One of the strangest features of Halley's comet discovered by the probes is the shape of the nucleus. Much longer than the approximately spherical object that had been theorized, it was really an irregular body which some scientists have compared to a potato, others to a peanut and still others to a banana. The probe on-instruments showed that it is about 15 kilometers long and no more than 8 kilometers wide. But the most surprising detail is that it is not a "dirty snowball", even though it has a very special shape, like astronomers believed, but rather a very dark object, which could be compared to black velvet, reflecting only 4% of the sunlight. This means that the ice of which it is formed is completely covered with a layer of non-volatile material, probably a carbon-like substance similar to what forms the rings of Uranus and covers the dark hemisphere of Iapetus, the eleventh moon of Saturn. If this layer, which acts a little bit like a protective shell, were not there, the nucleus would lose gas and dust to a much greater extent with each passage close to the Sun and comets would certainly have a much shorter life span. But the close-ups also showed other unexpected features, which cannot be observed from the Earth because of the luminosity of the coma. One of these is the many hollowed out structures similar to craters, which seem to be site of gaseous flows that are smaller than the two larger jets of dust and gas exhibited in a spectacular manner by the false-color pictures by Giotto. These ring-like structures, rather than the craters, might instead be the almost completely sealed openings of the ancient main jets which were active during previous passages near the Sun. And here is one of the mysteries destined to keep researchers busy for a long time with the data transmitted by the probes. First of all, the strange radio emissions captured by Sakigake at a distance of 10 000 000 kilometers and which got stronger as the Japanese probe got closer to the comet, a sign that they must be closely related: the mechanism is not yet clearly understood, but some scientists believe that they are the product of the interaction of the solar wind with the coma particles. Then this is another very important question relative to the composition of the comet's nucleus which, due to the large amount of data to be studied, will remain unanswered for a long time. For now, the probes have measured a large amount of hydrogen, carbon, nitrogen and oxygen in the dust of the tail, elements which would strengthen the hypothesis of the presence of organic substances in the nucleus, in particular in the mysterious dark coating that was found to be a very typical feature of the outer bodies of the solar system. If this means that life is "riding" around in the space, frozen in its fundamental "building blocks" on board a comet, as some astronomers state, it is still too early to say for certain. And perhaps other missions will be needed to try to "land" on the nucleus of other comets; but some clues are available and certainly molecules of life may be found even farther away, scattered among the interstellar clouds which will one day be the birthplace of other suns and other planets.
One of the strangest features of Halley's comet discovered by the probes is the shape of the nucleus. Much longer than the approximately spherical object that had been theorized, it was really an irregular body which some scientists have compared to a potato, others to a peanut and still others to a banana. The probe on-instruments showed that it is about 15 kilometers long and no more than 8 kilometers wide. But the most surprising detail is that it is not a "dirty snowball", even though it has a very special shape, like astronomers believed, but rather a very dark object, which could be compared to black velvet, reflecting only 4% of the sunlight. This means that the ice of which it is formed is completely covered with a layer of non-volatile material, probably a carbon-like substance similar to what forms the rings of Uranus and covers the dark hemisphere of Iapetus, the eleventh moon of Saturn. If this layer, which acts a little bit like a protective shell, were not there, the nucleus would lose gas and dust to a much greater extent with each passage close to the Sun and comets would certainly have a much shorter life span. But the close-ups also showed other unexpected features, which cannot be observed from the Earth because of the luminosity of the coma. One of these is the many hollowed out structures similar to craters, which seem to be site of gaseous flows that are smaller than the two larger jets of dust and gas exhibited in a spectacular manner by the false-color pictures by Giotto. These ring-like structures, rather than the craters, might instead be the almost completely sealed openings of the ancient main jets which were active during previous passages near the Sun. And here is one of the mysteries destined to keep researchers busy for a long time with the data transmitted by the probes. First of all, the strange radio emissions captured by Sakigake at a distance of 10 000 000 kilometers and which got stronger as the Japanese probe got closer to the comet, a sign that they must be closely related: the mechanism is not yet clearly understood, but some scientists believe that they are the product of the interaction of the solar wind with the coma particles. Then this is another very important question relative to the composition of the comet's nucleus which, due to the large amount of data to be studied, will remain unanswered for a long time. For now, the probes have measured a large amount of hydrogen, carbon, nitrogen and oxygen in the dust of the tail, elements which would strengthen the hypothesis of the presence of organic substances in the nucleus, in particular in the mysterious dark coating that was found to be a very typical feature of the outer bodies of the solar system. If this means that life is "riding" around in the space, frozen in its fundamental "building blocks" on board a comet, as some astronomers state, it is still too early to say for certain. And perhaps other missions will be needed to try to "land" on the nucleus of other comets; but some clues are available and certainly molecules of life may be found even farther away, scattered among the interstellar clouds which will one day be the birthplace of other suns and other planets.
