Asteroids are through and through cold, lifeless bodies. In the distant past, their bowels could be warm and even hot due to radioactive or some other heat sources. Since then, they have long cooled off. However, the internal heat never warmed the surface: the heat flux from the bowels was imperceptibly small. The surface layers remained cold, and only collisions from time to time caused a short-term local heating.
The only constant source of heat for asteroids remains the Sun, distant and therefore heating is very bad. A heated asteroid emits thermal energy into outer space, and the more intense it is, the stronger it is heated. Losses are covered by the absorbed part of the solar energy incident on the asteroid.
If we average the temperature over the entire illuminated surface, we get that for asteroids of a spherical shape, the average temperature of the illuminated surface is 1.2 times lower than the temperature at the sunflower point.
Due to the rotation of asteroids, their surface temperature changes rapidly. The surface areas heated by the Sun quickly cool down due to the low heat capacity and low thermal conductivity of the substance composing them. As a result, a thermal wave runs along the surface of the asteroid. It quickly fades with depth, without penetrating even a few tens of centimeters deep. Deeper, the temperature of the substance is almost constant, the same as in the bowels of the asteroid – several tens of degrees below the average temperature of the surface illuminated by the Sun. For bodies moving in an asteroid ring, it can roughly be taken equal to 100-150 K.
No matter how small the thermal inertia of the surface layers of the asteroid, nevertheless, to be very strict, it should be said that the temperature does not have time to take on an equilibrium value with changing lighting conditions. The morning side, not having time to warm up, is always slightly colder than it should, and the evening side is slightly warmer, not having time to cool. Relative to the sunflower point, slight asymmetry in the temperature distribution occurs.
The maximum thermal radiation of asteroids lies in the region of wavelengths of the order of 20 μm. Therefore, their infrared spectra should look like continuous radiation with an intensity that monotonically decreases in both directions from the maximum. This is confirmed by observations made by O. Hansen in the range of 8–20 μm. However, when Hansen tried to determine the temperature of asteroids based on these observations, it turned out to be higher than the calculated one (about 240K), and the reason for this is still not clear.
The low temperature of bodies moving in the asteroid ring means that diffusion in the asteroid substance is “frozen.” Atoms are not able to leave their places. Their relative position has remained unchanged for billions of years. Isolation can cause diffusion to life only for those asteroids that are very close to the Sun, but only in the surface layers and for a short time.
The composition of the asteroid substance.
Meteorites are extremely diverse, as are their parent bodies – asteroids. At the same time, their mineral composition is very poor. Meteorites are composed mainly of iron-magnesium silicates. They are present in the form of small crystals or in the form of glass, usually partially recrystallized. The other main component is nickel iron, which is a solid solution of nickel in iron, and, as in any solution, the nickel content in iron is different – from 6-7% to 30-50%. Nickel-free iron is also rare. Sometimes significant amounts of iron sulfides are present. Other minerals are in small quantities. It was possible to identify only about 150 minerals, and although even now they are discovering more and more, it is clear that the number of meteorite minerals is very small compared to their abundance in the rocks of the Earth, where more than 1000 are detected. This indicates a primitive, undeveloped meteorite character substances. Many minerals are not present in all meteorites, but only in some of them.
The most common among meteorites are chondrites. These are stone meteorites from light gray to very dark in color with an amazing structure: they contain rounded grains – chondras, sometimes clearly visible on the surface of the fault and easily tinted from the meteorite. The sizes of the chondras vary from microscopic to centimeter. They occupy a significant amount of meteorite, sometimes up to half of it, and are weakly cemented by interchondral matter – the matrix. The composition of the matrix is usually identical with the composition of the chondras, and sometimes differs from it. There are many hypotheses about the origin of chondras, but they are all debatable.