Comets, messengers of life

Comets are icy, Solar System bodies, generally with diameters of a few kilometers (1 to 30 kms) which, when passing close to the Sun, warm and release gases, a process called outgassing.

This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.

As of July 2018 there are 6,339 known comets, a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion (1,000,000,000,000).

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. 

Comet nuclei with radii of up to 30 kilometres (19 mi) have been observed, but ascertaining their exact size is difficult.

The nucleus of the comet 322P/SOHO is probably only 100–200 metres (330–660 ft) in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330 ft) across.[27] Known comets have been estimated to have an average density of 0.6 g/cm3 (0.35 oz/cu in).[28] Because of their low mass, comet nuclei do not  become spherical under their own gravity and therefore have irregular shapes.

The objection that life can't survive in space needs examination. A serious problem for Svante Arrhenius's theory in 1908 was that spores in empty space would be subject to radiation damage, especially in the vicinity of a star. In 1978, Hoyle and Wickramasinghe observe in Lifecloud that if a cloud of bacterial matter were dense enough, the inner contents would be protected from radiation by the outer layers. Other scientists have recently observed that a coating of dust only half a micrometer thick would adequately protect a bacterium from ultraviolet radiation in space.

Hoyle and Wickramasinghe also discuss another means of space travel which solves the radiation problem: comets. And even before that danger was known, the idea that comets could contribute to life on Earth was afoot. Among others, Isaac Newton endorsed it. "Newton considered the continual arrival of cometary material to be essential for life on Earth".

Comets, as astronomer Fred Whipple figured out, are made largely of ice. Much of the ice in comets is frozen water, but ices of other compounds such as carbon monoxide and carbon dioxide are also present. And comets contain, we have recently learned, a large amount of more complex organic compounds. These organic compounds may be limited to a mixture of molecules such as the original Miller - Urey experiment was able to produce, or they may be even more closely related to life; we can't be sure from here, yet. In the interior of a comet, under layers of opaque organic material, viable cells would be shielded from radiation. Of course, freezing slows or stops metabolism, so cells could exist there in suspended animation.

A few larger comets such as Halley's comet have orbits that bring them as close to the sun as Earth is. Even fewer comets, called "sungrazers," actually strike the sun, or pass so close that they are destroyed by it. Most comets reside at distances far beyond that of Pluto, in orbits not confined to the plane in which the planets' orbits lie. They are so numerous that the total mass of comets in solar orbit may be as great as the total mass of the planets. Slight gravitational disturbances caused by the outer planets or neighboring stars can change a comet's orbit completely, steering some closer to the sun, others completely away.

When a comet nears the sun, some of its surface material ablates away, making the comet's "tail." This process usually begins somewhere between the orbits of Jupiter and Mars. Some of the discharged material is gas, some of it is dust. Each makes a different kind of cometary tail. Dust and larger debris left by comets remain for a while in solar orbit. Earth often passes through the orbits of cometary debris, causing meteor showers such as the Perseid meteor shower around August 10 every year, when we cross the orbit of comet Swift-Tuttle.