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Aki Roberge
April 21, 1998
Second Year Seminar Paper
Johns Hopkins University


The Planets After Formation




Life in the Universe:

Life is most easily identified as a process, rather than a thing. The characteristics are 1) that life takes in molecules and turns them into new molecules, 2) the new molecules are turned into new structures, 3) unused material is ejected, and 4) life reproduces itself. However, as one can see, all these characteristics could be said to apply in some sense to fire, which we do not consider to be alive. Thus, the identification of life can be a tricky business.

A) Life on the Earth

The basic components of life as we know it are carbon, hydrogen, oxygen, and nitrogen (CHON elements), plus liquid water. Obviously, water is made up of some of the preceeding elements, but it seem to be particularly necessary for life on Earth. It is thought that water is required to provide a medium in which organic molecules may be brought into contact and react with one another. A famous experiment from the 1950's tried to see just how life might have arisen on the early Earth. The Miller Experiment simulated the primitive Earth atmosphere as a combination of hydrogen, ammonia, methane gases, and water vapor. This was placed in a chamber above a pool of liquid water. Electric sparks were then produced in the vapor, simulating lightning, and providing energy. When the chamber was opened several days later, the pool of water was found to be full of amino acids, the building blocks of proteins. The experiment was repeated with varying conditions, and it was found that these amino acids would arise with many different atmospheres and energy sources. Scientists have found extraterrestrial amino acids in several meteorites as well. Thus, they conclude that the building blocks of life may arise easily. This is also consistent with the fact that life on Earth arose very soon after the planet's formation, before 3.5 billion years ago.

The range of conditions in which life survives on the Earth is very broad.

Temp. range in nature: 228 K (Antarctic ponds) - 363 K (Yellowstone hot vents)
Temp. range in lab: 228 K - 435 K (a 90% variation in temperature)
Pressure range in nature: 0.2 bar - hundreds of bars

B) Life in the Solar System

1) Mars:

The consensus seems to be that there is no life on Mars now. It's too dry and too cold (no liquid water on the surface). Also, the atmosphere is too thin; too much ultraviolet radiation from the Sun penetrates to the surface and breaks apart molecules. But there is strong evidence for liquid water on the surface of Mars in the past. And there is some highly controversial evidence that life may have existed at that time. This evidence is found in a meteorite discovered in Antarctica. There are several points that are indicitave of ancient microbial life in the rock, but I must emphasize, they do not constitute proof. They are listed below, roughly in order of decreasing certainty/relevance.

ALH 84001
(Allen Hills meteorite, Antarctica)


1) The meteorite was infiltrated by liquid water long ago.

2) The meteorite is from Mars.

3) Researchers can see shapes that resemble bacteria, although they are smaller than typical Earth bacteria.

4) They see magnetic mineral grains similar to some produced by bacteria on Earth.

5) Polyaromatic hydrocarbons (PAH's) are present. These organic molecules can be produced by decomposition of bacteria. However, they are also seen in non-biological contexts (i.e in the interstellar medium).

There is a strong possibility that even if the meteorite contains evidence of life, it may not be extraterrestrial; the rock could have been contaminated with Earth bacteria. However, the studies done on this sample have at least clarified the problems and procedures in the search for life beyond our planet.

2) Europa:

This moon of Jupiter has an icy crust that is thought to cover a liquid water ocean. There is certainly also geothermal activity at the bottom of this ocean, driven by the same tidal heating that drives Io's volcanoes. On Earth, dense animal populations have been found around deep sea geothermal vents, at depths of 2550 meters below the surface and pressures of 260 bars. The life does not require sunlight; they satisfy all their energy needs with the heat of the volcanic activity released at the vent. This leads researchers to hope that life could exist in Europa's ocean.

3) Titan:

This moon of Saturn has no water. A dense atmosphere prevents researchers from viewing the surface, but it is though that there may be liquid nitrogen and/or liquid methane on the surface. There are lots of organic molecules present, so it has been speculated that perhaps a different chemistry of life could arise there.

Life Outside the Solar System

Given the abundance of life on Earth, researchers think that if liquid water and CHON elements exist together with the right conditions, then life is likely to arise. So what do we think are the right conditions?

Conditions for a Habitable Planet

1) The central star of the planetary system should have a mass less than or equal to 1.5 times the mass of our Sun. This is to ensure that the star lives long enough for life to evolve on one of its planets.

2) The central star should have a mass greater than or equal to 0.3 times the mass of our Sun. Otherwise, the band of orbital distances at which a planet is temperate is very small.

3) The planet must orbit at the right distance, such that surface temperatures allow liquid water. Also, the orbit should not be too eccentric, or the planet's surface temperatures will vary too extremely.

4) The planet should be large enough so that it has enough gravity to hold a substantial atmosphere. An atmosphere protects life from the harmful effects of ultraviolet radiation from the star, cosmic rays, and bombardments.

Although we have now begun to discover many planets beyond the Solar System, we still do not know what fraction of stars have planets. We suspect that 1% - 30% of stars have planets; this leads to the wild estimate of 6% of stars having habitable planets.

A famous equation in the study of life in the Galaxy is the Drake equation. This equation is highly speculative, but is intended to allow estimation of the number of civilizations in our galaxy capable of making contact with us. I emphasize that there is currently no observational foundation for the factors in the table below (except for the first two). This is not that surprising, since we only inventoried the number of stars in our galaxy in the last several decades or so and only discovered the first planet outside the Solar System in 1995.

Drake Equation


N = # of civilizations within the Galaxy capable of making contact with us


N = n*(fp*fh*ft*fl*fc*fi*fs*fd)


Constants in above equation Pessimistic estimate Optimistic estimate
n : number of stars in Galaxy 10^11 10^11
fp : fraction of stars with planets 0.01 0.3
fh : fraction of stars with planets ever having habitable conditions on at least one planet 0.1 0.7
ft : fraction of planets on which habitable conditions last long enough for life to evolve 0.1 1
fl : fraction of planets on which life evolves 0.1 1
fc : fraction of planets on which habitable conditions last long enough for intelligence to evolve 0.001 0.9
fi : fraction of planets on which intelligence evolves 0.1 1
fs : fraction of planets on which intelligent life endures 10^-7 0.1
fd : fraction of duration of intelligent life during which it retains an interest in contact with Earth-like civilizations 0.001 1
Fraction of stars with planets that have contactable civilizations 10^-19 0.02
N 10^-8 2 x 10^9
Distance to nearest civilization 3 x 10^8 light years (not in our galaxy) 15 light years (right next door)


Thus, one can see the huge range between the optimistic guess and the pessimistic guess. One says that we should be surrounded by contactable civilizations, the other that we are alone in the galaxy. However, bearing in mind that there are billions of galaxies, even the pessimistic estimate still predicts contactable civilizations somewhere in the Universe. (Of course, this is leaving out any consideration of the mechanics of that contact. It assumes that all have sufficient technology to allow communication.) The Drake Equation is not really intended to be a quantitative prediction, but a qualitative thought exercise. There may be many other factors that should be included in the equation, but this is the best that our current understanding of life and our imaginations can do so far.

Conclusion:

I hope this introduction to the richness and diversity of planetary studies has been informative and enjoyable. For those who wish to study further, here are some other good web sites.


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