Which primary conditions are necessary
for life as we know it?

Saskia J. Bruining

This is one of several subquestions related to the second year astronomy groupproject.

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Going to extremes

Life can exist in the strangest inhospitable places on earth. It are mostly bacteria or algae that apperently choose extreme lifeconditions.
Especially in the deep-seas live so-called "super-thermophilic" (=extremely-heatloving) micro-organisms, these not only exist, but thrive at temperatures even beyond 150 ° C (Baross & Deming, 1993). They inhabit pressurized enviroments beneath deep-sea hydrothermal vents. At this temperatures you might expect the water to boil, but it doesn't because of the immense pressure. Recently bacteria were discovered which live at an astonishing 169 ° C!
It's not only the temperature, lots of organisms at places where no sunlight ever comes use chemicals like hydrogen sulfite as their energy source. The bacteria inturn sustain larger organisms in the ventcommunity.

Also well into the Earths crust, the toplayer of the earth, microbes are found. The mounting pressure has little direct effect on them even at several kilometres below groundlevel. It is the increasing temperature that limits the depth of life beneath the surface. In the oceanic crust the temperature rises about 15 ° C per kilometre. So microbial life extends on average about 7 kilometres below the sea floor. For continental crust the microscopic life should reach almost 4 kilometres into the earth, for the surface temperature is approximately 20 ° C and it rises with 25 ° C per kilometre. However the amount of micro-organisms will vary from place to place.


Bacteria
From biology-courses we know that bacteria have an optimum curve, which means that at certain temperatures they have a peak where they thrive most. At too cold temperatures they go into somesort of stasis, you could compare it with hibernation, and they do not show any characteristics of life. If the temperatures increase they become active again. At too hot temperatures, however, they are damaged too severly that the damage is irretrivable.

Conditions necessary for life


If we want to investigate the possibility of life on other planets we have to figure out what exactly we are looking for. You cannot go marching off looking only for planets that resemble Earth as it is nowadays. We shall have to go back to the formation of the Earth and life's origin, things were very different then...

Origin of life


Earth formed about 4.6 billion years ago (= 4.6 Giga years) as third planet in a series of nine circling around a star, the Sun. The Sun and the planets are made from stellardebris of stars that came to the end of their existance. Everything we see around us is actually recycled stardust.
After the formation Earth was a pockmarked planet of roughly uniform composition and had an early atmosphere of mainly hydrogen (H2). Then radioactive heating began to melt the interior and the core was formed. Now this heating had as a consequense that degassing from the planets interior created a second atmospere rich in water (H2O), carbon dioxide (CO2), methane (CH4) and Ammonia (NH3). When the surface had cooled enough intense rains began to fall and created the oceans.

It is generaly believed in science that this "prebiotic soup" as it is called is where life originated. The famous Miller-Urey experiment duplicated the conditions of early Earth in a laboratory (Orgel, 1994). In a self-contained apparatus an "atmosphere" consisting of hydrogen, water, methane and ammonia was created above an "ocean" of water. These gasses were subjected to "lightning" in the formof an electrical discharge. They found that 10% of the carbon (C) in the system had converted into organic compounds and 2% of this carbon went on to make amino-acids, the buildingblocks of our carbon-based life.
Although doubt has arisen because recent investigations indicate that Earth's early atmosphere may have contained more gasses than in the experiment, like CO2 (Orgel, 1994)(De Jager, 1995). But it is still the best theory we have.
These amino-acids that formed on Earth are very important, they are the buildingblocks of the nucleic-acids RNA and DNA which in their turn carry the genetic information of organisms. Whether RNA arose spontanous or replaced some earlier geneticsystem is not quite clear. But its development was probably the key in the development of life. It very likely led to the synthesis of proteins, the formation of DNA and the emergence of a cell that could be the ancestor of all current lifeforms as theory implies. (Orgel, 1994).

Nowadays one of the vital needs for survival of complex organisms is the presence of oxigen (O2). Yet if one looks at Miller-Urey's experiment one sees that no free oxigen has been included in the initial mixture of gasses. Free oxigen was little or not present at the creation of life on the Earth. For it is an agressive element, it oxidizes other chemicals; it subtracts hydrogen from existing molecules. Therefore under oxidizing conditions amino-acids do not or very little form.
The atmosphere plays a major part in the creation of life and sustaining it

More about life and atmosphere.

Development of life



What has to be said, is that simple models of the evolution of life and its development do not apply. Webs and chains in the past are so intricate and full of random and chaotic events. All scientists have to make assumptions especially on this matter.

As stated before, Earth formed approximately 4.6 billion years ago. It had a to endure heavy bombardments of cosmicdebris. These heavy bombarments ended about 3.8 billion years ago (Kasting, 1993). We know this because the oldest rocks date from that period. The large impacts before this date melted the Earth's surface so no solid rock could exist. Still the oldest rocks to hold cellular fossils date from approximately 3.5 billion years ago(Gould, 1994). Which means that life on Earth evolved quickly and is really old. Those first fossils were of bacteria. Bacteria represent more or less the simplest forms of life, so the only way to expand was in width and height. Or to put it differently, to expand in diversity and in complexity.

After the bacterial cells the cells belonging to the plant- and animal kingdom began to to evolve around 2 billion years ago. Yet, life remained unicellular for the first five sixths of its history. Some of the multicellular algae evolved a bilion yars ago.
But no record can be found of multicellular animal organization for the span of 3 bilion years. Even more surprisingly, all major stages organizing animal life's multicellular design occured in a very small timespan. It began less than 600 milion years ago and lasted until 530 million years ago, but the steps are not gradually, they're discontinous. Though it actually took only five million years of intense creativity to develop, called the Cambran explosion,followed by 500 million years of variation.
It is not known how nature came up with these anatomical designs so quickly. But this first period of both internal and external flexability gave a range of invertabrate anatomies that may have outnumbered the full range of animal form in all Earth's enviroments today.

The question is why did most of these early experiments die out, while others survived. It's more by luck than by a predictable struggle for existance that those organisms we know survived. For mass extinction mark the boundaries of divisions of geologic-timescale. It is thought that these extinctions were mainly caused by impacts of large extraterrestrial objects which smashed into the Earth (the last of these, about 65 million years ago, is thought to have wiped out the dinosaures). Mass extinctions are not randomly distributed in their inpact on life. Some descendants die and others survive as a practical outcome on presence or absence of evolved characteristics. But if the triggering cause of the extinction is sudden catastrophic, reasons for death or life may lie very close to eachother, they may be random. (Gould, 1994).
Extended evolutionary theories go beyond the scope of this project.

What do we call life?

A reasonable biological definition of life:
Living systems are capable of: metabolism, growth, reaction to stimuli, reproduction, mutation and reproduction of its mutations.


Necessity of water


It seems that for life on Earth water was and is of the utmost important to us. Earth is clearly distinct from other (terrestrial) planets by its wetness.We will now look at the right distance from a star to allow a planet to have liquid water.
We have a planet orbiting at a star. This star has a radius Rs and a temperature Ts. The planet has an albedo (=reflectance) A of its surface. Then its equilibrium temperature will be Tp. The distance between the star and the planet we call a.

Tp = { (1-A) / sqrt2}power 1/4 • (Rs / a)power 1/2 • Ts   (1)     (J. Schneider, 1995).

If we fill in the values known for Earth we find that A=0.39, Tsun =5770 K, so Tp=280 K (which is very close to the actual 287 K)

So from equation (1) we have a planet having a temperature of approximately 300 ± 20 K to allow for liquid water must be located at a distance from the star given by

a = Rs ( Ts / 300)²   (2)

where the albedo is A=1.
This distance depends on the type of centralstar. So it ranges from approximately 0.1 Au (1Au is the distance between the Sun and the Earth) for cool stars with Ts = 3000 K to about 2 Au for hot stars with Ts = 6500 K.
in the next section we will see that there is more to be dealth with in calculating the habitable distance .

However some assumptions were made in forwarding the two equations we just saw. The one of main interest to us, is that it has we assume to have a solid planet, which would exclude giant gaseous planets like Jupiter. But is this assumption correct? What do we know about Jupiter anyway?

Moreover in 1995 an anouncement was made that large ammount a gassious alcohol had been discovered around a star in its initial formationfase. The temperature of the gas was 125 K, very warm for conditions in interstellarspace. The alcohol and other complicated molecules had probably been formed on dustparticles, when the star got larger and heated the dustparticles, the precipitated gasses evaporated.
In meteorites, some even older than the solarsystem itself, complicated organic molecules were found aswell. (De Jager, 1995).
It seems that no prebioticsoup was needed to create those. But wether life would be able to originate in anyotherway, without the aid of water remains a mystery.


Are we just lucky?


Its very easy to state that Earth is apparently in the right spot to be suitable for life. But this would be very simplistic, there are a lot more factors to be reckoned with. As we saw in the previous paragraph one is those is the albedo or reflectance of a planet. Another major important role is being played by the greenhouse effect. If one takes the planet Venus for example the contribution of the greenhouse effect adds no less than 521 K of warming.

It seems that some important feedbacks exist that control Earth's climate. This we need for tackling the "faint young Sun paradox".The radiation-intensity of the Sun 4 billion years ago was much less then it is now. To be exact about 70% of the current strength (De Jager, 1995). With an atmosphere as Earth has now, the temperature would inevitably have dropped below freezing.
The relative size of our planet has kept its internal heat from leaking away too rapidly, and plate-tectonic activity and vulcanic outgassing have maintained recycling of carbondoixide (CO2) a greenhouse gas. This CO2 is responsible for the greenhouse effect which prevented the Earth from freezing.
The build in temperature dependence feedback effectively removed CO2 from the atmosphere as the Sun luminosity increased. Thus warmer surface temperatures on Earth resulted in more removal of CO2 and cooling the planet's surface (and vice versa).
This feedback runs on "water-mediated silicate rock weathering". Earth is wet, not merely on the outside, but on the inside aswell. Two main reasons for the increased weathering as temperature rises are , first of all, rainfall and runoff increase, exposing more solidrock to erosion. And secondly, respiration of soil organisms. So balance is maintained probably with the help of life additional feedbacks related to life.
But if Earth had experienced an extreme ice age before approximately 3 billion years ago, the situation might heve been irreversible. Since CO2-ice-clouds have a high albedo and would thus block the sunlight.
(Kasting, 1993), (Rampino & Caldeira, 1994).
More about life and atmosphere.


Habitable distances



The region in which planetary temperature are neither too high nor too low for life to develop are is called the Habitable Zone (HZ). Whereas the region around a main sequence star within which planetary temperatures remain within the temperature constraints for habitability. And taking into account the evolution of the stars's luminosity in time is called the Continous Habitable Zone (CHZ).
Taking into consideration many variables some of which we looked at in above text (like albedo, rockweathering, etc.) these distances can be calculated. During the Sun's evolution the HZ evolves outward as the Sun's luminosity increases. It has been estimated that the CHZ over 4.6 billion years ranges from 0.95 to 1.15 Au, a width of about 0.2 Au. The HZ in the Solarsystem is conservatively estimated at 0.95 Au and 1.37 Au. This width will be slightly greater for planets larger than Earth. (Rampino & Caldeira, 1994)
More on the distances in the Solarsystem

Conclusion



The problem with science is that one has to make models and assumptions. These stand until proven wrong. In dealing with life's history and the atmosphere we have made loads of assumptions and either of them may not be true. Fact is that these are the best ideas we have at the moment.
We saw that for the origin of carbon-based life an atmosphere consisting of the gasses H2, NH3, H2O and CH4 above an "ocean" of liquid water is necessary. Oxigen (O2), however, would have disrupted the formation of life. Yet its presence later remains a major key in sustaining life. Mass extiction had a great impact too on the evolution of life.Ofcourse possition, reflectance and greenhouse effect are very important. Is a planet large enough to keep its atmosphere? And is a feedback loop created which keeps the planet from either becoming a frozen world or a hothouse?
All of these things have to be taken into account if we go into the immense regions of space, searching for LIFE (AS WE KNOW IT)!


A very helpful website is the Astrobiologyweb especially Life in Extreme enviroments.

Go to astronomy groupproject index
More about the second year astronomy groupproject.


Literature




Acknowledgements


I wish to thank the following people:

Ms. Penny D. Sackett, whose enthusiasm, dedication and help made this project an interesting and enjoyable one!!!!

The other secondyear astronomy students: guys it may have been a lot of stress, but it was fun!!
Michiel H. Nieuwenhuis thanks for helping me out, this page wouldn't have looked so good if it wasn't for you!
Sybren Bruining, my brother, thank you for the pictures!

My fellow-editors of Muon (our faculty-paper) for whatever they like to be thanked for.
Anneke and Jojenneke (with whom I share an apartment), who had to deal with my scientific gibberish and pretend it was interesting.
Send your mail to Saskia