Sikhote-Alin: The Heart of an Asteroid

The Event
Nothing could have prepared the residents of far eastern Siberia in the Sikhote-Alin Mountains for what they were going to see and experience that day. It was 10:38 am on a clear frosty morning when the huge iron meteoroid first penetrated the atmosphere at an altitude of some 60 miles above the Earth from the Northeast and at an incline of about 41 degrees. The main mass was later estimated to weigh approximately three hundred tons. It hurtled towards earth at a cosmic velocity estimated to be 30,000 miles per hour or more. The momentum for this meteoroid was simply unimaginably immense (momentum being the product of its mass times its velocity).

Sharp eyes on the ground might have picked up the beginning of the fire in the sky as the tumbling mass tore through the atmosphere and became incandescent at a temperature of about 1500 degrees C. Moments later, the superheated air surrounding the meteoroid became ionized (in which case, the atmospheric molecules surrounding the object lost their electrons and then almost immediately regained their lost electrons). At this point, the object became a true bolide, a fireball many times the size of the object itself competing with the sun in brightness to observers on the ground. So bright was the object that it cast its own shadows that eerily and soundlessly moved across the ground as the mass progressed on its downward path through the chilly morning sky.

The intense heat at the leading edge of the object ablated or melted the outer surface of the mass and the tiny metallic droplets the size of dust motes sloughed off and cooled into microscopic bits of metallic smoke that formed a huge roiling and churning smoke train that survived in the sky for most of the remainder of the day. The process of ablation is destructive to the main mass but simultaneously serves the purpose of preserving it by allowing the building heat to pass away along with the melting surface. It was later estimated that the total weight of the meteoric material in the resulting smoke train was around two hundred tons and thus consisted of around two thirds of the original mass of the object. It was estimated to be as much as twenty miles long.

The huge meteoroid continued on its path until, at an altitude of just under 20,000 feet, it exploded catastrophically into countless pieces weighing as little as a fraction of a gram up to just under two tons (the largest single individual known to have survived). Due to its hypervelocity, the atmospheric pressure on the front of the object had risen to an unimaginable level of thousands of pounds per square inch while a virtual vacuum trailed behind it. Even a mighty solid ball of iron such as this could not resist these huge opposing forces and the explosion was the inevitable result. The concussive shock wave of this explosion broke windows and rattled buildings as much as 100 miles away from the event.

Everything up to this point, from initial entry into the atmosphere to the explosion about three and a half miles above the Earth’s surface, had taken place in a matter of just a few seconds and, with the exception of those directly in the vicinity of the explosion (most likely foresters), it had taken place without a sound — at least during the time it could be seen. For most observers, the lightshow itself was completely soundless. Several seconds or even minutes passed before the sounds of the explosion and sonic booms traveling at (a relatively much more sedate) 1,130 feet per second finally arrived.

At the point of the explosion (also called the retardation point), the cosmic velocity essentially reached zero and the many pieces fell to Earth at terminal velocity under the force of Earth’s gravity. However, this was notwithstanding the acceleration of the various pieces created by the explosion itself. The distribution ellipse was later measured at approximately 1.2 miles long (the Major Axis) by 0.6 miles wide (the Minor Axis) and, as such, is one of the smallest known strewn fields for an event of this magnitude.

The explosion of the main mass took place over an uninhabited subarctic forest called a taiga within the Sikhote-Alin Mountains, and it would have been witnessed only by foresters working in the area who were likely instrumental in helping to estimate the altitude of the point of explosion. Because of the remoteness, the actual impact point of the various masses was not discovered until the next day when it was spotted by two pilots who had seen the event itself. It still took several more days for an expedition to be mounted and finally discover the impact area in the thickets and brambles of the forest.

What they found were three types of uniquely shaped meteorites that were so different from each other that they may as well have come from different falls.

The Meteorites
The members of the expedition(s) found a total of 122 craters and penetration holes during their survey. The largest crater was some 87 feet in diameter and almost 20 feet deep. It soon became apparent that none of the craters had been formed by explosive forces but rather solely by the physical impact and shattering of the meteorites themselves. Craters formed in this manner are called penetration holes. The members of the expedition team soon learned that there was an inverse relationship between the size of the craters and the size of the meteorites in them. The largest craters held the smallest fragments and the larger unbroken meteorites were in the smaller craters. This was counter-intuitive to what they had expected.

Shrapnel
The larger craters were naturally the first to be investigated and contained large amounts of fragmented meteoritic material that looked like it had been through a huge shredder. It was very similar to the twisted and torn metal remnants from an exploding bomb. This was shrapnel. Most meteorites get their shape from forces created by heat. Shrapnel is the exception in that it is shaped without heat as a major factor. It became evident that large masses of meteoritic material cast off by the explosion had smashed into the frozen earth with massive and violent force causing these larger masses to splinter and shred into thousands of smaller pieces. They had even dug deep below the frozen earth into the bedrock. It’s hard to imagine the kind of energy involved in tearing cold metal apart such as occurred here. The kinetic energy of an object is half its mass times its velocity squared. So, a three hundred ton iron meteoroid traveling at several hundred miles per hour would smash into the frozen hard ground with immense force and the iron itself would shatter like glass with the release of this kinetic energy. This is what happened.

Larger pieces of shrapnel from the Sikhote-Alin fall is often curved and convex on the “outside” and somewhat concave on the reverse, often with a large protruding bump. The convex side usually has flow lines and the edges are sharp and ragged and generally curled inward (Figure 1). Shrapnel almost never has a fusion crust confirming that it is not formed by aerodynamic forces in the same way as the other types of meteorites. Due to the density of the surrounding forest, many pieces of the shattered shrapnel tore into the vegetation. In fact, many pieces of the shrapnel were found imbedded in tree trunks and branches.

Sikhote-Alin

Figure 1

Sculptural
It is easy to imagine that the main mass was most likely rotating rapidly as it approached the retardation point where it exploded. After all, whatever event sent it on its collision course with Earth was not a gentle one! Globules of molten metal likely sloughed off the main mass during its approach and similar globs were most likely cast off by the explosion itself at the retardation point. These globs of molten metal were then shaped into exotic smooth shapes by aerodynamic forces as they cooled and became sculptural types of meteorites. These neither have the pitted regmaglypts of individuals nor the ragged torn appearance of shrapnel but are often satin smooth both in form as well as in touch. In addition, they tend to have a lighter very glossy fusion crust (Figure 2).

Sikhote-Alin

Figure 2

Sikhote-Alin

Figure 3

These sculpted meteorites also are most likely to manifest indications of orientation such as nose cones, flow lines or rollover lips. (Figure 3) Orientation is the result of a meteorite stabilizing itself in flight so that it presents a constant face in the direction of flight – it therefore becomes oriented during its fall (as opposed to tumbling). Sculptural meteorites from Sikhote-Alin tend to be somewhat limited in size generally weighing less than 500 grams. Fortunately, the two feet of snow on the surface of the fall area at the time of the fall tended to cushion the impact of these smaller objects and preserved their delicate oriented features.

Regmaglypted Individual
There are few meteorites that are more aesthetically beautiful than a Sikhote-Alin regmaglypted individual with their gun metal blue-gray fusion crust and they have given rise to the phrase “the Queen of Meteorites.” Regmaglypt is the scientific word for what are often called “thumbprints” on the surface of meteorites because they look like thumbprints pressed into soft clay. This characteristic is what the public expects to see on a meteorite probably because so many meteorites shown in museums are heavily regmaglypted (Figure 4).

Sikhote-Alin

Figure 4

Regmaglypts are formed during the process of ablation. Ablation is the heating and melting of the surface of the meteorite as it passes through the atmosphere. This melted material passes to the rear of the mass and is cast off cooling into the millions of tiny metallic dust particles that make up the smoke train. However, there are certain minerals in the meteorite that have a lower melting temperature than the surrounding material and thus these areas melt first in the ablation process leaving a surrounding ridge of material around each pit with a higher melting point. Troilite is an iron sulfide common to meteorites and is usually uniformly dispersed through the meteorite as tiny spherical nodules. Troilite has a lower melt point and is likely responsible for much of the regmaglypted formation.

A unique characteristic of regmaglypts is that their size is proportional to the size of the specimen itself. A meteorite weighing approximately ten pounds or so will have regmaglypts that are actually thumbprint sized. A specimen weighing only a few grams may have a full regmaglypted structure bit with much smaller “pits.” (Figure 5) Occasionally, regmaglypts will form on a larger mass and it will then fragment and the resulting smaller pieces will form a new fusion crust. These smaller pieces will then have disproportionately larger regmaglypts acquired from the larger original piece.

Sikhote-Alin

Figure 5

Regmaglypted individuals sometimes show cracks or fissure where they were beginning to break apart into smaller pieces but cooled before this took place (Figure 6). Other specimens exhibit what can only be interpreted as small impact craters which were the result of impacts with other pieces during the last moments of descent (Figure 7).

Sikhote Alin

Figure 6

Sikhote Alin

Figure 7

While some 200 tons of the original main mass were removed through the process of ablation and migrated to the smoke train, it was originally estimated that more than 70 tons reached the retardation point and subsequently fell to Earth. More contemporary estimates put the figure close to 100 tons of which possibly 30 tons have been recovered so far. It was not until the third expedition in 1950 and 1951 that the largest single individual was recovered. It weighed approximately 3,840 pounds and was of the regmaglypted form. Interestingly and true to form for this fall, it was found buried in a penetration crater that was less than 12 feet in diameter.

More than 300 reliable witnesses were found and interviewed during the investigation process. From their reports, the path of the meteorite was determined rather accurately and the orbit was then reverse extrapolated so that it was determined that at its apehelion (the point in the orbit where it was the furthest from the sun) its orbit and therefore the origin had been in the asteroid belt. However, some force such as a catastrophic collision with another asteroid or even the force of Jupiter’s gravity had upset the orbit of the meteoroid so that at its perihelion (the point in the orbit where it was the closest to the sun) it passed through to the inside of Earth’s orbit and, as such, it became an NEO or Near Earth Object. The collision with Earth was inevitable – it was just a question of when.

We have learned that given enough mass, asteroids go through a process called differentiation in which the heavier components migrate to the center of the asteroid and the lighter components stay near the surface. Thus, an asteroid becomes like a miniature planet with a crust, mantle and heavy iron core. Later, asteroids may collide with each other catastrophically and the lighter crust and mantle may be stripped away leaving the heavy iron core by itself. The mass that formed the Sikhote-Alin fall was either a part or whole such core and it therefore was the very heart of an asteroid.

The Legacy
The Sikhote-Alin fall of 1947 was the most witnessed and well-documented iron meteorite fall in modern history. Virtually all iron meteorites falls that resulted in the formation of a crater took place during prehistoric times and time has taken its toll on these sites through erosion, oxidation, etc. Even Meteor Crater in Winslow Arizona is considered to be a “recent” fall and that occurred some 50,000 year ago. Now, here was an opportunity to study and learn about a pristine fall site unaffected by terrestrial weathering, site deterioration and other such distracting factors. This was unprecedented in the annals of Meteoritic Science.

A Footnote
This story cannot be told without noting that an artist in the village of Iman by the name of P.I. Medvedev was in the process of setting up his canvas and easel to paint a landscape on that fateful morning. He was facing East at the time and witnessed the entire fall of Sikhote-Alin from the time it appeared as a bolide in the sky in the North to the time it disappeared into the namesake Sikhote-Alin Mountains. He would have been somewhat less than 100 miles from the point of impact. His trained artist’s eye took in a great deal of detail and he immediately began to paint what he had seen. He thus created a magnificent visual record of what was to be the most witnessed fall in modern history. His painting (Figure Eight) is on permanent display at the Mineralogical Museum in Moscow and a commemorative stamp (Figure Nine) using his painting was issued on the tenth anniversary of the fall.

Sikhote Aline

Figure Eight

Sikhote Alin

Figure Nine

References:
O. Richard Norton Rocks From Space Second Edition 1998
O. Richard Norton & Lawrence A. Chitwood Field Guide to Meteors and Meteorites 2008