In this article I would like to discuss the conditions that enabled life to evolve on Earth, and the possibility of such conditions existing elsewhere within the Universe.
First of all let us focus on the position of Planet Earth around the Sun. Earth is the third planet in a succession of the rocky planets within the inner Solar System, after Mercury and Venus respectively, and ahead of Mars.
That means that Earth is within the Sun’s Habitable Zone; the Earth’s orbit is just the right distance from the Sun to ensure the right conditions for liquid water to be present at its surface. But why is this important?
So far we haven’t found any life forms that could exist without at least some access to liquid water, since most biochemical reactions requires water as a solvent or reactant. It is believed, that life on earth orgnially originate in a primordial sea or ocean of liquid water as well as other biochemical compounds needed for life.
Another very important factor to make a planet habitable is a atmosphere capable of providing protection from deadly cosmic radiation, and to prevent water loss in to space (which is believed to be what happened on Mars).
In order to develop a life sustainable atmosphere, the planet has to have sufficient gravity, otherwise the atmosphere would escape in to space, therefore the planet needs to have a substantial mass.
In the case of Earth- the atmosphere originated by the degassing of the interior. Active volcanism played a huge role, by transferring large amounts of carbon dioxide in to the atmosphere, stabilizing the average surface temperature of our planet. This leads to another important feature – active geology. On planet Earth, active geology not only regulates the atmospheric carbon dioxide levels, but also provides all life with an additional shield- the Earth’s magnetic field.
The magnetic field produced by Earth’s liquid core prevents many highly energized particles from the Sun (Solar wind, flares and plasma), from colliding with Earth’s upper atmosphere, possibly damaging it, and eventually removing it all together over time!
Earth’s magnetic field
What about other candidates for hosting life?
The obvious choice, would be to look for Earth – like planets, with similar size, mass, distance from its parent star and composition, after all this is the best and only example we know so far.
What about other planets within the Solar System?
The most promising and famous is our smaller planetary neighbor – Mars.
It is believed that Mars, now a barren and cold world once had liquid water on its surface, and possibly life supporting conditions.Currently, there are signs suggesting that water still could be present, frozen within the Martian ice caps, or even, in a liquid form under the surface, mixed with soil.
Mars doesn’t have active geology, a thick atmosphere or strong magnetic field, and is just on the outer edge of the Sun’s habitable zone.
What about elsewhere?
There is one particularly interesting and intriguing place to look for life supporting conditions and even life itself… Europa, one of Jupiter’s 67 moons. Jupiter itself is well outside the Sun’s habitable zone, and Europa is a very cold and icy moon.
However, there is evidence that Europa’s surface is undergoing active geological changes, there are many grooves, ridges, rafts and faults visible, and it is also believed that under the thick layer of an icy surface, there might be an ocean of liquid water!
Possible source of energy needed to heat up Europa’s interior comes from an unusual source-not the Sun, but mighty Jupiter itself. The biggest planet in the Solar System, has such a massive gravity, that it stretches and contracts Europa’s interior, generating heat in a similar manner to a spoon being heated up by bending. It has been suggested, that within Europa’s hidden ocean, there might be hot vents similar to those found in Earth’s ocean, capable of supporting eco systems independently from the Sun’s energy.
Surface of Jupiter’s moon-Europa.
Currently they are many space missions either ongoing, or planned within the near future to investigate the possibility of life being present within the Solar System. There are also extensive efforts being conducted to look for Earth-like planets within our own galaxy, and the for possible traces of extraterrestrial life!