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In BriefTitan mission data now available The

European Space Agency (ESA) has made public the data from four of the six experi-ments on the Huygens probe, which landed on the surface of Saturn’s moon Titan in January 2005.

Data is available from the following instruments: the Huygens Atmospheric Structure Instrument, which measured the physical and electrical properties of the moon’s atmosphere; the Doppler Wind Experiment, which studied the direction and strength of Titan’s winds; the Aerosol Collector and Pyrolyser (ACP), which col-lected aerosols; and the Gas Chromato-graph and Mass Spectrometer, which ana-lyzed data from the ACP and measured the chemical composition of the atmo-sphere.

Data from the Descent Imager/Spectral Radiometer and the Surface Science Pack-ager will be released in September or October along with the probe’s entry and descent trajectory.

The data sets include the data as well as calibration information and documentation needed to process and analyze the data.

The data can be accessed from the ESA Planetary Science archive at http://www.rssd.esa.int/PSA

Report highlights climate change risk to oceans Human-induced changes in atmospheric carbon dioxide levels will have severe consequences for oceans, con-cluded a 31 May report from the German Advisory Council on Global Change (WBGU).

The WGBU wrote the report, which was released in English on 26 July, in order to draw attention to the need to develop cli-mate change mitigation and adaptation strategies and to contribute to the develop-ment of a European Union ocean policy.

The report identifies several threats to world oceans from climate change, includ-ing increasing ocean acidification, melting of sea ice, increasing destructive force of tropical cyclones, rising sea level, and nega-tive impacts on marine ecosystems.

For each of these items, WBGU established ‘guard rails,’ quantitative limits on damage that if breached would lead to intolerable consequences. For example, an increase in mean global surface temperature above 2°C would lead to climatic changes that have intolerable consequences for marine con-servation.

In addition, the report makes several recom-mendations to avoid exceeding these guard rails. They include decreasing anthropo-genic greenhouse gas emissions by 2050 to half of 1990 levels, establishing 20–30% of any marine ecosystem as a protected area, sequestering carbon dioxide under the sea floor only as a transitional strategy and not a permanent solution, and constructing international agreements to accommodate refugees from coastal areas affected by climate change.

The report is available in English at http://www.wbgu.de/wbgu_sn2006_en.html and in German at http://www.wbgu.de/wbgu_sn2006.html

Witnesses urge measures to protect oceans To better manage the nations oceans, the United States should establish a national

ocean policy and ratify the United Nations Convention on the Law of the Sea, several wit-nesses testified at a 3 August hearing before the Senate Commerce, Science, and Transpor-tation Subcomittee on National Ocean Policy Study.

“Our oceans are in crisis,” said Leon Panetta, co-chair of the Joint Ocean Commission Initiative. Panetta cited several issues threat-ening oceans, including ocean acidification, increasing amounts of pollution, lost wet-lands, depleted fisheries, invasive species, and a fragmented U.S. governance system.

Michael Orbach, director of the Duke Uni-versity Marine Laboratory, said, “If we are to be perceived as a leader in ocean science and policy, we have to demonstrate that we are, in fact, part of the international community.” He explained, “We cannot solve the ocean policy or scientific or human problems by ourselves, so accession to [the Law of the Sea] treaty is very important.”

The United States is the only industrial-ized nation that has not yet ratified the treaty. Although the convention has wide support in the Senate, a small group led by Sen. James Inhofe (R-Okla.) has prevented a vote on ratification.

Other countries are beginning to take advantage of the treaty. Several—including Russia, Canada, and Norway—are preparing or already have submitted applications to extend the limits of their continental shelf, said Paul Kelly, a member of the U.S. Com-mission on Ocean Policy. “With tight oil and gas supplies and growing demand, these countries are beginning to look at the Arctic for future exploration,” he said. The United States has the technology to map its conti-nental shelf and expand the limits, but “we have not even started the mapping,” Kelly noted.

—Sarah ZielinSki, Staff Writer

Studying the Earth’s deep conductivity structures, important for developing our understanding of the dynamics of the Earth, is complicated due to effects of the shallow conductive structures on the electromag-netic (EM) responses for periods larger than hours. The results of the deep EM soundings can be heavily distorted by the surface shell conductance, which varies from fractions of siemens (S) inland to up to tens thousand of siemens in the oceans. Thus, separating the effects caused by those variations and by deep conductivity structures is an impor-tant step during interpretation of the data.

This article reports on efforts to overcome these difficulties by providing high-resolu-tion, global maps of the Earth’s surface shell conductivity structure, from which deep conductivity can be interpolated. Using fine-scale regional surface schemes of conduct-ance for the shallow structures (S-maps) overlain and compiled into broader spatial maps, scientists will be able to use data products from these efforts to accomplish research goals of the currently running USArray (http://www.emscope.org) and for the planned Euro-Array (http://www.euroar-ray.org), projects that aim in part to region-ally map the conductivity structures at upper and middle mantle depths by using magnetotelluric (MT) and magnetovariation (MV) methods.

Changes in the outer part of the Earth’s magnetic field, usually caused by interac-tions between the solar wind and the iono-sphere and magnetosphere, induces an electric ‘telluric’ field in the Earth, and the strength of the telluric field is dependent on the conductivity (resistivity) of the medium. In the MT method, observing the magnetic and electric fields simultaneously, and deter-mining their ratios at varying periods, allows for the derivation of the conductivity distri-bution with depth. A similar process for determination of conductivity is used in the MV method, but just from magnetic compo-nents of the field.

Measuring conductivity is a useful tool for distinguishing different rock types, and measurements of the Earth’s subsurface conductivity can shed light on structural geology. The research described here seeks to characterize mantle inhomogeneity against the background of known surface

shell conductance distribution for the Euro-pean region. This study reveals a number of problems that must be overcome in order to collect reliable mantle soundings, and it highlights the necessity of improving of S-maps.

Improving S-maps

The most straightforward way to separate deep conductivity from surface shell con-ductivity is to numerically simulate EM fields in the frame of the Earth’s layered model, incorporating a surface conducting shell that consists of contributions from the sea water and from sediments (see Figure 1) Obviously, the accuracy of such a modeling depends essentially on the accuracy of the global S-maps used.

TRANSACTIONS AMERICAN GEOPHYSICAL UNION

The Newspaper of the Earth and Space Sciences

Editors

John W. Geissman: Dept. of Earth and Plan-etary Science,University of New Mexico, Albu-querque, USA; jgeiss@unm.edu

Wendy S. Gordon: Texas Commission on Environmental Quality; wgordon@tceq.state.tx.us

Manuel Grande: University of Wales, Aberystwyth; mng@aber.ac.uk

Hassan Virji: START; hvirji@agu.org

Editor in Chief

A. F. Spilhaus, Jr.: AGU, Washington, D.C., USA; fspilhaus@agu.org

Corresponding Editors

Roland Burgmann (Tectonophysics): University of California at Berkeley, USA; burgmann@seismo. berkeley.edu

William E. Carter (History): Dept. of Civil Engineering, University of Florida, Gainesville, USA; bcarter@ce.ufl.edu

Millard F. Coffin (Japan): Ocean Research Institute, University of Tokyo, Japan; mcoffin@ori. u-tokyo.ac.jp

Steven C. Constable (Geomagnetism and Paleomagnetism): Scripps Institution of Oceanog-raphy, La Jolla, California, USA; sconstable@ucsd.edu

Richard S. Gross (Geodesy): Jet Propulsion Laboratory, Pasadena,California USA; richard.gross@ jpl.nasa.gov

Marguerite Kingston (Public Affairs): USGS (retired); Mkingston9@aol.com

Stephen A. Macko (Education): Dept. of Environmental Sciences, University of Virginia, Charlottesville, USA; sam8f@virginia.edu

Louise Prockter (Planetary Sciences): Applied Physics Laboratory, Laurel, Maryland, USA; Louise.Prockter@jhuapl.edu

Paul Renne (VGP): Berkeley Geochronology Center, Berkeley, California, USA;prenne@bgc.org

Justin S. Revenaugh (Seismology): University of Minnesota, Minneapolis, USA; justinr@tc.umn. edu

Jeffery J. Roberts (Mineral and Rock Physics): Lawrence Livermore National Labora-tory, Livermore, California, USA; roberts17@llnl.gov

Sarah L. Shafer (Paleoceanography and Paleoclimatology): U.S. Geological Survey, Corvallis, Oregon, USA; sshafer@usgs.gov

David Sibeck (Space Physics and Aerono-my): NASA/Goddard Space Flight Center, Green-belt, Maryland, USA; david.g.sibeck@nasa.gov

Ramesh P. Singh (India): Dept. of Civil Engi-neering, Indian Institute of Technology, Kanpur, India; ramesh@iitk.ac.in

Maribeth Stolzenburg (Atmospheric and Space Electricity): Dept. of Physics and Astronomy, University of Mississippi, University, USA; mstolzen@phy.olemiss.edu

Jeffrey M. Welker (Biogeosciences): Environ- ment and Natural Resources Institute, University of Alaska, Anchorage, USA; afjmw1@uaa.alaska.edu

Assistant Editors

Peter Folger: AGU, Washington, D.C., USA; pfolger@agu.org

Catherine O’Riordan: AGU, Washington, D.C., USA; coriordan@agu.org Staff

Editorial: Randy Showstack, Managing Editor; Ayesha Badhwar, Production Coordinator; Sarah Zielinski, News Writer/Editor; Mohi Kumar, Science Writer/ Editor; Liz Castenson, Editor’s Assistant Advertising Carla Childres, Advertising Assistant; Tel: +1-202-777-7536; E-mail: advertising@agu.org Composition and Graphics: Habib Hastaie, Manager; Valerie Bassett, Carole Saylor, and Nancy Sims, Electronic Graphics Specialists

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www.agu.org/pubs/eos

SECTION newsUpdating the Map of Earth’s Surface Conductance

news

Fig. 1. The current global surface S-map with a global conductivity distribution of the Earth’s interior and extracted S-map for the European region. The initial S-map is obtained from sediment thicknesses (left) and the new corrected S-map is obtained by MT soundings in situ (right). Sedi-ment thicknesses are from Laske and Masters [1997].

Section News cont. on page 331

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The modern global S-maps are mainly based on the bathymetry, and global sedi-ment thicknesses (on land as well as under water) given by Laske and Masters [1997]. Additionally, improvement in the oceanic conductance data recently was achieved, taking into account the salinity, temperature and pressure of the sea water [cf. Manoj et al., 2006]. However, the methods used to create these global S-maps seem to be insuf-ficient do to the ambiguity introduced by applying heuristic procedures to convert sediment thicknesses into conductance val-ues [cf. Everett et al., 2003].

One way to improve the global S-map on the continents is to merge the existing regional S-maps that have been compiled all over the world for many years. These regional S-maps often have better resolution and, most probably, higher reliability since they use a pri-ori information, for example, the shal-low seismic and gravity investiga-tions, or electrical and EM prospecting [cf. Harinarayana and Naganjaneyulu, 2003]. The authors of this article started updating the global S-map on the continents by incorporating the existing regional S-maps, and welcome the contribution of regional conductance data in any form for subsequent correction of the global S-map. The data can be sent to article lead author Ján Vozár (geofSmap@savba.sk), Geophysical Institute, Bratislava, Slovak Republic. The modeling results for the regions of interest with the updated global S-map will be available on request.

Modeling EM Responses With a New Regional S-map

In a demonstration of the distorting effect of the nonuniform conductance distribution on EM responses in the European region, the layered model of the Earth shown in Figure 1, which incorporates a thin spherical shell, was used with updated regional con-ductance for northern, central, and eastern Europe. This S-map was compiled with spa-tial resolution of 1° × 1° as part of two proj-ects: Baltic Electromagnetic Array Research (BEAR) [Korja et al., 2001] and Central

Europe Mantle Geoelectrical Structure (CEMES) [Semenov et al., 2003]. The new regional S-maps are shown in Figure 1. Induction equations have been solved numer-ically according to the three-dimensional scheme by Kuvshinov et al. [2005].

Figure 2 presents examples of the theoret-ically calculated apparent resistivity and impedance phase values for the MT and magnetovariational geomagnetic deep soundings (GDS) methods at periods of 0.25 days (more results are available at ftp://gpi.savba.sk/Smap). The two methods respond in different ways to the near-surface inho-mogeneities, and moreover, the MT responses depend on the field polarization. The distor-tions of the apparent resistivity obtained by the MT method are much higher than the impedance phases, as expected. The appar-

ent resistivity values obtained for the GDS method (apart from the MT responses) are less sensitive to the sur-face conductance vari-ations than the MT method. Note that the inconsistency between the two methods due

to the lateral variability of the conductance decreases as the period increases.

These modeling results show that effects caused by surface shell conductance inho-mogeneities are significant and can be pre-dicted with required detail and accuracy if the reliable conductance maps are avail-able. Once these high-resolution S-maps have been made, it then would soon be pos-sible to model deep conductivity structures by subtracting shallow components from the overall signal. Hopefully, this would lead to a better understanding of mantle evolu-tion and plate tectonics.

Acknowledgments

CEMES Experimental Team [Semenov et al., 2003] members from the following institutions participated in the preparation of the S-map of central and eastern Europe: Institute of Geophysics, Warsaw, Poland; Geo-physical Institute, Prague, Czech Republic; Institute of Geological Sciences, Minsk, Belarus; Geodetic and Geophysical Research Institute, Sopron, Hungary; Geo-logical Survey of Romania, Bucharest;

Geophysical Institute, Bratislava, Slovakia; Institute of Geophysics, Kiev, Ukraine; and State University of Moscow, Russia. Thanks are due to the Polish Committee of Scien-tific Research, which has supported the investigations through grants 6P04D-01220 and 2P04D-02329, and to the Centre on Geophysical Methods and Observations for Sustainable Development, Warsaw, Poland, which has supported the joint work of the authors. Additionally, C. Manoj thanks V. P. Dimri, National Geophysical Research Insti-tute, Hyderabad, India, for permission to publish this article. J. Vozár is grateful to Vedecká Grantová Agentúra (VEGA) for support of this work through grant number 02/6045/26.

References

Everett, M. E., S. Constable, and C. G. Constable (2003), Effects of near-surface conductance on global satellite induction responses, Geophys. J. Int., 153, 277–286.

Harinarayana, T., and K. Naganjaneyulu (2003), Regional surface electrical conductance map of India. J., Geol. Soc. Ind.ia, 61(6), 724–728.

Korja, T., et al. (2002), Crustal conductivity in Fen-noscandia: A compilation of a database on crustal conductance in the Fennoscandian Shield, Earth Planets Space, 54, 535–558.

Kuvshinov, A., H. Utada, D. Avdeev, and T. Koyama (2005), 3-D modelling and analysis of the Dst EM responses in the North Pacific Ocean region, Geo-phys. J. Int., 160, 505–526.

Laske, G., and G. Masters (1997), A global digital map of sediment thickness, Eos Trans. AGU, 78(46), Fall Meet. Suppl., Abstract S41E-1.

Manoj, C., A. Kuvshinov, S. Maus, and H. Luhr (2006), Ocean circulation generated magnetic signals, Earth Planets Space, in press.

Semenov, V. Y., W. Jozwiak, and J. Pek (2003), Deep electromagnetic soundings conducted in Trans-European Suture Zone, Eos Trans. AGU, 84(52), 581, 584.

—Ján Vozár, Geophysical Institute of SAS, Bratislava, Slovak Republic; E-mail: geofjavo@savba.sk; VlAdiMir y. seMenoV, Institute of Geo-physics, Polish Academy of Sciences, Warsaw, Poland; Alexey V. kuVsHinoV, Danish National Space Center, Copenhagen, Denmark; and CHAndrA-sekHArAn MAnoJ, Magnetotelluric Division, National Geophysical Research Institute, Hyder-abad, India.

Section Newscont. from page 326

Fig. 2. (top) The apparent resistivities and (bottom) phases of impedances modeled for the Europe region by (left) the GDS and (center and right) MT methods (for two polarizations). The period is 0.25 days.

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