By David Bryant
A recent TV documentary (of the somewhat over-dramatic variety!) sought to explain several recent – and very damaging - falls of ice. Arriving at high speed from cloudless skies, these have battered roofs, cars and aircraft. The conclusion of the program was that these were examples of ‘mega-hailstones’, poorly-understood phenomena, more usually called megacryometeors by the Scientific community.
Around 50 of these have been recorded so far this century, varying in mass from 0.5 kg to real giants such as a Brazilian example of over 50 kg: a specimen with a mass of 200kg was reportedly seen falling in Scotland in the nineteenth century!
Meteorologists on the documentary spent much of the program establishing a mechanism by which huge chunks of ice could form in the upper atmosphere, other than in the conventional nursery of the convection currents of a cumulo-nimbus cloud. As most people will be aware, the powerful updraughts inside such clouds (which are typically associated with thunderstorms) allow the formation of hailstones. These may gyrate inside the cloud, accumulating mass until they are too heavy to remain aloft: hailstones the size of golf balls are not that uncommon.
However, they generally display a layered cumulate structure similar to that of an onion, while megacryometeors do not.
At the time of writing, no generally accepted mechanism for generating and supporting such large masses in the upper atmosphere has been forthcoming, although some of the theories put forward seem credible at first glance.
That chunks of ice occasionally fall from aircraft is undeniable and may be placed in two separate categories. The first (of
which I have personal experience) is generated by the dumping of liquid waste from on-board lavatories. Some years ago a
local radio station invited me to interview a lady who had been struck on the arm while hanging out her laundry. On arrival,
I asked to see the object that had hit her: to my amazement, she opened her freezer and took out a polythene bag, inside which I saw a bluish lump of ice similar to that in the photo:
The lady was less than thrilled when I told her she’d been storing lavatory waste from an aircraft among her fish fingers and
frozen chips! It had an obvious smell of disinfectant, so I’m at a loss to know how the lady for one minute thought it was a
meteorite…
Of course, icing on the fuselage and wings of aircraft still poses a threat to aviation safety, and flying through Supercooled Large Droplet (SLD) conditions can generate chunks of ice: these could fall to the ground and cause damage, but would not, I feel, be confused with genuine extraterrestrial ice meteors that should show signs of flowlines and ablation.
So then: is it possible that ice meteors could reach the Earth from space and pass through the atmosphere to the ground?
An online search will quickly discover a good number of learned publications that appear to answer this question with a resounding ‘no’. The majority maintain that a vast initial mass of tens of thousands of tonnes would be required for a football-sized chunk to reach ground level. But is this necessarily the case? The assumption is that frictional ablation would melt away most of the object’s mass, but this ignores certain factors:
1) Objects entering the atmosphere from deep space may hit the Earth head-on, at a combined velocity of 220,000kph or more. But equally, they may ‘creep up’ on the Earth from behind, with a closing velocity of just a few thousand kph, reducing frictional heating by a huge amount
2) Our putative ice meteor would be at a temperature just a few degrees above absolute zero (-273 degrees C) Whilst the outer layers would indeed become extremely hot, they would slough off like the heat shield of a re-entering Apollo spacecraft, taking heat energy with them.
Moreover, like the tiles on the five Shuttle Orbiters, ice is a pretty poor conductor of heat, so that the interior could be expected to survive better than, say, a piece of rock or iron.
Assuming, then, that some of the ice that falls from the sky may indeed originate in space, two questions immediately occur:
Where would ice meteorites originate? The Solar System is full of water-ice: billions of tons make up most of the mass of each of the trillions of comets in the Oort Cloud and Kuyper Belt.
Additionally, the Asteroid Main Belt must hold thousands of captured objects from these remote regions: these, we know, are
occasionally deflected into the inner Solar System.
I have written elsewhere that it is my belief that the majority of large craters on the inner planets and their satellites are the result of cometary (rather than asteroidal) impacts:
certainly, recent research has shown that to be the case with the Gilf-el-Kebir in Egypt:
How could we prove an extraterrestrial origin?
The relative concentrations of two isotopes of oxygen (17O, and 18O ) is used to assign an origin to planetary and asteroidal
meteorites. Cometary water should display relative oxygen isotope concentrations different to that of terrestrial water.
At the time of writing, just a few megacryometeors have been tested: these have all had terrestrial ROICs. But the samples tested were just a tiny fraction of the numbers that fall on the Earth: it is unscientific to discount the possibility of ice
meteorites on such a small sample. Should a sample be found to have an exotic ROIC, it could then be examined for evidence of presolar grains, interplanetary dust and regalith fragments it had picked up during its wanderings in space.
In conclusion, I suggest there is a high probability that chunks of cometary ice do reach the Earth’s surface from time to time:
I have written this article in the hope that not all astronomers will continue to dismiss a possible extraterrestrial origin for these.
David Bryant
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