AGU Journal highlights - 30 June 2005

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I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

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1. Listening to the 2004 Indian Ocean tsunami quake

Recordings of ocean sound waves taken during the earthquake that caused the Indian Ocean tsunami of 26 December 2004 offer clues to what happened under the ocean. The 9.0 magnitude earthquake resulted from two tectonic plates suddenly moving against each other. This rupture generated sound waves that were recorded by a network of five sensor arrays located around the Indian Ocean. De Groot-Hedlin tracked the movement of the rupture by comparing recordings at the various sensors and using them to triangulate the location of their source. The results indicated that the rupture first moved northwest at 2.4 kilometers per second [1.6 miles per second] along the Sunda trench, a submarine trench where three tectonic plates intersect, then slowed to 1.5 kilometers per second [0.93 miles per second] around 600 kilometers [400 miles] from the earthquake's epicenter. Although an island blocked several of the sensors from recording waves emanating from the rupture as it moved beyond 800 kilometers [500 miles] from the epicenter, some of sensors made recordings that suggest the rupture continued on northward. The author says that the slower speed of the rupture was unusual for an earthquake caused by a rupture close to the surface.

Title: Estimation of the rupture length and velocity of the Great Sumatra earthquake of Dec 26, 2004 using hydroacoustic signals

Author: Catherine D. de Groot-Hedlin, Scripps Institution of Oceanography, La Jolla, California, USA.

Source: Geophysical Research Letters (GL) paper
10.1029/2005GL022695, 2005

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2. Building a better virtual raindrop

A new way of mathematically modeling the formation of rain drops in clouds may improve understanding of Earth's climate, cloud formation and movement, and the effect that small airborne particles have on rainfall. In the first step in the formation of raindrops, small cloud droplets combine to form larger drops, in a process known as autoconversion. Liu et al. say that the mathematical representation of this process, which is used in simulating cloud activity and global climate patterns, has in the past been oversimplified and vague, because some of the terms in the equation lacked a physical basis. To address this problem, they developed a new model for raindrop formation that takes into account the limited size range of droplets that can interact to create rain drops. The new model also accounts for the amount of liquid water present and the concentration of droplets in a cloud. The authors say that their model avoids guesswork by being better grounded in physics and is as easy to use as other models.

Title: Size truncation effect, threshold behavior, and a new type of autoconversion parameterization

Authors: Yangang Liu, Peter H. Daum, and Robert L. McGraw,
Brookhaven National Laboratory, Upton, New York, USA.

Source: Geophysical Research Letters (GL) paper
10.1029/2005GL022636, 2005

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3. Looking to Europe's past to predict future climate

Computer simulations of Earth's climate are important tools for researchers attempting to predict future climate patterns. Kaspar et al. compared the output of a climate model called ECHO-G to a reconstruction of Europe's past climate based on fossil evidence, in part to test the model's effectiveness. The climate model simulated ocean and atmospheric circulation at a warm point between ice ages (around 125,000 years ago) during the Eemian period, and was based on previously established greenhouse gas concentrations and Earth's orbit around the Sun. Since climate has an influence on the species and growth of plants present in a region, the authors were also able to reconstruct climate patterns during the period, using pollen and plant fossils with a newly developed method. They found that the computer simulation and the fossil reconstruction painted similar pictures of Eemian climate: summers that were hotter than now and varying winter conditions from west to east across Europe. Thus, the ECHO-G model was successful in simulating the Eemian climate. The results also indicated that changes in Earth's orbit during the period explain the temperature patterns seen in the climate reconstruction.

Title: A model-data comparison of European temperatures in the Eemian interglacial

Authors: Frank Kaspar, Norbert Kühl, Ulrich Cubasch, and Thomas Litt,
Max Planck Institute for Meteorology, Hamburg, Germany.

Source: Geophysical Research Letters (GL) paper
10.1029/2005GL022456, 2005

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4. A new tool in the search for water on Mars

A major goal of Mars exploration is to search for underground water. One tool scientists may soon use in this search is MARSIS, an instrument on the Mars Express spacecraft, which is currently in orbit around Mars. MARSIS is a 40-meter [100-foot] antenna that can transmit and receive ground penetrating radar signals, which researchers hope will allow them to search for water under Mars' polar ice caps. It is estimated that MARSIS signals can penetrate 0.1-5 kilometers [300 feet to three miles] below the planet's surface. To narrow the estimate of how deep in the crust the radar system might be

able to find lakes, Farrell et al. ran a computer simulation of MARSIS signals penetrating an ice mass. The simulation showed that the disruptive effect ice has on radar signals limited the maximum depth at which sub-glacial lakes could be detected to 2.5 kilometers [1.6 miles]. The authors note that the model is simplistic and ignores factors such as the presence of unfrozen water in the ice caps or atmospheric interference of the signals, but that it does give insight into MARSIS' ability to detect underground water.

Title: Detecting sub-glacial aquifers in the north polar layered deposits with Mars Express/MARSIS

Authors: W. M. Farrell, J. J. Plaut, D. A. Gurnett, and G. Picardi,
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.

Source: Geophysical Research Letters (GL) paper
10.1029/2005GL022488, 2005

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5. Surface gases found in deep-mantle volcanic rock

The composition of rocks and minerals varies from place to place in Earth's mantle, the layer of hot rock that comprises most of the planet's mass. The source of some of this variation may be the recycling of surface rock back into the mantle as one tectonic plate subducts, or moves down, under another. Fischer et al. collected lava, volcanic ash, and xenoliths (pieces of anomalous rock trapped in lava) from volcanoes at several subduction zones around the world. They analyzed the samples to determine the ratio of different nitrogen isotopes present in the rocks. The results indicated that the composition of nitrogen isotopes varies from place to place in the mantle. In some of the rocks, they found a nitrogen isotopic composition characteristic of Earth's surface. Ruling out intrinsic mantle processes as the source of this composition, the authors argue that surface rock that had at one time been exposed to ocean water or air had been recycled deep into the mantle.

Title: Nitrogen isotopes of the mantle: Insights from mineral separates

Authors: Tobias P. Fischer, Naoto Takahata, Yuji Sano, Hirochika Sumino, and David R. Hilton,
University of New Mexico, Albuquerque, New Mexico, USA.

Source: Geophysical Research Letters (GL) paper
10.1029/2005GL022792, 2005

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6. Ground tilt gives clues to crater activity inside a volcano

A resurgence of activity at a West Indies volcano has allowed researchers to observe the cycle of growth and destruction of lava domes inside its crater. The Soufrière Hills Volcano on the island of Montserrat began erupting on 18 July 1995, its first recorded eruption in history, and has remained active since. Researchers have used the opportunity to make detailed observations of factors involved in the formation of lava domes in the volcano's crater. Widiwijayanti et al. used measurements of ground tilt made in May 1997 by sensors placed on the side of a lava dome to estimate the depth, size, and pressure of a shallow region under the dome. The tilt measurements indicated that pressure in the lava dome cycled about every 3-30 hours and were due to a magma-influenced pressure source of radius 200-340 meters [660-1,100 feet] located 740-870 meters [2,400-2,900 feet] below the dome. This source is larger than the dimensions of the magma conduit, and the authors suggest pressurization of a fluid-saturated fractured rock mass surrounding a narrow magma-filled conduit.

Title: Geodetic constraints on the shallow magma system at Soufrière Hills Volcano, Montserrat

Authors: Christina Widiwijayanti, Amanda Clarke, Derek Elsworth, and Barry Voight,
Pennsylvania State University, University Park, Pennsylvania, USA.

Source: Geophysical Research Letters (GL) paper
10.1029/2005GL022846, 2005

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7. Alaskan wildfires emitted copious carbon monoxide

The largest fires in Alaska's recorded history burned huge tracts of Alaskan and Canadian forest in the summer of 2004, releasing large amounts of carbon monoxide into the

atmosphere. Carbon monoxide plays an important role in atmospheric chemistry--in the formation of ozone, for example--and may be a key factor in global climate change. But determining the amount of carbon monoxide released into the air above a particular region has proved difficult. Pfister et al. were able to more accurately determine the amount of carbon monoxide emitted by the wildfires into the atmosphere over northern North America by combining recordings of atmospheric carbon monoxide levels taken from satellites and airplanes with simulations from a chemical transport model. They estimate that from June through August the wildfires emitted roughly 30 teragrams [30 million tons] of carbon monoxide, an amount similar to that released by human activities in the continental U.S. during the same period.

Title: Quantifying CO emissions from the 2004 Alaskan wildfires using MOPITT CO data

Authors: G. Pfister, P. G. Hess, L. K. Emmons, J.-F. Lamarque, C. Wiedinmyer, D. P. Edwards, G. Pétron, J. C. Gille, and G. W. Sachse,
National Center for Atmospheric Research, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022995, 2005

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8. Moonlets may create propellers in Saturn's rings

A new computer simulation may lead to the discovery of large bodies known as "moonlets" in Saturn's rings. The Cassini spacecraft, which entered orbit around Saturn in June 2004, has been sending back high-quality images of the planet's rings. The rings are made up of icy particles a few centimeters [inches] to several meters [yards] in size. Moonlets, larger bodies, perhaps as big as several kilometers [miles] in diameter, are also expected to be present in the rings, but only one has been found so far. Seiß et al. simulated the patterns that the gravity of different sized moonlets might create in the dense clouds of particles that compose the rings. They found that S-shaped density patterns, resembling two-bladed airplane propellers, formed in the clouds of particles around the virtual moonlets. The sizes and shapes of the propellers varied depending on the size of the moonlet. The authors suggest that it might be possible to discover actual moonlets in Saturn's rings by looking for these patterns in the data and images sent back from Cassini.

Title: Structures induced by small moonlets in Saturn's rings: Implications for the Cassini Mission

Authors: M. Seiß, F. Spahn, M. Sremcevic, and H. Salo,
University of Potsdam, Potsdam, Germany.

Source: Geophysical Research Letters (GL) paper
10.1029/2005GL022506, 2005

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II. Ordering information for science writers and general public

Journalists and public information officers of educational and scientific institutions (only) may receive one or more of the papers cited in the Highlights by sending a message to Jonathan Lifland [jlifland@agu.org], indicating which one(s). Include your name, the name of your publication, and your phone number. The papers will be e-mailed as pdf attachments.

Others may purchase a copy of the paper online for nine dollars:
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