The interior structure of Saturn, the depth of its winds, and the mass and age of its rings constrain its formation and evolution. In the final phase of the Cassini mission, the spacecraft dived ...between the planet and its innermost ring, at altitudes of 2600 to 3900 kilometers above the cloud tops. During six of these crossings, a radio link with Earth was monitored to determine the gravitational field of the planet and the mass of its rings. We find that Saturn's gravity deviates from theoretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ± 0.49) × 10
kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 10
to 10
years ago.
How deep do Saturn's zonal winds penetrate below the cloud level has been a decades‐long question, with important implications not only for the atmospheric dynamics but also for the interior density ...structure, composition, magnetic field, and core mass. The Cassini Grand Finale gravity experiment enables answering this question for the first time, with the premise that the planet's gravity harmonics are affected not only by the rigid body density structure but also by its flow field. Using a wide range of rigid body interior models and an adjoint based optimization for the flow field using thermal wind balance, we calculate the flow structure below the cloud level and its depth. We find that with a wind profile, largely consistent with the observed winds, when extended to a depth of around 8,800 km, all the gravity harmonics measured by Cassini are explained. This solution is in agreement with considerations of angular momentum conservation and is consistent with magnetohydrodynamics constraints.
Plain Language Summary
Observations show strong east‐west flows at the cloud level of Saturn. These winds are strongest at the equatorial regions, reaching up to 400 m/s, about 4 times stronger than tornado strength winds on Earth. Yet until now we had no knowledge on how deep these winds penetrate into the interior of the gas giant. The gravity experiment executed during the Grand Finale stage (May–August 2017) of the NASA Cassini mission helps answering this question. It is well established that any large‐scale motion of the fluid would have a signature in the density distribution and therefore in the planet gravity field. If we can estimate the internal structure and shape of the planet, we might be able to decipher the depth of the winds from its signal in the gravity measurements. Moreover, the rigid‐body and flow contribution to gravity field are entangled together, therefore it is necessary to use a wide range of rigid‐body models in order to define the wind‐induced gravity signal. In this work we propose a solution to the problem. We find that the gravity measurements can be explained with a flow pattern, similar to that observed at the cloud level, penetrating to depths of more than 8,000 km into the planet interior. This has important implications not only for the atmospheric dynamics but also for the interior density structure, composition, magnetic field, and core mass.
Key Points
Cassini gravity measurements point to deep differential flows within Saturn
Using a wide range of rigid body internal structure models, the required wind‐induced gravity signal is defined
With a conservatively modified cloud‐level wind and an optimized vertical profile, extended to a depth of 8,800km, all gravity measurements are explained
The Juno spacecraft reached the mid‐point of its nominal mission in December 2018, after completing 17 perijove passes. Ten of these were dedicated to the determination of the gravity field of the ...planet, with the aim of constraining its interior structure. We provide an update on Jupiter's gravity field, its tidal response and spin axis motion over time. The analysis of the Doppler data collected during the perijove passes hints to a non‐static and/or non‐axially symmetric field, possibly related to several different physical mechanisms, such as normal modes or localized atmospheric or deeply‐rooted dynamics.
Plain Language Summary
Jupiter's gravity field has been updated with the use of Juno's data collected up to the mid‐point of its mission. The field is largely symmetric about the rotation axis, and shows conspicuous north‐south asymmetry. Possible non‐static and/or non‐axially symmetric field is compatible with the data.
Key Points
Juno updates Jupiter's gravity field halfway through its mission, revealing a largely axially symmetric, north‐south asymmetric field
Hints to a non‐static and/or non‐axially symmetric field, possibly related to several different physical mechanisms, appear in the data
The tidal response is evaluated and compared to interior model predictions
Since its arrival at Saturn in 2004, Cassini performed nine flybys devoted to the determination of Titan's gravity field and its tidal variations. Here we present an updated gravity solution based on ...the final data set collected during the gravity-dedicated passes, before Cassini's plunge into Saturn's atmosphere. The data set includes an additional flyby (T110, March 2015, primarily devoted to imaging Titan's north polar lakes) carried out with the low-gain antenna. This flyby was particularly valuable because the closest approach occurred at a high latitude (75°N), over an area not previously sampled.
Previously published gravity results (Iess et al., 2012) indicated that Titan is subject to large eccentricity tides in response to the time varying perturbing potential exerted by Saturn. The magnitude of the response quadrupole field, expressed in the tidal Love number k2, was used to infer the existence of an internal ocean. The new gravity field determination provides an improved estimate of k2 of about 0.62, accurate to a level of a few percent. The value is higher than the simplest models of Titan suggest and the interpretation is unclear; possibilities include a high density ocean (as high as 1300 kg/m3), a partially viscous response of the deeper region, or a dynamic contribution to the tidal response. The new solution includes higher degree and order harmonic coefficients (up to 5) and offers an improved map of gravity anomalies. The geoid is poorly correlated with the topography, implying strong compensation. In addition, the updated geoid and its associated uncertainty could be used to refine the gravity-altimetry correlation analysis and for improved interpretation of radar altimetric data.
•Cassini's final data set updates Titan's gravity solution to degree and order 5.•The power spectrum of the degree > 2 gravity coefficients follows a Kaula's power law.•The geoid is poorly correlated with the topography, implying good compensation.•A strongly differentiated interior implies a large non-hydrostatic degree-2 field.
The gravity harmonics of a fluid, rotating planet can be decomposed into static components arising from solid-body rotation and dynamic components arising from flows. In the absence of internal ...dynamics, the gravity field is axially and hemispherically symmetric and is dominated by even zonal gravity harmonics J
that are approximately proportional to q
, where q is the ratio between centrifugal acceleration and gravity at the planet's equator. Any asymmetry in the gravity field is attributed to differential rotation and deep atmospheric flows. The odd harmonics, J
, J
, J
, J
and higher, are a measure of the depth of the winds in the different zones of the atmosphere. Here we report measurements of Jupiter's gravity harmonics (both even and odd) through precise Doppler tracking of the Juno spacecraft in its polar orbit around Jupiter. We find a north-south asymmetry, which is a signature of atmospheric and interior flows. Analysis of the harmonics, described in two accompanying papers, provides the vertical profile of the winds and precise constraints for the depth of Jupiter's dynamical atmosphere.
Understanding the processes that led Venus to its current state and will drive its future evolution is a major objective of the next generation of orbiters. In this work we analyze the retrieval of ...the spin vector, the tidal response and the moment of inertia of Venus with VERITAS, a NASA Discovery-class mission. By simulating a systematic joint analysis of Doppler tracking data and tie points provided by the onboard synthetic aperture radar we show that VERITAS will provide accuracies (3σ) in the estimates of the tidal Love number k_2 to 4.6×10^(-4), its tidal phase lag to 0.05°, and the moment of inertia factor to 9.8×10^(-4) (0.3% of the expected value). Applying these results to recent models of the Venus interior, we show that VERITAS will provide much improved constraints on the interior structure of the planet.
Deciphering the genesis and evolution of the Martian polar caps can provide crucial understanding of Mars' climate system and will be a big step forward for comparative climatology of the terrestrial ...planets. The growing scientific interest for the exploration of Mars at high latitudes, together with the need of minimizing the resources onboard landers and rovers, motivates the need for an adequate navigation support from orbit. In the context of the ARES4SC study, we propose a novel concept based on a constellation that can support autonomous navigation of different kind of users devoted to scientific investigations of those regions. We study two constellations, that differ mainly for the semi-major axis and the inclination of the orbits, composed of 5 small satellites (based on the SmallSats design being developed in Argotec), offering dedicated coverage of the Mars polar regions. We focus on the architecture of the inter-satellite links (ISL), the key elements providing both ephemerides and time synchronization for the broadcasting of the navigation message. Our concept is based on suitably configured coherent links, able to suppress the adverse effects of on-board clock instabilities and to provide excellent range-rate accuracies between the constellation's nodes. The data quality allows attaining good positioning performance for both constellations with a largely autonomous system. Indeed, we show that ground support can be heavily reduced by employing an ISL communication architecture. Periodic synchronization of the clocks on-board the constellation nodes with terrestrial time (TT) is enabled through the main spacecraft (the mother-craft), the only element of the constellation enabling radio communication with the Earth. We report on the results of numerical simulations in different operational scenarios and show that a very high-quality orbit reconstruction can be obtained for the constellation nodes using a batch-sequential filter or a batch filter with overlapping arcs, that could be implemented on board the mother-craft, thus enabling a high level of navigation autonomy. The assessment of the achievable positioning accuracy with this concept is fundamental to evaluate the feasibility of a future positioning system providing a global coverage of the red planet.
•Polar constellation of small satellites on Mars.•Provide navigation services for Martian landers, rovers, and orbiters.•Quasi-autonomous constellation ephemeris reconstruction.•Novel tracking technique for inter-satellite link.
Recent models of Titan's interior predict that the satellite contains an ocean of water and ammonia under an icy layer. Direct evidence for the presence of an ocean can be provided on the Cassini ...mission only by radio science determination of Titan Love number
k
2
. Simulations that use the five flybys T11, T22 T33, T45, and T68 (the latter two belonging to the extended mission) lead to the result that in the elastic case, where the Love number is real,
k
2
will be determined with a one-sigma accuracy of 0.1. In the viscoelastic case, where
k
2
is complex, the real and imaginary parts of
k
2
will be determined with one sigma accuracies of 0.138 and 0.115, respectively. Ocean and oceanless models that include a viscoelastic rheology are built. In the viscoelastic case, there is a 93% probability to correctly predict the presence or absence of an ocean; this probability improves to 97% in the elastic case.
Deciphering the genesis and evolution of the Martian polar caps can provide crucial understanding of Mars' climate system. The growing scientific interest for the exploration of Mars at high ...latitudes, and the need of minimizing the resources onboard landers and rovers, motivates the need for adequate navigation support from orbit. We propose a novel concept based on a constellation that can support autonomous navigation of different kind of users devoted to scientific investigations of those regions. We study two constellations, that differ mainly for the semi-major axis, composed of 5 small satellites (based on the SmallSats design being developed in Argotec), offering dedicated coverage of the Mars polar regions. We focus on the architecture of the inter-satellite links (ISL), the key elements providing both ephemerides and time synchronization for the broadcasting of the navigation message. Our concept is based on suitably configured coherent links, able to suppress the adverse effects of on-board clock instabilities and to provide excellent range-rate accuracies between the constellation's nodes. The data quality allows attaining good positioning performance for both constellations with a largely autonomous system. Indeed, we show that ground support can be heavily reduced by employing an ISL communication architecture.
To assess whether timing of administration can influence the antihypertensive effect of quinapril, 18 patients with hypertension were studied with noninvasive ambulatory blood pressure monitoring. ...Quinapril, 20 mg, was given at 8 AM or 10 PM for 4 weeks in a double‐blind crossover fashion. To study the pattern of angiotensin converting enzyme (ACE) inhibition with the two treatment regimens, plasma ACE activity was measured in seven subjects 2,4, 8, 12 and 24 hours after quinapril administration. The 24‐hour blood pressure profiles showed a more sustained antihypertensive action with the evening administration of quinapril compared with the morning administration of quinapril; as with the morning administration, a partial loss of effectiveness was observed during nighttime hours. Measurement of ACE activity showed that evening administration caused a less pronounced but a more sustained decline of plasma ACE. These findings show that 20 mg quinapril given once daily is effective in lowering blood pressure levels throughout a 24‐hour period. The evening administration seems to be preferable because it causes a more favorable modulation of ACE inhibition and therefore determines a more homogeneous 24‐hour blood pressure control.
Clinical Pharmacology and Therapeutics (1992) 52, 378–383; doi:10.1038/clpt.1992.158
PDF
