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The well investigated size-frequency distributions (SFD) for lunar craters is used to estimate the SFD for projectiles which formed craters on terrestrial planets and on asteroids. The result shows the relative stability of these distributions during the past 4 Gyr. The derived projectile size-frequency distribution is found to be very close to the size-frequency distribution of Main-Belt asteroids as compared with the recent Spacewatch asteroid data and astronomical observations (Palomar-Leiden survey, IRAS data) as well as data from close-up imagery by space missions. It means that asteroids (or, more generally, collisionally evolved bodies) are the main component of the impactor family. Lunar crater chronology models of the authors published elsewhere are reviewed and refined by making use of refinements in the interpretation of radiometric ages and the improved lunar SFD. In this way, a unified cratering chronology model is established which can be used as a safe basis for modeling the impact chronology of other terrestrial planets, especially Mars.

Geological mapping and establishment of stratigraphic relationships provides an overview of geological processes operating on Mars and how they have varied in time and space. Impact craters and basins shaped the crust in earliest history and as their importance declined, evidence of extensive regional volcanism emerged during the Late Noachian. Regional volcanism characterized the Early Hesperian and subsequent to that time, volcanism was largely centered at Tharsis and Elysium, continuing until the recent geological past. The Tharsis region appears to have been largely constructed by the Late Noachian, and represents a series of tectonic and volcanic centers. Globally distributed structural features representing contraction characterize the middle Hesperian. Water-related processes involve the formation of valley networks in the Late Noachian and into the Hesperian, an ice sheet at the south pole in the middle Hesperian, and outflow channels and possible standing bodies of water in the northern lowlands in the Late Hesperian and into the Amazonian. A significant part of the present water budget occurs in the present geologically young polar layered terrains. In order to establish more firmly rates of processes, we stress the need to improve the calibration of the absolute timescale, which today is based on crater count systems with substantial uncertainties, along with a sampling of rocks of unknown provenance. Sample return from carefully chosen stratigraphic units could calibrate the existing timescale and vastly improve our knowledge of Martian evolution.

The first five Galileo flybys have provided image data up to two orders of magnitude better in resolution than the best Voyager data. Measurements of crater distributions can now reliably be extended down to sizes of about 100 m crater diameter. Crater distributions show steep slopes (cumulative index about −3) at smaller diameters on each satellite and are shallower at larger diameters, as seen on the Moon and the asteroids Gaspra and Ida. Crater densities differ about a factor of 10 (at D = 1km) between average dark terrain of Galileo Regio and youngest bright resurfaced areas on Ganymede. Crater densities on the most heavily cratered regions on both Ganymede and Callisto are fairly comparable. Crater densities on Europa have turned out to be higher than anticipated but are factors of about 10 lower than the youngest bright terrain in the Uruk Sulcus region of Ganymede. The similarity to crater distributions in the inner solar system suggests that the impactor population during early heavy bombardment preferentially was derived from the asteroid belt and that the impact rate on the Galilean satellites may have had a lunar-like decay with time. Making use of the idea of the “marker horizon” of 3.8 b.y. for the end of the heavy bombardment, i.e. formation of the youngest basin, also in the Jovian system, the frequency superimposed on the Gilgamesh basin can be related to the formation age of 3.8 b.y. and thus an impact rate derived for this point in time. In this way, assuming a lunar-like cratering chronology scenario, absolute ages can be determined for the Galilean moons from crater statistics. Thus, the most densely cratered dark terrains on both Ganymede and Callisto have a likely age of about 4.1 – 4.3 b.y. Some areas counted on Europa high-resolution images may be as old as 3 – 3.3 b.y. Other scenarios based on values proposed for the present-day comet impact rate in the Jovian system with non-lunar-like flux decays are conceivable and would result in generally younger ages, possibly as young as 10 m.y. These young ages and impact rates would result in ages for Ganymede and Callisto which are in serious conflict with the geologic evolution of these two moons as we understand it now.

A control network is a series of identifiable points on a surface and a table of their coordinates (latitude, longitude, radius). To create a planetary control network, points (usually craters) are identified on pictures and their image coordinates are measured in pixels. Each control point must be measured on two or more images. The navigation team supplies discrete spacecraft positions and approximate camera-pointing angles. An analytical triangulation program is used to compute the coordinates of the control points and to improve the camera-pointing angles.

In situ observations of comet Halley provided the first photographs of a cometary nucleus and yielded information about its environment, including the emitted gas and dust. The relation between these measurements and properties of and processes on the nucleus is established by theoretical modelling, while laboratory experiments may provide some of the physical parameters needed. In addition, laboratory tests can stimulate new ideas for processes that may be relevant to cometary physics. Processes to be studied in detail by large-scale laboratory experiments may include: (1) heat transport phenomena during sublimation of porous ice-dust mixtures, (2) material modification and chemical fractionation caused by the sublimation processes, (3) buildup and destruction of dust mantles, (4) detailed studies of gas release from mixtures of volatile ices, and (5) the investigation of ice and dust particle release mechanisms. The KOSI-team (Kometensimulation) carried out sublimation experiments with ice-mineral mixtures in a large Space Simulator. During initial experiments, cylindrical samples of 30-cm diameter and 15-cm thickness were irradiated with up to 2700-W/m2 light energy. The samples consisted of water-ice or water- and CO2-ice mineral mixtures. The experiments showed the importance of advection for heat transport into the interior. It was found that the sublimation of CO2 advances into the sample at a higher speed than that of water vapor release. Therefore, emission of volatile gases responded to insolation changes with a time lag of several hours. The ratio of the emitted gas species, as well as the dust-to-gas mass ratio, differs significantly from the values within the sample. A partly permeable refractory mantle of minerals and carbonaceous material developed with time. Dust and ice particle emission has been observed to occur from irradiated dirty ices as well as from dust mantles.