Showing papers by "Frank G. Lemoine published in 2008"
Abstract: [1] Using densities derived from precise orbit determination of the Mars Global Surveyor (MGS) spacecraft from 1999 to mid-2005, the response of Mars' exosphere to long-term solar change is established and compared to that of Earth and Venus. At Mars, exosphere temperatures (weighted towards high-latitude Southern Hemisphere daytime conditions) change only 36–50% as much as those at Earth as solar activity increases from solar minimum to solar maximum, whereas the response at Venus is one-fifth that at Mars. General circulation models suggest that this difference may be strongly influenced by adiabatic cooling associated with the thermosphere general circulation. However, other processes such as differences in CO2 cooling rates may also be playing a role.
79 citations
Massachusetts Institute of Technology1, Goddard Space Flight Center2, Carnegie Institution for Science3, Southwest Research Institute4, University of California, Santa Barbara5, Brown University6, Case Western Reserve University7, Johns Hopkins University Applied Physics Laboratory8, German Aerospace Center9, Arecibo Observatory10
TL;DR: A 3200-kilometers-long profile of Mercury studied by the Mercury Laser Altimeter on the MESSENGER spacecraft spans ∼20% of the near-equatorial region of the planet, implying a variation of equatorial shape that is at least partially compensated.
Abstract: A 3200-kilometers-long profile of Mercury by the Mercury Laser Altimeter on the MESSENGER spacecraft spans approximately 20% of the near-equatorial region of the planet. Topography along the profile is characterized by a 5.2-kilometer dynamic range and 930-meter root-mean-square roughness. At long wavelengths, topography slopes eastward by 0.02 degrees , implying a variation of equatorial shape that is at least partially compensated. Sampled craters on Mercury are shallower than their counterparts on the Moon, at least in part the result of Mercury's higher gravity. Crater floors vary in roughness and slope, implying complex modification over a range of length scales.
54 citations
TL;DR: In this paper, a method to analyze the observations of relative distance change between two low Earth orbiting satellites of the Gravity Recovery And Climate Experiment (GRACE) mission after removing the effects caused by the mean gravity field and better-known temporal mass redistribution is presented.
Abstract: [1] We present a method to analyze the observations of relative distance change between two low Earth orbiting satellites of the Gravity Recovery And Climate Experiment (GRACE) mission after removing the effects caused by the mean gravity field and better-known temporal mass redistribution. The gravitational acceleration exerted by the block mean mass within a region on the Earth surface is formulated by point mass approximation and by spherical harmonic expansion. In addition to the regional mass parameters, the arc-dependent parameters (initial relative state vectors) is simultaneously modeled to remove the signals not associated with the mass variation within the focus area. While a certain level of approximation, that is committed when locally formulating the gravitational acceleration vector for numerical integration, causes model error, we benefit from regionally estimating time-variable mass with improved spatial and temporal resolutions. In addition, various temporal parameterizations depending on geographical areas and expected signals can be applied for the regional analysis in a straightforward manner and it helps to retrieve even sub-daily time-variable signals such as tides from many years of GRACE data. We present the results of hydrology and ocean tide recovery in South America, Africa, North America, and Antarctica from the analysis of 3.5 years of GRACE tracking data and compare them with the traditional global harmonic approach. The algorithm development thoroughly described here will be helpful for the science community to exploit fundamental data (range rate) of the GRACE mission in addition to monthly gravity products.
29 citations
TL;DR: Mazarico et al. as discussed by the authors estimated the density of the Martian exosphere near 250 km altitude with much better temporal (and therefore spatial) resolution than previously possible with this method.
Abstract: [1] Based on the perturbations of the trajectory of the Mars Reconnaissance Orbiter due to atmospheric drag, we estimate, using Precision Orbit Determination, the density of the Martian exosphere near 250 km altitude with much better temporal (and therefore spatial) resolution than previously possible with this method. Small longitudinal density variations are observed to be consistent over long time periods. Although they appear fixed with respect to the planet, we interpret them as non-migrating tides because of the Sun-synchronous spacecraft orbit. Similar tides were observed previously by MGS at much lower altitude (125 km), and our results agree to first order with the MGS findings. Citation: Mazarico, E., M. T. Zuber, F. G. Lemoine, and D. E. Smith (2008), Observation of atmospheric tides in the Martian exosphere using Mars Reconnaissance Orbiter radio tracking data, Geophys. Res. Lett., 35, L09202, doi:10.1029/ 2008GL033388.
19 citations
TL;DR: In this paper, four different data sources have been compared with respect to observations of the annual water storage variations in the region of Bangladesh and the annual signal has been estimated from a common dataset spanning the period 2003 and 2004.
Abstract: Four different data sources have been compared with respect to observations of the annual water storage variations in the region of Bangladesh. Data from satellite altimeters and river gauges estimates the variation in surface water storage in the major rivers of Bangladesh. The GRACE satellites measure the integrated mass change and hence the terrestrial soil moisture variations, which can also be estimated by a hydrological model (GLDAS). These types of observations enable the derivation of the integrated water storage in the entire region of Bangladesh. For all data types, the annual signal has been estimated from a common dataset spanning the period 2003 and 2004. All four different data observe that water storage in Bangladesh is largely dominated by an annual signal with a phase peaking in early September. The annual variations in river level peaks roughly two weeks earlier than terrestrial soil moisture observations by GRACE observations and GLDAS model output.
19 citations
18 Aug 2008
TL;DR: In this article, the authors present improvements to the modeling of non-conservative forces affecting Marsorbiting spacecraft and assess the benefits of these updated models by studying two spacecraft, Mars Odyssey and Mars Reconnaissance Orbiter.
Abstract: We present improvements to the modeling of non-conservative forces affecting Marsorbiting spacecraft. Recent high-resolution gravity fields enable the recovery of smaller signals in the radio tracking data, previously obscured by mismodeled gravitational anomalies. In particular, we show that the estimation of the atmospheric drag experienced by the spacecraft benefits from the new force models. More precise calculations of the spacecraft cross-sectional areas entering the equations for the atmospheric drag and direct solar radiation pressure are possible after accounting for the inter-plate self-shadowing of the spacecraft physical model. The relevant surface areas can vary by as much as 20% on average, and the effects can be very variable within one orbit (±10%). We assess the benefits of these updated models by studying two spacecraft, Mars Odyssey and Mars Reconnaissance Orbiter. We study the effects of modeling on the magnitude and characteristics of the accelerations, on the reconstructed spacecraft trajectory and on the estimated atmospheric density. We plan to use these model improvements with the upcoming Lunar Reconnaissance Orbiter.
3 citations