409 research outputs found
Librations and Obliquity of Mercury from the BepiColombo radio-science and camera experiments
A major goal of the BepiColombo mission to Mercury is the determination of
the structure and state of Mercury's interior. Here the BepiColombo rotation
experiment has been simulated in order to assess the ability to attain the
mission goals and to help lay out a series of constraints on the experiment's
possible progress. In the rotation experiment pairs of images of identical
surface regions taken at different epochs are used to retrieve information on
Mercury's rotation and orientation. The idea is that from observations of the
same patch of Mercury's surface at two different solar longitudes of Mercury
the orientation of Mercury can be determined, and therefore also the obliquity
and rotation variations with respect to the uniform rotation. The estimation of
the libration amplitude and obliquity through pattern matching of observed
surface landmarks is challenging. The main problem arises from the difficulty
to observe the same landmark on the planetary surface repeatedly over the MPO
mission lifetime, due to the combination of Mercury's 3:2 spin-orbit resonance,
the absence of a drift of the MPO polar orbital plane and the need to combine
data from different instruments with their own measurement restrictions. By
assuming that Mercury occupies a Cassini state and that the spacecraft operates
nominally we show that under worst case assumptions the annual libration
amplitude and obliquity can be measured with a precision of respectively 1.4
arcseconds (as) and 1.0 as over the nominal BepiColombo MPO lifetime with about
25 landmarks for rather stringent illumination restrictions. The outcome of the
experiment cannot be easily improved by simply relaxing the observational
constraints, or increasing the data volume.Comment: 30 pages, 6 figures, 2 table
Exoplanet interiors and habitability
More than 1000 exoplanets with a radius smaller than twice that of the Earth are currently known, mainly thanks to space missions dedicated to the search of exoplanets. Mass and radius estimates, which are only available for a fraction (∼ 10%) of the exoplanets, provide an indication of the bulk composition and interior structure and show that the diversity in exoplanets is far greater than in the Solar System. Geophysical studies of the interior of exoplanets are key to understanding their formation and evolution, and are also crucial for assessing their potential habitability since interior processes play an essential role in creating and maintaining conditions for water to exist at the surface or in subsurface layers. For lack of detailed observations, investigations of the interior of exoplanets are guided by the more refined knowledge already acquired about the Solar System planets and moons, and are heavily based on theoretical modelling and on studies of the behaviour of materials under the high pressure and temperature conditions in planets. Here we review the physical principles and methods used in modelling the interior and evolution of exoplanets with a rock or water/ice surface layer and identify possible habitats in or on exoplanets
Titan's Obliquity as evidence for a subsurface ocean?
On the basis of gravity and radar observations with the Cassini spacecraft,
the moment of inertia of Titan and the orientation of Titan's rotation axis
have been estimated in recent studies. According to the observed orientation,
Titan is close to the Cassini state. However, the observed obliquity is
inconsistent with the estimate of the moment of inertia for an entirely solid
Titan occupying the Cassini state. We propose a new Cassini state model for
Titan in which we assume the presence of a liquid water ocean beneath an ice
shell and consider the gravitational and pressure torques arising between the
different layers of the satellite. With the new model, we find a closer
agreement between the moment of inertia and the rotation state than for the
solid case, strengthening the possibility that Titan has a subsurface ocean.Comment: 11 pages, 4 figure
Nonradial oscillations in classical Cepheids: the problem revisited
We analyse the presence of nonradial oscillations in Cepheids, a problem
which has not been theoretically revised since the work of Dziembowsky (1977)
and Osaki (1977). Our analysis is motivated by a work of Moskalik et al. (2004)
which reports the detec tion of low amplitude periodicities in a few Cepheids
of the large Magellanic cloud. These newly discovered periodicities were
interpreted as nonradial modes.} {Based on linear nonadiabatic stability
analysis, our goal is to reanalyse the presence and stability of nonradial
modes, taking into account improvement in the main input phys ics required for
the modelling of Cepheids.} {We compare the results obtained from two different
numerical methods used to solve the set of differential equations: a matrix
method and the Ricatti method.} {We show the limitation of the matrix method to
find low order p-modes (), because of their dual character in evolved
stars such as Cepheids. For higher order p-modes, we find an excellent
agreement between the two methods.} {No nonradial instability is found below
, whereas many unstable nonradial modes exist for higher orders. We also
find that nonradial modes remain unstable, even at hotter effective
temperatures than the blue edge of the Cepheid instability strip, where no
radial pulsations are expected.Comment: Accepted for publication in A&A; 7 pages, 8 figure
Effect of core--mantle and tidal torques on Mercury's spin axis orientation
The rotational evolution of Mercury's mantle and its core under conservative
and dissipative torques is important for understanding the planet's spin state.
Dissipation results from tides and viscous, magnetic and topographic
core--mantle interactions. The dissipative core--mantle torques take the system
to an equilibrium state wherein both spins are fixed in the frame precessing
with the orbit, and in which the mantle and core are differentially rotating.
This equilibrium exhibits a mantle spin axis that is offset from the Cassini
state by larger amounts for weaker core--mantle coupling for all three
dissipative core--mantle coupling mechanisms, and the spin axis of the core is
separated farther from that of the mantle, leading to larger differential
rotation. The relatively strong core--mantle coupling necessary to bring the
mantle spin axis to its observed position close to the Cassini state is not
obtained by any of the three dissipative core--mantle coupling mechanisms. For
a hydrostatic ellipsoidal core--mantle boundary, pressure coupling dominates
the dissipative effects on the mantle and core positions, and dissipation
together with pressure coupling brings the mantle spin solidly to the Cassini
state. The core spin goes to a position displaced from that of the mantle by
about 3.55 arcmin nearly in the plane containing the Cassini state. With the
maximum viscosity considered of if the coupling is
by the circulation through an Ekman boundary layer or for purely viscous coupling, the core spin lags the
precessing Cassini plane by 23 arcsec, whereas the mantle spin lags by only
0.055 arcsec. Larger, non hydrostatic values of the CMB ellipticity also result
in the mantle spin at the Cassini state, but the core spin is moved closer to
the mantle spin.Comment: 35 pages, 7 figure
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