Periods of glaciation and thawing have been so regular during the course of the earth's history that it is possible to unambiguously identify at least two major cycles, one with a period of about 41,000 years and another of about 100,000 years. In the early twentieth century mathematician Milutin Milankovitch was able to build a theory that argued that these cycles were directly caused by the mechanics of planetary motion.

The earth's orbit is characterised by three major cycles:

• Eccentricity: over a period of 100,000 years the shape of the earth's orbit varies from an almost perfect circle to a mildly elliptical one causing a 6% variation in radiation from the sun.
• Obliquity: over a period of 40,000 years, the tilt of the earth's axis varies between 22.1 and 24.5 degrees. The greater the tilt to more marked are the seasons: colder winters and warmer summers
• Precession: over a period of about 20,000 years, the elliptical orbit itself rotates around the sun (the earth actually traces out something a bit like a spirograph pattern around the sun). This has the effect of changing the timings of when the earth is at its closest and furthest points from the sun and biasing the seasons differently in the different hemispheres (i.e. making one more or less severe than the other).
  

Linking climate change to planetary mechanics has been so successful in explaining the cyclical nature of the earth's glaciation periods that since the 1970s the Milankovitch theory has become the standard model for climatologists. A closer reading of the historical record, however, has cast some doubts over whether these variations could be solely due to changes in solar heating alone.

Apart from light and heat the sun also beams to earth colossal quantities of changed particles known as solar winds. These winds would strip the earth of its atmosphere in a comparatively short time if they weren't being constantly deflected by the earth's magnetic field. New research by Jasper Kirkby of CERN, Augusto Mangini of the University of Heidelberg and Richard Muller of the University of California at Berkeley is suggestive of the possibility that while these cycles are still almost certainly orbital in their nature, the main mechanism driving glaciation is not the regular fluctuation in sunlight but rather changes in the flux of cosmic radiation reaching the atmosphere.


The earth's magnetic field also fluctuates in a cyclical manner and Kirky et al argue that this also correlates with the orbital precession cycle. They also argue that the earth's atmosphere which is prone to make more cloud cover when exposed to more solar wind and thus cooling the climate is extraordinarily sensitive to variations in flux. This phenomena may even go as far as having the earth's climate affected by interstellar winds due solar system's position relative to the nearest spiral arm of the Milky Way(!)

That kind of sensitivity may be a demonstration of just how connected events on earth really are with the rest of the universe. Enough to give pause when thinking about the Heliopause.

Beheira is a region of rich farming land located in the middle of the Nile delta. Its densely populated and fertile landcsape is dotted with mud huts and every other square inch is under intense cultivation with fields growing a wide variety of crops such as wheat and cotton as well as strawberries, okra and cucumbers. It is perhaps best known for its production of a fine variety of tobacco which is used in huqqah smoking.

On the 28th of June, 1911 at nine o'clock in the morning while standing in one of those fertile fields, a farmer by the name of Mohammed Ali Effendi Hakim observed something strange in the skies about his village of Denshal, near Nakhla (40 kilometres south-east of Alexandria). First he heard an tremendous explosion in his field and then looking up he saw that the sky had been streaked with a "fearful column" of white smoke. His dog, which had been there only moments before had been transformed into a mound of smoldering cinders.

To be born a dog in the modern Middle East would not, I think it's a reasonable to say, be the luckiest thing in the world. That seems especially so in a country like Egypt where in ancient times dogs had been considered sacred (and were even mummified) but were now, at least since the advent of Islam, thought of as unclean and lowly creatures fit only for hunting and guard duty. But the fate of Hakim's dog would have to be considered a extremely unlucky case of being at the wrong place at exactly the wrong time.

Sometime before he crawled out from his resting and venturing out into Egyptian summer heat (that "sometime" being anything between 11 and 1,300 million years before) a massive meteor plummeted into the surface of the planet Mars. Its impact was so great that it made a crater of more than 100 kilometres in diameter and launched into space several tons of volcanic and sedimentary material.

This material went into orbit around the Sun where it collided with other rocks, broke into smaller particles and got bathed in cosmic radiation. At the time when the fragment of rock that later became known as the Nakhla meteorite broke away from the other material, Hakim's dog's wolf-like ancestors had only just diverged from their fox-like cousins (Mohammed Ali Effendi Hakim's ancestors meanwhile had not yet differentiated themselves from orangutans or gorillas).

This rock and others like it crossed the earth's path many times eventually entering its atmosphere at widely different times and places.

It is often said that the death of the dog was the only fatality ever recorded for a meteor impact. It was certainly this angle of the story that led the Egyptian newspaper "Al Ahali" to send a reporter to Denshal to interview the farmer who showed him a greenish fragment covered in pitch. He said that numerous shards had plummeted to earth and buried themselves to depths of up to one metre.The news when it was translated into English prompted a few British scientists, who were residents of Egypt, to visit the strewn area. They succeeded in recovering about 10 kilograms of the meteorite.

Curiously, none of them visited Denshal or communicated any further with Hakim whose eyewitness account was the very first report of the event. John Ball of the Egyptian Survey Department stated that the meteorite would probably have been lost to science altogether "but for the action of a farmer, Mohammed Ali Effendi Hakim, who communicated a note of the occurrence to the Arabic Newspaper El Ahali". But after discussions via telegram with an official in Denshal, Ball concluded that no meteor fall had actually occurred in the area and no column of smoke had really been seen there. He ended up dismissing the dog story as the likely "product of a lively imagination".

Poor pooch. As if being vaporised by a scorching ball of Martian lava wasn't indignity enough. Still, short and novel deaths, whether apocryphal or not, do tend to lend the victim a certain kind of immortality.

Nakhlite is now the generic name for meteorites thought to have originated from this ancient Martian impact and their extreme rarity has led them to be referred to as the "crown jewels" of meteorites amongst the cognoscenti. They have found application in providing a base for comparison with results being sent back by the Martian rover missions and some scientists have convinced themselves that traces of fossilized Martian bacteria can be found within them. This week saw the report of a new Nakhlite fragment weighing 0.7 kilograms recently discovered in the ice of Antarctica.

or you will go blind... and insane.

I know Jason hates these things but I made this three dimensional view of the sun by grabbing two frames from this video. You can (hopefully) see the 3D effect by sitting back and letting your eyes cross until they form a single image. It may take a little while before you can get your eyes to focus.

(Note: no liability whatsoever will accepted for any irrepairable eye strain that may or may not be caused following or not following these instructions).

These frames were taken by the SOHO Extreme ultraviolet Imaging Telescope. As the dates show they were taken ten hours apart during late October 2003. This was a period when the sun was undergoing a peak of intense sunspot activity. Here's a bigger one taken from the day before.

This week has also been a big week for sunspots. Check here for the latest images and movies.

While we're on the subject of Martian meteorites, it occurred to me the other day that given the pock-marked surface of the Moon, it's surprising that you don't hear more about lunar rocks arriving here the same way. Well, it turns out of course that they do but, perhaps surprisingly, lunar meteorites are exceedingly rare.

Only 30 have been found mainly in the icy wastes of Antarctica or the stony deserts of Oman. Apparently, it is these extreme climates that provide the best places to search for meteorites because the stones suffer the least from weathering and erosion. These lunar rocks can tell interesting stories about the Moon's evolution but one of the meteorites recently discovered in Oman tells what I reckon is a fair rip-snorter of one (despite not even having a caninocidal angle to it).

When the researchers got hold of the meteorite SaU 169 in the lab they discovered that it was made of an easily identifiable lunar material known as KREEP (that K for Potassium, REE for rare-earth elements and P for Phosphorus). This type of rock was first encountered during the Apollo missions and subsequent surveying by NASA's Lunar Prospector Orbiter in 1998-9 established that this mildly radioactive material was essentially localised to one place on the Moon, the Mare Imbrium (for Northern Hemisphere readers that the right eye of the Man in the Moon or alternatively the belly of the Jade Rabbit who busily pounds the Elixir of Immorality with his mortar and pestle).

This rock formed as a result of a melting caused by a massive impact nearly 4 billion years ago. This impact, which created an impression the size of the 1160 kilometre-wide Mare Imbrium, was one of the last of its kind and marked the end of the epoch of devastating planetoid-sized collisions that had characterised the early solar system (it was catcylsmic events such as this one but on a much larger scale that lead to the creation of the Moon itself). This cessation of massive impacts, incidentally, was also the time when life really started to get cracking on Earth - surely more than just a coincidence.

Now this is where the story starts to get quite convoluted. As can be seen in the cross-section picture of the meteorite to the left, the rock is made up of two components, the older and lighter coloured KREEP material which makes up 87% of the rock and the layer of regolith or lunar soil which has adhered to it. This was a serendipitous occurrence because it permitted researchers to pinpoint the times and places in this rock's history to an astonishing degree of accuracy.

It is known that the KREEP material formed at the interface of the Moon's crust and mantle and would have laid deep under the surface for a long time. The regolith layer, on the other hand, indicates that it must have later been brought up close to the surface. Furthermore, the KREEP layer shows evidence of undergoing a shock some 2.8 billion years ago and an examination of the regolith indicates that the rock had been exposed to solar winds for a long time implying that it had been near the surface for aeons.

This is evidence that the rock had been involved in a second meteorite impact about 2.8 billion years ago which caused it to be dug out of the ground and thrown somewhere onto the surface. Researchers scoured the NASA lunar survey database in search of regions of surface regolith that matched the meteorite's chemical signature and were in the vicinity of Mare Imbrium. Then they looked for craters that were of the correct age.

A crater named for the French astronomer Lalande (1732-1807), which is about 25 kilometers in diameter and not far south of Mare Imbrium, fitted these requirements to a tee (see map). This feature is believed to have been formed about 2.8 billion years ago. Lalande is known as a "thorium hotspot", with high concentrations of this element providing a strong indication of its connection with the Imbrium impact. This is something that is observable even from Earth.

It is within the lighter coloured ground surrounding the Lalande impact site (left) that the rock is thought to have stayed for another 2.6 billion years before being involved in yet another impact (possibly at sites marked either A or B) that excavated the rock and dumped it into soil only 50 centimetres deep (possibly at the site marked as C).

To get from there into orbit is not all that difficult and requires only a modest impact capable of creating a 3 kilometre crater (such as the one at C). Analysis of isotopic data which gauges how long the rock was in space and directly exposed to solar winds indicates that this impact (its last before striking Earth) that launched the rock happened up to 340,000 years ago.

The rock then went into orbit around the Earth before finally descending as a ball of fire into the atmosphere and landing in Omani desert about 10,000 years ago.

Several research firsts have come out of this study of the meteorite. As already mentioned, it was the lucky combination of two different rock types that enabled the accurate pinpointing of its place origin to a remarkable degree. The rock is also the largest specimen found from the Imbrium impact (the Apollo astronauts were only able to bring back minute quantities) and this has enabled much more detailed analysis and the accurate dating of the end this important epoch in the solar system's history. It also provides a dating of when massive impacts stopped happening to Earth and this must have had an important positive effect on the evolution of life on this planet.

This impact even has a tie in to one of my favourite images from the Apollo missions, the descent of the Apollo 12 lunar module.