What makes our planet different from the others?
Our planet has plenty of water in liquid form. There's water elsewhere in the solar system, plenty of it. The satellites of Jupiter and of Saturn contain water in the form of ice, because of their very low temperatures. Recent measurements by the Galileo satellite suggest that liquid water may be present on the surface of Europa. Venus also has water in the form of vapor, because, being second closest to the Sun, Venus has extremely high temperatures. The Earth's orbit keeps us just far enough from the Sun to allow water to remain liquid.
Mars also used to have liquid water, as the so-called canals and dried-up wadis that spacecrafts have shown seem to indicate.
As recently confirmed by the Mars mission Pathfinder, torrents of water did flow on the surface of Mars some billion years ago. But for a long time now, there has been none. Why? We really don't know. Given its relatively small mass, its tectonic activity is now very weak.
But where does Earth's water come from?
Let's go back to those torrents of matter projected into space at the death of the stars. Dust was formed - literally stardust - on which ice and frozen carbon dioxide came to rest. When agglomerations of dust grew large enough to give birth to planets, the ice volatilized and escaped outside in the form of geysers. What's more, comets, which are made up largely of frozen water, fell on the planets, bringing water with them.
And the Earth retained that water?
Its field of gravity is strong enough to retain the water molecules on its surface, and its distance from the Sun allows it to retain water in liquid form, at least in part. In these early days of the Earth's formation, it was constantly bombarded by ultraviolet rays emitted by the young Sun.
The Gift of Water
Why didn't the same evolution take place on Venus?
We don't quite know. The two planets are so much alike they have virtually the same mass and contain the same amount of carbon. On Venus, however, this carbon is in the atmosphere, whereas on Earth, it is to a large extent in the ocean. Yet the atmospheric compositions of the two planets were very much alike in the early stages of their formation.
(Esta imagem publicada pela Nasa mostra como veríamos Vênus, com sua camada de espessas nuvens de dióxido de carbono. Por ser coberto com estas nuvens brancas, o planeta reflete muita luz do sol e se torna muito visível para nós. O brilho de Vênus nos faz confundir o planeta com uma estrela, que até ganhou o nome de Estrela Dalva.)
(Mas por debaixo das nuvens brancas, reina um planeta extremamente quente, como mostra esta imagem publicada pela Nasa como "Vênus desvelada".
What does the difference stem from?
We think that water in the liquid state on the surface of our planet played a crucial role. Thanks to this blanket of water, the carbon dioxide in Earth's early atmosphere was dissolved and wound up at the bottom of the oceans in the form of carbonates. Venus is slightly closer to the Sun than we are. The difference in temperature was in all likelihood responsible for the absence of liquid water in the planet's early stages. Its atmospheric envelope of carbon dioxide created an enormous greenhouse effect, which kept its surface temperature in the vicinity of five hundred dregrees. So it was that two planets, alike in many respects, evolved in two very different ways.
Without water - liquid water - it's safe to say that it would have been the end f our story.
I think so. Water played a primordial role in the appearance of cosmic complexity. Within the ocean blanket, sheltered from the ionizing rays from outer space, intense chemical reactions would occur. By means of various encounters and associations, those chemical reactions would produce molecular structures that were increasingly large. In the early stages of prebiotic evolution, carbon, born of the red giants, would play a major role.
(O nosso querido planeta Terra e seus lençóis de água.)
An Atmospheric Face
Why is carbon so successful?
It's the ideal atom for molecular constructions. It has what we call a valence of four, meaning that it has four electron "holes" that can act as harnesses for numerous other atoms. The links it creates are sufficiently supple to allow easy and quick association or disassociation, which is indispensable to life. Silicon also has a valence of four, but the links it makes are much more rigid. It creates stable structures, such as sand, but it has no capability to yield to the constrains of metabolism.
It's therefore absurd to imagine that somewhere out there in the universe there is life based on silicon.
It's highly unlikely. In our galaxy, as in the neighboring galaxies, the various molecules of more than four atoms that we've been able to identify by radio telescope always contain carbon, never silicon. This observation strongly suggests that if life does exist elsewhere in the universe, it is also made out of carbon.
Once Earth's atmosphere was formed, life soon followed, isn't that so?
When Earth was born roughly four and a half billion years ago, the conditions were scarcely favorable. The temperature on the surface was too high. In addition, at that time, space was rife with countless small celestial bodies that would later be absorbed by more massive planets (the solar system was cleaning up its own house). The constant bombardment of meteorites
and comets was extremely violent. Studies of various comets revealed the presence of a considerable quantity of hydrocarbons. The collisions of the first billion years in all likelihood brought, in addition to water, an important quantity of complex molecules to the surface of the Earth. These comets, which in ages past were generally thought to e harbingers of death and destruction, probably played a beneficial role in the appearance of life. Less than a billion years after the birth of Earth, its oceans were swarming with living organisms, including the first blue-green bacteria. This view was strongly confirmed by the rich harvest of organic molecules left behind in the tail of the comet Hale-Bopp in 1997, including formaldehyde, various cyanides, and methanol.
