ASTROBIOLOGY PART VI-PLANETARY NEIGHBOURS

 

Looking at all the habitats of life on the earth, 3 factors emerge as the essential prerequisites for life as we know it.

1.       Supply of energy with which organisms can exist. In the Earths case this is the Sun. In a few cases for some organisms, it can also be the Geothermal energy originating from within the Earth itself.

2.       Stock of basic elements needed to build Amino acids and Nucleotides—Carbon, Hydrogen, Oxygen & Nitrogen. All these elements are common throughout the Solar system in the form of Carbon dioxide, Methane & Ammonia.

3.       It is the 3^rd one that sets apart Earth from the other planets and moons. It is the presence of liquid water. It provides a solvent for biochemical reactions to take place. It is possible that other liquids may also play this role but as of now we do not know.        

It is very difficult to find liquid water on the other planets of the Solar system. If we look at our neighbors Venus and Mars, they are rocky planets of a similar composition to earth with Venus 5% smaller than Earth and Mars about half its size.

Mars gets lesser solar heating than the earth being 50% far away to the Sun than the earth, while Venus gets more solar heating being 25% near to the Sun than the Earth.

The problem on Venus is it has an atmosphere of Carbon dioxide which produces the Greenhouse effect by trapping the Suns radiation without letting it dissipate into space. This heats up Venus to a temperature of 460 degrees Celsius which of course is too hot for water. In the past before the Greenhouse effect took hold Venus would have had rudimentary form of single celled life and it is probable that such life may even now survive in the upper atmosphere of Venus about 50 Km from its surface where the temperature & pressure are similar to the earth. The dense clouds there contain all the ingredients for life including tiny droplets of liquid water. So life on Venus is still a possibility, however in a most rudimentary form.

On Mars the scientists do expect to find life, if not today, perhaps later. Climatic conditions of Mars were more Earthlike when life appeared on our Earth, but since then have gone in the opposite direction. Mars now has a very thin atmosphere having lost most of it on account of its smaller size and weaker gravity. The thin atmosphere coupled with the higher distance from the Sun makes the Temperatures on Mars rarely exceed 0 Degrees Celsius. Low temperature and low atmospheric pressure means no chance of liquid water on its surface.

The Martian surface shows many signs of being shaped by water having valleys carved out by ancient rivers and dried up lake beds. But according to current estimates they were formed 3 billion years ago when water was abundant on Mars. If we remember that at that time Earth already had life for 1 billion years in the form of primitive Prokaryotes, so the situation would have then been similar on Mars.

A dramatic incident occurred in August 1996 that seemed to support this possibility. The then US President Bill Clinton has announced about microscopic bacteria found in a meteorite. It was the first rock to be studied in 1984 by the government funded scientists of the US and the rock is named ALH84001. This rock weighed slightly less than 2 Kgs. We know that it came from mars because of its distinctive composition. Radiometric dating revealed that the rock was sitting on Mars, it was fractured and permeated by flowing water somewhere between 4 and 3.6 billion years ago.

It was then claimed by the scientists who unravelled that the fractures were colonized by Martian bacteria. Photographs taken with an Electron microscope revealed structures that looked like fossils of tiny bacteria. The magnetite obtained from the rock also looked like bacterial magnetite that occurs on the Earth. The meteorite also contained amino acids. I am posting a pic of the electron microscope at the end of this article. However, based on this it is very difficult to conclude if they are fossilized remains of Martian bacteria.

So far there have been many missions to mars, however, there is no conclusive proof that life exists there.

Now, it is defined that water is a must for life to originate. The inner planets Mercury, Venus & Mars have very less of it. But then, so also on Earth, mass wise. Water comprises of only one fiftieth of 1% or just 0.02% of our planets mass.

As one goes past the orbit of Mars, the picture changes drastically. The outer solar system consists of enormous amounts of water, but in frozen condition because the temperatures there are well below freezing point of water.  

Jupiter’s largest moon Ganymede and Saturn’s largest moon Titan are almost 50% water, but all frozen. However, in both these moons and the other moons in the outer solar system there is huge gravitational attraction acting as all the outer planets are huge. On the side of the moon facing the planets the attraction is particularly high and this causes volcanic activity in them which forms a good source of heat.   

The first moon of Jupiter “Io” is the most volcanically active in the entire solar system on account of the huge gravitational force of Jupiter acting on it. The next moon of Jupiter Europa has frozen seas of water. However, it is postulated that liquid water would be present in its depths where life can exist.

NASA is launching a spacecraft “Clipper” to explore Europa in about 2024, to be followed by “JUICE” from European Space Agency to explore the next moon ‘Ganymede” which also has ice oceans.

NASA has tentative plans for Europa lander which would touch down on its surface and conduct experiments. However, there are reservations for this program. However much we may sanitize a craft, millions of microbes still cling to it. If we transport these Earth microbes to Europa one does not know what the repercussions would be. If it has a thriving ecosystem, then it may get destroyed by the invasions of earthly microbes.

Jupiters moon “Enceladus” is another candidate for having icy oceans on its surface. It is just 500 Km in diameter. Scientists found that it shoots out water in the form of Geysers, sometimes as far as 900 Km into space. That would enable a spacecraft to analyze its composition without actually landing on it.

The final promising candidate in the soalr system is “Titan” the largest moon of Saturn which is the only moon in the Solar system which has a significant atmosphere. In fact its atmospheric pressure is 50% more than that of earth. The commonest element in it is Nitrogen with the rest consisting of Methane and other Hydrocarbons. These hydrocarbons form smoggy clouds and hide its surface forever to view.

The Cassini space mission of NASA revealed that Titan consists of mountains of frozen ice and the lakes and rivers are formed by methane.  It has methane rain falling from methane clouds. It is possible that a different sort of life has evolved in Titan using Methane as a solvent than water.               

In 2009, Sarah Horst of the University of Arizona ran the Miller experiment tailored to conditions on Titan. This she had done without any liquid water. Yet the experiment yielded not just amino acids but also the nucleotides. In all likelihood the life either on Titan, Europa, Ganymede or Enceladus would only be microscsopic.