Billions & Billions (8 page)

Any astronomer’s choice of the four most interesting problems will be idiosyncratic and I know many would make choices different from mine. Among other candidate mysteries are what 90 percent of the Universe is made of (we still don’t know); identification of the nearest black hole; the bizarre putative result that the distances of galaxies are quantized—that is, galaxies are at certain distances and their multiples but not at intermediary distances; the nature of gamma ray bursters, in which the equivalent of whole solar systems episodically blow up; the apparent paradox that the age of the Universe may be less than the age of the oldest stars in it (probably resolved by the recent conclusion, using Hubble Space Telescope data, that the Universe is 15 billion years old); the investigation in Earth laboratories
of returned cometary samples; the search for interstellar amino acids; and the nature of the earliest galaxies.

Unless there are major cuts in the funding for astronomy and space exploration worldwide—a doleful possibility by no means unthinkable—here are four questions
^*
of enormous promise:

1.
Was There Ever Life on Mars?
The planet Mars is today a bone-dry frozen desert. But all over the planet there are clearly preserved ancient river valleys. There are also signs of ancient lakes and perhaps even oceans. From how cratered the terrain is, we can make a rough estimate of when Mars was warmer and wetter. (The method has been calibrated by cratering on our Moon and radioactive dating from the half-lives of elements in lunar samples returned by
Apollo
astronauts.) The answer is about 4 billion years ago. But 4 billion years ago is just the epoch in which life was arising on Earth. Is it possible that there were two nearby planets with very similar environments, and life arose on one but not the other? Or did life arise on early Mars, only to be wiped out when the climate mysteriously changed? Or might there be oases or refuges, perhaps subsurface, where some forms of life linger into our own time? Mars thus raises two fundamental enigmas for us—the possible existence of past or present life, and the reason that an Earth-like planet has become locked into a permanent ice age. This latter question may be of practical interest to us, a species that is busily pushing and pulling on its own environment with a very poor understanding of the consequences.

When
Viking
landed on Mars in 1976, it sniffed the atmosphere, finding many of the same gases as in the Earth’s atmosphere—carbon dioxide, for example—and a paucity of gases prevalent in the Earth’s atmosphere—ozone, for example.
What’s more, the particular variety of molecule, its isotopic composition, was determined and was in many cases different from the isotopic composition of the comparable molecules on Earth. We had discovered the characteristic signature of the Martian atmosphere.

A curious fact then transpired. Meteorites—rocks from space—had been found in the Antarctic ice sheet, sitting directly on top of the frozen snows. Some had been discovered by the time of
Viking
, some after; all had fallen to Earth before the
Viking
mission, often tens of thousands of years before. On the clean Antarctic ice shelf, they were not difficult to discern. Most of the meteorites so collected were brought to what in the
Apollo
days had been the Lunar Receiving Laboratory in Houston.

But funding is very meager at NASA these days, and not even a preliminary look at all these meteorites had been performed for years. Some turned out to be from the Moon—a meteorite or comet impacting the Moon, spraying Moon rocks out into space, one or some of which land in Antarctica. One or two of these meteorites come from Venus. And astonishingly, some of them, judging by the Martian atmospheric signature hidden away in their minerals—come from Mars.

In 1995—96, scientists at NASA’s Johnson Space Flight Center finally got around to examining one of the meteorites—ALH84001—that proved to come from Mars. It looked in no way extraordinary, resembling a brownish potato. When the microchemistry was examined, certain species of organic molecules were discovered, chiefly polycyclic aromatic hydrocarbons (PAHs). These are not in themselves all that remarkable. Structurally they resemble the hexagonal patterns on bathroom tiles with a carbon atom at each vertex. PAHs are known in ordinary meteorites, in interstellar grains, and are suspected on Jupiter and Titan. They do not by any means indicate life. But the
PAHs were arranged so that there were more of them deeper in the Antarctic meteorite, suggesting that this was not contamination from Earthly rocks (or automobile exhaust), but intrinsic to the meteorite. Still, PAHs in uncontaminated meteorites do not indicate life. Other minerals sometimes associated with life on Earth were also found. But the most provocative result was the discovery of what some scientists are calling nanofossils—tiny spheres attached one to another, like very small bacterial colonies on Earth. But can we be sure that there are no terrestrial or Martian minerals that have a similar form? Is the evidence adequate? For years I’ve been stressing with regard to UFOs that extraordinary claims require extraordinary evidence. The evidence for life on Mars is not yet extraordinary enough.

But it’s a start. It points us to other parts of this particular Martian meteorite. It guides us to other Martian meteorites. It suggests the search for quite different meteorites in the Antarctic ice field. It hints that we search not just for other deeply buried rocks obtained from or on Mars, but for much shallower rocks. It urges upon us a reconsideration of the enigmatic results from the biology experiments on
Viking
, some of which were argued by a few scientists to indicate the presence of life. It suggests sending spacecraft missions to special locales on Mars which may have been the last to surrender their warmth and wetness. It opens up the entire field of Martian exobiology.

And if we are so lucky as to find even a simple microbe on Mars, we have the wonderful circumstance of two nearby planets, each with life on it in the same early epoch. True, maybe life was transported by meteorite impact from one world to another and does not indicate independent origins on each world. We should be able to check that by checking the organic chemistry and morphology of the life-forms uncovered. Maybe life arose on only one of these worlds, but evolved separately on both. We
then would have an example of several billion years of independent evolution, a biological bonanza available in no other way.

And if we are most lucky, we will find really independent life-forms. Are they based on nucleic acids for their genetic coding? Are they based on proteins for their enzymatic catalysis? What genetic code do they use? Whatever the answers to these questions, the entire science of biology is the winner. And whatever the outcome, the implication is that life may be much more widespread than most scientists had thought.

In the next decade there are vigorous plans by many nations for robot orbiters, landers, roving vehicles, and subsurface penetrator spacecraft to be sent to Mars to lay the groundwork for answering these questions; and—maybe—in 2005 a robotic mission to return surface and subsurface samples from Mars to Earth.

2.
Is Titan a Laboratory for the Origin of Life?
Titan is the big moon of Saturn, an extraordinary world with an atmosphere ten times denser than the Earth’s and made mainly of nitrogen (as here) and methane (CH
₄
). The two U.S.
Voyager
spacecraft detected a number of simple organic molecules in the atmosphere of Titan—carbon-based compounds that have been implicated in the origin of life on Earth. This moon is surrounded by an opaque reddish haze layer, which has properties identical to a red-brown solid made in the laboratory when energy is supplied to a simulated Titan atmosphere. When we analyze what this stuff is made of we find many of the essential building blocks of life on Earth. Because Titan is so far from the Sun, any water there should be frozen—and so you might think it is at best an incomplete analog of the Earth at the time of the origin of life. However, occasional impacts by comets are capable of melting the surface, and it looks as if an average place on Titan has been underwater for a millennium, more or less, in its
4.5 billion year history. In the year 2004, a NASA spacecraft called
Cassini
will arrive in the Saturn system; an entry probe built by the European Space Agency called
Huygens
will detach itself and slowly sink through the atmosphere of Titan toward its enigmatic surface. We may then learn how far Titan has gone on the path to life.

3.
Is There Intelligent Life Elsewhere?
Radio waves travel at the speed of light. Nothing goes faster. At the right frequency they pass cleanly through interstellar space and through planetary atmospheres. If the largest radio/radar telescope on Earth were pointing at an equivalent telescope on a planet of another star, the two telescopes could be separated by thousands of light-years and still hear each other. For these reasons, existing radio telescopes are being used to see if anyone is sending us a message. So far we have found nothing certain, but there have been tantalizing “events”—signals recorded that satisfy all the criteria for extraterrestrial intelligence, except one: You turn the telescope back and point at that patch of sky again, minutes later, months later, years later; and the signal never repeats. We are only at the beginning of the search program. A really thorough search would take a decade or two. If extraterrestrial intelligence is found, then our view of the Universe and ourselves is changed forever. And if after a long and systematic search we find nothing, then we may have calibrated something of the rarity and preciousness of life on Earth. Either way, this is a search well worth doing.

4.
What Is the Origin and Fate of the Universe?
Astonishingly, modern astrophysics is on the verge of determining fundamental insights on the origin, nature, and fate of the entire Universe. The Universe is expanding; all the galaxies are running away from each other in what is called the Hubble flow, one of three main pieces of evidence for an enormous explosion
at the time the Universe began—or at least its present incarnation. The gravity of the Earth is strong enough to pull back a stone thrown up into the sky, but not a rocket at escape velocity. And so it is with the Universe: If it contains a great deal of matter, the gravity exercised by all this matter will slow down and stop the expansion. An expanding Universe will be converted into a collapsing Universe. And if there is not enough matter, the expansion will continue forever. The present inventory of matter in the Universe is insufficient to slow the expansion, but there are reasons to think that there may be a great deal of dark matter that does not betray its existence by giving off light for the convenience of astronomers. If the expanding Universe turns out to be only temporary, ultimately to be replaced by a contracting Universe, this would certainly raise the possibility that the Universe goes through an infinite number of expansions and contractions and is infinitely old. An infinitely old Universe has no need to be created. It was always here. If, on the other hand, there is insufficient matter to reverse the expansion, then this would be consistent with a Universe created from nothing. These are deep and difficult questions which every human culture has one way or another tried to grapple with. But only in our time do we have a real prospect of uncovering some of the answers. Not by guesses or stories—but by real, repeatable, verifiable observations.

—

I think there is a reasonable chance that startling revelations in all four of these areas can be expected in the next decade or two. Again, there are many other questions in modern astronomy that I could have substituted, but the prediction I can make with the highest confidence is that the most amazing discoveries will be ones we are not today wise enough to foresee.

I
n December 1995, an entry probe, detached from the
Galileo
Jupiter orbiter, entered the turbulent, roiling atmosphere of Jupiter and sank to a fiery death. Along the way it radioed back information on what it found. Four previous spacecraft had examined Jupiter as they raced by. The planet has also been studied by ground-based and space telescopes. Unlike the Earth, which is made mainly of rock and metal, Jupiter is made mostly of hydrogen and helium. It is so big that a thousand Earths could fit inside. At depth, its atmospheric pressure gets so large that electrons are squeezed off atoms and the hydrogen becomes a hot
metal. This state of affairs is thought to be the reason that twice as much energy comes pouring out of Jupiter than Jupiter gets from the Sun. The winds that buffeted the
Galileo
probe at its deepest entry point probably arise not from sunlight but from the energy originating in the deep interior. At the very core of Jupiter there seems to be a rocky and iron world many times the mass of the Earth, surmounted by the immense ocean of hydrogen and helium. Visiting the metallic hydrogen—much less the rocky core—is beyond human abilities for at least centuries or millennia to come.

The pressures are so great in the interior of Jupiter that it is hard to imagine life there—even life very different from our own. A few scientists, myself among them, have tried, just for fun, to imagine an ecology that might evolve in the atmosphere of a Jupiter-like planet, somewhat like the microbes and fish in the Earth’s oceans. The origin of life might be difficult in such an environment, but we now know that asteroidal and cometary impacts transfer surface material from world to world, and it is even possible that impacts in the early history of the Earth transferred primitive life from our planet to Jupiter. This, though, is the merest speculation.