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Monday, 20 December 2010

Exploring Krypton....

Note: All links are numbered and listed in order at the end. On this occasion I apologise for the number of links to pages that require subscription for full access, where possible I have tried to find PDF versions of papers published in conferences earlier this year. actually I can't back that up -[1] Superman's home planet is not on my hiking itinerary this month. I was going to post on the subject of the upcoming NASA mission [2]Juno.....but [3]a slew of recently published papers in  the journal 'Meteoritics and Planetary Science' have allowed a surprisingly  detailed exploration of a real life destroyed world, and its just too good to pass up. Although it is unclear if [4]general Zods irresponsible mining operations were also to blame for the cataclysm that claimed this long vanished place.

Image above: Superman's home planet Krypton. Image courtesy of 'Concept Art of Return to Krypton'.

The world that has suddenly come into a clearer focus is the lost home of a meteorite known to science as 2008 TC3, or the [5]Almahata Sitta meteorite. 2008TC3 is exceptional for a bunch of reasons: Firstly [6]it was found while it was still in space, by the [7]University of Arizona, on October the 6th 2008. From the brightness and course of the object they knew that this was a small chunk - around 2 meters across - with a very good chance of hitting Earth. This wasn't cause for the ringing of even the tiniest alarm bells as things this small hit earth on a regular basis, and although they can explode with the force of a small nuclear device they always do so long before they get close to the ground.
But it was a cause for celebration bells (if UoA has such things) as this was the very first time an asteroid  on a collision course had been detected prior to colliding with Earth! Not only that but enough astronomical data was gathered prior to impact to make a [8]rough determination of its size, shape and spin.

Oh and for reference: An object is an asteroid before it hits Earth, and a meteorite after, so both terms can justifiably be used for 2008 TC3/Almahata Sitta.

Image left: Infra red satellite image of the Almahata Sitta fireball. Image courtesy of ESA

This ancient piece of rock hit the Earths atmosphere over the Sudanese Nuberian desert, and as predicted exploded harmlessly. But not without being observed: at about quarter to 3 in the morning (UTC) a KML airliner reported the flash of the explosion, and the Kenyan Infrasonic Array picked up the infra-sound pulse as it passed over about 2 hours and 20 minutes later, which allowed the force of the blast to be estimated at 1 to 2 kilotons. So it's a good thing these baby's go bang way up in the atmosphere. Satellites recorded the fireball, and onlookers took pictures of the dust cloud when the morning sun illuminated it -  and meteorite hunters were already gearing up to look for pieces.

Image above: The remnants of the Almahata Sitta/2008TC3 dust cloud caught in the early morning sunlight. Image courtesy of the  University of Khartoum

Image above: A fragment of Almahata Sitta sitting on the hard desert soil. Image courtesy of APOD, NASA.

Over the next few months expeditions scoured the Nuberian desert for fragments of the new arrival, and took them back for classification. And this is when things began to get really interesting from an ancient solar system perspective.
The rocks were found to be of a rare type of meteorite known as a [9]ureilites. They are a rare type of space rock, and seem to have come from a place, possibly a developing [10]protoplanet, with a dramatic history. They have a high percentage of carbon, about 3%, but unlike the carbon in [11]carbanaceous chondrites this hasn't been gently warmed and processed by running water. Ureilite carbon comes [12]in the form of graphite and nanodiamonds, a good indicator that the growing object they hailed from was victim of explosive high temperature events, most likely [13]planetesimal impacts and collisions with other protoplanets.

Image right: Photograph of a slice surface of NWA 2353 showing graphite in between olivine and pyroxene grains. Image width = 5 mm. Image courtesy of T.E. Bunch, 2005

Science takes time, so teasing out the knowledge from these fragments of a battered world has been years in the coming, but fortunately people like geophysical scientist [14] Doug Rumble (authour of one of the papers)from the [15]Carnegie institute have both the skills and the patience. Rumble used tiny fragments of  11 recovered  sections of Almahat Sitta to measure the oxygen isotopes in  2008 TC3. Oxygen isotopes are a unique fingerprint that allows a meteorite to be traced back to its source. The relative abundances of oxygen 16, oxygen 17and oxygen 18 to are unique to every world and large asteroid in the solar system, as the solar nebular was not totally well mixed.  Past ureilites have been found to have [16]different but similar isotope ratios, leaving room for doubt as to whether or not they all hailed from the same place. [17]Rumbles finding showed that the fragments of 2008 TC3 include all the ratios so far found in ureilites, making the idea that they are fragments of the same world much more compelling

Image right: locals crowd around a fragment of Almahata Sitta / 2008 TC3. Image courtesy of [18]'The Register'.
Search for a lost world:
The search for 'ureilite prime' still goes on, but five families of asteroids, also the result of larger bodies being pulverized in huge impacts, [19]can now be ruled out. This is done by comparing [20]the infrared spectroscopic signature of 2008 TC3 to  the main members of these families and looking for a match, as well as comparing the orbits of known members to the orbital path of our new arrival. The significance of 2008TC3 is that as it was seen as a [21]fresh fall and quickly collected its surface is relatively pristine, and so its surface should be a good match for whatever family of objects it belongs to. Other ureilites were exposed to our atmosphere and weather for much longer, distorting their spectral signature and muddying the waters. But it seems the waters are [22]already pretty murky -  the fragments have only weak spectral lines, or in CSI (CSI deep space anyone?) terms the fingerprint is faint, and for many ureilites smudged. The search goes on...

Story of a wanderer:
What about the process that these meteorites have gone through? The blast that destroyed 'ureilite prime' [23]shattered the protoplanet into chunks tens to hundreds of meters cross, birthing a massive swarm of rocks, that began to collide with each other, and to an extent reform into daughter asteroids made of the pieces - a process in which some [24]non ureilite material was mixed in. This material is proving to be fascinating in its own right, as at least one piece is from a [25]previously unknown family of [26]chondritic (stony) meteorites.

[27]A close up look at the structure of the samples reveals what a complex piece of rock this was -  with an equally complex history. The evidence tells an involved tale of  birth as part of an asteroid or protoplanet, which was shattered by a devastating impact, heating and partly vaporizing the rocks, which then came back together under gravity to form daughter asteroids. And then things get really interesting: This fragment of space rock, which was born in the inferno of a dying protoplanet, [28]houses the barest trace of amino acids -  the [29]building blocks of living proteins!

Image right: The well known Murchison meteorite, is loaded with organic chemical thought to be pre-cursors of life. Could fragments like this be the source of Almahatta Sitta/2008TC3 amino acids and PAHs? Image courtesy of

How could these pre-biological chemicals, thought to form only on objects with moderate temperatures and some liquid water beneath the surface, survive the fires that smelted and re-smelted the Almahat Sitta meteorite to its core? How did they form when there is no sign of aqueous alteration in the meteorite? We know they are not contamination from Earth, as Earth amino acids are mainly left handed ([30]known as chirality), and these are an equal mix of left and right. So there are three possibilities being looked at:

1: The Amino acids are from 'ureilite prime', and survived the cataclysm - this would certainly explain why they are so low in abundance. There is plenty of thermally altered carbon there that could have been amino acids and other organic material at some point.

2: They were introduced into the Almahata Sitta meteorite from the chondrite components not from the original protoplanet that were mixed into the daughter asteroids

3: They were formed by a poorly understood process after the protoplanet was destroyed -  this could be potentially revolutionary if true, as until now it was widely believed that amino acids were largely the results of chemical reactions involving liquid water.

Polycyclic aromatic hydrocarbons, (PAHs for short) a type of compound [31]sometimes implicated in the origin of life, have also been found, and similar questions apply to their origin.

[32]Looking at the radioactive isotopes in the samples has allowed researchers to make an estimate of when the blast that created Almahata Sitta occurred around 20 million years ago. Now that's a long time, but not that long when compared to the 4.5 billion year [33]history of the solar system. Since most of the major protoplanet on protoplanet collisions ceased before the solar system was fully formed this may raise an intriguing possibility; perhaps the Almahata Sitta parent body was hit by a big impact, but not a cataclysmic one. Perhaps 'ureilite prime' is still out there in some form, with a huge crater blasted into it!

Image Left: Vesta, a possible analogue for Almahatta Sittas home world? Vesta has also suffered at the hands of a huge impact, giving it a deformed, off spherical, shape. Image courtesy of JPL/NASA

Whether or not it is still around the abundance of rare isotopes of chromium in the meteorites suggests that the parent world may have been far from dull - it seems to have [34]similarities with the HED meteorite parent world. The [35]HED meteorites are widely thought to be from the once lava smothered giant asteroid [36]Vesta, so the ureilite parent body may well have been a violent, active little place for millions of years after its formation. How this squares with the presence of amino acids and PAHs isn't clear, but if 'Ureilite prime' was as active as Vesta is believed to have been it almost certainly rules out the prebiotic organics forming there via low temperature water based processes .
The meteorite evidence also suggests that the parent body was [37]still quite warm, over a thousand degrees Celsius in its mantle, before something caused abrupt cooling. If that something was the impact that produced Almahat Sitta then this is hard to reconcile with the results that suggest the world shattering occurred only 20 million years ago -  a protoplanet should have been long cooled solid by then. But this is an ongoing investigation, there is still lots of scope for theories to change and new ideas to emerge. Such mysteries are the bread and butter of science.
One possible explanation is that the journey from 'ureilite prime' to Earth may not have been as straightforward as 'got walloped, bits fell off, floated around and landed on Earth'. [38]An ultra fine detail study of the pyroxene in  Almahata Sitta by electron microscope suggests that it cooled at a different rate than other ureilites - it may be that after the initial impact Almahata Sitta was part of a swarm of daughter asteroids, each of which had enough internal heat to have their own geological histories, and then another impact split the daughter asteroid resulting in our 2 meter visitor from space...

....and the story will continue, for months and years, as Holmsian scientists unravel the clues that allow us to understand and explore a world lost, not just in space, but in the deeps of time. And all we had to do to get to know this place, far distant to us, was look at stones strewn across the ground in the desert.

Rosetta begins to unravel an enigma asteroid:

Image above: Asteroid 21 Lutetia, as seen by the Rosetta spacecraft. Image courtesy of NASA/JPL.

One other [39]intriguing abstract has surface recently: Results from the [40]Rosetta space probes [41]flyby of the asteroid [42] 21 Lutetia. This 132 km long rock has been an enigma for years now: Its high albedo suggests it is and [43]M-class asteroid, mainly nickel and iron and a fragment of some destroyed worlds core. But its density is closer to that of a [44]C-class, the kind believed to originate carbanaceous chondrite meteorites like the famous [45]Murchison meteorite. Spectral measurements suggest that the asteroid may in fact be an amalgamation of different kinds of smaller asteroids - an asteroid rubble pile. Rosetta [46]mass spectrometers, which sniff out trace gasses in the [47]interplanetary medium, may have detected the faintest of faint traces of water coming from this enigmatic little world, and if true this adds weight to the idea that at least part of Lutetia is carbanceous chondrite as these can be up to 1% water bey weight.

Next ( I swear I mean it this time) Juno, cracking the mysteries of Jupiter.

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