Simulations of planet migration driven by planetesimal scattering

Evidence has mounted for some time that planet migration is an important part of the formation of planetary systems, both in the Solar System [Malhotra, R., 1993. Nature 365, 819-821] and in extrasolar systems [Mayor, M., Queloz, D., 1995. Nature 378, 355-359; Lin, D.N.C., Bodenheimer, P., Richardson, D.C., 1996. Nature 380, 606-607]. One mechanism that produces migration (the change in a planet's semi-major axis a over time) is the scattering of comet- and asteroid-size bodies called planetesimals [Fernandez, J.A., Ip, W.-H., 1984. Icarus 58, 109-120]. Significant angular momentum exchange can occur between the planets and the planetesimals during local scattering, enough to cause a rapid, self-sustained migration of the planet [Ida, S., Bryden, G., Lin, D.N.C., Tanaka, H., 2000. Astrophys. J. 534, 428-445]. This migration has been studied for the particular case of the four outer planets of the Solar System (as in Gomes et al. [Gomes, R.S., Morbidelli, A., Levison, H.F., 2004. Icarus 170, 492-507]), but is not well understood in general. We have used the Miranda [McNeil, D., Duncan, M., Levison, H.F., 2005. Astron. J. 130, 2884-2899] computer simulation code to perform a broad parameter-space survey of the physical variables that determine the migration of a single planet in a planetesimal disk. Migration is found to be predominantly inwards, and the migration rate is found to be independent of planet mass for low-mass planets in relatively high-mass disks. Indeed, a simple scaling relation from Ida et al. [Ida, S., Bryden, G., Lin, D.N.C., Tanaka, H., 2000. Astrophys. J. 534, 428-445] matches well with the dependencies of the migration rate:

(1)     

Structure of self-gravity wakes in Saturn's A ring as measured by Cassini CIRS

The CIRS infrared spectrometer onboard the Cassini spacecraft has scanned Saturn's A ring azimuthally from several viewing angles since its orbit insertion in 2004. A quadrupolar asymmetry has been detected in this ring at spacecraft elevations ranging between 16° to 37°. Its fractional amplitude decreases from 22% to 8% from 20° to 37° elevations. The patterns observed in two almost complete azimuthal scans at elevations 20° and 36° strongly favor the self-gravity wakes as the origin of the asymmetry. The elliptical, infinite cylinder model of Hedman et al. [Hedman, M.M., Nicholson, P.D., Salo, H., Wallis, B.D., Buratti, B.J., Baines, K.H., Brown, R.H., Clark, R.N., 2007. Astron. J. 133, 2624-2629] can reproduce the CIRS observations well. Such wakes are found to have an average height-to-spacing ratio H/λ=0.1607±0.0002, a width-over-spacing W/λ=0.3833±0.0008. Gaps between wakes, which are filled with particles, have an optical depth τG=0.1231±0.0005. The wakes mean pitch angle ΦW is 70.70°±0.07°, relative to the radial direction. The comparison of ground-based visible data with CIRS observations constrains the A ring to be a monolayer. For a surface mass density of 40 g cm⁻² [Tiscarino, M.S., Burns, J.A., Nicholson, P.D., Hedman, M.M., Porco, C.C., 2007. Icarus 189, 14-34], the expected spacing of wakes is λ≈60 m. Their height and width would then be H≈10 m and W≈24 m, values that match the maximum size of particles in this ring as determined from ground-based stellar occultations [French, R.G., Nicholson, P.D., 2000. Icarus 145, 502-523].     

Did Saturn's rings form during the Late Heavy Bombardment?

The origin of Saturn's massive ring system is still unknown. Two popular scenarios—the tidal splitting of passing comets and the collisional destruction of a satellite—rely on a high cometary flux in the past. In the present paper we attempt to quantify the cometary flux during the Late Heavy Bombardment (LHB) to assess the likelihood of both scenarios. Our analysis relies on the so-called “Nice model” of the origin of the LHB [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461; Morbidelli, A., Levison, H.H., Tsiganis, K., Gomes, R., 2005. Nature 435, 462-465; Gomes, R., Levison, H.F., Tsiganis, K., Morbidelli, A., 2005. Nature 435, 466-469] and on the size distribution of the primordial trans-neptunian planetesimals constrained in [Charnoz, S., Morbidelli, A., 2007. Icarus 188, 468-480]. We find that the cometary flux on Saturn during the LHB was so high that both scenarios for the formation of Saturn rings are viable in principle. However, a more detailed study shows that the comet tidal disruption scenario implies that all four giant planets should have comparable ring systems whereas the destroyed satellite scenario would work only for Saturn, and perhaps Jupiter. This is because in Saturn's system, the synchronous orbit is interior to the Roche Limit, which is a necessary condition for maintaining a satellite in the Roche Zone up to the time of the LHB. We also discuss the apparent elimination of silicates from the ring parent body implied by the purity of the ice in Saturn's rings. The LHB has also strong implications for the survival of the saturnian satellites: all satellites smaller than Mimas would have been destroyed during the LHB, whereas Enceladus would have had from 40% to 70% chance of survival depending on the disruption model. In conclusion, these results suggest that the LHB is the “sweet moment” for the formation of a massive ring system around Saturn.     

Orbital stability of systems of closely-spaced planets

An investigation of the stability of systems of 1 M_⊕ (Earth-mass) bodies orbiting a Sun-like star has been conducted for virtual times reaching 10 billion years. For the majority of the tests, a symplectic integrator with a fixed timestep of between 1 and 10 days was employed; however, smaller timesteps and a Bulirsch-Stoer integrator were also selectively utilized to increase confidence in the results. In most cases, the planets were started on initially coplanar, circular orbits, and the longitudinal initial positions of neighboring planets were widely separated. The ratio of the semimajor axes of consecutive planets in each system was approximately uniform (so the spacing between consecutive planets increased slowly in terms of distance from the star). The stability time for a system was taken to be the time at which the orbits of two or more planets crossed. Our results show that, for a given class of system (e.g., three 1 M_⊕ planets), orbit crossing times vary with planetary spacing approximately as a power law over a wide range of separation in semimajor axis. Chaos tests indicate that deviations from this power law persist for changed initial longitudes and also for small but non-trivial changes in orbital spacing. We find that the stability time increases more rapidly at large initial orbital separations than the power-law dependence predicted from moderate initial orbital separations. Systems of five planets are less stable than systems of three planets for a specified semimajor axis spacing. Furthermore, systems of less massive planets can be packed more closely, being about as stable as 1 M_⊕ planets when the radial separation between planets is scaled using the mutual Hill radius. Finally, systems with retrograde planets can be packed substantially more closely than prograde systems with equal numbers of planets.     

Ice lines, planetesimal composition and solid surface density in the solar nebula

To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2–3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the solar nebula. By combining a viscously evolving protostellar disk with a kinetic model of ice formation, which includes not just water but methane, ammonia, CO and 54 minor ices, we calculate the solid surface density of a possible giant planet-forming solar nebula as a function of heliocentric distance and time. Our results can be used to provide the starting planetesimal surface density and evolving solar nebula conditions for core accretion simulations, or to predict the composition of planetesimals as a function of radius. We find three effects that favor giant planet formation by the core accretion mechanism: (1) a decretion flow that brings mass from the inner solar nebula to the giant planet-forming region, (2) the fact that the ammonia and water ice lines should coincide, according to recent lab results from Collings et al. [Collings, M.P., Anderson, M.A., Chen, R., Dever, J.W., Viti, S., Williams, D.A., McCoustra, M.R.S., 2004. Mon. Not. R. Astron. Soc. 354, 1133–1140], and (3) the presence of a substantial amount of methane ice in the trans-saturnian region. Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62–85] by a factor of 3–4 throughout the trans-saturnian region. We also discuss the location of ice lines and their movement through the solar nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments.     

Galileo in-situ dust measurements in Jupiter’s gossamer rings

Galileo was the first artificial satellite to orbit Jupiter. During its late orbital mission the spacecraft made two passages through the giant planet’s gossamer ring system. The impact-ionization dust detector on board successfully recorded dust impacts during both ring passages and provided the first in-situ measurements from a dusty planetary ring. During the first passage—on 5 November 2002 while Galileo was approaching Jupiter—dust measurements were collected until a spacecraft anomaly at 2.33R_J (Jupiter radii) just 16 min after a close flyby of Amalthea put the spacecraft into a safing mode. The second ring passage on 21 September 2003 provided ring dust measurements down to about 2.5R_J and the Galileo spacecraft was destroyed shortly thereafter in a planned impact with Jupiter. In all, a few thousand dust impacts were counted with the instrument accumulators during both ring passages, but only a total of 110 complete data sets of dust impacts were transmitted to Earth. Detected particle sizes range from about 0.2 to 5 μm, extending the known size distribution by an order of magnitude towards smaller particles than previously derived from optical imaging [Showalter, M.R., de Pater, I., Verbanac, G., Hamilton, D.P., Burns, J.A., 2008. Icarus 195, 361-377; de Pater, I., Showalter, M.R., Macintosh, B., 2008. Icarus 195, 348-360]. The grain size distribution increases towards smaller particles and shows an excess of these tiny motes in the Amalthea gossamer ring compared to the Thebe ring. The size distribution for the Amalthea ring derived from our in-situ measurements for the small grains agrees very well with the one obtained from images for large grains. Our analysis shows that particles contributing most to the optical cross-section are about 5 μm in radius, in agreement with imaging results. The measurements indicate a large drop in particle flux immediately interior to Thebe’s orbit and some detected particles seem to be on highly-tilted orbits with inclinations up to 20°. Finally, the faint Thebe ring extension was detected out to at least 5R_J, indicating that grains attain higher eccentricities than previously thought. The drop interior to Thebe, the excess of submicron grains at Amalthea, and the faint ring extension indicate that grain dynamics is strongly influenced by electromagnetic forces. These findings can all be explained by a shadow resonance as detailed by Hamilton and Krüger [Hamilton, D.P., Krüger, H., 2008. Nature 453, 72-75].     

Density waves in Cassini UVIS stellar occultations: 1. The Cassini Division

We analyze density waves in the Cassini Division of Saturn's rings revealed by multiple stellar occultations by Saturn's rings observed with the Cassini Ultraviolet Imaging Spectrograph. The dispersion and damping of density waves provide information on the local ring surface mass density and viscosity. Several waves in the Cassini Division are on gradients in the background optical depth, and we find that the dispersion of the wave reflects a change in the underlying surface mass density. We find that over most of the Cassini Division the ring opacity (the ratio of optical depth to surface mass density) is nearly constant and is ∼5 times higher than the opacity in the A ring where most density waves are found. However, the Cassini Division ramp, a 1100-km-wide, nearly featureless region of low optical depth that connects the Cassini Division to the inner edge of the A ring, has an opacity like that of the A ring and significantly less than that in the rest of the Cassini Division. This is consistent with particles in the ramp originating in the A ring and being transported into the Cassini Division through ballistic transport processes. Damping of the waves in the Cassini Division suggests a vertical thickness of 3–6 m. Using a mean opacity of 0.1 cm²/g we find the mass of the Cassini Division, excluding the ramp, is 3.1×¹⁰¹⁶ kg while the mass of the Cassini Division ramp, with an opacity of 0.015 cm²/g, is 1.1×¹⁰¹⁷ kg. Assuming a power-law size distribution for the ring particles, the larger opacity of the main Cassini Division is consistent with the largest ring particles there being ∼5 times smaller than the largest particles in the ramp and A ring.     

Discriminating contamination from particle components in spectra of Cassini's dust detector CDA

The Chemical Analyser subsystem of the Cosmic Dust Analyser (CDA) aboard the Cassini spacecraft performs in situ measurements of the chemical composition of dust in space. The instrument records time-of-flight mass spectra of cations, extracted from the impact cloud that is created by high-velocity particle impacts onto the detector target. Thus, the spectra not only show signals of particle components but also of ions from the target material and target contamination. The aim of this work is to determine which non-particle ions are to be expected in the spectra obtained in space operation at Saturn.We present an analysis of the contamination state of the instrument's impact target. Beside investigations of the purity of the rhodium target surface, spectra from CDA calibration experiments at the dust accelerator facility are evaluated with regard to contamination signatures. Furthermore, contamination mass lines in spectra obtained by impacts of Jovian and Saturnian dust stream particles are analysed. Due to their small size and high speed, stream particle impacts predominantly produce ions from the target material and therefore the spectra are excellent probes of the contamination state of the target operating in space. With the exception of adsorbed hydrogen and carbon, the level of contamination is very low.Implications for CDA spectra of Saturnian E ring particle impacts are derived. The findings confirm the published interpretations. The low level of alkali metal contamination implies a significant sodium contribution in the composition of E ring ice particles. Additionally, ionisation thresholds for the occurrence of contamination mass lines can be utilised to set limits for the impact velocity.     

Capability of the penetrator seismometer system for lunar seismic event observation

We developed a seismometer system for a hard landing “penetrator” probe in the course of the former Japanese LUNAR-A project to deploy new seismic stations on the Moon. The penetrator seismometer system (PSS) consists of two short-period sensor components, a two-axis gimbal mechanism for orientation, and measurement electronics. To carry out seismic observations on the Moon using the penetrator, the seismometer system has to function properly in a lunar environment after a hard landing (impact acceleration of about 8000 G), and requires a signal-to-noise ratio to detect lunar seismic events. We evaluated whether the PSS could satisfactorily observe seismic events on the Moon by investigating the frequency response, noise level, and response to ground motion of our instrument in a simulated lunar environment after a simulated impact test. Our results indicate that the newly developed seismometer system can function properly after impact and is sensitive enough to detect seismic events on the Moon. Using this PSS, new seismic data from the Moon can be obtained during future lunar missions.     

TandEM: Titan and Enceladus mission

TandEM was proposed as an L-class (large) mission in response to ESA’s Cosmic Vision 2015–2025 Call, and accepted for further studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini–Huygens mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is designed to build on but exceed the scientific and technological accomplishments of the Cassini–Huygens mission, exploring Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time). In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in situ investigation components, i.e. a hot-air balloon (Montgolfière) and possibly several landing probes to be delivered through the atmosphere.