Laramie V. Potts

Validation of Jason-1 Nadir Ionosphere TEC Using GEONET

The Jason-1 dual-frequency nadir ionosphere Total Electron Content (TEC) for 10-day cycles 1–67 is validated using absolute TEC measured by Japans GPS Earth Observation Network (GEONET), or the GEONET Regional Ionosphere Map (RIM). The bias estimates (Jason–RIM) are small and statistically insignificant: 1.62 ± 9 TECu (TEC unit or 1016 electrons/m2, 1 TECu = 2.2 mm delay at Ku-band) and 0.73 ± 0.05 TECu, using the along-track difference and Gaussian distribution method, respectively. The bias estimates are –3.05 ± 10.44 TECu during daytime passes, and 0.02 ± 8.05 TECu during nighttime passes, respectively. When global Jason-1 TEC is compared with the Global Ionosphere Map (GIM) from the Center for Orbit Determination in Europe (or CODE) TEC, the bias (Jason–GIM) estimate is 0.68 ± 1.00 TECu, indicating Jason-1 ionosphere delay at Ku-band is longer than GIM by 3.1 mm, which is at present statistically insignificant. Significant zonal distributions of biases are found when the differences are projected into a sun-fixed geomagnetic reference frame. The observed biases range from –7 TECu (GIM larger by 15.4 mm) in the equatorial region, to +2 TECu in the Arctic region, and to +7 TECu in the Antarctica region, indicating significant geographical variations. This phenomena is primarily attributed to the uneven and poorly distributed global GPS stations particularly over ocean and near polar regions. Finally, when the Jason-1 and TOPEX/Poseidon (T/P) TECs were compared during Jason-1 cycles 1–67 (where cycles 1–21 represent the formation flight with T/P, cycles 22–67 represent the interleave orbits), the estimated bias is 1.42 ± 0.04 TECu. It is concluded that the offset between Jason/TOPEX and GPS (RIM or GIM) TECs is < 4 mm at Ku-band, which at present is negligible.

Crustal analysis of Venus from Magellan satellite observations at Atalanta Planitia, Beta Regio, and Thetis Regio

We investigated an improved quantitative method for interpreting attributes of the Venusian crust from Magellan spacecraft imagery and spherical harmonic topographic and gravity models. We used spectral correlation theory to compare gravity effects of topography against free-air gravity anomalies for three tectonically disparate crustal regions. Adjusting terrain gravity effects for correlated free-air anomalies yields compensated effects that we used to estimate the Moho and crustal thickness variations. For Atalanta Planitia, the crust may be less than 10 km thick, but thickened peripherally at concentric ridge belts and tessera terrain. Crustal thickness may increase rapidly between the interior of this lowland plain and the peripheral ridge belts. For Beta Regio, the crustal thickness may exceed 40 km. Of the three regions considered, the crust of Beta Regio may be the most poorly compensated because its free-air anomalies are largest relative to corresponding terrain gravity effects. For Thetis Regio, crustal thicknesses may exceed 50 km. The thickened crust of Thetis Regio may also have been rifted and embayed by plains forming lavas. The lavas appear to embay and cover complexly ridged areas except where these units remained above the reach of the plains forming lavas. Crosscutting relationships, which are evident from Magellan imagery, together with our crustal thickness estimates, suggest that the complexly ridged terrain (or tesserae) may be the oldest crustal units within the three study regions. Hence these units may represent relict surfaces that predate emplacement of the plains.

Lunar crustal analysis of Mare Orientale from topographic and gravity correlations

We investigated the use of spectral correlation analysis for modeling the crustal features of Mare Orientale from lunar 70th degree spherical harmonic topographic and gravity field models derived from Clementine satellite and earlier investigations. The analysis considered a 64°-by-64° region of the Moon centered roughly on Mare Orientale at an altitude of 100 km. The topography of the study region, which includes over 11 km of relief, was modeled for its gravity effects in lunar spherical coordinates by Gauss-Legendre quadrature integration assuming a terrain density of 2.8 g/cm3. We observed substantial positive and negative correlations between terrain gravity effects and free-air gravity anomalies that seriously limit the utility of simple Bouguer gravity anomalies for subsurface studies. Using the wavenumber correlation spectrum between the two data sets, we designed correlation filters to extract the common features. Possible interpretations for the terrain-correlated free-air gravity anomalies include isostatic crustal mass imbalances that may be equilibrated by radial adjustments of the Moho of up to 44 km, assuming Airy-Heiskanen compensation and a mantle density contrast of 0.5 g/cm3 with the crust. These Moho adjustments define mass variations that account for most of the mascon and flanking negative free-air gravity anomalies. Furthermore, their remarkable correlation with the topographic rings of Mare Orientale points to the possible influence of a strong local stress field of the crust in the development of the ring structures. Subtracting the terrain-correlated free-air anomalies from the free-air gravity anomalies and terrain gravity effects yielded terrain-decorrelated free-air and isostatically compensated terrain gravity anomalies, respectively, that show zero correlation. This lack of correlation may be interpreted for a Moho that involves over 100 km of relief assuming Airy-Heiskanen compensation of the crust. Beneath Mare Orientale, we observed a minimum crustal thickness of about 17 km. Corresponding terrain-decorrelated free-air gravity anomalies of Mare Orientale may be related to a central cone-shaped body of 0.5 g/cm3 density contrast with apex extending nearly 5 km below the surface, which is surrounded by a ringed-shaped body of −0.5 g/cm3 density contrast that may extend about 7 km below the surface. These bodies resulted possibly from meteorite impact that produced a roughly circular region of breccia and highly fractured crust with a higher density core where some remelting of the rocks about the impact site may have occurred.

Effect of Varying Crustal Thickness on CHAMP Geopotential Data

To determine the effect of crustal thickness variation on satellite-altitude geopotential anomalies we compared two regions of Europe with vastly different values, South and Central Finland and the Pannonian Basin. Crustal thickness exceeds 44 km in Finland and is less than 26 km in the Pannonian Basin. Heat-flow data indicate that the crust of the Pannonian Basin has a value nearly three times that of the Finnish Svecofennian Province. A positive CHAMP gravity anomaly (∼4 mGal) is quasi-coincidental with the CHAMP magnetic anomaly across the Pannonian Basin. CHAMP gravity data indicates a minimum of 3 mGal in southwest Finland. CHAMP magnetic data reveal elongated semicircular negative anomalies for both regions with South-Central Finland having larger amplitude (<−6 nT) than that over the Pannonian Basin, Hungary (<−5 nT). In this latter region subducted oceanic lithosphere has been proposed as the anomalous body. In the former the central part of the negative gravity anomaly covers the northern part of the Baltic Sea basin and Gulf of Finland and underlying two rapakivi provinces plus it coincides with an area of lower crustal thickness. The magnetic anomaly directly correlates with the crustal thickness and inversely with the heat flow and, hence, may be caused either by variation of concentration of magnetite or by the elevated Curie-isotherm of magnetite in the lower crust — upper mantle region.

Recovery of Isostatic Topography over North America from Topographic and CHAMP Gravity Correlations

We investigate North American crustal structure and mass loads from spectral correlation analysis of topographic, CHAMP and terrestrial gravity data. We use free-air and terrain gravity correlations to isolate tectonically driven vertical motions and mass imbalances of the crust and lithosphere. Specifically, we apply correlation filters to decompose the free-air gravity anomalies into terrain-correlated and terrain-decorrelated components to yield compensated terrain gravity effects that we evaluate for crustal thickness variations. Our results compare quite favourably with the seismically inferred global crustal thickness model Crust5.1 and a 3.4 km rms difference with LITH5.0 over North America. Terrain-correlated anomalies reveal mass excesses and deficits that are interpreted as uncompensated elements of the crust. For Hudson Bay, the average terrain-correlated free-air anomaly suggests that the crustal topography is depressed by about 400 m. Because glacial isostatic adjustment considerations can only marginally account for the depression, we speculate that it may reflect other effects such as a preglacial impact.

CHAMP Gravity Anomalies over Antarctica

Before CHAMP, the Antarctic gravity field was constrained predominantly by satellite altimetry-derived gravity measurements over the oceans and the variations in satellite orbits at altitudes of about 700 km and higher. CHAMP free-air gravity estimates at 400 km altitude suggest that previous gravity models may have regionally overestimated the anomaly field of Antarctica by roughly 5 mGals. The free-air anomalies can be separated into terrain-correlated and terrain-decorrelated components using the correlation spectrum with the computed terrain gravity effects. Analysis of the terrain-correlated anomalies together with the terrain effects reveals anomalously thinned crust beneath East Antarctica between the Gamburtsev and Transantarctic Mountains that includes Wilkes Land. These results suggest tectonically extended crust for roughly a third of East Antarctica as important new constraints on Gondwana dynamics. Additionally, the terrain-decorrelated free-air anomalies reveal mass variations of the mantle and core to constrain the thermodynamics of the subcrust, the origin of the geomagnetic field, and standard precession and nutation models. With CHAMP, we also have for the first time co-registered complementary magnetic and gravity observations. Hence, we can identify and study regionally correlative lithospheric mass and magnetization variations via Poisson’s relationship for new constraints on regional petrological, structural, and thermal variations of the Antarctic lithosphere.