Our planet is a paradise for life. But it wasn't always this way. There are two mass extinctions that are fairly well known to most people. The Permian and Cretaceous Mass Extinctions. The Cretaceous is the most well known. It's the "Dinosaur Killer Asteroid" event. The Permian could be due to several factors: Glaciation, Volcanic Eruptions, Possible Impactor, Continental Drift etc.. However, there have really been at least 8 Mass extinctions of life on Earth. There have also been around 3 minor extinction events.
Yup. Eight Mass extinction events. Some of these effected life-forms in certain environments such as the oceans. Others were truly destructive, killing life down to the microbial level. The mechanisms behind all of these are not completely understood. However, even though the severity of some of these is hotly debated, the implications are well understood in my opinion: Life is tenacious and very hardy. However, only within a certain, limited, range of environmental conditions. Change these conditions and life will cease to thrive. Any dramatic change has catastrophic implications. "Life" may very well be a "paper tiger." It may appear all-pervasive and tenacious. But when faced with sever and rapid global change, life may not be able to adapt.
So, while life is certainly vibrant and hardy, it does require a narrow range of conditions. How do we apply that knowledge to extraterrestrial life? It's difficult, but I suppose it can be done. Exobiologists seem to enjoy trying at least!
If we look at Mars we see a dead planet. However, recent evidence points to a dynamic planet which may have had abundant reserves of liquid water. We believe, as evidenced on Earth, that liquid water is required for "life." (The truth is, thinking about life without water is, well, very difficult. Let's go with what we know.) Other recent findings suggest that life on Earth developed rapidly. Far more rapidly than was first thought. There is good evidence to suggest that life took hold on Earth over 3.9 billion years ago. It's very possible that life developed on Earth over 4 billion years ago. Basically, as soon as the house was ready, life may have moved in.
What would this say about Mars? Well, if Mars had a history which included liquid water, it is possible that life took hold. We know from our own history that life seems to grab at the first chance it gets. (We have a humongous representative sample of "1" ) If Mars' life-giving environment was stable enough, for long enough, then life should have taken root.. somewhere. But, there is a catch. Mars has problems. It doesn't have the gravity to hold on to it's atmosphere. It does not have a protective magnetic field similar to Earth's. Radiation from solar winds could cause problems. (Mars does not exhibit the "dynamo" effect that Earth does. It's mag field is different. This is fairly rare planets in our system.) These factors combine to make Mars, presently, an inhospitable planet. The water seems to either have frozen underground or been vaporized due to low pressures and leached out into space. What could have happened to life?
Life could have developed on Mars. But, it would have been fated to die. As conditions slowly changed, the very small range within such life could have survived would have been drastically effected. As we know from our own history, if such changes last long enough or take place quickly enough, life can neither survive nor adapt. We only survived 8 mass extinctions and 3 minor ones because conditions on Earth stabilized rapidly enough or environmental changes did not cover a large spectrum. If that hadn't of happened, we wouldn't be here.
It is possible that there could be some form of life left on Mars. If there had ever been any to begin with that is. I agree that it is somewhat difficult to destroy all life on a planet. However, Earth has never been subject to the conditions that Mars exhibits. There could very well be a "breaking point" where life is doomed to oblivion. If life still exists on Mars, the best bet, imho, would be microbial life beneath the surface. Beyond that, I would say that Mars may be a dead rock. If liquid water existed on Mars long enough for life to take hold, then I believe that there is a 100% chance that we will discover evidence for life when/if the Mars Mission gets there.
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but I don´t think we´ll live to see other life forms.
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) I can't help but comment on.
