PAOLO MUSSO

Presentation given at the

"2⁰ Simposio Mondiale sulla Esplorazione dello Spazio e la Vita nel Cosmo"

sul tema

"Intelligenze extra-terrestri e frontiere della Bioastronomia e del SETI"

Repubblica di San Marino, 16 Marzo 2001

One of the main tools for the evaluation of the possible number N of technological civilizations with which we could enter in touch via radio-waves is the famous Drake Equation

where:

n_e = number of suitable planets per ecosphere;

L = mean longevity of a technological civilization.

Anyway, it is possible to find some constraints on this process, which, on the contrary, can be scientifically evaluated. One of them (maybe the main one) is, in my opinion, the problem of energy sources.

The starting point is the consideration that any technological civilization needs, for its development, a large amount of free energy. Obviously, we cannot know what kind of energy sources could be used by another, more advanced civilization. However, it is possible to make some reliable considerations basing on what we can deduce about this subject looking at our own history, because we are able to communicate via radio-waves, consequently our present degree of technological development, as primitive as it could be, is a good touchstone for our purposes.

If we consider carefully the energy sources we know, we’ll find a surprising circumstance: they are all strictly related with life. For example:

1) All the simplest energy sources (that is wood and fossil fuels) are organic and derive directly from living beings.

2) Uranium is much more abundant in the Earth’s surface than in the inner because of its transformation into soluble UO₂, due to the oxidant atmosphere, which is produced by plants. On the other side, it plays a crucial role in keeping the Earth warm both with its radioactivity and its taking part to the CO₂ cycle.

3) Deuterium (which is indispensible for nuclear fusion) is abundant and easily available on Earth due to the presence of a large amount of liquid water, which is indispensible also for life and can still exist because the Earth itself has just the right size and temperature, which are also the right ones for life.

4) Renewable energy sources (as solar, aeolian, hydro-electric, hydrothermal and, obviously, biomasses) are all involved in the basic phenomena of life (i.e. photosynthesis, the presence of great masses of water and a not too rarefied atmosphere, the existence of a warm core keeping the planet at a suitable temperature)...

...and we could go on further.

In conclusion, it’s very likely that, if somewhere there is life, then there will be also energy sources enough to develop technology, because the same processes seem to be involved in generating both.

What we have said implies also that many of the "cosmic coincidences" related with life in the so-called "Anthropic Principle" (as the "fine-tuning" of many cosmic constants, the "carbon cycle" in the stars, the creation and diffusion of heavy elements in the supernovae, the crucial role of comets in bringing water on planets and so on) hold for the energy sources, too. But there are also others, more specific. For example:

1) Most of fossil fuels became available only about 100 millions years ago (that is last week, making the Earth’s age equal to a year), just in time to be at our disposal.

2) Their abundance is almost exactly the one we need to learn how to use the other energy sources before they run dry: only a 10 factor less would be probably enough to make it impossible.

3) In a just a little higher-density universe than ours the abundance of deuterium would be exponentially decreasing, so that fusion would be impossible.

4) The total amount of solar energy per year reaching the Earth is about 100.000 times (2.301x10²⁴ J vs. 1.833x10¹⁹ J) the total amount of energy needed by our technological civilization, and this enables it to exist: with a smaller rate, indeed, our energy production would be too great, in proportion, to be absorbed by the environment without causing terrible unbalances...

Once more, we could go on further.

So, we could even speak about a "Techno-Anthropic" Principle, meaning by this that:

"Our universe is shaped in such a way that its laws allow the birth of technological civilizations".

But, from another point of view, just from a very rough observation we can easily see that, while the total amount of fuel needed to develop a given amount of energy tends to decrease with the development of technology, on the contrary the size of the "match" needed for it tends to increase. The reason is that at every step we are going deeper and deeper into the inner structure of the matter. We don’t need a very deep knowledge, nor a complex technology to get thermal energy from wood or coal, being enough to burn them; but we need something more to build up a steam engine or a ydro-electric central, and something much more to get nuclear fission or, worse, fusion; finally, renewable energies, such as, e.g., solar, in which there is no "fuel" in the proper sense, require (at least at present) even much wider apparatuses to be exploited efficiently.

This relation seems to hold universally, because it is based on the laws of nature. So, it is very unlikely that a technological civilization could ever arise without needing to use, at least for a while, the simplest energy sources, among which a crucial role is played by fossil fuels.

Thus, on one side we should admit that every intelligent species in the universe should have a very high a priori probability (very likely equal to 1) to develop technology. On the other, some precise constraints are implied by what we have said, both on the rising and the duration of a technological civilization.

1) Intelligent life must not evolve too soon, otherwise it could become extinct before a sufficient amount of fossil fuels have formed. Further analyses will be needed to evaluate in a quantitative way, as far as possible, this range of time.

2) Intelligent life must evolve on dry land, because the simplest ways to get energy from nature always imply the need of burning something: so, maybe a technological civilization could develop also under the sea, but certainly it could never begin down there.

3) Just for the same reason, intelligent life must evolve in beings of a suitable size (that is not too small) and provided with something like hands, otherwise they could never be able to manage fire, as requested at the previous point.

4) Scientific progress and the development of technology cannot work under a critical speed, otherwise it would become impossible to reach the degree of "know how" needed to use other energy sources before fossil fuels run dry (or generate a devastating "greenhouse effect"). Further studies will be needed in order to determine the width of this "window", which, however, does not seem likely to be very large (probably no more than some centuries): so, our fastest technological development, about which we are so worried, could turn out to be, in reality, the only possible way for every civilization, with all the related risks we know very well.

5) Technological progress does not necessarily imply a moral one and vice versa. A more "altruistic" attitude than ours, which made energy sources accessible to all people just from the beginning in a more equalitarian way, indeed, would have unavoidably the effect of shortening the "window" we have spoken about at point 4, asking them for a greater (and therefore more difficult) effort in order to develop their scientific knowledge faster. While a more "egoistic" civilization could be even favoured from this point of view, but would run a greater risk of self-destruction.

* In 2000 Peter D. Ward and Donald Brownlee published "Rare Earth: Why Complex Life Is Uncommon in the Universe" (Copernicus, Springer-Verlag, New York).

* In their book the authors pointed out that if you take in consideration all the factors needed to allow the birth, long-term preservation and evolution of multicellular beings (i.e. animals) it could turn out that they are so many, fine-tuned and improbable, that Earth is indeed a very peculiar and rare place in the Universe, and so complex life, too.

* Ward and Brownlee indicated not less than 18 factors (largely independent from each other) responsible for about 40 features, each being relevant (and often crucial) to allow complex life to evolve until intelligence. Most of them are very unlikely an their relative independence implies that their probabilities must tactorize, making the resulting probability for intelligence to evolve elsewhere than Earth really very, very low.

1) Right distance from star

2) Right mass of star

3) Stable planetary orbits

4) Right planetary mass

5) Jupiter-like neighbor

6) A Mars

7) Plate tectonics

8) Ocean

9) Large Moon

10) The right tilt

11) Few giant impacts

12) The right amount of carbon

13) Atmospheric properties

14) Biological evolution

15) Evolution of oxygen

16) Right kind of galaxy

17) Right position in galaxy

18) Wild Cards (random events)

This is the way Ward and Brownlee summarized "Our Rare Earth" features:

"Our planet coalesced out of the debris from previous cosmic events at a position within a galaxy highly appropriate for the eventual evolution of animal life, around a star also highly appropriate -a star rich in metal, a star found in a safe region of a spiral galaxy, a star moving very slowly on its galactic pinwheel. Not in the center of the galaxy, not in a metal-poor galaxy, not in a globular cluster, not near an active gamma ray source, not in a multiple-star system, not even in a binary, or near a pulsar, or ear stars too small, too large, or soon to go supernova. We became a planet where global temperatures have allowed liquid water to exist for more than 4 billion years -and for that, our planet had to have a nearly circular orbit at a distance from a star itself emitting a nearly constant energy output for a long period of time. Our planet received a volume of water sufficient to cover most -but not all- of the planetary surface. Asteroids and comets hit us but not excessively so, thanks to the presence of giant planets such as Jupiter beyond us. In the time since animals evolved over 600 million years ago, we have not been punched out, although the means of our destruction by catastrophic impacts are certainly there. Earth received the right range of building materials -and the correct amount of internal heat- to allow plate tectonics to work on the planet, shaping the continents required and keeping global temperatures within a narrow range for several billion years. Even as the Sun grew brighter and atmosphere composition changed, the Earth’s remarkable thermostatic regulating process successfully kept the surface temperature within livable range. Alone among terrestrial planets we have a large moon, and this single fact, which sets up apart from Mercury, Venus and Mars, may have been crucial to the rise and continued existence of animal life on Earth." (pp. 282-283)

* What about all that and my 5 "energetic constraints"? All of them are concerned by the "Rare Earth Hypothesis", at least at some degree, but the most matched is certainly point n. 2.

We could object, indeed, that Ward and Brownlee’s theory is too much anthropocentric, even though I don’t think so. Anyway, they admit that submarine life is less affected by climate changes and cosmic catastrophes due to its water shield. So, we could imagine that complex animal life, and intelligence, too, are not so rare, after all, often evolving into the sea.

* But what about intelligence able to develop technology? We have seen that, due to energetic reason, it cannot evolve but on dry land.

Now, making continents is not a simple matter. It is not enough having a surface full of holes and bumps and some water to throw into them: erosion, indeed, would quickly disrupt any bump or even mountain you can conceive.

* Thus, continents need to be continuously built up. And the only known continent-building gear is plate tectonics.

*In the subduction zones lower, heavy basalt goes down remelting once more, upper hydrated basalt forms a liquid magma and goes up, forming the lighter, floating andesite and granite continental crust.

*In the oceanic ridges magma, carried up by gigantic convection cells, spread off and forms the heavy, basaltic oceanic crust.

1) Right distance from star…

…to allow the presence of liquid water

4) Right planetary mass…

…to retain water gravitationally and to start plate tectonics with its inner heat and its solid/molten core

7) Plate tectonics…

…,to form stable, non-volcanic dry land

8) Ocean (not too much, not too little)…

… to "soften" the crust, allowing it to drift, and to create light rocks forming continents able to "float" over

9) Large Moon…

…to provide during its formation the heat to start plat tectonics early

16) Right kind of galaxy (rich in heavy elements)…

…to provide the right composition of the core for plate tectonics

* If Ward and Brownlee are correct, it could seem as a sunset of darkness goes down upon the SETI dreams. Their conditions, indeed, are very severe and restrictive.

Among all planets and moons of Solar System, Earth alone has plate tectonics. Furthermore, its "engine" is so complex and strictly related to Earth’s size and chemical composition, that even very little variations of them could make it impossible.

Despite what one could think, ocean could be an even more complex problem.

It’s easy to say, indeed, "not too much, not too little", but it’s not so easy to make it so. Just a 100-meter-deeper ocean, indeed, would be enough to submerge a huge percent of our present dry land. And, in fact, on Earth there is such an ocean, but, for our good luck, part of it is frozen (mainly in the Antarctic Icecap). So, the existence of large continents on Earth is the result of very complex and fine-tuned interrelations of both chemical relative abundances and atmospheric balances, which are likely to be very rare.

The worst, however, is that complex life on dry land is very less protected than submarine one. So, at least for technological intelligent beings we could be forced to accept many (or even all) of the other Rare Earth constraints.

* A "not-so-bad" bad new for SETI.

If intelligent life were really extremely rare, this would be certainly a bad new for SETI scientists and fans. At the same time, it would also mean that SETI is our only chance to establish a contact with any other civilization.

* A new horizon.

Furthermore, if Ward and Brownlee were right, it would mean that SETI is much more difficult than we’ve ever thought. But at the same time it would mean that SETI is also much more simple than we’ve ever thought. It would turn out, indeed, that our search should be directed only toward a very little number of very peculiar stars, which scientific progress will identify better and better in time.

I want to thank very much Dr. Carlo Sozzi (Institute of Plasma Physics of CNR, Milano), who has suggested me the idea for the first part of my talk and has given me also a great aid in adjusting some technical points.