suggest that the subsurface temperature increases make geothermal energy a more attractive option in areas surrounding Winnipeg and near other cities. The authors further note that urban heating can affect past climate reconstructions based on subsurface temperature measurements; they conclude that gleaning such information on past climates is only possible in locations where the underground temperatures are not significantly affected by urban heat islands.
Title: Subsurface heat flow in an urban environment
Grant Ferguson, Allan D. Woodbury, University of Manitoba, Winnipeg, Manitoba, Canada.
Source: Journal of Geophysical Research--Solid Earth (JB) paper 10.1029/2003JB002715, 2004
7. Coastal water pollution's effects on ocean dynamics
Pollution and assorted waste at the surface of the coastal ocean has a significant effect on the heating rate and heat budget of waters below. Chang and Dickey investigated the influences of surface material on the reflectivity and thermodynamics of shallow coastal waters off New Jersey as part of a 2001 ocean dynamics experiment. The authors found that chlorophyll and dissolved organic matter from upper ocean geochemistry have the greatest impact on diminishing the depth that sunlight reaches into water, which affects the physical properties of coastal waters. Previous studies have examined the water temperature effects from clouds and solar radiation in the open ocean. The current study shows, however, that coastal waters have far more short-term, and a wider variety of, inputs than the ocean. Such inputs, ranging from freshwater addition to assorted detritus and sewage from nearby urban areas, affect the water column temperature structure and can limit the normal movement of nutrients and particles between warmer and cooler waters.
Title: Coastal ocean optical influences on solar transmission and radiant heating rate
Grace C. Chang, Tommy D. Dickey, University of California--Santa Barbara, Goleta, California.
Source: Journal of Geophysical Research--Oceans (JC) paper 10.1029/2003JC001821, 2004
8. New method uses ground sensors to analyze clouds
A new method to analyze cloud characteristics using readily available data from ground-based sensors can help researchers determine whether solar radiation changes caused by cloud cover have caused any long-term global climate changes. Dutton et al. studied seasonal and annual cloud occurrence frequency averages over nearly 30 years in four remote, climactically diverse areas. Their method uses already-recorded surface radiation observations to describe daytime cloud characteristics, and then examines the cloud's effect on solar heating. The technique also uses solar radiation monitors to identify long-term trends in climate variations that can be attributed to changes in solar radiation during times of cloud cover. The researchers' findings can help determine the conditions that are responsible for changes in solar transmissions and cloudiness, providing a ground-based source for comparison to current satellite observations and numerical climate models, which require more detail about basic cloud properties.
Title: Long-term variations in the occurrence and effective solar transmission of clouds as determined from surface-based total irradiance observations
Ellsworth G. Button, Donald W. Nelson, Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado; Alessio Farhadi, Imperial College, London, United Kingdom; Robert S. Stone, Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, and Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado; Charles N. Long, Pacific Northwest National Laboratory, Richland, Washington.
Source: Journal of Geophysical Research--Atmospheres (JD) paper 10.1029/2003JD003568, 2004
9. First measurement of ozone-destroying chlorine compound
A team of researchers has made the first atmospheric measurements of the dimer of chlorine monoxide, a principal compound involved with ozone destruction in the wintertime polar stratosphere. The observations confirm that chlorine monoxide must play a key role in destroying ozone, which scientists have theorized about for years based on laboratory investigations and atmospheric models. Stimpfle et al. acquired the measurements of the dimer during a joint U.S. and European field campaign in winter 1999-spring 2000 over northern Sweden. The improved understanding of the chlorine monoxide compound helps better explain ozone losses that have been observed over the Arctic and Antarctic regions during the winter and aid in developing improved predictive models for ozone loss over the poles.
[Note: See also AGU Press Release 04-07: http://www.agu.org/sci_soc/prrl/prrl0407.html]
Title: First measurements of ClOOCl in the stratosphere: The coupling of ClOOCl and ClO in the Arctic polar vortex
Richard M. Stimpfle, D. M. Wilmouth, J. G. Anderson, Harvard University, Cambridge, Massachusetts; R. J. Salawitch, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California. Source: Journal of Geophysical Research--Atmospheres (JD) paper 10.1029/2003JD003811, 2004
10. New method to predict flood timing
A new measure to accurately identify flood seasons can help refine flood occurrence models and assist in grouping areas likely to share similar risks. Cunderlik et al. used a number of hydrological and water management principles to determine the variability of seasonal flooding and create maps to note the distribution of flood-prone areas. They then applied their method to data from sites across Great Britain in order to estimate the type and identify the time of flooding. The authors note that their newly created seasonal flood maps could be compared to topographical maps to depict how downstream areas could be affected by the timing of such water flow and to generate predictions for future flood timing. They report that there was no previous technique to reliably identify flood seasons and that the new method can be used in streamflow forecasting, reservoir operations and floodplain protection.
Title: On the objective identification of flood seasons
Juraj M. Cunderlik, Taha B. M. J. Ouarda, Bernard Bobee, National Institute for Scientific Research: Water, Land, and Environment, University of Quebec, Sainte-Foy, Quebec, Canada.
Source: Water Resources Research (WR) paper
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