MY pronouncements on planet Mars, including the absence of any life forms on it, may have contributed to rub some sheen off the long romanticised planet. The presence of some water, permafrost and ice could have led to a thin veneer of life (prokaryotes and bacteria at best, but highly suspect) but nothing, nothing at all more than that.

For the absence of a sufficiently strong gravitational field, and the incessant bombardment of cosmic rays from outer space as well as the ultraviolet and gamma rays, with no atmosphere to protect the planet despite the massive distance from the Sun (over 778 million km) contributed to creating conditions where life just could not prosper nor proliferate on life’s own terms.

Having surveyed planet Mars in detail we shall now take off to an all together different region of the Solar System: from the rocky world to the realm of gaseous planets, also called the gas giants. But before we go any further we are sure to encounter the tenuous belt of asteroids which lies irrepressibly between planets Mars and Jupiter. All those asteroids frighteningly close to one another.

These rocky pieces of stone come in all sizes: from tiny grains to boulders to mountains bigger than a range on the Earth. There is no respite for the unwary traveller for he (or she) must exercise extreme caution to avoid them lest an asteroid may ram into the vehicle at break-neck speed and cause extensive, in fact mortal damage. These chunks of rock are present in the form of a diffuse belt and orbit the Sun in a uniform and dignified manner. It takes each one of them almost equal time to orbit the Sun, though they number at least 100,000 of the major ones and millions of tiny pebbles and grains the size described above.

Regular collisions between them cause impact craters as well as the grazes lead to those fractions and grains as they break up in the process. Their collisions with each other also result in veering off the prescribed path and crashing into a planet or a moon that happens to lie in their path. Occasionally, once in a hundred million years or so, an asteroid may attain a trajectory which may lead to its flying off the Solar System altogether and abandon its family forever.

This game of unruly asteroids colliding with planets or their moons has been going on ever since, let us say for well over three billion years until this day. One such crash (besides many others) could well have been responsible for the near total annihilation of dinosaurs on the Earth 65 million years ago.

But we shall have more to say about asteroids later on. Let us concentrate on the gas giants for now. There are four of them: Jupiter, Saturn, Uranus and Neptune.

Rightly named for the king of gods, planet Jupiter is indeed the mighty ruler of a vast kingdom: of planets, moons, comets, meteors and asteroids, a miniature version of the Solar System.

But before we forge ahead, the first question that comes into play is: how gas giants are different from the so-called rocky planets? What is so special about them? Why are they gaseous in the first place? And why are the rocky planets as they are?

These are tricky questions that defy easy answers. But we shall still find them. Let us consider the first one first. The one way these planets, all four of them, differ from the rocky planets is that they are bigger, much bigger. The “smallest” of them (Neptune – 49,530km) is bigger than all the rocky planets put together — their total diameter: just over 38,800km. That’s how massive they are! There is yet another difference. All these planets are gaseous i.e. above their rocky cores, mantles and crusts they are covered by a thick envelope of gasses.

Let us take Jupiter first: the gas giant, first viewed closely through a self-made telescope by the great Italian scientist and observer, Galileo Galilae (popularly called, Galilee) in the year 1609, is also known as Jovian moon. It is by far the largest planet: 142,980 km, (Earth by comparison is 12,756km), lying at a distance of just about 778.3 million kilometres from Sun. That is its average distance, of course.

For, as you know, like everything else in space, it keeps moving in its orbit and in doing so, changes its position every second. Jupiter is 318 times more massive than the Earth, and its density (relative to water’s is 1.3, which means that it is heavier than water. Its period of revolution (year) is 11.9 Earth years, and the period of rotation (day, is nine hours and 55 minutes, which is very fast considering its diameter. It appears that by the latest count, it has some 21 moons which must keep the night on it pretty well-lit, fully illuminated!

But four of its moons gained prominence after Galilae’s discovery which landed the grand old man of science into deep trouble at the hands of the church. But that is another story…

The four big moons came to be called Galilean moons in his honour. All gas giants have a rocky surface, covered with thick clouds, which give to them their appearance and main characteristics.

Now the answer to the last two questions. Why are they gaseous? And why are the others rocky? It has taken scientists a century of sustained study of various branches of science to find the answers.

Here they are. The planets came into being some 4.6 billion years ago. In the beginning, they were soggy and fluid, subject to all sorts of large-scale chemical reactions. Also, there were impacts from outer space — much more than now. The entire Solar System was yet unsettled. Neither planets nor their moons, much less asteroids and comets had settled into their orbits. Consequently, they were all rogues for one another, bashing and ramming into each other at regular intervals, causing upheavals knowing well to all “solar systems” in their infancy.

Wide cracks and crevices appeared in the crust of planets. Yes, the planets had begun to “grow” crusts. For heavier materials, above all metals, gradually sank to the bottom forming mantle and cores while the lighter elements ‘swam’ above forming the crusts. But another thing happened which led to the formation of a wide variety of gasses.

What was it that resulted in the birth of gasses, and eventually led to trillions of tonnes for some, and none at all for the rest? For a complete answer see the next issue! i

The writer is a professional astronomer and a former head of PIA Planetarium. He can be reached at astronomerpreone@hotmail.com