This week I am going to talk to you about dust. Some of you may think that in picking this topic I am scraping the bottom of the barrel! But not so. Dust is important stuff. It is a prominent part of the structure of the universe. It has played an important role in our past, and we will meet it again in our futures - "Dust thou art, and unto dust thou shalt return", as the Bible gloomily reminds us. Dust in the atmosphere plays an important part in temperature regulation of the planet and contributes various interesting and beautiful effects to our skies.
Dust also creates problems. Dust can cause allergies. If enormous amounts of dust get into the atmosphere this will have big effects on climate. In certain industrial situations, dust explosions are a potential problem. And, of course, dust is the perennial enemy of the housekeeper.
My personal interest in dust dates back to November 29th, 1979, when we had an extensive fall of fine reddish dust in Ireland. The fall caused a considerable amount of disquiet and we received many calls in the university from the public inquiring as to the nature of the dust and if it posed a health hazard.
Dr Peter Vernon of the geology department at UCC and I carried out a detailed analysis of the red dust phenomenon. We collected samples and made a detailed examination of its structure and composition. We examined meteorological records of conditions over Europe and North Africa around the time of the 1979 dust-fall and previously reported falls.
The results were clear. The dust comes from the Sahara Desert. The structure and composition of the dust is consistent with a desert origin. Wind storms raise the dust high into the atmosphere and meteorological records show that it is carried up over Europe where it gradually falls back to earth.
The size of the dust grains is small enough to allow them to be carried aloft for such large distances before falling out. When the dust cloud intercepts precipitation, i.e., rain, snow etc., it is washed down in copious amounts. If any readers are interested in looking into the details of this phenomenon I have copies of our publications on the topic and can send them to anyone who requests them.
Dust is of the most fundamental importance to the structure and evolution of the universe. The space between stars is not a perfect vacuum but contains gas, mostly hydrogen, and dust particles.
The dust particles are very tiny (less than a thousandth of a millimetre wide) and are spread out very thinly. A volume of space the size of a large cathedral would contain only one particle. But there are regions where the gas and dust concentrate into denser clumps called interstellar clouds.
The larger and more numerous dust grains in space are silicates (chips of rock) covered with ices (frozen water, ammonia, methane, carbon monoxide and dioxide) and organic molecules (based on carbon). The smaller grains are composed of carbon. On the other hand, the British astronomer Fred Hoyle has claimed that most interstellar dust grains are freeze-dried viruses and bacteria that act as the source of life on various planets. Few scientists accept this theory.
The largest and densest interstellar clouds can collapse under their own gravity to form stars. Surrounding the young star is a swirling disc of gas and dust that may become a system of planets. Our solar system was born like this about 4.5 billion years ago.
The solar system is now comparatively free of dust grains. Most of the original grains were incorporated into planets and most of the rest have been "blown away" by solar winds. Nevertheless there is a small amount of dust between the planets.
Some of this dust comes from collisions between asteroids (minor planets between Mars and Jupiter) and also the jets of gas and dust emitted by comets that enter our solar system.
Dust is raised into our atmosphere by various surface disturbances. In the absence of this dust, the Earth would be a less beautiful and less moderate planet.
The visible light emitted by the sun is a mixture of seven different colours that in combination is colourless. The gas and dust in our atmosphere preferentially scatters out blue-coloured light to illuminate the sky and give it its familiar blue colour. The sun itself with some of the blue component scattered out looks somewhat redder than it would if no atmosphere was present.
This effect is accentuated at sunset when the sun is on the horizon, as the light must then pass through a greater thickness of atmosphere to reach our eyes. A considerable proportion of the sun's light can now be scattered out, particularly if the atmosphere is very dusty, giving the sun a beautiful deep red colour.
The evening atmosphere will be particularly dusty if the day has been hot and dry and sunsets are particularly spectacular in such conditions. This light, reflected from broken clouds, can produce most beautiful effects.
Speaking of clouds, dust also plays a part here. Without dust particles to serve as nucleation (seeding) sites rain clouds would not readily form, and consequently our days would be hotter and the nights extremely cold.
Too much dust in our atmosphere can have very bad effects. Dust from the volcanic eruption in 1883 at Krakatoa (an island east of Java) rose more than 16 miles into the atmosphere, circled the Earth four times and produced darkness at noon 150 miles away.
Sunsets were particularly beautiful for many months afterwards. Also, a prominent theory holds that the extinction of the dinosaurs was caused by the dust from an exploding meteorite which blotted out the sun and killed the edible plants.
Various industrial processes can generate dust clouds large enough to present a threat to life. These dust clouds include pharmaceuticals, cosmetics and metal powder, and even homely substances such as flour and sugar can present a danger when handled in factories.
Dust clouds can burn with explosive violence even when the material in them is not itself a fire hazard. The explanation is that oxygen gains easier access to a mass in a suspended powdery form than it can to a solid mass.
At this point I must cease my musings on dust. Now when next you notice that the living room has become noticeably dusty I expect you to greet the sight with a cry of joy. But for dust there would be no Earth; but for dust the mammals would never have thrived and replaced the dinosaurs.
William Reville is a senior lecturer in biochemistry at UCC