Vol 57, No 2 (2023)

Based on the photogeological and stratigraphic analysis of Venusian coronae, we have found that (1) late manifestations of volcanic activity on Venus, lobate plains, are weakly related to the formation of coronae. The small number of coronae–sources of lobate plains (~17% of the total population of coronae) indicates that the volcanic activity of the coronae ceased mainly in the pre-Atlian time. Thus, the coronae associated with manifestations of late volcanism in the form of lobate plains are either long-lived volcano tectonic complexes or structures of the final phases of volcanic activity; their locations mark the regions of long-term volcanism; (2) the small number of coronae formed by rift structures (~14% of the total popula tion) indicates that rifting in the Atlian period did not lead to mass formation of coronae. The majority of Venusian coronae was probably formed in the Fortunian–Guineverian periods of the geological history of Venus. A sharp decrease in the number of coronae formed in the Atlian period may be associated with an increase in the thickness of the lithosphere and an increase in its role as a rheological barrier.

In front of the bow shock with a quasi-parallel configuration of the interplanetary magnetic field, there exists a region called a foreshock, in which many nonstationary processes take place, the largest of which are collectively named “foreshock transients.” The size of these formations can reach tens of Earth radii, which significantly influences the nature of the solar wind interaction with the magnetosphere. Some types of foreshock transients are also observed at other planets, including those without their own global mag netic field, which indicates the universality of these phenomena. This article lists the most well-known non stationary processes occurring in the foreshock, as well as provides current ideas about the formation mech anisms of the largest foreshock transient.

The reflectance spectra of active asteroids (AAs) measured in the visible and near-UV ranges exhibit unusual features, which are apparently caused by the light scattering in an exosphere formed under active processes on an asteroid. To estimate the prospects for a quantitative interpretation of these features, we numerically simulated reflectance spectra of an AA enveloped by an exosphere composed of aggregate submicron particles of various composition and morphology, as well as homogeneous submicron particles. We assumed that the sizes of aggregates’ constituents correspond to those of grains in agglomerates of come tary and interplanetary dust. It has been shown that the scattering on aggregates of submicron grains produces interference features at wavelengths shorter than 0.6 μm, and the positions of these features are determined by both the sizes of these grains (rather than the aggregates themselves) and the real part of their refractive index. The structure of an aggregate and variations (up to ±20%) in the sizes of constituents weakly influence the position of these features. The shape of the spectrum at longer wavelengths also depends on the sizes of grains in the aggregates and serve as an additional criterion for estimating this parameter. Calculations per formed for aggregate particles absorbing in a short-wavelength range (which is typical of many materials that one may expect to find on AAs) show that the absorption significantly weakens the interference details appeared in this range. Hence, the attempts to detect strongly absorbing particles in the exosphere and to esti mate their properties by these spectral features cannot yield reliable results, as opposed to the simulations for weakly absorbing particles. The presence of homogeneous weakly absorbing submicron particles in the exo sphere of an AA results in a steady growth of the intensity at wavelengths shorter than 0.4−0.5 μm. Spectral measurements at the wavelengths shorter than 0.35 μm may help to estimate more reliably the properties of weakly absorbing particles, both aggregate and homogeneous, in the exospheres of AAs.