UPPSALA UNIVERSITET : Inst. för fysik och astronomi, avd. för astronomi och rymdfysik
Uppsala universitet
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Institutionen för fysik och astronomi

Avdelningen för astronomi och rymdfysik

Planetgruppen

Swedish National Space Board (Rymdstyrelsen)

Physics and dynamics of Solar System bodies

Within the Planetary group at the Astronomical observatory, a number of projects concerning physics and dynamics of solar system objects are carried out. On this web page you will find short descriptions of projects financed by the Swedish National Space Board.

Prof. Claes-Ingvar Lagerkvist
Prof. Hans Rickman
FD Johan Warell
FD Björn Davidsson
Fil lic Ola Karlsson
M Sc Anna Önehag

The BepiColombo mission

Scheduled for launch in 2013, the ESA/JAXA BepiColombo mission to Mercury will reveal cosmogonically important information about the innermost planet. In situ measurements will clarify the composition and types of geologic units of the surface, its thermal history, the vertical structure of the crust, and explain the enormous and massive core, the strong differentiation between the crust and the mantle, and the Earth-like magnetic field. Lagerkvist and Warell are co-Investigators for the thermal infrared imaging spectrometer MERTIS and the X-ray imaging spectrometer MIXS, which will determine the mineralogical and chemical composition of the surface and provide input for internal structure models.

The Rosetta mission

The ESA cornerstone mission Rosetta encounters comet 67P/Churyumov-Gerasimenko in 2014. The spacecraft will orbit the comet nucleus for an extended period and investigate its physical and chemical properties, and a lander will perform detailed in situ measurements on the cometary surface. This thorough characterization of a primitive Solar System body will have a profound impact on our understanding of Solar System formation and evolution. Rickman is Lead Scientist, and he and Davidsson are both co-Investigators for the optical imaging system OSIRIS, which will produce high-resolution images and photometric data for the nucleus and coma. They are also providing modeling support for the mission, for instance through their membership in an International Space Science Institute (ISSI, Bern) Comet Team.

The Rosetta spacecraft (white dot among colored star trails) made a close passage of the Earth on March 4, 2005, while on its way to the comet Churyumov-Gerasimenko. Image with the Westerlund Telescope by Karlsson.

The Gaia mission

The ESA spacecraft Gaia, to be launched in 2011, has a primary mission of measuring positions and radial velocities of one billion stars in the Milky Way. However, an important secondary mission is to obtain spectra for 300,000 asteroids. Lagerkvist, Warell, and Önehag are involved in the taxonomic classification of these asteroids and the characterization of stars with properties very similar to the Sun, which undoubtedly will revolutionize our understanding of, e.g., compositional properties and interrelations of bodies in the asteroid belt.

Mercury: composition and surface properties

Observational and modeling studies of Mercury and the Moon in preparation for the BepiColombo mission, have been carried out along several lines and a number of new results have been obtained. High-resolution multicolor imaging and global mapping of Mercury's regolith albedo at a spatial scale of 200 km indicates that the light scattering properties and chemistry, as well as the distribution of geologic units, are remarkably homogeneous over the planet's surface, and that the surface properties are similar to, though in a more extreme sense, to the iron-poor lunar highland crust. Radiative-transfer based light-scattering modeling of Mercury's optical--near infrared reflectance spectrum has allowed the determination of a strongly matured, glass-rich and anorthosite-dominated surface with minor pyroxene in the surface rocks, with a composition of about 1.2 wt% FeO, ~0 wt% TiO2, 0.2 wt% Fe0, and an optically active average grain size of about 30 micrometers or half that of the Moon. A near-infrared absorption band at 1.1 micrometer wavelength has been discovered in Mercury's spectrum, attributed to crystalline Ca-poor clinopyroxene, which supports results from microwave and thermal infrared spectroscopy. We have also studied the dynamical stability of possible mercurian satellites and objects in the vicinity of Mercury via numerical integrations and find that certain orbiting objects can be dynamically stable for 5 Myr or more. We have made a follow-up observational search for such objects which set an improved upper limit of 0.6 to 1.5 km for the size of any mercurian satellite.

Mercury imaged at close range by the NASA MESSENGER mission on January 14, 2008 (left), and as seen from the Earth with the Swedish Vacuum Solar Telescope by Warell on November 24, 1997 (right), at similar illumination and viewing conditions. In the middle, the MESSENGER image has been blurred to simulate a mid-disk spatial resolution of 250 km. Many of the albedo features seen ten years ago are now confirmed by spacecraft images.

Thermophysical modeling of cometary nuclei

Cometary nuclei are porous ice-rich bodies which evaporates when heated by the Sun. Thermophysical models combine radiative transfer, heat transfer, gas diffusion, sublimation theory and kinetics of non-equilibrium gases, to describe how observables such as the water production rate, depend on fundamental nucleus parameters. The thermophysical models developed in the Planetary System Group are among the most sophisticated in the world. Rockman was the first to utilize non-gravitational forces calculated within thermophysical models, to measure the cosmogonically important bulk densities of cometary nuclei. E.g., for the spacecraft target Comets 19P/Borrelly, 67P/Churyumov-Gerasimenko, 81P/Wild 2, and 9P/Tempel 1, Davidsson and Guiterrez (Granada) consistently obtain bulk densities substantially below that of compacted material, in accordance with grain agglomeration growth models of protoplanetesimals. Interpretation and analysis of data delivered by Rosetta will rely heavily on thermophysical nucleus modeling, hence, continued efforts are made to improve modeling capabilities.

Cometary activity in distant objects

Centaurs are objects belonging to the outer Solar System, in orbits outside that of Jupiter and generally crossing those of other giant planets. Though far outside the limiting distance for water sublimation, nine of them have shown cometary activity - probably driven by CO that leaks from subsurface layers inside these bodies. Observations by an international team involving Rickman and former group member Gunnarsson have yielded a strong indication that this is caused by the crystallization of amorphous ice. Rickman, Gunnarsson and Davidsson are involved in modeling of this process and observing a larger sample of Centaurs using the ESO APEX and ALMA telescope facilities.

Origin of the Earth's water

The Earth appears to have accreted most of its water during the giant impact phase, when planetary embryos came from the region where the asteroid main belt now has its outer parts. Rickman, together with Piskunov, Lyra, Regandell and Valsecchi (Rome), studies by means of Monte Carlo simulations, how a combination of gravitational scatterings and gas drag and gravity effects of the primordial Solar Nebula could have emplaced large quantities of icy planetesimals from the accretion zone of the giant planets into this region.

The Oort Cloud and the scattered disk

Recent advances in understanding the dynamics of distant cometary reservoirs are leading to a new picture of the extreme outskirts of the Solar System, i.e., the Oort cometary cloud. Rickman, Fouchard (Paris), Froeschle (Nice) and Valsecchi (Rome) have developed the most advanced tool so far available for simulating the history of this population under the influence of the Galaxy and passing stars. Their studies have clarified how the stars and the Galactic tide act together in a synergetic way to feed new comets into planet-crossing orbits over the age of the Solar System. The work continues in order to reveal the true size of the Oort Cloud population and how its infeed may be supplemented by a leakage of comets from the transneptunian 'scattered disk'.

Observations of asteroids

A number of imaging surveys of the ecliptic region has resulted in the discovery of 1464 asteroids and 3 comets. Spectroscopy and photometry of NEAs, Cybele, Hilda, and Trojan asteroids are made to characterize such objects in terms of taxonomy, size distribution, and rotational parameters. Combined with orbital evolution studies, such fundamental physical parameters are needed to understand the evolutionary history of the Solar System.

Orbital evolution and stability of minor body orbits

Numerical integration is used both to study the stability of particular orbit types using fictitious objects, as well as the past or future evolution of real objects using a number of clones to account for uncertainties in measured orbital elements. These techniques have been used to study, e.g., the orbital history of a newly discovered rare class of comets being gravitationally dominated by Saturn, as well as the orbital stability of Trojan asteroids librating around Jupiter's triangular Lagrangian points.