Note: All links are numbered and listed at the end.
[1]Hayabusa 2, a successor to the [2]beleaguered but triumphant JAXA mission [3]Hayabusa, has had its budget green lighted by the Japanese government. For those with some skill in Japanese [4]here is the original story.
Hayabusa was great science, not to mention being a story of success against the odds, but Hayabusa 2 will be a an even greater humdinger from a 'how did we and our world get here' perspective.
Like Hayabusa, Hayabusa 2 will travel to an asteroid using [5]ion propulsion, collect samples and return them to earth. But two things are different in Hayabusa 2 :
First the technology is more mature, and JAXA have had a lot of experience dealing successfully with every conceivable problem from Haybusa. So the odds are good that this will be a successful (and hopefully drama free) mission.
Image above: The target of Hayabusa 2, asteroid 162173 1999JU3, moves through space. Image courtesy of JAXA.
C-type asteroids are thought to be the sources for [10]carbanaceous chondrite meteorites, which are known to have [11]complex organic chemistry. This chemistry is thought to be similar to the kind that may have preceded the start of life on Earth. One famous carbanaceous chondrite, the Murchison meteorite, has been found to contain an [12]incredible an variety of amino acids (the building blocks of proteins) and [13]nucleobases (the building blocks of DNA). Chemical compounds from the same meteorite have been shown to [14]self assemble into membranes and [15]form boundary structures. It is certainly likely that carbanaceous chondrites contributed some of pre-biotic Earths supply if organic compounds.
What is more carbanaceous chondrites show definite signs of coming from bodies with subsurface, [16]pore space, water. That is: water flowing through minute gaps and fissures in the subsurface rock. 1999JU3 also shows signs of having interacted with liquid water, [17]bearing patches of phyllosilicates, although given its size it is likely 1999JU3 is itself a fragment of a larger body which had pore space water.
Image right: A coal black carbanaceous chondrite -the dark colouring is due to the abundance of carbon compounds. Image courtesy of meteors-for-sale.com
So the next Hayabusa will return samples from a body that has had organic chemistry, interacting with liquid water., in a rocky environment.... those are conditions close to the ones life is thought to have started in, but preserved and uncontaminated by actual life for over 4 billion years. To a guy like me, fascinated by the idea of how chemical systems evolve towards life, that's a mouth watering prospect!
Hyabusa 2 is currently slated to launch in 2014, return around 2020, and will be patterned after the first Hayabusa. When at the target asteroid it will not only collect samples, but will deploy a small landing robot, MINERVA-2 to explore the asteroids surface.
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.
Well...no 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...
[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 Meteor-center.com
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.
Note: All links are numbered, and listed in order at the end.
Before anyone points this out, yes I know that most of what I'm reporting today is not directly ancient solar system related, but as a space geek some things I just can't let pass without mention. The events of last week are high on that list - it was a roller coaster. I have a piece on the upcoming [1]Juno mission that I will be posting Friday night, but before I do that....
....first the bad news:
My heart goes out to the team behind the [2]Akatsuki (Literally: dawn) mission. This is the Japanese space agency's (JAXA) first attempt to put a space ship in orbit around Venus. It was scheduled to fire its main engine, and go into orbit around this [3]mysterious and [4]deadly sibling to earth on Wednesday, but [5]something has gone wrong...and quite badly to.
Image left: Akatsuki under construction. Image courtesy of Ohio Wesleyan University.
At 11:49 pm UT the engine began its main burn. As the crafts path took it behind Venus from earths point of view contact was lost. JAXA expected to get contact back after around 20 minutes. but those of s following the mission knew something was wrong ; Ten minutes after the blackout was meant to end.... no cheering, no applause.. no contact with Akatsuki. JAXA engineers laboured to re-establish contact, and eventually got an intermittent signal from the crafts [6]Low Gain Antenna. Akatsuki was in '[7]safe mode' (think of it as survival mode, basic functions only), out of position, and spinning, meaning that even the LGA was only talking to Earth for 40 seconds out of every 10 minutes. As the team slowly downloaded drabs of data a picture o what had happened - one that still has many blanks- began to emerge.
The moment he engine fired up then the internal fuel tank pressure began to drop - for [8] 2 minutes 32 seconds the crafts acceleration dropped away from what it should have been then it nosedived, recovered a little, and Akatsuki's computer decided to switch [9]attitude control from the reaction control system to the internal reaction wheels. When this happened the computer also shut off the valve supplying fuel to the engine, and the engine stopped. Akatsuki ran like this for 3 minutes and 37 seconds, until the computer brain realised it couldn't recover the maneuver and put the craft into safe mode.
Image right: the orientation of Akatsukis X, Y and Z axis with respect to time (time along the horizontal, orientation in degrees along the vertical). As you can see something caused the craft to begin wobbling. Image courtesy of JAXA .
What does this mean for Akatsuki? Well the probe is now on an orbit slightly inside that of Venus, so it is slightly faster and at present the probe is drifting slowly away from Venus. Once it is a half orbit ahead then it will start drifting towards Venus (the video below is clearer), but it will take six years for the injured bird to return to the vicinity of Venus.
Video above: the orbit of Akatsuki and Venus, illustrating how it will take six years to return to the vicinity of Venus. Video courtesy of JAXA.
Exactly what will be done with Akatsuki now depends on a lot of factors that aren't yet clear: She still has 80% of her fuel in the tank, but [10]if the engine has been damaged by the failed orbit insertion then that won't do our lady a whole lot of good unless a workaround can be found. However the craft is otherwise healthy,an the status of the engine is not confirmed, so it is possible [11]they could try again in six years. Even if the engine is inoperable JAXA showed with the [12]Hayabusa mission how inventive they can be - we can hope that they will find a way to still make use of Akatsuki, perhaps by changing the mission to study asteroids and comets. If the damaged engine is partially operational or another means of changing Akatsukis path, like [13]solar sailing ([14]as was done with Hayabusa), can be found they may be able to station her at one of Venus [15]Lagrange points and conduct long term studies of Venus from farther away than planned. But knowing the reputation of JAXA engineers we can expect heroic efforts and out of the box thinking to salvage as much use from Akatsuki as humanly possible
Image above: Venus as seen from Akatsuki as it drifts away from the evening star. Its a sad image, with so much undone. But we can still hope that all is not lost. Image courtesy of JAXA
....And in another kick to the ribs it seems that [16]Nanosail-D may have failed to eject from its fast-sat mother ship. This was to be NASA's first successful attempt at unfurling a solar sail in space.... but for solar sailing fans at least JAXAs solar sail mission [17]Ikaros, launched on the same rocket as Akatsuki, is in good health and performing flawlessly. In fact while Akatsuki was fighting to survive [18]Ikaros also flew by Venus!
Image above: The fully deployed Ikaros sail, with its variable reflectivity attitude control system. Image [19]courtesy of centauri-dreams.org
And some good news: Commercial orbital space flight takes to space!
Video above: The launch of the Dragon spacecraft on its maiden flight, atop its Falcon 9 booster.
Wednesday night, British time, [20]SpaceX launched its privately developed [21]Dragon space capsule, atop it privately developed Falcon 9 rocket. The ship orbited our cerulean Earth for about two hours, before making a flawless splashdown. Its easier to write reams about bad news than it is about good, but this is a truly historic flight. SpaceX is owned by South African born American citizen Elon Musk. At the post mission conference, and admittedly looking like sleep hasn't been huge on his agenda recently, Mr Musk made a lot of interesting statements. Near the top of the list was that while profit was needed for any company to survive, his main goal was to help establish humanity as a space faring civilization - how cool is that?
Video above: The post mission debrief for the maiden flight of the Dragon space capsule. Video courtesy of nasatv.
Earlier in the week a test firing of the [23]Falcon 9 had revealed a damaged engine nozzle. An engineer, although Elon describes him as more like an artist, named Marty Anderson went truly above and beyond to come to the launch site and conduct the repairs: He is afraid of flying! Despite this he was on the first available flight from California, which gives an insight into how passionate and dedicated Musks team are. There's a lot more, so check out the press conference above. Oh and he Monty Python related secret cargo Mr Musk mentions? It was wheel of cheese!
Image Above: Cheese from space! The wheel of cheese flown on the Dragon space capsules maiden flight. Image courtesy of spaceflight now.
Finally, something that is directly ancient solar system related:
Image above : Earths Moons, still mysterious! Image courtesy of NASA
Ever heard of [24]sidereophile elements? These are chemical elements, like gold for example, that according to the [25]giant impact hypothesis of the Moon formation should have been relegated to Earths core when a titanic collision reduced Earths entire surface to a magma ocean. That collision spewed debris into space that formed the Moon. However there is an obvious hitch; sidereophile elements are actually fairly plentiful in Earths near surface. So is the model wrong?
Not necessarily. [26]A new study has suggested that huge, but not quite planet destroying impacts could have replenished the young Earths stocks of sidereophile elements. These collisions would have been almost the last stage of terrestrial planet formation, and the Earth, Mars and our Moon would each have got clobbered with different sizes of debris: A few hundred kilometers across in the case of the Moon, up to a thousand kilometers across for Mars, and up to 1500 km across for the Earth. Good news indeed if you are looking for a gold engagement ring! Not so good if you were standing on any of those worlds at that time. Although the giant impact model is widely accepted, there are still plenty of kinks and questions about it, so it is good to see scientists working on refining this idea.
Next : Juno, A new frontiers mission to crack the secrets of Jupiter.
When the solar system was 2 to 3 million years old, [1]well into its construction, the main kind of object going around the sun was a small but vigorous type of world known as a [2]protoplanet.
Today the surviving inner solar system protoplanets are [3]small and cold places; their surfaces no longer see any event other than daybreak and the odd meteorite strike, even the largest of them would have struggled to [4]hold enough heat to keep a little liquid near its core. [5]But this was not always the case....
Small worlds with warm hearts:
Image above: Ceres and Vesta, the two largest remaining protopplanets in the inner solar system. Image courtesy of Physorg.com
As the [6]planetesimals, those kilometer scaled planetary building blocks, had different makeups so the protoplanets will have done, and different makeups would give each a unique face. From [7]meteorite samples we know that some at least had enough internal heat to [8]differentiate (melt and separate into layers) despite their small size. From these same samples we also know that the material of the early solar system was humming with short lived radioisotopes, [9]such as aluminium 26 and Iron 60. These put out a lot of heat compared to today's longer lived isotopes, allowing [10]even a world too tiny to have a spherical shape to warm its innards towards melting point.
And molten innards is another way of saying [11]active geology- a world with lava flows, a crust, a mantle and a core, vents of gas, [12]vents of explosive magma, geysers, subterranean lakes of fluids, lava tubes, crystals condensing from vapour, perhaps even something like an atmosphere- a dynamic world with all the vibrant energy only full planets, or [13]a handful of lucky moons have in the modern epoch. It has even been suggested that the nomadic comets, pieces of rock and ice from tens of kilometers to only hundreds of meters across, could have held [14]enough internal heat for cryo volcanism, involving materials like ammonia and water.
The sky was full of them: not tens but [15]hundreds, each with a unique character and history. Each with its own geology, atmosphere and chemistry. Each under constant bombardment from the sparkling reams of [16]planetesimals separating their orbital tracks. The sky from anywhere in the [17]ecliptic plane would seem a nonstop barrage of hectic movement. The death of one protoplanet by collision with another would shower the solar system in their guts: shallow gravity wells would make exchanges of material between these little worlds much easier than between todays gargantuan worlds.
Many of these worlds will only ever be known to us a shattered fragments of them drop to Earth today as meteorites, but some survive to this day, and their surfaces reveal what character of world they were before their short lived isotopes decayed; vibrant, active, volcanic worlds. And some of them have survived the passage of time, and by meteroite finds, by tlescope, and even by space probe, we are beginning to explore them.
Ceres: An ocean world?
Video above: The Hubble space telescope put together this animation of Ceres as it rotates, showing enigmatic light and dark areas. Courtesy of NASA/JPL
At close to a thousand kilometers across and heavy enough to pull itself into a spherical shape, Ceres earns the title of [18]dwarf planet. Discovered by Giuesppe Piazi in 1801, Ceres is bigger than some moons. The surface is covered in phyllosilicates and hydrated minerals, suggesting abundant water in the interior. Water that [19]at some point was almost certainly liquid.
Image left : The proposed structure of Ceres interior. Image courtesy of solstation.com.
[20]Ceres holds a fascination for astronomers and explorers, an enigmatic world on the very cusp of planet hood that is [21]often the focus of mission proposals. Computer modelling, and [22]analysis of its shape, suggests that Ceres is a differentiated body: it was warmed enough to separate into layers like a planet, with core crust and mantle. Ceres formed close to the [23]frost line, and so its abundant water and other volatiles would have been heated by the hearth of radioactive decay during its first few million years. The core would have cooled, but ever more slowly as time went on, and [24]subsurface water could have persisted here for billions of years . Conceivably some trace of that ancient ocean could persist today, near the core, if the water holds ammonia to lower its freezing point. [25]Hints of the hydroxyl (OH) molecule being vented from the south pole have further whetted appetites- perhaps a distant hint that some trace of activity indeed remains there.The spectral similarities of the surface to carbon compound bearing meteorites, coupled with the tantalizing prospect of an ancient ocean, has even led to [26]speculation of Ceres bearing life!
Image right: A mysterious bright spot moves with Ceres 9 hour rotation in these series of Hubble UV light images.Image courtesy of nasaimages.org
The idea that Ceres may have been a habitable world is still unproven, and the idea that it may have supported life is only speculation - and likely to remain so for a long time.It may well be that we will need to wait for a Ceres surface mission to nail down some of the little worlds more compelling mysteries. But we will soon be taking the first steps in answering the many questions surrounding the king of the asteroids: In 2015 we'll be able to stop speculating and start getting to know this enigmatic survivor- the [27]Dawn deep space mission will reach Ceres, its second stop, in that February.
Vesta: A history of violence.
Video above: Vestas rotation, as viewed by Hubble. Courtesy of NASA/ESA/STScl
Vesta is another kettle of fish- or [28]molten rock to be accurate. The surface of Vesta is covered in [29]basaltic lava, suggesting that its interior went through an intense internal heating that made it vomit its fiery innards onto its skin.
Image above: Basaltic lava oozes stickily onto Earths surface. Image courtesy of the United States Geological Survey.
Vesta probably formed close to the sun, and never contained much in the way of low temperature material, like water or ammonia. While its volcanoes have been silent for billions of years, its existence has not been dull- for billions of years [30]it was hammered by huge pieces of space debris. One cataclysmic blast deleted its entire south pole, and showered the rest of the solar system with a percent of its matter- some of these pieces [31]may have found there way to earth as a class of meteorites called [32]Howardite Eucrite Diogenite (HED) meteorites, and these are our main source of knowledge on Vestan geology. HED meteroites believed to be from various depths into its crust have been recovered, and telescopes have analysed [33]v-type asteroids believed to be fragments of its south pole still adrift in space.
From this we have built up a crude cross section of Vesta: The surface is probably [34]composed of compacted regolith, beneath that lies the true crust of basaltic lava, around 10 km thick. Below the crust lies a mantle of [35]pyroxene and other [36]plutonic rocks, and at the core lies a metal rich nugget of [37]orthopyroxene.
Astrogeologists have also built up a simple history of Vesta as an active world: After around [38]three million years of accretion the pummeling rain of planetesimals slowed, leaving Vesta as one small hot world amongst a skyfull. Over the next one or two million years Vesta melted almost completely, becoming a carmine droplet of lava against the backdrop of the evolving soar system.
Image right: A massive volcanic plume cuts into the space above the volcaninc moon Io: Is this what Vesta might have looked like in its youth? Image courtesy of space.com.
This fiery teardrop had a [39]molten mantle, and a structure like a true planet. In the center a metal rich core formed, and the mantle convected as Earths does today. Over time the radioactive heartburn died down , and Vesta cooled, first forming a solid crust over [40]its lava ocean. Then when only a fifth of the little world was still molten the remaining lavas spewed onto the surface, coating the little world in basalt. Over billions of years the little world cooled completely, but when Dawn arrives at Vesta next year its a good bet that the scars of its ferocious past will still be visible.
Pallas: A young face.
Image left : This marshmallow looking blob is the best view of 2 Pallas we have to date. But that doesn't mean we can't tease information out of such images.Image courtesy of NASA/HST.
Pallas is an altogether different character again- wider than Vesta, but also lighter, and less developed seeming than either Vesta or Ceres. Pallas is 600km across, and the Hubble space telescope has revealed [41]curiously varied areas of subtle colour on its surface. Its low density suggest a object that formed rich in water ice, and colour variations across its surface suggest that [42]Pallas may have gone partway through the kind of internal change and differentiation that Ceres and Vesta did- then stopped. A spectrum of its surface suggests that there are strong similarities to [43]CM class carbonaceous chondrite meteorites, and [44]hydrated silicates, similar to Ceres. Its low mass and density mean it has stayed more asteroid like than either Ceres or Vesta, and is still irregular in shape. But it seems almost certain that 2 Pallas had an active geology for a brief time, making it more of a missing link between planets and asteroids than either Vesta or Ceres. Still, this little conundrum will have to stay mysterious for many years longer: while there was a rumour circulated that the DAWN mission might do a flyby of Pallas, there is no spacecraft due to visit the secretive little world at the moment.
Hygiea: A Dark Horse
Image above: Based on its light curve a simple model of Hygiea can be built up. Image courtesy of comcast.net.
Hygiea is in may ways a sister world to Pallas. Despite being 500km across at its widest the giant of the carbon bearing [45]c-class asteroids was only discovered nearly 50 years after Ceres, the first asteroid to be found. In part this is because of its stealthy skin: A black mantle of carbon deposits keeps it from reflecting much of the suns radiation. [46]By spectroscopic comparison to the carbonaceous chondrite meteorites Hygiea is a good match - but this is no surprise as c-type asteroids are the most common in then main asteroid belt- indeed beyond the Jupiter induced chasm of [47]the kirkwood gap c-classes seem the norm.. Some signs of water altered mineral are present, suggesting that water may have been heated there at some point, but what really marks Hygeia out is just how little it has been altered. Of all the putative protoplanets in the inner solar system Hygeia is the most pristine- making it incredibly ancient, even by asteroid standards. And Hygiea does not orbit its distant track alone: It is the largest of the Hygiea [48]asteroid family: A group of asteroids all of the same type, ranging up to as large as 70km across (after Hygiea) and seemingly all from the same parent object, set free by yet another titanic collision. Whether this object was Hygiea itself is uncertain: at 70 km the next largest members are very big to have been blasted free yet still leave the parent body intact.
Strikingly these miniature planets seem to cross the entire spectrum of internal heating, from Vesta wearing its own innards to Hygiea almost untouched by geological activity, to Ceres with its long vanished ocean. From studying these we can get just a glimpse of a solar system crammed with a stunning variety of active worlds.
How do we know these things?
As these worlds are hard to get to, being often tilted away from the main plane (the ecliptic) in which the planets move, we have to let the universe come to us:
Video above: A large meteorite explodes with the force of a small nuclear weapon in the skies above Canada. Courtesy of the Canadian Police.
We can learn by [49]meteorite falls, most meteorites being fragments of a protoplanet or planetesimals, and by light, which we collect and interpret with our telescopes. In the case of meteorite falls the challenge is matching the meteorite to the parent body, which may be known only as a fuzzy blob in a telescope lens. But [50]infra red spectroscopy can help: We can start by getting the infra red spectrum of a distant protoplanet. If that spectrum is closely matched by the surface of a recovered meteorite it is a reasonable guess that the one came from the other. If the path of the meteorite before it hit Earths atmosphere can be determined (usually only when the fall is widely seen) then [51]its orbit can be reconstructed and backtracked to see if it might have begun as apart of another body.
The kind of material the meteorites is composed of speaks volumes: Primitive, unaltered material must have come from a place that saw at best only mild heating - such as the [52]Murchison meteorite. But stony-iron or [53]iron meteorites must have come from places that were intensely heated, as these materials only occur when the primitive solar material has [54]been heated to a high temperature and allowed to change and separate out.
Image above: A nickel-iron meteorite, next to a 1cm cube. Image courtesy of Astro.wsu.edu
Close examination of tiny features of a meteorite can reveal [55]an even more detailed story: For example if water has been passed through the pore spaces of the rock in liquid form then it will have dissolved certain chemicals and leached them out of some places, depositing them in others they could not otherwise have been. [56]Some materials need a watery environment to form, many even include the water molecule in their structures, so finding any of these in a sample is a good indicator of liquid water. There are many other clues that can be looked for, but just from these we can see how the beginnings of a crude scale of heating might form: [57]Relatively primitive material - stayed cold. Material with water bearing compounds and [58]evolved organic chemistry - warm enough for water but too cool for anything else. Metallic or igneous rock sample - came from a place with intense heating, and probably volcanism.
Telescopes, especially [59]space borne ones like the [60]Hubble, are a great means of exploring these places even though the resolution of their surfaces at such a great distance from Earth isn't amazing. [61]Spectroscopy is, as ever [62]a great ally, but simply observing the general shape, orbit and colour of these objects can be very informative. For example painstaking observations of the asteroid belt allowed the various members of the Hygiea family to have their orbits backtracked and found to have a common source. Observations of the shape of Ceres allowed how much [63]its material has relaxed under its mild gravity to be found, which in turn informs us on its internal structure. And as our understanding of these places improves we can refine our ideas with ever greater accuracy.
The arrival of the Dawn mission to Vesta and Ceres will accelerate our learning about these worlds immeasurably. No Longer distant fuzzy blobs, they will leap into focus as crisp little worlds int heir own rights. And I for one can hardly wait!
