Category: Planetary, extraterrestrial geology, and meteoritics

One of the annoying features of the Earth as a planet is that it engages in a kind of Saint Vitus’ dance.  The best known of its wandering are those involving variations in the eccentricity of its orbit, and the tilt and precession of its axis of rotation.  These follow from the gravitational influences of massive planets elsewhere in the Solar System, and are implicated in the modulation of climatic change through the last 2.5 Ma.  Rather less well-known, and even more aggravating are far more rapid, but geometrically quite small deviations from good behaviour.  One of these is the habit of the spin axis to wander around the geographic poles within a circle roughly 3 to 6 metres across.  It does this every 14 months.  It takes a certain degree of dedication to chart such a tiny planetary tic.  Chandler Wobble is the single claim to fame of its eponymous discoverer.  Seth Carlo Chandler Jr, an American businessman and amateur astronomer, discovered the quirk in 1891 by observing stars with a degree of single-mindedness that might have put a lesser mortal on the couch.  He set out to verify the famous Swiss mathematician Leonhard Euler’s prediction that the Earth ought to wobble every year, and he did.

So minuscule is Chandler Wobble, that keeping it going is something of a vexing problem, for a single jostle’s effect ought to fade away in a few years.  There are innumerable ways of nudging the Earth, and deciding which is sufficiently regular and just right to maintain the wobble is no easy task.  Following in the great tradition of Seth Chandler, Richard Gross of the Jet Propulsion Laboratory compared Wobbling between 1985 and 1996 with the continual but inconstant motions of atmosphere and oceans, as simulated by super-computer modelling of climate.  The forces of winds and currents are simply insufficient to induce the Wobble, but variations in atmospheric and deep-water pressure, together with their positional shifts are, in the manner of Goldilocks and the little bear’s porridge, just about right.  Because changes in water depth are wind-driven (as for instance with the wandering hump in the Pacific’s surface, linked with El Niño), ‘weather’ is the ultimate driving force for Chandler Wobble.

Why devote time to this picayune curiosity?  The answer is to chart more accurately the position of distant spacecraft; not easy when the measuring platform is behaving like a Womble.

Source:  Richard A. Kerr, 2000.  Atmosphere drives earth’s tipsiness.  Science, v. 289, p. 710.

Up until 10 years back, I was under the impression that as individuals we run little risk of being struck by objects falling on us from between the orbits of Mars and Jupiter.  A slim chance, but one tempered by a recollection of my father’s news clip of a small meteorite landing in the sidecar of a 1930’s biker on his way from Hull to Hornsea.  The biker finished his journey.  These days aliens seem to be falling thick and fast.

Late last year, the sleepy hamlet of Guyra, Australia, about 400 kilometres north of Sydney had a heavenly visitor, or so it seemed. On December 7, an object the size of a cricket ball slammed into the town water supply. In recent months, town officials have been pondering how to exploit their near misfortune.

In early July, a local businessman pledged AU$3,000 to dredge the rock out of the reservoir’s bed so it could be put on display, given to a local university or donated to the Australian Museum in Sydney.  Intrepid snorklers discovered that the object had drilled a 1 metre hole in the mud, after penetrating the reservoir itself.  Because such a small meteorite should have slowed to terminal velocity on entering the atmosphere from space, it is highly unlikely that it would have had enough remaining energy after ploughing through water to have buried itself that deep.  Experts have cautioned the amateur meteorite collectors to leave the object well alone, pending more cautious examination.

If Mars is ever to visited by astronauts, and for there to be any chance of finding living things there, water close to the surface is vital.  Not surprisingly, the search for Martian water, albeit not in a network of canals, is becoming a thriving cottage industry. The last week of June 2000 saw a leaked report from research using images from the Mars Global Surveyor spacecraft, publicised in New Scientist and Science for that week..  Some of these showed systems of V-shaped gullies on steep sides of valleys and craters, which are extremely sharp.  Several workers claim that they were cut by running water in the recent past.  That they are young features is clear, because they are not blurred by dust blown across the Martian surface by it nightmarish winds, and none are cut by craters.    How water might have flowed freely a short time ago is not too clear.  The Martian surface is well below freezing point for most of the time (average temperature -50°C).

The explanation given by the researchers is that a layer of frozen pore water a few hundred metres below the surface can melt because of  built up of pressure.  Where the layer meet the surface in valleys cut through it, the pore water remains frozen, and acts as a dam.  When this becomes breached, water simply squirts out to form the peculiar runnels seen at more then 150 sites.  Several of the gullies lie below signs of collapse on the slopes above, suggesting that water release has removed support for debris on the steep slopes.

There a number of reasons to take these accounts with a pinch of salt.  Sure, increased pressure depresses the melting point of water, but at -50°C it would have to be pretty high.  In permafrost areas on Earth, waterlogged soil freezes from the top down in winter, thereby trapping the last dregs of water.  This becomes pressurised, to remain liquid in a supercooled state.  If it breaks out it does not flow, but forms ice almost instantly.  As well as forming the famous pingoes (ice cored mounds) of Arctic alluvial plains, this phenomenon almost caused a bizarre disaster during one of the Yukon gold rushes.  High-pressure water jetted into a public bath house – the warmth of the building had created a trough of melt water directly beneath – and filled the entire edifice with ice.  Fortunately, this happened at night and no prospector was encased.  Much the same would probably happen to any such water escape on Mars, unless it was preternaturally warm.  Such was the case for the truly huge and unmistakable water-cut valleys on Mars.  But they formed far back in Martian history, perhaps as a result of energy introduced by large impacts.

It is tempting to look to other explanations for the gullies.  Very dry sand flows down the lee slopes of dunes, often to form runnels with collapse features above them.  Perhaps some attention to the physics of dry sand – Mars is a sandy and silty place – under near-airless conditions and suitably reduced gravity, might offer an alternative explanation.

Even more optimistic is the notion that Mars once has seas, based on the discovery of various salts in an Egyptian meteorite that approximate the blend of dissolved ions in Earthly seawater (New Scientist, 1 July 2000, In Brief).  The evidence that the class to which this meteorite belongs comes from Mars rests on comparison of its noble-gas content with the extremely imprecise measurements or Mars’ air by the Viking mission in the 1970s.  Why the chemistry of Martian ‘seas’, or any of its water for that matter should bear comparison with that for waters derived from a planet with both weather and highly evolved continents seems to demand an explanation.  Oh well, no doubt we will get answers when astronauts do get there – it is not inconceivable that all the papers suggesting it is important to go have some relation to NASA’s decades long fight for funds to do that.

Not only geologists are waking up to the influence that stray asteroids and comets have had on geological and biological evolution, but so too are politicians.  Despite the minuscule chances of a sizeable body hitting the Earth within our lifetime, the devastation would be awesome.  Insurance actuaries have calculated the risk from such rare events, taking into account the number of likely deaths in the same way as for airline disasters.  You or I are more likely to perish in the aftermath of an asteroid or comet strike than from botulism or a fireworks accident, and the risk is comparable with that of intercontinental flying.  Governments are beginning to find money to support systematic mapping of bodies that may pose a threat; not a lot, but sufficient to spot bad news and refine the risks.

On June 22, a French-US team released a first assessment of the near-Earth objects (NEOs) that pose the biggest threat; those more than 1 kilometre in diameter (Bottke, W.F. et al., 2000.  Understanding the distribution of near-Earth asteroids.  Science, 288, p. 2190-2194).  They estimate about 900 big asteroids in orbits that will pass eventually within a few moon distances of us. “Sometime in the future, one of these objects could conceivably run into the Earth,” warns astronomy researcher William Bottke at Cornell University. “One kilometer (about .6 of a mile) in size is thought to be a magic number, because it has been estimated that these asteroids are capable of wreaking global devastation if they hit the Earth.”  Much smaller objects caused the celebrated Meteor Crater in Arizona (20 000 years ago) and the Tunguska explosion (1905), and seem to pose the greatest hazard, being undetectable at present.

The Cambridge-Conference Network (CCNet) freely provides a regular electronic newsletter about research into short-lived catastrophic events, including climate change, the effects of supervolcanoes, and impacts, both in the geological record and possible in future from NEOs.  To subscribe, contact the moderator Benny J Peiser at b.j.peiser@livjm.ac.uk .

The K-T event is back for the death of the dinosaurs

Just when those palaeontologists who don’t like ‘whizz-bang’ theories for the fossil record had begun once more to feel comfortable, the geological record has bitten back.

One of the main planks against an impact cause for the extinction of all the dinosaurs at the end of the Cretaceous Period was the raraity of their remains in the top 3 metres of the Hell Creek Formation in the Great Plains of North America.  The Hell Creek Formation is noted for clear signs of the Chixculub bolide strike very close to its top, as well as for a rich dinosaur fauna.  Previous workers stated that a rarity of dinosaur signs just below this signified that they were under considerable evolutionary stress before any catastrophe; support for a gradualist notion of mass extinction.  A team of geologists and biologists from the US have just published the results of a painstaking survey of the Hell Creek (15 thousand hours of field survey of 11 million square meters of its outcrops in North Dakota and Montana) (Sheehan, P.M. et al., 2000.  Dinosaur abundance was not declining in a “3 m gap” at the top of the Hell Creek Formation, Montana and North Dakota.  Geology, 28, p. 523-526).  Their work finds that the top 3 metres are just as rich in dinosaur signs as any of the strata below it, right up to the layer immediately beneath the signal of Chixculub.  They do not report any findings from above the impactite, though dinosaur teeth are reported to be present by earlier workers.

As journalists say, this will run and run!