Friday, 27 January 2012

Emergence of the Sun’s Family

The planets, their moons, the asteroids and the comets – all are part of the Sun’s family. And they are just as ancient as their parent. Evidence suggests that the Solar System’s contents started to form even while the Sun itself was still only a protostar, almost as soon as the Solar Nebula was in place.

We have seen that, in some ways, the Sun formed in much the same manner in which a sculpture is made. What began as a single, large block of material – the giant molecular cloud – was gradually whittled away to reveal a smaller end product. But the planets’ origins are more like those of buildings. They grew bit by bit, from the bottom up, by accumulating steadily larger building blocks. The very first process in the planet-building production line is a familiar concept known as condensation. You can see it in action when somebody wearing spectacles enters a warm room after being outside in the cold. As soon as air-borne water molecules hit the cold lens surfaces, the molecules cool down and stick to the lenses one at a time to produce a thin – and very annoying – film of tiny water droplets. Exactly the same phenomenon was big business in the very earliest stages of the Solar Nebula. As more and more material spiralled from the Solar Nebula into the newly forming Sun, the disc grew less dense. Eventually it became so sparse that its infrared energy could pass through with less hindrance. Thus the heat leaked away into space, the disc began to cool, and its material started to condense – single atoms or molecules grouping together one at a time until they had grown into tiny grains or droplets less than a millionth of a metre across.

But it was not condensation alone that produced the Sun’s family. Condensation is only an e¤icient growth mechanism when the grains or droplets involved are small, because matter is added one atom or molecule at a time. Eventually, as will become clear, the process was replaced by agglomeration and accretion – the building of progressively larger fragments through the accumulation of other fragments, not atoms.

The planet-building processes themselves are reasonably well understood. And yet, even after decades of research, astronomers can agree neither on the timescales involved in the various stages, nor on the sequence in which the events took place. It seems fairly certain that the gas-rich planets Jupiter and possibly Saturn formed very quickly – shortly it will become evident why. The rest, though, is more uncertain. And so what follows represents only one possible sequence in which the various elements of the Sun’s family came into being. This, the second part of our story, begins in the Solar Nebula, after the onset of condensation. Time elapsed since the fragmentation and collapse of the giant molecular cloud: 2 200 000 years.

2 200 000 years
Planetesimals and Protoplanets

The Solar Nebula was a rich soup of many different components. Gases such as hydrogen, helium, carbon and oxygen were common. Thus the disc brimmed with molecules – water, ammonia and methane – made from these available gases. Atoms of silicon – the basis of rock – were also abundant, along with metals. But these metals did not exist uniformly throughout the disc. Close to the protosun, where the temperature was around 2000 Celsius, only the very densest materials, such as iron, could condense. So the grains that grew there had a significant iron content. A bit further out, where it was cooler, silicate particles condensed into grains of rock. And at about 5 AU from the centre, the current location of the planet Jupiter, ices began to gather. Here, at what astronomers call the ‘snow line’, the Solar Nebula was a lot colder – maybe less than _70 Celsius. It was here and beyond where the water, ammonia and methane finally condensed out and froze to form ice crystals.

Thus, with the onset of condensation in the Solar Nebula, the protoplanetary disc soon began to resemble a vast, swirling storm of sand, iron filings and snow, whizzing around the central star at speeds of tens of kilometres per second. Collisions between adjacent particles were of course inevitable. And yet, for the most part, these interactions were fairly gentle, not violent. One way to imagine the scenario is to picture racing cars speeding around their circuit. Naturally the cars travel very fast – relative to the road and the cheering spectators. But, relative to each other, their speeds are much less reduced, hovering around the zero mark. Occasionally one of the cars will nudge up alongside and touch one of the others.

And so it was with the condensed particles in the Solar Nebula. Even though they were moving around so quickly, they were still able to jostle up alongside their neighbours fairly gently. When that happened, many of the particles stuck together, bonded perhaps by electrostatic forces. This is known as agglomeration. Thus, through this process, the first fragments grew steadily larger still. And the results were extremely rapid. Within just a few thousand years of its appearance, the Solar Nebula teamed not only with dust, but also with countless pebble-sized chunks of rubble – rocky and metallic close in, icy beyond the snow line. The planet construction line was underway.

Gradually, through increasing collisions, the great majority of these primordial fragments were deflected towards the mid-plane of the disc. With the fragments thus concentrated into a thinner plane, the rate of collision and agglomeration in the disc then escalated drastically. After only another 1000 years or so, the primordial pebbles had grown to dimensions of several kilometres forming mountain-sized ‘planetesimals’. This marked a turning point in planet construction.

Because of their dimensions, the planetesimals now grew not only by collisions with other fragments, but also by virtue of their own gravity. The larger the planetesimals became, the more matter they attracted. And so, only 10 000–100 000 years after the appearance of the Solar Nebula, the inner disc overflowed with innumerable bodies ranging in size up to that of the modern Moon. These bodies, quite justifiably, are known as ‘protoplanets’.

2–3 million years
Gas Giants and Asteroids

Not all of the protoplanets grew at the same rate. On the snow line, ices were about ten times more abundant than the silicates and metals closer in. Ices are also very adhesive: calculations have shown that they are 20 times stickier than silicates at comparable impact speeds. Thus, with such a wealth of condensed, gluey materials to work with beyond 5 AU, the agglomeration process operated extremely efficiently there. The end product was the first planet to form: Jupiter.

In less than 100 000 years, a protoplanet larger than the modern Earth appeared on the snow line, a gigantic ball of ice and rock. But its growth didn’t stop there, such was the amount of ice. Eventually this icy protoplanet became so large, maybe 15 Earth mases, that it began to suck in even lightweight materials – the gases, principally hydrogen and helium, that still form the greatest part of it today. In this way, the proto-Jupiter gorged itself for several hundred thousand years, after which time it had swept a clear path for itself in the disc. As the planet orbited the Sun, it sucked in gas from either side of the gap it had created, and gradually the reservoir that spawned it began to run dry. What finally stopped Jupiter’s growth in its tracks, though, was not a lack of raw material. It was the Sun. After Jupiter had been growing for about one million years, maybe less, the contracting Sun entered the T-Tauri phase. Its powerful wind surged through the Solar Nebula like a tsunami and blasted the unused gas away, deep into interstellar space. At last Jupiter’s growth was quenched. But by now it had hoarded more than 300 Earth masses. Unable to grow any larger, the giant planet – by now surrounded by its own gigantic disc of gas and dust, similar to the Solar Nebula itself but on a smaller scale – settled down and began to cool. This was about 3 million years down the planetproduction line, long before any of the other planets appeared, with the possible exception of Saturn.

This early appearance of Jupiter spelt trouble for those nearby planetesimals that had not been swept up in the planet’s formation. Those that passed close to Jupiter experienced a tug due to the planet’s gravity. Over time, some of these planetesimals developed chaotic orbits and were flung out of the disc. Those that remained, unable to group together because of the constant bullying of Jupiter’s gravity, survived until the present day in the guise of the asteroids. We shall learn more about these bodies in Part 3.

Saturn, a gas giant similar to Jupiter, came about in a similar manner. But, being twice as far from the Sun, its ice and rock core took longer to form in the relatively sparse surroundings. By the time the solar wind turned on and blasted away the unused gas, Saturn had not had enough time to grow as large as its cousin. A similar fate would meet the next two planets to form, several million years later: Uranus and Neptune.

3–10 million years
Ice Giants and Comets

By about 3 million years, Jupiter and Saturn had formed and were cooling down. But the protoplanetary disc was still very active. Closer to the Sun, the rocky planetesimals were continuing to gather. And much further from the Sun – twice as far out as Saturn is, and beyond – so too were the last of the icy planetesimals. Despite the abundance of ice there, it took longer for icy protoplanets to accrete to the dimensions where, like Jupiter and Saturn, they could pull in gas directly from the disc, because the orbital speeds there were slower. Eventually, though, two more dominant protoplanets of ice and rock did develop. These would become the outermost giants, Uranus and Neptune.

In time these kernels of rock and ice, each about as massive as the modern Earth, began to stockpile hydrogen and helium, just as the larger cores of the gas giants had done a couple of million years earlier. But they had arrived on the scene too late. The Sun was by now past its T-Tauri phase, and very little gas remained in the protoplanetary disc. For a few more million years Uranus and Neptune seized what little gas they could from the ever-diminishing supply, but their growth ceased after about 10 million years – the exact time remains uncertain. The end result was a pair of planets a little over one-third the diameter of Jupiter and only 5 per cent of its mass. And yet, despite their diminutive statures compared with Jupiter, Uranus and Neptune are each still heavier than 15 Earths. They were more than capable of joining in the game of cosmic billiards demonstrated earlier by Jupiter and Saturn. While Uranus and Neptune were still forming, those icy planetesimals that they could not sweep up were instead tossed away like toys that no longer pleased. Today, these fragments, known as comets, surround the Sun in two extensive reservoirs. One, the Kuiper belt, extends a little beyond the orbit of Neptune and is constrained largely to the plane of the Solar System; these fragments are also known as trans-Neptunian objects. Meanwhile, much, much further out, trillions more comets orbit the Sun in a gigantic spherical shell known as the Oort cloud, perhaps more than a light-year in diameter.

In some respects, Uranus and Neptune are like Jupiter and Saturn, but without those planets’ gaseous mantles of hydrogen and helium. And so, with a much smaller gas content compared with the proportion of icy substances such as water, methane and ammonia, Uranus and Neptune are not true gas giants. They are best referred to as the ice giants.

3–10 million years
Regular Satellites

While the four giant planets were forming, they were not doing it alone. As each of the giant protoplanets stole gas from the Solar Nebula, the material had swirled around the icy kernels to form gas discs like the Solar Nebula on a much smaller scale. Exactly as in the Solar Nebula itself, the particles in these discs had begun to lump together into larger building blocks – and new, independent worlds had started to appear in orbit around the planets. These would become the giant planets’ satellite systems – their moons. Because these moons formed from discs, like the planets, they now tend to orbit their planetary hosts in a thin plane, each in the same direction as the others and in fairly circular paths. Moons with these orbital characteristics also tend to be large. They are known as regular satellites.

It is probable that the regular satellites grew to maturity very quickly, even before their planets did. Why? Simply a question of scale. The discs that surrounded the newly emerging giant planets were much smaller than the Solar Nebula, so they had correspondingly shorter orbital timescales. Their rich cargoes of icy volatiles grew to protoplanet dimensions much more quickly than the planets did. But not all of the moons formed at the same time. The Jovian disc, right on the snow line, would have been the richest. So Jupiter’s regular satellites – Io, Europa, Ganymede and Callisto – no doubt formed first, alongside their planet, at T-plus 2–3 million years. These are known today as the Galilean moons, after their discoverer. The next moons to form were the seven or eight largest satellites of Saturn, followed by Uranus’ biggest five, and finishing with the moons of distant Neptune several million years after the appearance of the Galileans. Today, however, Neptune does not have a regular satellite system. It is possible, as we shall see later, that its original moons were destroyed when Neptune’s gravity netted a rogue protoplanet called Triton. This worldlet went into a retrograde, or backwards, orbit around Neptune and collided with or gravitationally ejected those moons already present. Triton remains today as Neptune’s only large satellite, though it is not regular because it did not accrete in a disc around that planet. Triton is a so-called irregular satellite, one of many found in orbit not only around Neptune, but also around all of the other giants. Triton aside, these irregular moons are mostly small lumps of ice and rock that were captured by the planets long after they had formed.

At last, with the giant planets, the regular satellites, the asteroids and the comets in place, the outer regions of the Solar System quietened down. Ten million years had passed. But there was a long way to go. Closer to the Sun, the planet-building factory was still in full swing. There, playing catch-up, the terrestrial planets were emerging.

10–100 million years
Terrestrial Planets

The terrestrial planets were latecomers. Because ices could not condense near the Sun, the materials (rock and metal) from which these planets coalesced were a lot less abundant than those that formed the giants further out. So, while the gas planets had formed within a million years – or at most a few million years – and the ice giants took maybe ten million years, for the terrestrials the formation process was even longer.

At least the initial growth of the terrestrial planets, within a few astronomical units of the Sun, had been very fast. Once the first rocky planetesimals had appeared, they had begun gravitationally to attract smaller bits of nearby debris. As we have seen, these first planetesimals grew to dimensions of hundreds or thousands of kilometres in less than 100 000 years. After about one million years the innermost regions of the Solar Nebula were populated by several large rocky and metallic protoplanets approaching the size of Mercury. And by 10 million years these protoplanets had grouped together through gravitation so that only four dominant spheres remained. These, at last, were the primitive terrestrial planets: from the Sun outwards, Mercury, Venus, Earth and Mars. But even after all four of the giants and their satellites had emerged, the terrestrial planets had grown to only half their eventual masses. And they had a very long way to go to make up that missing half – because the supply of available fragments in the disc was now much lower. Moreover, the terrestrial protoplanets had become large enough for the addition of more planetesimals to have a smaller and smaller effect on their size as they continued to accrete. Thus the growth of the terrestrial planets slowed very significantly.

Tens of millions of years after Neptune and Uranus had formed in the frigid, far reaches of the protoplanetary disc, even after the Sun had started on the main sequence, the terrestrials kept on growing, more and more slowly. In total, it took perhaps 100 million years for the terrestrial planets to mop up the debris, double their masses and swell to their present diameters. But because they never did grow large enough to pull in discs of gas from the Solar Nebula, not one of the terrestrial planets has any regular satellites. (We shall see, however, that Mars did capture two planetesimal moons, and that the Earth’s Moon is a specialcase.) Earth and Venus ended up with roughly equal masses, while Mars acquired only a little more than a tenth of that mass – later we shall see why. Meanwhile, we shall learn that Mercury might have started off with more mass, but lost much of its outermost regions in a gigantic collision with another protoplanet.

100–1300 million years
The Heavy Bombardment

The planets had finally finished growing. Now they would begin their long process of evolution towards the way we see them today. By now, about 100 million years had passed and the Solar Nebula was relatively sparse. Yet its activity did not stop completely. For the Solar System was still littered with fragments of debris that had not yet been ejected from the system by the giants or been swept up by the terrestrials. It was at this point that the Solar System entered what astronomers call, quite justifiably, the heavy bombardment phase.

For hundreds of millions of years, leftover scraps continued to rain down on the planets and their satellites. This is the battering that shaped the planets’ and moons’ crusts, and the majority of it occurred in the first 600 million years or so of their creation. A glance at the surface of the Moon gives ample reminder of this violent phase in the Solar System’s history. Many of the craters there are well over 100 kilometres across. One of them is about 12 kilometres deep and 2500 kilometres across – greater than half the Moon’s diameter. Called the Aitken basin, it is the largest known impact structure in the entire Solar System, carved out when the Moon was struck a glancing blow from a piece of rock and metal some 200 kilometres across. This constant barrage meant that the crusts of the terrestrial planets and moons oscillated between molten and solid states for many hundreds of millions of years. The heaviest elements sank to their centres, while the lighter substances, buoyed up, stayed near the surfaces. In this way the terrestrial planets and the satellites developed differential structures: in the planets, crusts and mantles of rock now surround molten cores of denser metal; and in the moons, the central cores are primarily rocky, with lightweight ices fashioning the mantles and crusts.

The early Solar System saw troubled times. But gradually, as more and more planetesimals collided with the planets and satellites, and were thus removed from the scene, the cratering rate began to drop. Some 1200 million years after the last of the planets had appeared, the craters were occurring perhaps 30 or 40 times less frequently than they had been 400 million years earlier. This point in history, about 3300 million years ago, marked the end of the heavy bombardment phase. The cratering did continue after this, but at a more or less constant although substantially reduced rate.

It was during the last few hundred million years of the heavy bombardment that the planets and satellites of the newly formed Solar System, after aeons of turmoil, began to develop their atmospheres.

700–1300 million years
Building the Atmospheres

Most if not all of the planets developed primitive atmospheres while they were still forming. The giants, as we saw, got their hydrogen–helium atmospheres by pulling in these gases from the Solar Nebula, and these have remained essentially unchanged since. Similarly, the terrestrial planets scooped thin veils of hydrogen and helium from the protoplanetary disc as they moved around within it. But these planets, having much punier gravitational pulls than their giant cousins far from the Sun, were unable to retain these lightweight, primitive skies. Slowly, they slipped away into space, their loss hastened along by the Sun’s solar wind.

Gradually, though, as the rate of impacts in the inner Solar System dropped after several hundred million years, the terrestrial planets started to cool. It was during these cooling stages that they developed their secondary atmospheres, via a process known as outgassing. These new skies came from the planets themselves. How? All rocks contain traces of compounds such as water or carbon dioxide that are chemically sealed within the mineral structure of the rock. When these rocks are heated sufficiently, those chemical bonds begin to sever and the trapped gases are released. The terrestrial planets were molten and extremely hot after they had first formed. And so, over hundreds of millions of years during the heavy bom-bardment phase, these hot balls of rock began to release their locked up vapours through volcanic fissures as they started to cool. Carbon dioxide, carbon monoxide, nitrogen, water vapour, and perhaps hydrogen sulphide were released in this way. In addition to outgassing, planetesimals from the orbit of Jupiter and beyond ventured regularly into the inner Solar System, thrown inwards by the mighty gravities of the giant planets far from the Sun. These comets and asteroids no doubt added a significant water content to the planets’ atmospheres – and in fact helped to seed the oceans on Earth.

The secondary atmospheres were in place within several hundred million years of the formation of the planets, while they were still sustaining heavy bombardment. As a result of that bombardment, lightweight Mars ultimately lost 99 per cent of its original secondary atmosphere, which was blasted away into space. And neither Mercury nor the Moon could retain their secondary atmospheres because they did not have sufficient gravity to hold on to even the slow-moving, heavy gases. Over time, all of the planets’ atmospheres have evolved. Today, Venus’ atmosphere is 100 times more substantial than Earth’s, which in turn is 100 times more substantial than that of Mars. But these are stories for Part 3.

4500 million years?
Formation of the Ring Systems

With the emergence and subsequent evolution of the planetary atmospheres, the Solar System was almost complete. Only two things remained to be added: the rings of the giant planets, and some of the smaller, irregular satellites. The irregular satellites were probably acquired early in the history of the Solar System, when the giant planets captured icy planetesimals from the thinning Solar Nebula. Some are no doubt of more recent origin. The origins of the rings, however, are more difficult to pin down.

The most famous ring system is Saturn’s. Consisting of countless boulder-sized, and smaller, icy chunks in individual orbits about the planet, the rings are exceedingly thin – with relative dimensions like those of a sheet of paper the size of a football pitch. But Saturn is not alone, because each of the other giant planets has similar accoutrements, albeit with different characteristics. Indeed, research has shown that no two systems are alike: they differ from each other in terms of diameter, brightness, and in the sizes and compositions of the particles th t constitute them. This is a clue to their formation. But the biggest hint is that most of the rings surround their planetary hosts inside their respective ‘Roche limits’. This is the distance from a given planet at which gravitational forces tear apart any body held together mostly by gravity. These clues could mean that the rings are the unassembled ruins of moons that strayed within this danger zone and got ripped to shreds, or the remains of comets that got too close and suffered a similar fate. Such a hypothesis neatly explains the differences in the rings: they depend on the constituents of the bodies that were destroyed in their making. Alternatively, the rings could be relics from the discs that surrounded the giant planets in the early Solar System, from which the regular moons formed. But this is unlikely. First, Saturn’s icy particles would have evaporated long ago while the planet was still a hot ball of gas. Second, computer simulations of particle orbits suggests that ring systems are unstable over long periods of time.

If these dynamical studies are correct, then the rings are of relatively recent origin – probably dating to less than 100 million years ago. But even this has its problems. How is it that we are alive at just the right time to witness the existence of not just one, but four ring systems, if they are all transient? The best answer is that the rings have existed for longer, but their particles are continually replaced by the break-ups of small moons and comets.

4660 million years
The Modern Solar System

At last we come to the present day. We have journeyed over 4 billion years in time to get to where we are now. But as we peer out into the depths of the Solar System that is our home, we can easily see the evidence of its formation. We see near-circular orbits, most of which lie in the same plane – a relic of the Solar Nebula. We see worlds with battered surfaces – the scars that betray the long and troubled period of meteoritic rain known as the heavy bombardment. And, because of the way the Solar System was made, we can now count five distinct zones within it.

The first zone lies within 1.7 AU of the Sun. This is home to the four terrestrial planets, Mercury, Venus, Earth and Mars. These are small worlds of rock and iron, forged from the hottest fires of the Solar Nebula. The expanse from about 2–3.3 AU marks the second zone, that of the asteroids. Some of these stony or metallic bodies have not been modified extensively in over 4 billion years, which means that they contain some of the most primitive materials in the Solar System. Zone three is much larger, the realm of the giants. Its innermost boundary is marked by the planet Jupiter, almost twice as far from the Sun as Mars is; its outermost boundary lies at Neptune, fully six times further from the Sun than even Jupiter. All of the giant planets are far bigger than the terrestrials, with compositions of ices and gases – and comparatively little rock. The fourth zone is the Kuiper belt of comets, or the so-called trans-Neptunian objects. This extends from roughly the orbit of Neptune to an unknown distance, but perhaps as far as 1000 AU. This icy wasteland is also home to the tiny worldlet known as Pluto, which we will meet in Part 3. The fifth and last zone in the Solar System is the largest by three orders of magnitude. It is the spherical shell of icy comets called the Oort cloud that surrounds the Sun at a distance that might even exceed 50 000 AU – a large fraction of a light-year. The comets in both the Oort cloud and the Kuiper belt owe their presence to the gravities of Neptune and Uranus.

And so we come to the known boundary of the Sun’s family. Somehow it is fitting that the phantom Oort cloud now surrounds the Sun on such a vast scale. It is similar in scale to that of the frigid globule of gas and dust from which everything in the Solar System sprang so long ago.

Source : 

Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



Post a Comment