J. P. Barriot

A homogeneous nucleus for comet 67P/Churyumov–Gerasimenko from its gravity field

Cometary nuclei consist mostly of dust and water ice. Previous observations have found nuclei to be low-density and highly porous bodies, but have only moderately constrained the range of allowed densities because of the measurement uncertainties. Here we report the precise mass, bulk density, porosity and internal structure of the nucleus of comet 67P/Churyumov–Gerasimenko on the basis of its gravity field. The mass and gravity field are derived from measured spacecraft velocity perturbations at fly-by distances between 10 and 100u2009kilometres. The gravitational point mass is GMu2009=u2009666.2u2009±u20090.2 cubic metres per second squared, giving a mass Mu2009=u2009(9,982u2009±u20093)u2009×u2009109 kilograms. Together with the current estimate of the volume of the nucleus, the average bulk density of the nucleus is 533u2009±u20096u2009kilograms per cubic metre. The nucleus appears to be a low-density, highly porous (72–74 per cent) dusty body, similar to that of comet 9P/Tempel 1. The most likely composition mix has approximately four times more dust than ice by mass and two times more dust than ice by volume. We conclude that the interior of the nucleus is homogeneous and constant in density on a global scale without large voids. The high porosity seems to be an inherent property of the nucleus material.

Interior structure of terrestrial planets: Modeling Mars' mantle and its electromagnetic, geodetic, and seismic properties

[1]xa0We present a new procedure to describe the one-dimensional thermodynamical state and mineralogy of any Earth-like planetary mantle, with Mars as an example. The model parameters are directly related to expected results from a geophysical network mission, in this case electromagnetic, geodetic, and seismological processed observations supplemented with laboratory measurements. We describe the internal structure of the planet in terms of a one-dimensional model depending on a set of eight parameters: for the crust, the thickness and the mean density, for the mantle, the bulk volume fraction of iron, the olivine volume fraction, the pressure gradient, and the temperature profile, and for the core, its mass and radius. Currently, available geophysical and geochemical knowledge constrains the range of the parameter values. In the present paper, we develop the forward problem and present the governing equations from which synthetic data are computed using a set of parameter values. Among all Martian models fitting the currently available knowledge, we select eight candidate models for which we compute synthetic network science data sets. The synergy between the three geophysical experiments of electromagnetic sounding, geodesy, and seismology is emphasized. The stochastic inversion of the synthetic data sets will be presented in a companion paper.

Assessment of the Martian gravity field at short wavelength with Mars Express

[1]xa0The gravity part of the Mars Express Radio Science Experiment consists in measuring gravity data near pericenter above selected target areas of geophysical interest. The low altitude of the Mars Express at pericenter (263–329 km) makes it a very sensitive gravity sensor at small wavelengths which can give new constraints on the local structure of the crust and lithosphere. Mars Express gravity data can also be used to check the quality of the existing global gravity solutions. In this paper, we show that Mars Express gravity data confirm in the Tharsis area the validity of existing global gravity solutions. In particular, the resolution of the most recent global gravity solution is excellent up to harmonic degree 73 and the amplitude at short wavelength is not biased toward zero by the power law regularization up to the same degree.

Network science landers for Mars

Abstract The NetLander Mission will deploy four landers to the Martian surface. Each lander includes a network science payload with instrumentation for studying the interior of Mars, the atmosphere and the subsurface, as well as the ionospheric structure and geodesy. The NetLander Mission is the first planetary mission focusing on investigations of the interior of the planet and the large-scale circulation of the atmosphere. A broad consortium of national space agencies and research laboratories will implement the mission. It is managed by CNES (the French Space Agency), with other major players being FMI (the Finnish Meteorological Institute), DLR (the German Space Agency), and other research institutes. According to current plans, the NetLander Mission will be launched in 2005 by means of an Ariane V launch, together with the Mars Sample Return mission. The landers will be separated from the spacecraft and targeted to their locations on the Martian surface several days prior to the spacecrafts arrival at Mars. The landing system employs parachutes and airbags. During the baseline mission of one Martian year, the network payloads will conduct simultaneous seismological, atmospheric, magnetic, ionospheric, geodetic measurements and ground penetrating radar mapping supported by panoramic images. The payloads also include entry phase measurements of the atmospheric vertical structure. The scientific data could be combined with simultaneous observations of the atmosphere and surface of Mars by the Mars Express Orbiter that is expected to be functional during the NetLander Missions operational phase. Communication between the landers and the Earth would take place via a data relay onboard the Mars Express Orbiter.

Modeling of the Eros gravity field as an ellipsoidal harmonic expansion from the NEAR Doppler tracking data

[1]xa0The gravity field for asteroid 433 Eros has been determined in terms of ellipsoidal harmonic functions by processing the Doppler tracking data of the NEAR spacecraft while it was in orbit about the asteroid. Using the same set of NEAR spacecraft Doppler tracking data, comparative descriptions of the Eros gravity field are provided for both the ellipsoidal and the traditional spherical harmonic models. It is shown that for elongated bodies, like the asteroid Eros, the ellipsoidal harmonics model permits a better representation of the gravity signature than does the spherical harmonics model. Eros has a nearly uniform density but there are negative gravity anomalies near the ends of Eros and positive gravity anomalies near the Psyche crater and the Himeros depression.

The Netlander Ionosphere and Geodesy Experiment

Abstract The NEtlander Ionosphere and Geodesy Experiment (NEIGE) of the Netlander Mission to Mars has two series of scientific objectives: (1) to determine Mars orientation parameters in order to obtain information about the interior of Mars and about the seasonal mass exchange between atmosphere and ice caps; and (2) to determine the total electron content (TEC) and the scintillation of radio signals in order to study the large- and small-scale structure of the ionosphere of Mars. These two sets of information will be derived from measurements of amplitudes and Doppler shifts of radio links at UHF and X-band between the Netlander microstations on the Mars surface and an orbiter and between this orbiter and the Earth (at X-band).

Venus gravity and topography: 60th degree and order model

We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978–1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free-air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar-Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.

Mars' time-variable gravity and its determination: Simulated geodesy experiments

[1]xa0The seasonal carbon dioxide (CO2) cycle on Mars results in a time-variable global redistribution of mass. These large-scale variations are associated with changes in the gravity field, mainly in the two zonal gravity coefficients and , which have been recently evaluated from Doppler tracking data of the Mars Global Surveyor (MGS) spacecraft. In the present study, we calculated these variations from the mass redistribution obtained from outputs of two general circulation models (GCM) as well as from CO2 thickness measurements by the High Energy Neutron Detector (HEND) instrument on board the Mars Odyssey spacecraft and compared them to the observations. Tracking observations provide one of the most direct measures of the global-scale atmospheric mass cycle. However, the associated uncertainties are relatively large, partly because the low-degree zonals obtained from a single orbiter tracking analysis are contaminated by higher-degree harmonics which are shown to have nonnegligible seasonal variations. Thus we investigated possibilities to improve the determination of the time-variable gravity field by means of simulated geodesy experiments. Additional radio tracking of a second spacecraft with suitable orbital characteristics was shown to be able to separate the higher-degree geodetic signatures. Radio links between landers on the Martian surface and a near-polar orbiter can further better estimate especially the even zonals.

Analytical modeling of the Doppler tracking between a lander and a Mars orbiter in terms of rotational dynamics

[1]xa0Radio Doppler shifts between a lander on Mars surface and an orbiter are modeled through a linearized analytical formulation. This simplified formulation allows us to understand the signature and interplay of the different geophysical parameters (nutations, polar motion, and rotation rate variations) on the lander-orbiter observable. The frequency band of the investigated periodic phenomena goes from some months to about one Martian year in their respective frames. Long-term effects like precession are taken into account but are not investigated. The satellite orbit is modeled as a precessing orbit because of the oblateness of Mars. We show that we are able to see the signature of a possible liquid core after less than 50 weeks of observation for a given precision of 0.1 mm/s for the Doppler tracking. We are also able to monitor the rotation rate variation and polar motion at a level of 5 mas over the same period. This study was done in the frame of the preparation of the Netlander Ionosphere and Geodesy Experiment (NEIGE).

The NEAR Radio Science investigations

The science objectives for the Near-Earth Asteroid Rendezvous (NEAR) Radio Science team include determining the masses of asteroids 253 Mathilde (to within 10%) and 433 Eros (<0.1%). While the NEAR spacecraft is in orbit about Eros, the radiometric spacecraft tracking data, the optical landmark tracking data, and the lidar altimetry measurements will be used to determine the gravity field of Eros. Comparisons of this gravity field with one determined from Eros shape models (assuming constant density) will provide constraints upon the internal structure of this asteroid. In addition, the spin state, principal axes, and moments of inertia will be determined as part of the Radio Science activities. A campaign to meet the radio science objectives will require the close cooperation of the NEAR Navigation, Imaging, and Lidar teams.