Snowballs from space
I heard there is a theory that all the Earth鈥檚 water was brought here by comets. Is this true? Surely the vast oceans can鈥檛 have been filled by snowballs from outer space.
鈥� The theory that much of the water in the oceans came from comets is still controversial, but there is increasing evidence to support it. The view you鈥檒l find in many textbooks is that the Earth鈥檚 water came from volcanic activity. Large comets, once thought of as a source of the ocean鈥檚 water, had been ruled out because the isotopic fingerprint of the water they contain (the ratio of deuterium to hydrogen) is different from that of the ocean.
Now, however, some scientists believe that the Earth is constantly being pelted by small 鈥渟nowball鈥� comets and that these may have a different isotopic signature. A source of 鈥渘ew鈥� water is certainly needed, because recent observations suggest that the seas are constantly leaking beneath the crust in ocean subduction zones.
Advertisement
This 鈥渟mall-comet鈥� theory is associated with Louis Frank, professor of physics at the University of Iowa, who wrote a book on the subject in 1990 called The Big Splash. His first observations of small comets were published in Geophysical Research Letters in 1986 and can be found at .
Evidence that there is a constant rain of small comets, each a loose snowball held together by a carbon shell, first came in the 1980s from satellites that looked down on Earth and observed 鈥渁tmospheric holes鈥�- dark spots 50 to 100 kilometres across- that were appearing in the dayglow of the atmosphere. The dayglow is ultraviolet light emitted when sunlight interacts with the oxygen in the upper atmosphere. Water is the only common substance that could create such holes, and each could be made by a small comet containing between 20 and 40 tonnes of water that broke up into a cloud of vapour as it hit the atmosphere. The case for a cometary origin for the atmospheric holes grew in the 1990s when the trail of a cosmic snowball was picked up as it vaporised over the Atlantic by the visible imaging cameras on polar satellites.
From the number of atmospheric holes, it鈥檚 possible to calculate that the atmosphere is being hit by a 20 to 40-tonne snowball comet every 3 seconds. According to Frank, this is enough to add an inch of water to the Earth鈥檚 surface every 20,000 years. Over the 4.5-billion-year lifetime of the Earth they could have provided all the water in the oceans.
There are plenty of other implications of the small comet theory. They might be pelting other planets, providing them too with a source of water. And it鈥檚 possible the comets carry more than water: they may also contain organic materials important to life. What鈥檚 really needed next is direct observation of the comets before they hit the Earth鈥檚 atmosphere. That requires a new spacecraft that can view them in detail. You can find more about the small comet hypothesis from the University of Iowa at .
Yuri Blochin
Moscow
Pluck for luck
I live on land recently reclaimed from dry bushland. I seeded the garden with grass and clover seed, and in the past three years I have found hundreds of four-leaved clovers and numerous versions with five, six, seven, eight and nine leaves. Sometimes there can be as many as 12 leaves all wound tightly round the stem so there is room for each leaf to grow. Is my garden exceptional? What conditions affect the number of leaves grown, bearing in mind that anything more than a three-leaved clover is considered out of the ordinary? And do I live on the luckiest patch of land in Australia?
鈥� You may well have the luckiest patch of land in Australia. But it is, of course, only four-leaved clovers that are considered to bring luck, as the four leaves are meant to represent fame, wealth, a faithful lover and glorious health.
There are in fact more than 250 different species of clover in the genus Trifolium. Under normal development and conditions, all will have just three leaflets. So the greater the number of leaflets on an individual plant, the more abnormal the development will have been- and, of course, the rarer the plants.
There are some species that appear to be particularly susceptible to producing more than three leaflets. For example, individual plants with between four leaflets and nine have often been observed in five species native to North America (T. andersonii, T. gymnocarpon, T. lemmonii, T. macrocephalum and T. thompsonii) and two species in eastern Europe (T. polyphyllum and T. lupinaster). But when you normally discover a clover with more than three leaflets in your back garden it is most likely to be one of the lawn weeds known as white clover (T. repens) or red clover (T. pratense).
Plants with more than the usual number of leaflets are thought to arise as a result of a mistake during development, which will occur if the plant is under any kind of stress. Environmental factors such as cold nights or warm days, disease, viruses, aphids, lack of moisture and herbicides can all be very stressful to a small plant, and may be contributing factors.
While four-leaved clovers are difficult to propagate in laboratory conditions, it is common to find more than one specimen with an abnormal number of leaflets in one area, for two reasons. The first is that they are all subject to the same environmental factors, the second is that Trifolium reproduces asexually, by spreading horizontal stems known as stolons that put down roots at the plant鈥檚 nodes. There is also evidence that in some extremely rare individuals there is a weak genetic link affecting the number of leaflets produced, and these plants consistently produce a small percentage of multiple-leaved versions.
It may be that extreme temperatures and lack of water are the most likely conditions that have led to your abnormal plants, given that you planted your clover seeds in a garden which was, until recently, dry bushland, and because Australia exhibits very different temperatures from those Trifolium is used to- Ed
With thanks to Terry Michaelson-Yeates, Legume Breeding Group, Institute of Grassland and Environmental Research, Aberystwyth and Aaron Liston, Department of Botany and Plant Pathology, Oregon State University
This week鈥檚 question
No more Moon
What would be the effect on the Earth if an alien spaceship came along and dragged the Moon away?