In preparation for the International Rosetta Mission (see International Rosetta Mission: the comet rendezvous mission), the potential target comets and asteroids have to be observed and monitored to provide crucial parameters for spacecraft and mission design: nucleus radius, rotation period, onset and evolution of activity, dust and gas production rates, and astrometric observations to be able to precisely determine the orbit of the nucleus (Schulz & Schwehm, 1996).
Organised out of the scientific community and led by H. Boehnhardt, Univ. München (D) and R. Schulz (SSD from November 1996), a ground-based observation campaign has been organised. Comet 46 P/Wirtanen, the target comet in the baseline mission, was recovered mid-1995 and has been regularly observed since March 1996 (Fig. 4.2.4.1). Recently, space-based observations with HST and ISO were added to the data base.
Figure 4.2.4.1: Comet 46 P/Wirtanen in September 1996. This
R-filter image was obtained at the European Southern Observatory,
ESO, La Silla, Chile, with the EFOSC2 instrument mounted on the
ESO/MPI 2.2 m Telescope. Observers: R. Schulz and G.P. Tozzi
(Oss. Astrophysico di Arcetri, Firenze, I).
G. Schwehm, a Co-I in the collaboration, participated in observing campaigns of potential back-up targets from Calar Alto, Spain and of Comet Wirtanen from ESO, La Silla, Chile.
Reference
Schulz, R. & Schwehm, G. (1996). Planet. Space
Sci. 44, 619.
The successful implementation of the Rosetta mission - very close orbits around the nucleus and the deployment of a lander on its surface are required - needs a good understanding of the evolution of the near-nucleus dust and gas environment and of the nucleus surface properties, e.g. strength, density and roughness. R. Orosei, during his stay at SSD, in collaboration with the Istituto di Astrofisica Spaziale, Rome, developed a numerical model simulating the physical evolution and chemical differentiation of a cometary nucleus (Orosei et al., 1995).
A comet nucleus is described as a porous aggregate of ices and dust in which solar heating causes the sublimation of volatiles and the ejection of dust particles lifted by the flow of the escaping gases. The numerical model solves the heat conduction and the gas diffusion equations throughout the nucleus made of H2O ice, CO2 ice and dust particles of different sizes. The equations are coupled via the source terms, which describe the sublimation and recondensation of ices as latent heat or mass exchanges. The ejection of dust particles is allowed when the drag exerted on a grain by the outflowing gas is stronger than the gravitational pull of the nucleus. Unbound grains that cannot be ejected form a layer of dust that darkens the nucleus, which, in turn, absorbs a larger fraction of the incoming solar energy. The calculations are performed under the assumption that the gas in the pore network acts as a perfect gas, and that sublimation and recondensation are instantaneous in order to keep local thermodynamic equilibrium. The results show that the fraction of large grains in the dust size distribution is critical in determining the final fate of the comet nucleus. The model calculations show that the nucleus can either disintegrate completely or turn into an inactive, asteroid-like object due to the formation of a thick, insulating dust crust. The variability of the albedo can determine a cyclic pattern in the comet activity, alternating phases of gas and dust emission with periods of dormancy.
Together with scientists from the Rosetta Lander Consortium and the Rosetta Interdisciplinary Scientists, the Rosetta Project Scientist is involved in an effort to create a working model of Comet Wirtanen. The group will establish a model of the nucleus that can provide the design and test criteria for the Rosetta Lander and its payload complement, by critically assessing the potential physical boundaries of crucial parameters that describe the nucleus and incorporating all available observational data.
A report will summarise the results of three workshops and a synthesis of the modelling efforts and approaches from scientists with different research backgrounds, ranging from ice physics, chemistry to solid state physics and mineralogy. It will be published in 1997.
Reference
Orosei, R. et al. (1995). A&A 301, 613.
Galileo and Ulysses Dust Detectors
For several
years, the twin dust detectors aboard Galileo and Ulysses (PI:
E. Grün, MPI für Kernphysik, Heidelberg, D) have
provided new data for the study of the properties and dynamics
of the interplanetary dust cloud and the population of
interstellar grains penetrating into the solar system.
G. Schwehm has collaborated with the group at MPI-K for many years. The international consortium analysing these data has grown to include nearly all the institutes working in this field. Data from both spacecraft are available to all scientists. In 1997 a similar instrument will be launched on Cassini to extend the spatial coverage to the regions up to Saturn and its system.
Collimated streams of submicron-sized dust were detected by the Ulysses dust detector coming from a source in the Jovian system. This was confirmed by Galileo's detector, which also detected dust streams in interplanetary space on its approach to Jupiter (Gr&$252;n et al., 1996).
Galileo observed at least two different sizes of dust particles: small submicron-sized particles during the approach to Jupiter all the way up to closest approach to Io, and bigger micron-sized dust grains just inside a distance of about 10 RJ. The measurements of the submicron-sized particles are consistent with dust grains emitted from Io, whereas the bigger micron-sized dust grains are particles in orbit about Jupiter or interplanetary grains that are gravitationally concentrated near Jupiter.
Hiten and GORID results
The Division is participating actively in the study of Cosmic
Dust and Space Debris by analysis of data provided by instruments
largely developed within the Division (H. Svedhem). The dust
instruments on Hiten and Bremsat have provided large data sets,
where a lot of analysis remains to be done. The Hiten dust
experiment sampled the regions between the Earth and the Moon for
2 years and the lunar region for 1 year as a lunar orbiter. The
majority of the ~500 particles detected during this time were
found to have more or less circular heliocentric orbits (Fig. 4.2.4.3/1).
The instrument showed the best evidence so far for
the existence of the beta meteoroids, small particles on
hyperbolic orbits, driven out of the solar system by solar
radiation pressure. An interesting result is the discovery of
interstellar particles at 1 AU heliocentric distance. These
particles have been found by Ulysses and Galileo instruments, but
only at larger heliocentric distances. The direction of the
influx of the interstellar particles corresponds fairly well to
what is seen by these two probes in spite of the large
electrodynamic forces the particles experienced on their inward
journey. The major forces on the particles are the gravitational
focusing by the Sun, solar radiation pressure and Lorentz forces
due to particle charge and the heliospheric magnetic field. The
two last forces, in turn, depend on time and location and on the
properties of the particles themselves. Therefore, a theoretical
treatment will be very sensitive to the models used and will
result in uncertain predictions. In situ measurements are
thus of high importance.
Figure 4.2.4.3/1: The flux of dust particles detected by the
Hiten experiment plotted as a function of detector pointing
direction in ecliptic longitude. The excess at 220° is
possibly due to interstellar particles.
The Bremsat dust experiment orbited on the small university satellite for 1 year in a low Earth, low inclination orbit before reentering in February 1995. During this year, several thousand impacts were detected, the vast majority being space debris particles like minor satellite fragments and rocket exhaust particles. The analysis of these data are somewhat complicated since the plasma environment in this low orbit presents a significantly higher noise level to the detector than in inter- planetary space. In addition, the average impact velocity for Earth-orbiting dust is lower than the interplanetary average impact velocity.
The most recent addition to the fleet of dust detectors is the GORID (Geostationary Orbit Impact Detector) experiment (Fig. 4.2.4.3/2), launched on the Russian geostationary telecommunication satellite Express 2 on 26 September 1996. The experiment, the Ulysses flight spare model, built by MPI-K, Heidelberg, was refurbished and recalibrated. It is operated by the Division, in cooperation with the ESA Technical Directorate, MPI-K and NPO-PM, Krasnoyarsk, Russia, and is the first instrument of this type measuring dust in GEO. It is performing well. Preliminary analysis shows an impact rate of 1-3/day. In addition to the characterisation of the GEO dust environment, the instrument will provide important data for long-term statistics of the interplanetary and, hopefully, interstellar dust (Express 2 lifetime is 7 years). Interesting correlation with the simultaneous measurements by the Ulysses and Galileo detectors will be attempted.
Figure 4.2.4.3/2: The sensor part of the GORID experiment.
Ions and electrons are generated by dust particle impacts on the
gold-covered hemispherical target seen in the inner back of the
detector. The aperture area is 0.1 m².
Reference
Grün, E. et al. (incl. G. Schwehm). (1996).
Science 274, 399.
The search for fullerenes and PAHs, and new evidence for
C60+ in space
Following the discovery of two Diffuse Interstellar Bands
(DIBs) at 9577 Å and 9632 Å that could be ascribed to
the cation C60+ (Foing & Ehrenfreund,
1994), these authors recently obtained new observations of the
C60+ bands from the Canadian-French
Hawaiian Telescope (CFHT) and ESO La Silla (Foing &
Ehrenfreund, 1997). These provide new evidence for interstellar
C60+ in addition to the earlier match with
laboratory spectra. The high quality of the spectra and reduced
telluric water absorption at these sites allowed us to confirm,
without doubt, the presence of the two DIBs and to measure their
strength, width and ratio reliably. The two DIBs correlate
perfectly in different environments. Together, they decrease in
a UV-shielded cold cloud but increase in a unique way among DIBs
in a region dominated by extreme UV radiation in Orion, with a
constant band ratio. As expected by arising from a common
carrier, both DIBs show the same width (3 cm-1),
consistent with the rotational contour broadening of
C60 fullerene molecules (Fig. 4.2.4.4).
Figure 4.2.4.4: Telluric corrected spectrum of HD183143
(spectral type B71ae) observed under very dry conditions at CFHT
with the two diffuse bands of 9577 Å and 9632 Å. The
star's spectrum has been divided, after instrumental corrections,
by the spectrum of a reference star of similar spectral type.
This allowed division of the stellar lines and limiting of flat
field variations. Residuals from telluric water lines are almost
absent.
Recent studies suggest carbon-containing molecules as stable molecules in space and as carriers of the unidentified DIBs. We have searched for coronene (C 24H12) and ovalene (C32H14) cations in the near-IR spectrum of DIBs, and derive limits on their presence to less than 0.05% of cosmic carbon (Ehrenfreund, Foing et al., 1995). To explain these limits, we proposed a possible selective destruction mechanism of PAHs (polycyclic aromatic hydrocarbons) through dication formation. We are now also searching for fullerene compounds, which are not expected to suffer from this dication destruction mechanism.
Discovery of substructures in DIBs as evidence for large
gas-phase molecule carriers
From high resolution R=70 000 and high signal/noise narrow
spectra, Ehrenfreund & Foing (1996) resolved two or three
peak substructures and wing asymmetries in the spectral profiles
of three narrow DIBs. The measured profiles show specific
similarities with branches of calculated rotational contours of
gas phase molecular spectra. We measured small changes of the
profile substructures with line of sight conditions. By
comparison with model calculations of PAHs and fullerenes, our
observations indicate that the molecular carriers of the DIBs
5797, 6379 and 6613 Å have rotational constants smaller than
0.004 cm-1 and would correspond to large PAH molecules
with more than 40 C atoms, chains of 12-18 C, 30 C rings or
60/C70 fullerenes. Recent higher resolution spectra (110 000) at
OHP and ESO obtained in 1996 have confirmed both these results
and a predicted increase of peak separation with temperature.
A new reference target for tracking the carriers of Diffuse
Interstellar Bands
Ehrenfreund, Cami, Dartois & Foing (1997) report the
discovery of a remarkable target (BD63 1964) for DIB studies. The
unusual DIB strengths allowed the confirmation of DIBs previously
denoted only as probable because of their weakness in most lines
of sight. We compared the DIB spectrum towards this star and the
well-studied earlier reference star HD183143. Line of sight
parameters, such as far-UV extinction and Ca I abundance,
indicate a denser environment that can shield molecules
efficiently from far-UV radiation. The relative enhancement of
many narrow DIBs was discussed in the context of neutral
fullerene compounds or large PAHs. The relative weakening of
broader strong DIBs indicates carriers with different
photoionisation or recombination rates.
References
Ehrenfreund, P. & Foing, B.H. (1996). A&A 307,
L25.
Ehrenfreund, P., Foing, B.H. et al. (1995). A&A29, 213.
Ehrenfreund, P., Cami, J., Dartois, E. & Foing, B.H.
(1997). A&A Lett. 318, L28.
Foing, B.H. & Ehrenfreund, P. (1994). Nature 369, 296
Foing, B.H. & Ehrenfreund, P. (1997). A&A 317, L59
Solar photons interact directly with the surface of Mercury due to the absence of an atmosphere. One hemisphere is therefore covered with a photoelectron layer that may play a role in the horizontal transport of charges and the coupling between the planet and its magnetosphere. The photoelectrons have a mean kinetic energy of about 1.6 eV and a current density of 0.1-0.2 mA/m2 at the subsolar point. The corresponding saturation current for the entire sunlit area is 1.7-3.7 GA. The conductance of the sheath is approximately 10-5 S, 30- 400 times larger than the height-integrated transverse conductivity of the exosphere. Photoemission can also ensure the electrical continuity between the planet and its exosphere, closing subsurface and magnetospheric currents (Fig. 4.2.4.5). This phenomenon should be taken into account in any modelling of the environment of Mercury. A similar process cannot take place at Earth, because its surface emits no photoelectrons and is electrically insulated from the magnetosphere by a neutral atmosphere.
Figure 4.2.4.5: Charge exchange between the surface of Mercury
and its environment.
Reference
Grard, R. (1997). Planet. Space Sci. 45, 67.
Planetary geology research in the Division is focusing on the study of potential landing sites for Mars missions and on the interpretation of terrestrial impact structures. In the framework of the Intermarsnet study (see 3.3.2.1), research on potential landing sites for a network of stations on Mars was carried out, including the methodology for selecting such sites according to scientific requirements and technical constraints (De Angelis & Chicarro, 1996). A network of stations on Mars addresses scientific objectives that are global in nature (seismology, meteorology, rotational dynamics), a fact that must be accounted for in the site evaluation. However, site characterisation (geology, geochemistry, mineralogy) from the surface, from orbit and during lander descent is important for understanding the evolution of the planet, and cannot be complete without additional detailed investigations (magnetic field, iron and volatile studies, exobiology and atmospheric structure). In practice, these scientific objectives dictate a number of requirements for the network, such as separation between stations in the order of 1000 km to allow regional and global internal structure studies, proximity to expected seismically active regions, variety of geological units (volcanic plains, cratered highlands, channel materials), variety of ages (determined from crater counting), variety of exposed materials (both igneous and sedimentary rocks), maximal latitudinal coverage to enable global meteorological studies, and regions of expected variations in radiation environment and volatile content.
Also, technical constraints limit the range of realistic landing sites. These include terrain homogeneity within the landing uncertainty ellipse, latitude and altitude constraints for lander delivery and solar cell power generation, and complementarity with surface stations from previous and future missions to Mars. After considering about 100 sites that satisfy most of the scientific requirements, a number of sets of technically feasible and scientifically meaningful landing sites were studied in depth to provide Intermarsnet with several realistic mission scenarios.
To diversify the scope of planetary geology activities in the Division, discussions, initiated by A. Chicarro, have begun to address a new theme in our research programme: the study of impact craters on Earth from a geological, geophysical and geochemical point of view, both from orbit and in the field. Impact craters on the Earth represent a unifying theme between planetary geology and terrestrial geosciences studies. Impact cratering is the most common geological process in the solar system. All planetary surfaces show the scars of the early heavy bombardment as well as the more recent one from meteorites and other bodies. Direct access to terrestrial impact craters, including sampling of deformed and surrounding materials, is by definition much easier than that for any other body in the solar system.
Reference
De Angelis & Chicarro, A. (1996). Planet. Space
Sci. 44, 1325.
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