THE SIKHOTE-ALIN METEORITE AND ITS SIGNIFICANCE FOR THE PROBLEM OF THE ORIGIN AND EVOLUTION OF THE SOLAR SYSTEM

V. G. Fesenkov

On February 12, 1947, an enormous iron shower fell in the coastal taiga, at a distance of about 150 km from the nearest railway station, Iman, midway between Vladivostok and Khabarovsk. This unusual phenomenon was investigated by an expedition of the Academy of Sciences of the USSR, which set out already in the following month, immediately after news of the fall had been received. Numerous witnesses in several dozen populated localities, who had been direct eyewitnesses of the meteorite’s flight through the atmosphere, were questioned, and the position of its radiant was established with complete certainty—that is, the direction of its motion in space at its encounter with the Earth, and the corresponding moment of time.

At the very site of the fall, in the spurs of the Sikhote-Alin range amid the dense coastal taiga, more than one hundred craters of various sizes were found, produced by the impacts of meteoritic masses on the rocky ground, composed mostly of inclined beds of gray porphyry; and numerous traces were discovered that characterize the dynamics of the phenomenon.

Although the large masses that produced craters by their fall had evidently fallen several seconds earlier and, consequently, possessed a greater velocity at the moment of impact than the small individual meteorites that sprinkled an area of several square kilometers like an iron shower, nevertheless the direction of their motion on striking the Earth’s surface was approximately the same—all the masses fell from the north, almost exactly in the meridian, at an angle of 60° to the horizon.

To determine the meteorite’s orbit in space it is necessary to know, in addition to the position of the radiant point, also the velocity of its encounter with the Earth. This velocity, however, cannot be determined with sufficient accuracy from the eyewitness accounts and therefore must be found by indirect but sufficiently reliable methods.

V. G. FESENKOV

The velocity of a meteorite’s motion in the atmosphere is connected with the overall curvature of its trajectory between the point of appearance, when this body had just appeared in the form of an incandescent sphere of dazzling brightness, and the point of impact on the Earth’s surface. Observations show that this curvature reached approximately \(30^\circ\). However, the picture is greatly complicated by the circumstance that the meteorite boiled intensely in the atmosphere, leaving a large part of its mass in the form of gases and fine dust and, in the end, broke up into a multitude of large and small fragments that fell to the ground. Determining the velocity of the meteorite in space, and consequently the elements of its orbit, depends also on knowledge of its initial mass and, to a much greater degree, on knowledge of the ratio of the initial mass to the final mass, which, upon falling, produced all the discovered destruction in the taiga and can be directly investigated.

To solve the latter problem it was necessary to make use of the results of all investigations carried out from 1947 up to the present time. After the first expedition mentioned above, which was of a general reconnaissance character, other expeditions followed, organized by the Committee on Meteorites of the Academy of Sciences of the USSR with the assistance of various organizations and institutions. These expeditions had at their disposal all possible modern means of investigation, varied equipment, a sufficiently suitable labor force, were outfitted annually, and each continued for a number of months.

A detailed aerial photographic survey of the fall site was made, as well as a topographic survey; several motion-picture films were shot, including from the air; the entire region of the fall of the iron masses was swept with mine detectors; magnetic surveys were carried out inside and outside the craters; and, in addition, several dozen small- and medium-sized craters were excavated down to undisturbed bedrock. Such a scale of work proved possible only thanks to substantial assistance from various organizations.

The most characteristic craters have been left untouched and protective pavilion-gazebos have been erected around them, with the aim of preserving this rare natural phenomenon for the future.

The first three expeditions of 1947–1949 collected and delivered to Moscow about 26 tons of meteoritic material. The results of the work of the last expedition, in 1950, have not yet been fully summarized, but it too collected eleven tons of meteoritic substance.

It may be supposed that the individual meteorites that acquired a fusion crust already in the air and are distinguished by their violet hue have been collected almost completely, although they covered the greatest area of several square kilometers. The situation is much more complicated with the meteorites that formed craters,

since such meteorites were completely shattered upon impact with the rock formations. Large meteorite blocks that were only partially broken up were found only in small craters and pits. The largest such block, weighing two tons, was discovered in a small pit located in the northern part of the crater field, in soft soil. The meteorite block pierced the surface layer and became lodged at a depth of 4 meters. A special cameraman was assigned to record its extraction and transport.

In the large craters the meteoritic material proved to have been fragmented into a multitude of pieces, ranging from fragments of several hundred kilograms down to the finest scales that saturated the soil. As a result, no significant masses of meteoritic material were found in the largest craters. The iron particles that abounded in the soil (they could be extracted with a magnet) had, over the several years that had passed since the fall, almost completely oxidized and in fact disappeared. This considerably complicates the assessment of the total meteoritic mass that fell to the Earth, especially since the largest craters have still not been fully excavated. However, the profiles of these craters and the deposits of ejecta material beyond their limits have been measured by our expeditions. In a number of cases, from marks on the surviving tree trunks, it was possible to judge the angle at which material was ejected from the craters and, on this basis, to estimate the energy expended in their formation. In addition, it is also possible to determine the approximate mass of the meteorite that went into forming a crater, since the velocity of fall of the meteoritic masses could not have exceeded 400–500 meters per second, but was hardly less than this value. Thus, for example, according to such calculations, the largest crater, No. 1, 28 meters in diameter, from which about 5,000 tons of clay and rock were thrown out in all directions within a few moments, must have formed when approximately 20–30 tons of meteoritic material fell into it. Taking into account the total energy expended in the formation of the crater field as a whole, all the meteoritic material actually collected, and the results of excavations of several dozen small, medium-sized, and partly large craters, it may be considered that the total mass of the meteorite that actually fell to the Earth was about 100–150 tons.

Nevertheless, this mass undoubtedly constituted only a small part of the original meteoritic body that encountered the Earth on February 12, 1947. Even a simple examination of individual meteorites that broke away from the main mass at a comparatively low altitude above the Earth’s surface reveals deep depressions caused by air eddies in front of the flying masses, which in a number of

cases, internal structural features. This indicates that even along the final segment of the meteorite’s trajectory there occurred a considerable loss of its mass.

From the observations of many eyewitnesses it was possible to establish the volume of the smoke trail left after the meteorite, which remained in the air for several hours and was so dense that the Sun either did not shine through it at all or appeared as a red disk of insignificant brightness. The total mass of the fine iron particles that formed this trail must, as our calculations show, have been at least 200 tons. In addition, a significant amount of matter in a semi-molten state was blown from the surface of the flying masses in the form of droplets of larger size, as shown by the investigations of E. L. Krinov. However, the greatest loss of mass occurred in the higher layers of the atmosphere as a consequence of the greater velocity of the meteorite’s motion and, correspondingly, the greater work of the resistance force. Let us note that the linear dimensions of the head of the Sikhote-Alin bolide reached 600 meters, and the effective temperature of this head, consisting of meteoritic masses mixed with gases, was of the order of 6000° (the head of the flying bolide did not differ in color from the Sun and by most eyewitnesses was compared with the color of electric welding). Taking this into account, it can be calculated that, for a flight duration even of several seconds, the initial mass of the meteorite must have amounted to several thousand tons under all possible assumptions concerning the velocity of its encounter with the Earth. In any case, all these estimates show that only a small fraction of the initial mass, within the limits of 5 to 20%, should have fallen to the terrestrial surface.

The final determination of the initial mass of the meteorite was carried out by us by successive approximations, together with the determination of the initial velocity of its encounter with the Earth. For this purpose it was necessary to construct a general theory of the motion of cosmic bodies in the Earth’s atmosphere at super-sound velocity, when the entering cosmic body experiences enormous resistance to its motion, becomes incandescent, melts on the surface, and continuously expends its mass. In addition, it was necessary to connect this theory with the general phenomena of the motion in the atmosphere of meteoric bodies already sufficiently studied.

It turned out that in fact several thousand tons entered the atmosphere, of which about 10% was able to reach the Earth’s surface at the angle indicated above. The velocity of encounter with the Earth proved to be quite definite, namely, lying within the limits of 14–15 km/sec. Having found the velocity of the meteorite at the moment of its encounter with the Earth, it was possible to proceed to determining its orbit in space.

As is known, every orbit—its dimensions, shape, the position of its plane, the orientation of the orbit itself in its plane, and the position of the body in the orbit—is determined by 6 parameters, the so-called orbital elements. To find these elements, which are unknown quantities, one must know six independent initial data, namely: 1) the position of the meteorite at a given instant in space, determined by three coordinates; 2) the direction of its motion, determined by two angles—the coordinates of the radiant; and 3) the magnitude of the velocity of the motion itself. The position of the meteorite in space at the moment of collision with the Earth is simply the position of the Earth itself at that moment and, consequently, is well known, provided only that the moment of collision itself has been determined, as we succeeded in doing. The direction of the meteorite, i.e., the position of that point on the celestial sphere from which it flew out—the point of radiation—was determined quite well from the testimony of numerous eyewitnesses in many populated localities, and this supplies two more independent parameters. The velocity of encounter of the meteorite with the Earth is the last necessary parameter, but it presents the greatest difficulties. As a result, we first made various trial determinations of the orbit under different assumptions regarding its velocity. Naturally, different sizes and orientations of the orbit are obtained in this way. Nevertheless, for all possible assumed velocities it turned out that the meteorite encountered the Earth near the moment of its passage through perihelion, i.e., near the point of its orbit closest to the Sun.

Thus, even without knowing the velocity, one can draw the undoubted conclusion that the meteorite had never approached the Sun appreciably closer than the Earth and therefore could not have been subjected to any heating that might have affected its structure. It is therefore quite possible that its structure is primary and reflects the very conditions of its formation.

Knowing the velocity, one can immediately determine the orbit by the usual methods of theoretical astronomy. It turned out that the plane of the orbit of the Sikhote-Alin meteorite made an angle of only \(9^\circ.4\) with the plane of the Earth’s orbit, and that the semimajor axis of its orbit exceeded the radius of the Earth’s orbit by only a factor of 2. If the meteorite’s orbit is plotted on the scale of the nearest planets of the solar system, then we clearly see that it came from the asteroid zone—the belt of minor planets, which revolve in great numbers between the orbits of Mars and Jupiter. Such, consequently, is the place in the solar system from which this meteorite came—the asteroid belt. In essence this body was a typical small asteroid and could have been observed as a separate body approaching the Earth while still at the distance of the lunar orbit.

In recent years, with the development of means of observation, an ever greater number of very small bodies—true asteroids—have been discovered, passing almost in the immediate vicinity of the Earth’s orbit and therefore also capable of colliding with the Earth. Some of them have even smaller orbits, penetrating still farther into the depths of the orbits of the planets nearest the Sun than the Sikhote-Alin meteorite. Thus, for example, in 1949 an asteroid was discovered that received the provisional designation MA 1949, and later the definitive name Icarus. This asteroid approaches the Sun to a minimum distance of 34 million km, i.e. considerably closer than Mercury and five times closer than the Earth, and each time it is heated thereby to the temperature of red heat, approximately to 700° abs.

The number of known similar small asteroids, closely approaching the Earth’s orbit, increases with each year. Thus, the Sikhote-Alin meteorite is essentially the first asteroid that can be studied directly in our laboratories. As has already been pointed out, there is no reason to think that its substance underwent recrystallization in interplanetary space under the action of solar heating.

What, then, are the results of its laboratory investigation?

In the past years the Sikhote-Alin meteorite has been subjected to a many-sided investigation. Academician Zavaritskii, Corresponding Member of the Academy of Sciences of the USSR Vinogradov, Senior Research Associate of the Institute of Geochemistry Trofimov, Corresponding Member of the Academy of Sciences of the Ukrainian SSR Burkser, Scientific Secretary of the Committee on Meteorites Krinov, Senior Research Associate of the Institute of Astronomy and Physics of the Academy of Sciences of the Kazakh SSR Yavnel, and others investigated its chemical composition, structure, age (by the helium method), the ratios of iron isotopes, changes during flight in the atmosphere, and other features. Thus, for example, it has been shown that iron is represented in this meteorite by the same isotopes and in the same ratio as on the Earth; that the meteoritic masses consist not of a single crystal, as was the case with Boguslavka (a meteorite of 1916, also fallen in the Far East), but of a multitude of individual crystals of iron-nickel, irregularly oriented with respect to one another and therefore poorly bound together. In addition, the meteorite contains numerous foreign inclusions—compounds of iron-nickel with phosphorus and sulfur—schreibersite and troilite, which further contribute to the brittleness of the meteoritic substance. Therefore a sufficiently strong impact of the meteoritic mass upon falling was enough for this mass to fly apart into small pieces, carrying terrestrial rocks with it in all directions. This accounts for the great destructive action of the meteorite and its ability to form craters. It is interesting, moreover, that polished sections of the meteorite show Neumann lines intersecting at right angles, so that mi

the structure of the meteorite is hexagonal: the crystallization planes intersect at right angles. On the other hand, its macrostructure turns out to be octahedral—eight-faced. This is clearly visible in a number of specimens showing the arrangement of beams intersecting at angles of 60°.

It is interesting that all specimens of this meteorite belonging to the much larger original mass have one and the same composition and structure. Whatever piece of the meteorite one takes, it contains 94% iron, 5.4% nickel, 0.16% cobalt, and negligible admixtures of other elements in the very same proportion, within the limits of error of determination.

A senior research associate of the Institute of Astronomy and Physics of the Academy of Sciences of the Kazakh SSR, A. A. Yavnel, also investigated the content in the Sikhote-Alin meteorite of a number of admixtures of noble metals—ruthenium, rhodium, palladium, silver, platinum, and gold. This content, amounting to several grams per ton, varies somewhat in different meteorites that fell in different places and at different times, but in all specimens of the Sikhote-Alin meteorite examined by Yavnel it proves to be one and the same. This circumstance is extremely indicative. The uniformity of the structure, of the chemical composition, and the constancy of the admixtures show that this meteorite must have formed by crystallization within a sufficiently large molten mass, in which migration of the most diverse elements and their combination according to their atomic affinity was possible. Such enrichment of some elements by others is observed in many terrestrial ores, is well known to geochemists, and is explained, as Goldschmidt and others pointed out, by the same causes.

Thus, the Sikhote-Alin fall clearly confirms the opinion expressed several years ago by Academician Zavaritsky, our greatest authority on the question of the structure of meteorites, that meteorites could have formed inside a sufficiently large body, perhaps of planetary dimensions, and at high temperature. This is also indicated by Yavnel’s investigation, in which he observed the diffusion of phosphides and, from its degree, drew a conclusion about the temperature at which it could have taken place.

The Sikhote-Alin meteorite is also a typical representative of meteorites in general, differing from them only by its enormous dimensions. It may therefore be supposed that other meteorites too are related to asteroids and had the same origin as they did. It has long been assumed that the average composition of meteorites corresponds very closely to the average composition of the Earth and, in general, of a terrestrial-type planet such as Venus, Mars, and Mercury. Iron meteorites, like the Sikhote-Alin meteorite, correspond to the purely iron composition of the presumed terrestrial core; stony-iron—

to the intermediate layers of the terrestrial globe, and the purely stony ones to the more external layers. Finally, rare meteorites, like the Mighei or the carbonaceous meteorite Staroe Boriskino, with a considerable content of water in a bound state, correspond, according to Academician Zavaritsky, to the crust of the parent planet.

Let us dwell briefly on the characteristic properties of asteroids.

As of January 1, 1950, 1565 such bodies with determined orbits were known in all. The more massive and bright asteroids have already been fully discovered. This is evident from the fact that all newly discovered bodies of this kind are very faint and accessible only to large instruments. At the same time, newly discovered asteroids hardly increase the total mass of these bodies, which, as may be supposed, does not exceed \(1/400\) of the mass of the Earth. All asteroids are devoid of any atmospheres—they are too small for this; but in those cases where one can judge their diameters, they have quite varied reflecting power. It may therefore be assumed that, like meteorites, they also have differing compositions.

Further, it may be considered established that small asteroids do not have a round shape and are simply irregular fragments. As a consequence of this, as a rule, small asteroids change their brightness, sometimes in a very noticeable manner, namely by several stellar magnitudes. In some cases one can directly see the irregular shape of these bodies. Thus, for example, the well-known asteroid Eros, which approaches the Earth very closely and for this reason serves for determining the linear scale of the entire solar system, turns out to have the appearance of an irregular bar: its length, \(10\)—\(12\) km, exceeds its width by at least a factor of two. The very small asteroids are all the more simply irregular fragments. It has further been established that, although these bodies have been subjected for hundreds of millions or even billions of years to the perturbing action of the planets of the solar system, they have nevertheless to this day preserved traces of some mutual connection. Already Newcomb, at the end of the 19th century, discovered interesting relations in the spatial arrangement of the major axes of their orbits, and these relations at the present time, with much greater material, not only have not been smoothed out but, on the contrary, have appeared with still greater definiteness. Hirayama and then N. M. Stau[[unclear: final letters of surname]] showed that about \(20\%\) of the asteroids are united, by the properties of their orbits, into definite families. There exist, moreover, very close groups of asteroids united by common properties. Thus, for example, there is a remarkable group of the so-called Trojans—asteroids which are simultaneously satellites of the Sun and, at the same time, of Jupiter. This is possible because they are always located at an equal

at distances from both bodies, namely at the vertices of equilateral triangles formed with the Sun and Jupiter. It seems obvious that the asteroids could not have formed each separately. All their properties indicate that they were formed by the disintegration into parts of a much larger body.

Thus, from the example of asteroids, meteorites, and especially clearly from the example of the Sikhote-Alin meteorite, it is evident that in the solar system there once occurred what would seem to be an incredible event—the breakup into parts of a planet that had previously existed. The Sikhote-Alin meteorite clearly shows that it comes from the region of the asteroidal ring, so that it is there that the mysterious planet must be sought. One must think that the asteroids themselves arose through the disintegration of such a planet and represent only its larger fragments in comparison with meteorites.

Before we dwell on this problem, let us point out that in the solar system there also exist other small bodies with an insignificantly small mass, whose origin until recently essentially remained without any explanation. What, for example, are comets? We know that their masses are insignificantly small, but they are saturated with gases. These gases, as a result of solar heating, are released at each approach of the comet to the Sun. Therefore each comet can exist as such only for a limited number of its revolutions around the Sun. All comets may therefore be considered young bodies. At the same time they belong to the solar system and do not come to us from interstellar space. Where, then, do they come from? S. K. Vsekhsvyatskii has for about 25 years been defending the hypothesis that comets are products of eruption from the large planets—Jupiter and Saturn—or even from their large satellites. Vsekhsvyatskii is not embarrassed by the fact that the velocity of ejection from the surface of Jupiter must be at least 60 km/sec and that the planet’s dense atmosphere, several thousand kilometers thick, must offer strong resistance to such an ejection. Besides the purely physical improbability of such gigantic eruptions of millions of tons, one may bring against this hypothesis the further objection that, with such ejections oriented arbitrarily with respect to the planet’s surface, nothing similar to the actual orbits of nonperiodic comets is obtained. True typical comets move in extremely elongated orbits with periods of revolution of tens and hundreds of thousands of years. The so-called short-period comets, with short periods of revolution, always form part of planetary families, and the properties of their orbits are well explained on the assumption that they were captured by means of large perturbations on the part of the large planets. Meanwhile, the ejected products of eruptions

must in the overwhelming majority of cases move in extremely short orbits, and this is in sharp contradiction with reality. A quite similar objection may also be raised against S. V. Orlov’s hypothesis, according to which comets arise as the result of the currently occurring fragmentation of asteroids when they collide with meteorites. Further, the origin is unknown of the so-called irregular satellites of the large planets, which find no explanation for themselves in any cosmogonic hypothesis. These irregular satellites are very small, are usually situated far from the planet, and moreover are extremely disorderly and have both direct and retrograde motions.

Meanwhile, the solar system as a whole is distinguished by a great regularity in its motions.

It was already mentioned above that there exist “Trojans,” which, as 14 bodies, form equilateral triangles with Jupiter and the Sun. At present their position is quite stable, and they cannot leave it. But how could they have entered it?

It can be shown that the supposition of the unusual event indicated above—the disintegration of a planet that had formerly revolved between the orbits of Mars and Jupiter—provides an explanation not only for the origin of asteroids and meteorites, but also for comets and the irregular satellites of the large planets.

Thus, let us suppose that there once existed a planet which exploded, and that its fragments scattered without any order in different directions. Naturally, the largest fragments must have received correspondingly lower velocities and remained entirely within the limits of the asteroid belt. These are the present-day more or less large asteroids. The small asteroids must have spread through a considerably greater region in the solar system.

All these asteroids move in the direct direction. Not a single asteroid with retrograde motion is known. This shows that the force of the explosion that occurred was not very great. Even the very smallest fragments, not to mention the larger ones, acquired a velocity much smaller than the velocity of the planet itself in its orbit, i.e. of the order of 5–7 km/sec. This only slightly exceeds the velocity of modern projectiles. If the very smallest fragments, which moved fastest of all, had acquired a velocity of only about 40% of the orbital velocity of the planet itself, which was 15–17 km/sec, then they could have gone beyond the limits of the solar system to an enormous distance, for example, to tens of thousands of astronomical units.

At such enormous distances the perturbation from the nearest stars already makes itself felt. The solar system, strictly

...speaking, cannot be considered isolated in space; on the contrary, it is in various interaction with the surrounding universe. Very weak lateral perturbing forces, arising from the nearest stars, are already sufficient to deflect somewhat the ejected asteroid from its radial direction toward the Sun and to make it, on returning to the Sun, pass by the latter at a considerably greater distance.

A detailed analysis carried out by us shows that secular perturbations from the nearest stars affect the parameter of an almost parabolic orbit to a much greater degree than its major axis. As a result, the orbit of such a small fragment, ejected to a great distance, will remain within the limits of the same complex of similar bodies, but, depending on the character of the perturbations, it will at times enter and at times leave the region of visibility, limited by the nearest neighborhood of the Sun with a radius of 2–3 astronomical units.

Entry into this region of visibility, where the asteroidal body, experiencing heating by the Sun, releases the gases occluded in it, is perceived by us as the appearance of a new comet. It is clear that such comets may come to us both in the direct and in the reverse direction. Their masses may be predicted in agreement with reality on the basis of the observed dispersion in the distribution of asteroids of known sizes.

It is much more difficult to understand the problem of the origin of the Trojans and of the irregular satellites of the major planets, including the satellites of Mars, which have always been a stumbling block in every cosmogonic hypothesis. It is possible, however, apparently, to prove that under certain conditions asteroids, scattered as a result of the explosion of a planet throughout the whole space of the solar system, could have been captured by planets and formed a system of irregular satellites, sharply differing from the ordinary regular satellites, apparently organically connected with the planets. This problem is more complex and requires the introduction of an additional factor—a resisting medium. At the present time one may assert that the origin of the Trojans, and still more of the irregular satellites of the major planets, can be qualitatively explained by the fact that they are small asteroids captured by the planet near which they passed under the action of a resisting medium. The detailed theory of this phenomenon must still be refined.

If such a picture is correct, if meteorites and asteroids were indeed formed in one and the same act—the explosion of some planet—then the age of all these bodies must be the same. By the age of meteorites one should understand the interval of time elapsed from the moment of their solidification, when the elements entering into their composition lose the possibility of moving freely

and in each meteorite sample the initial and final products of radioactive decay are jointly preserved, the ratio between them changing regularly over time. Until now the age of meteorites has been determined exclusively by the helium method.

However, helium is not generally retained in stony meteorites, and even in iron meteorites it can escape into space, especially when the temperature rises and when the porosity is high, as is usually characteristic of meteorites. This produces an apparent “rejuvenation” of them. On the other hand, cosmic rays, irradiating a meteorite in interplanetary space, cause the release of additional amounts of helium through the destruction of the nuclei of various elements and thereby lead to greatly overestimated values for their age. As a result, the use of the helium method leads to an enormous scatter in age estimates, ranging from several tens of millions to 8–10 billion years. Consequently, the helium method must be rejected as fundamentally incorrect.

Recently, at the Radium Institute of the Academy of Sciences of the USSR, E. K. Gerling determined the age of two meteorites supplied for this purpose by the Meteorite Committee of the Academy of Sciences of the USSR—namely Saratov and Zhovtnevy Khutor—by an entirely new argon method. As is known, one of the isotopes of potassium, namely \(K_{40}\), by losing an electron, is transformed into argon. By finding the amount of argon in stony meteorites in relation to this isotope of potassium, one can thereby determine their corresponding age. The enormous advantage of this method is that argon, possessing much larger molecular dimensions, is well retained in meteorites and does not diffuse into space. On the other hand, its amount is not changed under the action of cosmic rays.

For both meteorites, within the limits of the accuracy of the determinations, one and the same value was found for their age, namely 3.0 billion years. If this is also confirmed for other samples, then with high probability it will be possible to assert that all meteorites have the same age, corresponding to the epoch of the breakup of the hypothetical planet.

In conclusion, we should try to answer the question: in what way could this unusual event have occurred—the breakup into parts of a planet that had formerly revolved between the orbits of Mars and Jupiter? I note that this event seems incredible to us only because we do not imagine the physical properties of the Earth, or else because we mistakenly think that the Earth, like any other planet of sufficiently great mass, does not differ in its properties from an ordinary lump of matter such as we are accustomed to dealing with at the Earth’s surface.

This is entirely incorrect. The Earth as a whole consists, of course, of the same chemical elements that exist generally in the universe and occur in one quantity or another in the Earth’s crust, but, owing to its great mass and the enormous pressure connected with this, new physical properties arise within the Earth. In the present case we see a new example of how quantity passes into quality. The Sun also consists of the same elements, among which hydrogen predominates. However, because the solar mass is of an entirely different order than the mass of the Earth itself, the Sun is also distinguished by new properties, above all by the ability to sustain atomic reactions spontaneously.

Thus, it would be unfounded to assert that planets under no circumstances can disintegrate with an explosion. It is first necessary to investigate more closely the properties of their inner layers.

There is no doubt that all chemical reactions at very great pressures, of hundreds of thousands and millions of kilograms per \(\mathrm{cm}^2\), proceed predominantly in the direction of decreasing volume. It is for this reason that metals unknown in pure form on the Earth’s surface may occur inside the Earth. When pressure decreases, the reactions must proceed in the reverse order. It is known, moreover, that heat capacity increases to a high degree with pressure. When pressure decreases, the heat capacity of any body decreases sharply, and this means that its temperature must at once rise greatly. An increase in temperature leads to the formation of superheated gases at the center of the planet and, perhaps, to an explosion.

In order for the pressure inside a planet to be able to decrease, it is sufficient for it to approach another perturbing body—for example, a planet, in the present case Jupiter—to a certain distance at which the difference in attractions exerted by the latter on the various layers of the planet could already make itself felt. There is no need to suppose any direct collision. The approach of the planet to Jupiter may occur at a minimum distance of many radii of the latter. Nevertheless, this may prove sufficient for some redistribution of pressures to take place inside the planet, for the pressure at its center to decrease and, as a consequence, for its central temperature to increase sharply and for an explosion to occur. Although these considerations are of a purely qualitative character, there is no doubt that the disintegration of a planet in such a way is quite possible. The whole question is only at what minimum distance the planet approaches Jupiter.

For such an approach to take place, the planet must move around the Sun in an eccentric orbit. The circumstance that close approaches could also have been facilitated by the fact that

that in the remote past, namely several billion years ago, the dimensions of the solar system were several times smaller than at present. At present such close approaches of ordinary planets are impossible. Let us note, however, that even Mars, a comparatively massive planet, moves in a rather eccentric orbit \((e = 0.0933)\), as a result of which its distance from the Earth may vary from 54 to 102 million kilometers. The orbit of Mercury is still more eccentric and even at first glance differs sharply from a circle. Most eccentric of all is the orbit of the planet farthest from the Sun—Pluto, which is capable of approaching the Sun even more closely than its neighbor, the massive planet Neptune. These planets do not collide with one another only because they move in different planes in space. If, however, the planes of motion of Pluto and Neptune approximately coincided with one another, then the approach of the two planets to a small distance, and even their direct collision, would be possible, with all the destructive consequences resulting from this.

We do not know where exactly the orbit of the vanished planet was located and what it was like. It is obvious, however, that this planet was in the most dangerous neighborhood possible in the solar system, namely with the most massive planet—Jupiter, whose mass exceeds the mass of the Earth by more than 300 times. In any case, the disintegration of a planet into parts under certain specific conditions cannot be regarded as impossible either from the standpoint of physics or from the standpoint of celestial mechanics.

Let us mention that, according to Ramsey’s new theory, the Earth and the planets consist throughout their entire mass of one and the same substance, with only a slight increase toward the center in its molecular weight, which, however, occurs in different phase states depending on pressure (a metallic phase with an abundance of free electrons and increased electrical conductivity in the central regions). At certain critical dimensions a planet may find itself in an unstable state and explode even without the action of an external body upon it. The possibility of this process must be carefully examined.

In any case, there is no need to do violence to common sense in order to explain such diverse phenomena as meteorites, asteroids, comets, Trojan families, and the irregular satellites of the large planets. It is precisely these phenomena that until now have essentially received no explanation. All cosmogonists have confined themselves to explaining only the most general features of the structure of the solar system—the distribution of the planets in one plane, their motion in circular orbits in a definite order and exclusively in the direct direction, and to some extent also

their rotation about their axes. However, these general structural features of the solar system can be explained by the most diverse theories and therefore cannot serve as a touchstone for what actually took place. On the contrary, the examination of the small bodies of the solar system, above all meteorites, reveals far better the true events that once occurred in the solar system and makes it possible to judge its past state.