David C. McAdoo

GRAVITY FIELDS OF THE SOUTHERN OCEAN FROM GEOSAT DATA

In August 1990, the U.S. Navy declassified all Geodetic Mission (GM) radar altimeter data acquired by the Geosat satellite over oceanic regions south of 60°S. We have used these GM data in conjunction with the unclassified, lower-resolution Geosat Exact Repeat Mission (ERM) altimeter data to construct high-resolution gravity fields on a 5-km grid covering the annular region of the southern ocean, which lies between 60°S and 72°S and encircles Antarctica. During the GM a complete mapping of the marine geoid (between 72° and 72°N) was accomplished. The GM produced more densely spaced ground tracks (typically 2 or 3 km at 60°S) than those of either the ERM or Seasat. Consequently, we were able to use the GM data to map the marine gravity field at a higher resolution than was previously possible using satellite altimeter data. This paper describes the techniques we used to derive these gravity fields and image them. These techniques involve (1) computing along-track sea surface height slopes, (2) gridding of these ascending and descending slopes, (3) converting the slopes to conventional deflections of the vertical, (4) transforming the deflections to gravity anomalies in the frequency domain, and (5) imaging. The resulting images of the marine gravity field reveal much that is new about the seafloor and the tectonic fabric of the southern ocean: a region which includes large expanses of seafloor that have never been surveyed by ships.

A First Assessment of IceBridge Snow and Ice Thickness Data Over Arctic Sea Ice

We present a first assessment of airborne laser and radar altimeter data over snow-covered sea ice, gathered during the National Aeronautics and Space Administration Operation IceBridge Mission. We describe a new technique designed to process radar echograms from the University of Kansas snow radar to estimate snow depth. We combine IceBridge laser altimetry with radar-derived snow depths to determine sea ice thickness. Results are validated through comparison with direct measurements of snow and ice thickness collected in situ at the Danish GreenArc 2009 sea ice camp located on fast ice north of Greenland. The IceBridge instrument suite provides accurate measurements of snow and ice thickness, particularly over level ice. Mean IceBridge snow and ice thickness agree with in situ measurements to within ~ 0.01 and ~ 0.05 m, respectively, while modal snow and ice thickness estimates agree to within 0.02 and 0.10 m, respectively. IceBridge snow depths were correlated with in situ measurements (R = 0.7, for an averaging length of 55 m). The uncertainty associated with the derived IceBridge sea ice thickness estimates is 0.40 m. The results demonstrate the retrieval of both first-year and multiyear ice thickness from IceBridge data. The airborne data were however compromised in heavily ridged ice where snow depth, and hence ice thickness, could not be measured. Techniques developed as part of this study will be used for routine processing of IceBridge retrievals over Arctic sea ice. The limitations of the GreenArc study are discussed, and recommendations for future validation of airborne measurements via field activities are provided.

Satellites provide new insights into polar geophysics

A revolution in polar geophysics is under way thanks to altimeter data, which the ERS satellites have been collecting since 1991. Geophysical surveys in the polar regions have long been hampered by inaccessibility, particularly in areas that are covered yearround by sea ice or land ice. As a result the major remaining uncertainties in global tectonic models of the Mesozoic and Cenozoic tend to lie in the Arctic and Antarctic regions. In fact, major tectonic plate boundaries have been hypothesized, but not confirmed, for both regions. In the Arctic, a divergent plate boundary associated with the Mesozoic opening of the Canada Basin has been proposed [e.g., Lawver et al., 1990] while in the Antarctic a divergent boundary, active during the late Cretaceous in the Amundsen Sea, has been hypothesized [Cande et al., 1995; Stock and Molnar, 1987]. Due to the acute sparseness of seafloor surveys in these areas, however, no one has been able to prove that these plate boundaries actually existed, nor has anyone been able to locate extinct remnants of the boundaries. High-resolution marine gravity fields (Figures 1 and 2) derived from satellite altimeter data are now redressing this problem of sparse surveys.

New gravity data in the Arctic Ocean : Comparison of airborne and ERS gravity

New gravity fields from airborne gravimetry and from ERS-1 and -2 satellite altimetry cover extensive portions of the Arctic Ocean. These two data sets may constitute as much as 60% of the data contributions to the Arctic Gravity Project compilation. Here we evaluate the accuracy and resolution of these data and quantify their impact on the compilation. Both gravity determinations compare favorably with Geological Survey of Canada surface measurements in the Beaufort Sea (airborne, 1.86-2.09 mGal rms; ERS, 2.64-3.11 mGal rms). Comparisons between the airborne and ERS data over the Chukchi Borderlands reveal a 4.38 mGal rms difference over the smoother region of the field and 7.36 mGal rms over the rugose field generated by the shallow ridges and deep troughs. Coherency between the two data sets in the Chukchi region implies a resolution of 19 km. Comparison with Science Ice Expedition submarine measurements over Chukchi Plateau suggests that the ERS field resolves even shorter-wavelength signal than the airborne data, whereas in the Beaufort Sea the airborne data showed better coherence to ground truth data. Long-wavelength differences exist between the two data sets, expressed as a 2-3 mGal offset over the Chukchi region. This study highlights the respective strengths of the two data sets. The ERS gravity field has the advantage of ubiquitous coverage of the ocean south of 81.5 degreesN, a denser sampling of the gravity field, and a recovery of signal down to similar to 15 km. The airborne data cover a significant portion of the polar hole in the satellite coverage, have lower measurement noise, and recover somewhat higher anomaly amplitudes in the 25-100 km wavelength range.

Arctic Ocean gravity field derived from ICESat and ERS-2 altimetry : Tectonic implications

A new, detailed marine gravity field for the persistently ice-covered Arctic Ocean, derived entirely from satellite data, reveals important new tectonic features in both the Amerasian and Eurasian basins. Reprocessed Geoscience Laser Altimeter System (GLAS) data collected by NASAs Ice Cloud and land Elevation Satellite (ICESat) between 2003 and 2005 have been combined with 8 years worth of retracked radar altimeter data from ESAs ERS-2 satellite to produce the highest available resolution gravity mapping of the entire Arctic Ocean complete to 86 degrees N. This ARCtic Satellite-only (ARCS) marine gravity field uniformly and confidently resolves marine gravity to wavelengths as short as 35 km. ARCS relies on a Gravity Recovery and Climate Experiment (GRACE)-only satellite gravity model at long (> 580 km) wavelengths and plainly shows tectonic fabric and numerous details imprinted in the Arctic seafloor, in particular, in the enigmatic Amerasian Basin (AB). For example, in the Makarov Basin portion of the AB, two north-south trending lineations are likely clues to the highly uncertain seafloor spreading history which formed the AB.

Comparison of observed and predicted gravity profiles over Aphrodite Terra, Venus

The authors compare observed Pioneer Venus orbiter (PVO) gravity profiles over Aphrodite Terra to profiles predicted from models of thermal isostasy, mantle convection, and Airy compensation. Similar approaches are used in order to investigate how well the models can be distinguished with the PVO data. Topography profiles across Aphrodite are compared to model spreading ridge profiles in order to further assess this model. Airy compensation depths and convection layer thicknesses are greater under eastern Aphrodite than western Aphrodite. Compensation depths in the east are greater than most estimates of lithospheric thickness, suggesting that this part of the ridge is dynamically supported. In parts of western Aphrodite, the spreading ridge model gravity provides a better fit to the data than either Airy compensation or mantle convection. Best-fit spreading rates are between 0.3 and 1.6 cm/yr. Airy compensation and mantle convection cannot be distinguished in most places using only PVO data.

Marine gravity from Geosat and ERS-1 altimetry in the Weddell Sea

Abstract A high-resolution gravity field covering virtually all of the Weddell Sea has been derived using a combination of Geosat and ERS-1 data. This field encompasses the entire Weddell Sea region, including areas which are seasonally, as well as areas which are perpetually, covered by sea ice, but excludes areas covered by ice shelves. Permanent sea-ice cover has precluded exploration of large areas of the Weddell Sea by ships and, until now, by satellite altimeters. Complex radar echoes from sea ice confuse trackers onboard altimeter satellites and produce noisy height estimates. As a result, altimeter data over sea ice have usually been removed before marine gravity is computed. We have now, however, reprocessed or ‘retracked’ sea ice echo waveforms from ERS-1, and determined marine gravity fields over ice-covered as well as ice-free ocean. This new gravity map permits us to view tectonic details imprinted in the ocean floor by the complex history of divergence and relative motion between the South American and Antarctic plates as well as motions between crustal blocks comprising West Antarctica. These details include: (1) gravity lineations which are the gravitational expression of fracture zones that trace the history of seafloor spreading in the Weddell Sea; (2) gravitational expression of an ocean-continent boundary in the western Weddell Sea flanking the east coast of the Antarctic Peninsula (Graham Land); (3) a scarp-like gravity anomaly which coincides with the magnetically expressed ‘Orion anomaly’ at about 71°S; (4) a linear, relative gravity high in the southeastern Weddell Sea which parallels the coast and roughly coincides with the (failed) Weddell Rift/Explora Wedge; (5) adjacent linear gravity lows which directly overlie narrow buried basement ridges known as the Explora and Andenes escarpments.

Spectral Analysis of Marine Geoid Heights and Ocean Depths: Constraints on Models of Lithospheric and Sublithospheric Processes

Cross-spectral analysis has been used to study the relationship between geoid and bathymetry in 16°×16° blocks in the oceans. The admittances resulting from this analysis have been compared with thermomechanical models of the lithosphere and sublithosphere in order to determine modes of topographic compensation in different parts of the oceans. Peak admittances at short wavelengths (λ<800 km) indicate that loads are supported by the mechanical strength of the lithosphere, while peak admittances at long wavelengths (λ>800 km) are indicative of lithospheric cooling or dynamic sublithospheric processes. Models of upper mantle convection predict higher admittances at long wavelengths than do models of lithospheric cooling. In most areas the observed admittances can be explained by models of the thermomechanical properties of the lithosphere, but in the eastern Pacific Ocean, the northern Indian Ocean, and over the Cape Verde Rise high long-wavelength admittances are evidence for the existence of upper mantle convection.

Validating ICESat Over Thick Sea Ice in the Northern Canada Basin

Only in the past eight years has the feasibility of using satellite-borne altimeters to estimate sea ice freeboard and thickness been demonstrated, and these estimates still have uncertainties primarily associated with limited knowledge of snow loading on sea ice. Because accurate estimates of Arctic-wide sea ice thickness and volume are fundamental inputs to global climate models, validation of satellite-derived thickness estimates using independent data is required. A detailed assessment of freeboard retrieved by the Geoscience Laser Altimeter System (GLAS) aboard the Ice, Cloud, and land Elevation Satellite has been carried out using high-resolution laser altimetry from the National Aeronautics and Space Administrations Airborne Topographic Mapper (ATM), the Delay-Doppler radar altimeter, and digital photography collected along a 300-km segment of sea ice in the Canada Basin. Exploiting the repeat coverage of the aircraft flight line, a correction was applied to GLAS footprint geolocations to adjust for sea ice drift that occurred during the time between satellite and aircraft acquisitions. Comparisons of GLAS and ATM measurements over sea ice show excellent agreement (about a 0.00-m mean) with no apparent bias between data sets. Freeboard estimates were examined using data from GLAS and ATM independently, employing measurements over refrozen leads to estimate local sea surface heights (SSHs). The results demonstrate the sensitivity of freeboard and thickness calculations to an accurate estimation of local SSH. Snow depth derived by differencing laser and radar data was combined with the freeboard estimates to yield a mean sea ice thickness of ~ 5.5 m over a 250-km subsection of the flight track.

Improvements in Arctic Gravity and Geoid from CHAMP and GRACE: An Evaluation

The near-polar CHAMP and GRACE satellites are now acquiring vitally important new information on the geoid and gravity field of the polar regions. This investigation demonstrates that CHAMP and GRACE data are dramatically reducing the large gaps in our knowledge of the Arctic region, constraining the long wavelength geopotential (>300 km) and beginning to yield the high accuracy marine geoid which is needed for Arctic oceanographic and sea ice studies. Using a detailed Arctic surface gravity field and an independent altimetric gravity field as benchmarks we have evaluated the intermediate-to-long wavelength (> 300km) component of seven CHAMP and two GRACE satellite-only gravity models such as the GFZ EIGEN, the NASA PGS and UT/CSR. We evaluate, spectrally, the errors in - and differences between - these satellite-only models in the Arctic at wavelengths from 300 to 2500 km. The GRACE models accurately resolve Arctic gravity to full wavelengths as short as 500 km while the CHAMP models do so to full wavelengths as short as 1000 km. However the CHAMP models continue to show improved resolution as more and better (e.g. lower elevation) data are incorporated. The best CHAMP models agree well with the detailed Arctic ARC-GP model to an rms (error of commission) of better than 2.06 mGal (gravity)and 31 cm (geoid) for all wavelengths (full) longer than 1100 km. GRACE-only geoids are precise to 40 cm or better (all wavelengths) over large areas of the Arctic. CHAMP and GRACE-based geoids could have the accuracy required to detect (together with altimetry) the poorly known dynamic topography of the Arctic Ocean. As an example, a GRACE/detailedgravity hybrid geoid is presented.