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Landslides have been identified on several solar system bodies, and different mechanisms have been proposed to explain their runout length. We analyze images from the Rosetta mission and report the global characterization of such features on comet 67P/Churyumov‐Gerasimenko's surface. By assuming the height to runout length as an approximation for the friction coefficient of landslide material, we find that on comet 67P, this ratio falls between 0.50 and 0.97. Such unexpected high values reveal a rocky‐type mechanical behavior that is much more akin to Earth dry landslides than to icy satellites' mass movements. This behavior indicates that 67P and likely comets in general are characterized by consolidated materials possibly rejecting the idea that they are fluffy aggregates. The variability of the runout length among 67P landslides can be attributed to the different volatile content located in the top few meters of the cometary crust, which can drive the mass movement. Comet 67P Churyumov‐Gerasimenko has been imaged with unprecedented spatial detail thanks to the high‐resolution OSIRIS camera (Optical, Spectroscopic and Infrared Remote Imaging System) on board the Rosetta spacecraft. 67P is characterized by an extremely diverse morphology comprising different surface features such as rough consolidated terrains, smooth plains, unconsolidated mantles, pits, fractures, cliffs, cuestas, ubiquitous boulders, and layers. The peculiarity of 67P is also reflected by the widespread presence of landslides. By using high‐resolution images, we analyze the shape and aspect ratio of the landslides located on comet 67P finding a mechanical behavior of the cometary material that is more akin to Earth dry landslides than to icy satellites' mass movements. These results make 67P a very peculiar object, mainly composed by ices and refractory materials but characterized by rocky‐type properties rather than icy‐type characteristics. In addition, the considerable variability among the different landslides of 67P suggests that different volatile contents located in the top few meters of the cometary crust play a fundamental role on mass movement, hence being a general indicator for the subsurface cometary heterogeneities. The height to runout length (H/L) of comet 67P/Churyumov‐Gerasimenko landslides ranges between 0.50 and 0.9767P landslide reveal a rocky‐type mechanical behavior indicating that comets are made by consolidated materialsThe H/L variability is an indicator of the different volatile content located in the top few meters of the cometary crust

The cameras carried onboard the flyby missions to comet P/Halleyin 1986 imaged the near nuclear jet activity fromseveral spatial directions. The observed, very structured near nucleardust jets were considered at that timeas the result of dust emission from well localized active surface regions(without supporting 3-D model computations, however).Based on the first, recently developed 3-D gas dynamical model ofP/Halley's activity,we have been shown that jet features can be reproduced assuming ahomogeneous dusty icenucleus surface. The dust in the collisional near nuclear comais concentrated along the gas flow discontinuities resulting from thecomplicated surface orography, creating the visual impression ofdust jets. We present here the results of these calculations forthe near nucleus dust distributions,and we compare them with the direct observations made during thethree Halley flybys (Vega 1, Vega 2, and Giotto).

The inverse dust tail model (M. Fulle 1989, Astron. Astrophys.217, 283–297) is applied to a dust tail image of short-period Comet 46P/Wirtanen obtained by K. Jockers et al. (1998, Astron. Astrophys.335, L56–L59) to provide information on the dust environment which will be probed by the ESA Rosetta space mission. The obtained results are consistent with those coming from dust coma photometry during 2 months before perihelion, so that we provide a dust environment model consistent with all available data concerning 46P/Wirtanen dust. Although the dust coma brightness was increasing up to perihelion, we show that most probably during the month preceding the comet perihelion the dust mass loss rate decreased from about 50 to 20 kg s−, for an assumed dust albedo of 4%. This is due to a probable time evolution of the dust size distribution power index γ, dropping from values close to −3 a month before perihelion, to values close to −4 at perihelion. These results point out how dangerous are arbitrary interpretations of the results of dust coma photometry, which, on the other hand, provide first-order quality constraints to test all dust environment models. These, however, remain indispensable to correctly interpret the collected data. The results of this paper are compared with those obtained by L. Colangeli et al. (1998, Icarus134, 35–46) applying the same tail model to ISOCAM data, so that we obtain an upper limit of the dust albedo equal to 4% and we provide parametric expressions of the time evolution of the dust ejection velocity, mass loss rate, and size distribution over all the comet orbit section relevant for the Rosetta mission (i.e., from 2.5 to 1.0 AU inward). These expressions closely agree with those derived by M. Fulle et al. (1997a, Astron. Astrophys. Suppl. Ser.126, 183–195) from the results of hydrodynamical models of the 46P/Wirtanen coma (J. F. Crifo and A. V. Rodionov 1997, Icarus127, 319–353), but the perihelion decrease of the mass loss rate.

A great deal of attention has been given to the production and spatial distribution of sodium in comets after the discovery of the sodium tail, by Cremonese et al. (1997a), on Hale-Bopp. The sodium has been observed in several comets in the past, but the Hale-Bopp represent the first time where it will be deeply analyzed considering the several data and scientists working on that. The sodium tail stimulated different studies trying to explain the mechanism source and provided the new lifetime for photoionization of the neutral sodium atom. We took into account other sodium observations performed in this century and we focalized our attention to comet Hale-Bopp to understand the main sources responsible for the sodium features observed.