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:

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European Venus Explorer (EVE): an in-situ mission to Venus

The European Venus Explorer (EVE) mission was proposed to the European Space Agency in 2007, as an M-class mission under the Cosmic Vision Programme. Although it has not been chosen in the 2007 selection round for programmatic reasons, the EVE mission may serve as a useful reference point for future missions, so it is described here. It consists of one balloon platform floating at an altitude of 50–60 km, one descent probe provided by Russia, and an orbiter with a polar orbit which will relay data from the balloon and descent probe, and perform science observations. The balloon type preferred for scientific goals is one which oscillates in altitude through the cloud deck. To achieve this flight profile, the balloon envelope contains a phase change fluid, which results in a flight profile which oscillates in height. The nominal balloon lifetime is 7 days—enough for one full circumnavigation of the planet. The descent probe’s fall through the atmosphere takes 60 min, followed by 30 min of operation on the surface. The key measurement objectives of EVE are: (1) in situ measurement from the balloon of noble gas abundances and stable isotope ratios, to study the record of the evolution of Venus; (2) in situ balloon-borne measurement of cloud particle and gas composition, and their spatial variation, to understand the complex cloud-level chemistry; (3) in situ measurements of environmental parameters and winds (from tracking of the balloon) for one rotation around the planet, to understand atmospheric dynamics and radiative balance in this crucial region. The portfolio of key measurements is complemented by the Russian descent probe, which enables the investigation of the deep atmosphere and surface.     

Clumps in the outer disk by disk instability: Why they are initially gas giants and the legacy of disruption

We explore the initial conditions for fragments in the extended regions of gravitationally unstable disks. We combine analytic estimates for the fragmentation of spiral arms with 3D SPH simulations to show that initial fragment masses are in the gas giant regime. These initial fragments will have substantial angular momentum, and should form disks with radii of a few AU. We show that clumps will survive for multiple orbits before they undergo a second, rapid collapse due to H₂ dissociation and that it is possible to destroy bound clumps by transporting them into the inner disk. The consequences of disrupted clumps for planet formation, dust processing, and disk evolution are discussed. We argue that it is possible to produce Earth-mass cores in the outer disk during the earliest phases of disk evolution.     

A new relation between the central spectral solar H I Lyman α irradiance and the line irradiance measured by SUMER/SOHO during the cycle 23

The spectral irradiance at the center of the solar H I Lyman α (, referred to as Lyα in this paper) line profile is the main excitation source responsible for the atomic hydrogen resonant scattering of cool material in our Solar System. It has therefore to be known with the best possible accuracy in order to model the various Lyα emissions taking place in planetary, cometary, and interplanetary environments. Since the only permanently monitored solar irradiance is the total one (i.e. integrated over the whole Lyα line profile), Vidal-Madjar [1975. Evolution of the solar Lyman alpha flux during four consecutive years. Solar Phys. 40, 69-86] using Orbiting Solar Observatory 5 (OSO-5) satellite Lyα data, established a semi-empirical formula allowing him to deduce the central spectral Lyα irradiance from the total one. This relation has been extensively used for three decades. But, at the low altitude of the OSO-5 orbit, the central part of the solar line profile was deeply absorbed by a large column of exospheric atomic hydrogen. Consequently, the spectral irradiance at the center of the line was obtained by a complex procedure confronting the observations with simulations of both the geocoronal absorption and the self-reversed shape of the solar Lyα profile. The SUMER spectrometer onboard SOHO positioned well outside the hydrogen geocorona, provided full-Sun Lyα profiles, not affected by such an absorption [Lemaire et al., 1998. Solar H I Lyman α full disk profile obtained with the SUMER/SOHO spectrometer. Astron. Astrophys. 334, 1095-1098; 2002. Variation of the full Sun Hydrogen Lyman α and β profiles with the activity cycle. Proc. SOHO 11 Symposium, ESA SP-508, 219-222; 2004. Variation of the full Sun Hydrogen Lyman profiles through solar cycle 23. COSPAR 2004 Meeting], making it—for the first time—possible to measure the spectral and total Lyα solar irradiances directly and simultaneously. A new relation between these two quantities is derived in an expression that is formally similar to the previous one, but with significantly different parameters. After having discussed the potential causes for such differences, it is suggested that the new relation should replace the old one for any future modeling of the numerous Lyα absorptions and emissions observed in the Solar System.     

月球轨道的胀缩效应——引力质量与惯性质量不相等假说的推论

等效原理对天体是否成立?是近年来人们关心的重大课题之一。1981年作者提出的引力质量与惯性质量不相等假说是等效原理失效理论中形式比较具体的一种,本文则是应用它探讨月球的异常运动。按文[1]提出的假说,地球引力质量与它在太阳引力场中的位置和运动状态有关,从而引起月球轨道的大小有季节性变化。地球在近日点附近月轨最小,在远日点附近月轨最大,二者相差86.87厘米。现代激光测距技术和铯原子钟相结合,理应可以对此结果作出明确的判断。     

Comment on “Constraints on the source of lunar cataclysm impactors” (Cuk et al., 2010, Icarus 207, 590-594)

Cuk et al. (Cuk, M., Gladman, B.J., Stewart, S.T. [2010]. Icarus 207, 590-594) argue that the projectiles bombarding the Moon at the time of the so-called lunar cataclysm could not have been mainbelt asteroids ejected by purely gravitational means, in contradiction with a conclusion that was reached by Strom et al. (Strom, R.G., Malhotra, R., Ito, T., Yoshida, F., Kring, D.A. [2005]. Science 309, 1847-1850). We demonstrate that Cuk et al.’s argument is erroneous because, contrary to their arguments, the lunar highlands do register the cataclysm impacts, lunar class 1 craters do not represent the size distribution of the cataclysm craters, and the crater size distributions on the late-forming basins are quite similar to those of the highlands craters, albeit at a lower number density due to the rapid decline of the impact flux during the cataclysm.     

Thermal desorption of CH4 retained in CO2 ice

CO₂ ices are known to exist in different astrophysical environments. In spite of this, its physical properties (structure, density, refractive index) have not been as widely studied as those of water ice. It would be of great value to study the adsorption properties of this ice in conditions related to astrophysical environments. In this paper, we explore the possibility that CO₂ traps relevant molecules in astrophysical environments at temperatures higher than expected from their characteristic sublimation point. To fulfil this aim we have carried out desorption experiments under High Vacuum conditions based on a Quartz Crystal Microbalance and additionally monitored with a Quadrupole Mass Spectrometer. From our results, the presence of CH₄ in the solid phase above the sublimation temperature in some astrophysical scenarios could be explained by the presence of several retaining mechanisms related to the structure of CO₂ ice.     

Oligarchic growth with migration and fragmentation

In the core-accretion model, giant-planet cores form by oligarchic growth from a population of planetesimals prior to the dispersal of the disk gas. Once a core reaches a critical mass of roughly 10 Earth masses, it begins to accrete a gaseous envelope, forming a giant planet. Collisions between planetesimals cause fragmentation. Planetesimal fragments are more easily captured by cores, speeding up growth, but fragments are also lost by radial drift, reducing the total solid mass in the disk. Interaction with the gas causes cores to undergo inward type-I migration. Migration allows a core to accrete planetesimals from a larger region, but migrating cores may be lost if they reach the star. Thus, migration and fragmentation have both a positive and a negative impact on core formation. Here we describe results of new simulations of oligarchic growth that include fragmentation and/or migration. In the absence of migration, cores grow until they reach their isolation mass, which increases with distance from the star, or until the disk gas disperses. Fragmentation increases the maximum core mass by increasing growth rates in the outer disk, allowing objects to reach their isolation mass during the disk lifetime. When migration is present, cores migrate inwards rapidly when they approach 1 Earth mass. Most migrating cores are lost. Migrating cores gain little extra mass since they are passing through regions that have been depleted by earlier generations of cores. For a disk viscosity parameter alpha=1e−3 and planetesimal radius = 10 km, the maximum core mass is roughly 4 and 0.5 Earth masses with/without fragmentation, respectively, with little dependence on the disk mass. Formation and survival of 10-Earth-mass cores, in the presence of migration, requires large alpha (1e−2) and a massive disk (0.1 solar masses). When alpha is large, type-I migration rates decrease rapidly with time, allowing large, late-forming cores to survive. The addition of a stochastic (random-walk) migration component makes little difference to the outcome, provided that stochastic migration affects only cores larger than 0.01 Earth masses. Stochastic migration becomes increasingly important if it also affects lower-mass objects.