Inter-planetary dust gathers on Earth

Press, 26 July 1984, Page 24

Inter-planetary dust gathers on Earth

___ by

F.M. Bateson

Every year the Earth accumulates dust from inter-planetary space. This comes mainly from meteors which have burnt up in the atmosphere and from tiny micro-meteorites that fall to the surface, usually brought down in rain. Estimates as to the quantity vary widely but the most reliable guesses place the figure at 5000 tonnes annually. That is, of course, spread over the entire Earth and much of the dust falls into the oceans. The Zodiacal Light provides evidence of dust that is slowly spiralling into the Sun from inter-planetary space. We see this light as a tapering cone of diffuse light rising obliquely from the horizon after sunset, or before sunrise. It lies along the ecliptic, or zodiac. It is brightest at its base, but an extension, the Zodiacal Band, is a faint extension that continues round the zodiac and connects the two ends. At the point directly opposite to the Sun the Zodiacal Band broadens into a diffuse patch of light, known as the Gegenschein. Dust grains, of microscopic size, lying along the plane of the Solar System produce the Zodiacal Light by reflection. It is the large number of these dust grains that render the light visible. It is estimated that the total mass of the light is about 30 million million tonnes. These dust particles can enter the Earth’s atmosphere as micro-meteorites. The Zodiacal Light and its extensions are best seen from the tropics. From our latitudes it is best seen in August for two hours after evening twilight and for the same time before dawn in April.

The Infra-red Astronomy Satellite (IRAS) has now thrown fresh light on the Zodiacal Light. This satellite was launched on January 26,1983. It was designed to detect infra-red radiation from cold objects. These include the planets, asteroids, and the dark, dusty regions in which stars are formed. IRAS found that the comparatively bright infra-red glow from the Zodiacal Light extended, with diminishing intensity, to about sixty degrees above and below the ecliptic. Besides the broad central band of infrared emission it was found that two smaller peaks were found at eight to ten degrees from the ecliptic. Solar radiation has two effects on these dust particles. It causes their orbits to become smaller so that the particles spiral into the Sun, while the very smallest specks of dust are blown out of the Solar System. The continual loss of particles means that the dust could not survive more than about 60 million years unless it was being replenished. The findings of IRAS has lead to the theory that replenishment of the dust comes from collisions between asteroids with orbital inclinations of about nine degrees. It is also probable that some of the dust comes from comets that have broken up. Support for this replenishment of dust from comets also came from IRAS. It found a 20 million kilometre tail of debris trailing behind Comet Tempel 2. This comet orbits the Sun every five years and this was the first time a tail has been seen following this comet. A number of comets were

found by IRAS. One such object was known as 1983 TB and appeared to be a fast moving asteroid rather than a comet. It was discovered on October 11, 1983. Later it was found that it had the same orbit as the Geminid meteor shower whose orbit intersects that of the Earth each December. No known comet shares the same orbit. However, the discovery of 1983 TB, now only two kilometres across, suggests that this is probably the remains of the comet that originally produced the Geminid meteors. The discoveries made by IRAS were not confined to the Solar System. One of the most surprising was the finding of a ring of solid particles around the bright star, Vega.. The ring particles are orbiting Vega and resemble the zodiacal dust mentioned earlier. It is possible that this ring consists of material left over after Vega formed about a billion years ago. Although IRAS examined about 9000 of the brightest stars Vega was the only one that revealed a ring of material around it. Fifty other stars showed enhancement of the infra-red emission. These could be caused through the chance encounter with a dust cloud, or could mean that the object had a cool stellar companion or they may result from stellar winds. It must not be assumed from these observations that solar systems are rare since IRAS could not produce any evidence that they exist. That is not surprising since if IRAS had been orbiting the Alpha Centauri system, the nearest to us, it would have failed to detect anything unusual about the Sun.

Amongst the stellar objects scanned by IRAS was the dark cloud known as Chamelon 1. There several features were found that are probably stars in the course of forming. These have dust grains with temperatures between 70deg and 200 deg. Observations of other dark clouds also revealed young stars in the course of forming. So far only a small percentage of the data gathered by IRAS has been analysed. It is likely that even more interesting discoveries will come from a closer look at much of its data. The first fortnight of August will show all the naked eye planets clustered id the evening sky. During this period Mercury will set about an hour after evening twilight ends. It can be found very low to the northwest as twilight ends. Its apparent disc can be seen to grow larger as it comes closer, while its shape changes from the half phase to a slender crescent. From mid-August Mercury will dip down sharply towards the Sun and become lost in the twilight before reaching inferior conjunction on August 28. ’ Venus can be glimpsed very low in the north-west-ern evening sky, setting a few minutes after twilight ends.

Mars, Jupiter, and Saturn all remain in good viewing positions during the evening; Saturn will be occulted by the Moon at 9.51 p.m. on August 31. It will reappear from behind the Moon about 35 minutes later. South of Christchurch disappearance will occur some minutes earlier and so it is best to start watching this event some time before the times given.