Volume 50, Nº 4 (2016)

Consideration is given to the estimates for the strength of the consolidated material forming the bulk of the nucleus of comet 67P Churyumov–Gerasimenko and those for the strength of the surface material overlying the consolidated material at the sites of the first and last contact of the Philae lander with the nucleus. The strength of the consolidated material was estimated by analyzing the terrain characteristics of the steep cliffs, where the material is exposed on the surface. Based on these estimates, the tensile strength of the material is in the range from 1.5 to 100 Pa; the shear strength, from ∼13 to ⩾30 Pa; and the compressive strength, from 30 to 150 Pa, possibly up to 1.5 kPa. These are very low strength values. Given the dependence of the measurement results on the size of the measured object, they are similar to those of fresh dry snow at –10°C. The (compressive) strength of the surface material at the site of the first touchdown of Philae on the nucleus is estimated from the measurements of the dynamics of the surface impact by the spacecraft’s legs and the geometry of the impact pits as 1–3 kPa. For comparison with the measurement results for ice-containing materials in terrestrial laboratories, it needs to be taken into account that the rate of deformation by Philae’s legs is four orders of magnitude higher than that in typical terrestrial measurements, leading to a possible overestimation of the strength by roughly an order of magnitude. There was an attemp to put one of the MUPUS sensors into the surface material at the site of the last contact of Philae with the nucleus. Noticeable penetration of the tester probe was not achieved that led to estimation of the minimum compressive strength of the material to be ⩾4 MPa4 This fairly high strength appears to indicate the presence of highly porous ice with grains “frozen” at contacts.

Within the formalism of Tsallis nonextensive statistics designed to describe the behavior of anomalous systems, systems with a strong gravitational interaction between their individual parts and the fractal nature of phase space, we have obtained linearized equations for the oscillations of a rigidly rotating disk by taking into account dissipative effects and give a derivation of the dispersion equation in the WKB approximation. Based on the previously derived modified Navier—Stokes hydrodynamic equations (the so-called equations of q-hydrodynamics), we have analyzed the axisymmetric oscillations of an astrophysical, differentially rotating gas—dust cosmic object and obtained modified Jeans and Toomre gravitational instability criteria for disks with a fractal phase-space structure.

Spectrophotometric observations of 145 Adeona, 704 Interamnia, 779 Nina, and 1474 Beira—asteroids of close primitive types—allowed us to detect similar mineralogical absorption bands in their reflectance spectra centered in the range 0.35 to 0.92 μm; the bands are at 0.38, 0.44, and 0.67–0.71 μm. On the same asteroids, the spectral signs of simultaneous sublimation activity were found for the first time. Namely, there are maxima at ∼0.35–0.60 μm in the reflectance spectra of Adeona, Interamnia, and Nina and at ∼0.55–075 μm in the spectra of Beira. We connect this activity with small heliocentric distances of the asteroids and, consequently, with a high insolation at their surfaces. Examination of the samples of probable analogues allowed us to identify Fe³⁺ and Fe²⁺ in the material of these asteroids through the mentioned absorption bands. For analogues, we took powdered samples of carbonaceous chondrites Orgueil (CI), Mighei (CM2), Murchison (CM2), and Boriskino (CM2), as well as hydrosilicates of the serpentine group. Laboratory spectral reflectance study of the samples of low-iron Mg serpentines (<2 wt % FeO) showed that the equivalent width of the absorption band centered at 0.44–0.46 μm strongly correlates with the content of Fe³⁺ in octahedral and tetrahedral coordinations. Our conclusion is that this absorption band can be used as a qualitative indicator of Fe³⁺ in the surface matter of asteroids and other solid celestial bodies. The comparison of the listed analog samples and the asteroids by parameters of the spectral features suggests that the silicate component of the asteroids' surface material is a mixture of hydrated and oxidized compounds, including oxides and hydroxides of bivalent and trivalent iron and carbonaceous-chondritic material. At the same time, the sublimation activity of Adeona, Interamnia, Nina, and Beira at high surface temperatures points to a substantial content of water ice in their material. This contradicts the previously existing notions on the C-type and similar asteroids as bodies containing water only in the bound state. Moreover, since the sublimation process simultaneously occurs in four primitive-type bodies at small heliocentric distances, we may suppose that this phenomenon is common for the main-belt asteroids.