Today the surviving inner solar system protoplanets are 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 hold enough heat to keep a little liquid near its core. 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 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 meteorite samples we know that some at least had enough internal heat to 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, such as aluminium 26 and Iron 60. These put out a lot of heat compared to today's longer lived isotopes, allowing 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 active geology- a world with lava flows, a crust, a mantle and a core, vents of gas, 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 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 enough internal heat for cryo volcanism, involving materials like ammonia and water.
The sky was full of them: not tens but 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 planetesimals separating their orbital tracks. The sky from anywhere in the 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 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 at some point was almost certainly liquid.
Image left : The proposed structure of Ceres interior. Image courtesy of solstation.com.
Ceres holds a fascination for astronomers and explorers, an enigmatic world on the very cusp of planet hood that is often the focus of mission proposals. Computer modelling, and 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 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 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. 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 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 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 molten rock to be accurate. The surface of Vesta is covered in 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 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 may have found there way to earth as a class of meteorites called 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 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 composed of compacted regolith, beneath that lies the true crust of basaltic lava, around 10 km thick. Below the crust lies a mantle of pyroxene and other plutonic rocks, and at the core lies a metal rich nugget of orthopyroxene.
Astrogeologists have also built up a simple history of Vesta as an active world: After around 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 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 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 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 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 CM class carbonaceous chondrite meteorites, and 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 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. 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 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 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 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 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 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 Murchison meteorite. But stony-iron or iron meteorites must have come from places that were intensely heated, as these materials only occur when the primitive solar material has 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 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. 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: Relatively primitive material - stayed cold. Material with water bearing compounds and 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 space borne ones like the 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. Spectroscopy is, as ever 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 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!
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