Faint Echoes, Distant Stars_The Science and Politics of Finding Life Beyond Earth

by Ben Bova

  The age of fossils of once-living creatures can be determined by radiocarbon dating, often abbreviated as carbon dating. When plants take up carbon dioxide from the air, the carbon they “inhale” includes a smattering of the radioactive form of carbon, carbon-14, as well as the normal, nonradioactive carbon-12. Carbon-14’s half-life is precisely known: 5,730 years. By comparing the amount of carbon-12 in a fossil to the remaining amount of carbon-14, the age of the fossil can be determined. Animal fossils (including human) can be dated this way because the animals eat plant matter. This technique only works to an age of about 120,000 years because beyond that time so little carbon-14 remains that it is impossible to measure it.

  Appendix 7

  DNA and RNA

  In the molecular world, DNA—deoxyribonucleic acid—truly is a giant. While complex organic molecules may contain thousands of atoms, DNA consists of tens of millions of atoms.

  The backbone of the DNA molecule consists of two long, spiraling strands that are composed of chains of phosphoric acid (compounds based on the element phosphorus) and sugars (carbohydrates built of carbon, hydrogen, and oxygen atoms), twining around each other like the balustrades of a long circular staircase.

  The phosphate-sugar strands form the twin backbones of DNA’s double helix, the two outer tracks of the twisting ladder. The interesting part of the molecule, though, is the cross-links that hold the two intertwining backbones together, the “rungs” that connect the tracks. These links are made of nitrogen compounds known as bases.

  There are some 3 billion bases in the forty-six chromosomes of each human cell, 3 billion rungs on the chromosomal ladders. Yet, these 3 billion–some bases come in only four varieties. The bases are called adenine, thymine, cytosine, and guanine. They are usually abbreviated by their initial letters: A, T, C, and G.

  Each of the bases is anchored to one of the coiling phosphate-sugar units of the molecule’s twin backbones. This combination is called by molecular biologists a nucleotide. Thus, a DNA nucleotide is composed of one phosphoric acid, one sugar, and one of the four bases: A, C, G, or T. Essentially, a nucleotide is a unit of one of the double helix’s outer tracks, plus one half of one of its interconnecting “rungs.”

  On the other ends of their bases each nucleotide is linked to another base that is anchored onto the opposite spiral backbone of the double helix. The end of one base latches onto the end of another, like two pieces of a jigsaw puzzle. They are called base pairs when they link together. The two spiraling phosphate-sugar “tracks” of each DNA molecule are joined to each other by some 60 million base pairs.

  However, the link between the base pairs is, chemically speaking, rather weak. Moreover, these base pairs are very specific in their choice of partners. Adenine will link only with thymine, and cytosine links only with guanine. Thus, in every cell of every creature that lives on Earth, the bases pair A-T, C-G, or T-A, G-C. No other base pairings are possible.

  On that simple yet fundamental linkage lies the secret of life.

  REPLICATION AND MANUFACTURING

  To reproduce itself, or replicate, the double helix unzips itself. The base pairs split apart and the molecule unravels into two long strands with unmated bases exposed. Meanwhile, unattached nucleotides are floating in the cell’s nucleus. These are linked to the unmated bases of the DNA molecule’s exposed strand in exactly the same order as the bases were originally paired up: A with T, C with G. No other combinations are possible. At the end of this replication we have two identical DNA molecules, each with one of its original spirals and a new one that is exactly like the one it formerly was linked to.

  That is the secret of genetic inheritance. It is how cells reproduce themselves. First the blueprint (the DNA) replicates itself. Then the cell can reproduce a faithful copy of itself. If there is a mistake in the DNA replication or if the DNA is damaged, a mutation results. The genes do not copy themselves exactly, and the daughter cell will be different from its mother—a mutant.

  In addition to replicating itself, DNA also directs the manufacture of new proteins. Proteins are the basic material of our bodies, as fundamental to us as concrete is to a building contractor. The cells of our bodies, the many hormones and enzymes that our glands produce, are all mostly proteins. Proteins ‘R’ Us.

  Genes are groups of bases along the DNA molecule. They vary in length: Some genes are a few thousand bases long, others a hundred thousand or more. The genes direct the cell’s production of proteins; the typical human cell contains about 10,000 different proteins. Genes carry the master blueprints for all the proteins that our bodies are constantly manufacturing, every moment of our lives.

  To build new protein, the double-helix molecule unzips only partially, exposing only a particular gene—a stretch of a few thousand to a hundred thousand or so of its millions of base pairs. The exposed bases are met and matched by free-floating nucleotides that have been carried into the nucleus. Following the pattern of the exposed bases, the nucleotides link together with them. Then the resulting string of nucleotides pulls away from the DNA.

  What has been formed is a molecule of messenger RNA (mRNA). RNA is ribonucleic acid, a molecule that is very similar to DNA, the main difference being that RNA has only a single strand of phosphate-sugar backbone instead of DNA’s double helix. The RNA molecule also has four bases anchored to its backbone. Three of the bases are the same as DNA’s: adenine, guanine, and cytosine. Instead of thymine, however, RNA contains a similar unit called uracil.

  In making a protein, the DNA’s guanine links with the mRNA’s cytosine, just as it does in DNA replication. The adenine in the DNA, however, links with the uracil of the mRNA.

  The sequence of DNA bases serves as a template that builds a new messenger RNA molecule by linking up bases like the units of a Tinkertoy set. Thus, a sequence of bases in the DNA molecule that reads C-C-G-T-A-A will produce a section of mRNA molecule that reads G-G-C-A-U-U.

  The mRNA molecule has transcribed the order of bases from the DNA and carries this as a blueprint for the manufacture of a specific protein. It leaves the cell’s nucleus and heads for one of the ribosomes. The ribosomes are the cell’s assembly shops. In the ribosomes are molecules of transfer RNA (tRNA), each of them toting an amino acid molecule. Amino acids are the building blocks of proteins.

  There are twenty amino acids, yet they can be combined to form thousands of different proteins. Amino acids can be thought of as the words of a very simple language that can be combined in many different ways to form thousands of different sentences.

  In the ribosome, tRNA carries in the “raw materials,” the amino acids. The mRNA brings in the blueprint for building a new protein molecule out of the amino acids. Following the blueprint etched into the mRNA, amino acid molecules are joined together to form a new protein, linking up in the order prescribed by the mRNA like the engine, coaches, and caboose of a train.

  Appendix 8

  Contamination: Sterilization and the Isolation Ward

  Sending spacecraft to land on other worlds brings up the question of contamination. If we are seeking evidence for life on another planet, we must make certain that Earthly life does not “stow away” aboard the spacecraft that lands on the alien planet. Remember the “little bacteria that lived” on the Surveyor 3 spacecraft for more than two and a half years on the airless, waterless, radiation-drenched surface of the Moon.

  Microscopic organisms might be able to ride a spacecraft to Mars or Jupiter’s moon Europa or even farther. Terrestrial organisms could possibly cause the spacecraft’s instruments to give false indications of finding extraterrestrial life. Even worse, terrestrial stowaways might alter or even destroy altogether any “local” life-forms that might exist on the planet being studied. The worst devastation wreaked by invading Europeans upon the Native Americans came not from guns, but from disease organisms for which the Native Americans had no immunities. Smallpox killed more Native Americans than the U.S. Cavalry ever did.

  As part of NASA�
�s planetary protection protocol, all spacecraft intended to land on other planets are carefully sterilized. Robotic landers such as Pathfinder/Sojourner are swabbed with bactericides and bathed in lethal levels of ultraviolet light before they are launched to other worlds. Then the spacecraft spend months in the hard vacuum and high radiation environment of space itself. Still, there is always some chance that some tough little “bug” will survive all that and arrive on the target planet alive and ready to screw up a new world.

  The problems of biocontamination will become much worse when human astronauts begin to travel to Mars or beyond. Although their spacesuits and equipment can be sterilized repeatedly, the procedures will be cumbersome and time-consuming—but vitally necessary.

  There is also the question of back-contamination. Suppose we find living organisms on Mars or Europa or elsewhere. Will they be so totally different from terrestrial life-forms that there is no possibility of their mixing with and altering Earthly organisms? Or might they be able to invade, infect, and infest our planet, possibly causing pandemic plagues for which we have no defenses? Will we be slaughtered like the Native Americans by invading alien bacteria?

  When the Apollo astronauts returned to Earth, both they and the samples of lunar rock and soil they brought back with them were quarantined until medical tests proved that they harbored no alien organisms. None were expected, since the Moon seems quite dead, but the quarantine was put in place anyway.

  Current NASA plans call for robotic missions to Mars that will send samples of Martian rock and soil back to Earth. The space agency is already developing plans to test the returned material for biohazards, to determine if the sample contains anything that might pose a threat to the teeming organisms of Earth’s biosphere (including us).

  Eventually, human explorers will go to Mars. Whether or not they find any evidence of life, they will be quarantined upon their return to Earth and examined thoroughly to see if they are harboring any biohazardous materials or organisms.

  The International Space Station would make an excellent site for the quarantine and examination of samples and people returned from Mars (or elsewhere). Orbiting several hundred kilometers above the Earth, the space station is in effect an isolation ward, unconnected to Earth’s biosphere. A special module, self-contained and flying alongside the ISS rather than connected to it, could be used for the quarantine station. Until studies prove that the returned samples (and astronauts) harbor no threat to Earth, the module could remain isolated.

  Appendix 9

  Death’s Calling Card

  Although Leonardo Da Vinci found fossils of marine animals high in the mountains of Italy, it was not until the nineteenth century that geologists and biologists began to understand that the gigantic fossilized bones of dinosaurs belonged to a vanished group of animals that had flourished millions of years earlier.

  Once they realized that the dinosaurs had become extinct some 65 million years ago, they began to wonder what killed them. After all, the dinosaurs had ranged all across the world, on land, in the sea, and in the air, for nearly 150 million years. What drove them into sudden, total extinction? Many possible explanations were examined: climate change, widespread disease, genetic defects. None of them proved satisfactory.

  In 1979, the father-son team of Luis and Walter Alvarez, together with colleagues from the University of California at Berkeley, found the answer.

  The Lords of the Earth were destroyed in the flash of a moment. The dinosaurs were wiped out by the crash of an asteroid.

  In that titanic explosion, a geologic era ended. One moment it was the Cretaceous Period, rife with Tyrannosaurus rex and other dinosaurs. Within an eyeblink, in geologic terms, the Cretaceous ended and the Tertiary Period began. No more dinosaurs. Mammals inherited an Earth in which fully one-third of all the previous species of land, sea, and air had abruptly become extinct.

  Paleontologists speak of it as the Cretaceous/Tertiary extinction event, or simply the K/T extinction.28

  Luis Alvarez (1911–1988) won the Nobel Prize for physics in 1968. His son, Walter, is a geologist. They realized that samples of sediments that mark the abrupt transition from Cretaceous to Tertiary are marked by a layer of the element iridium separating the deposits laid down in the two adjacent eras. They studied hundreds of such samples, from all around the world. Each had the layer of iridium sandwiched between the K and the T layers.

  The iridium in the K/T boundary layer is twenty times thicker than iridium deposits anywhere else on Earth. Certain types of meteorites are much richer in iridium than the Earth. Their conclusion: An iridium-rich asteroid struck the Earth, causing a worldwide catastrophe of blast shock, planet-spanning clouds of choking, Sun-blotting dust, wildfires, tidal waves, and a global cataclysm that wiped out all the larger animals on Earth.

  And leaving some 200,000 tons of iridium spread all across the world: death’s calling card.

  The crater left by that deadly asteroid was found on the coast of the Yucatán peninsula, buried beneath 65 million years worth of sediments in a site now called Chicxulub. The crater is immense, some 300 kilometers in diameter. If the asteroid had hit in what is now Connecticut, its crater would have spanned from Boston to New York City.

  Appendix 10

  The Nitrogen Fix . . . and Israel

  As life on Earth approached its 2-billionth birthday, it faced a growing crisis.

  All terrestrial life requires nitrogen. Nitrogen is an essential component of amino acids, for example, and amino acids are essential components of proteins.

  Fortunately, Earth’s atmosphere is 78 percent nitrogen. There are approximately 35.77 quadrillion tons of nitrogen in our planet’s air.

  Unfortunately, no living organism can use atmospheric nitrogen directly. As David Wolfe puts it, as far as living creatures are concerned “the immense supply of nitrogen in the air might as well be on Jupiter or Mars.”

  The nitrogen gas in the atmosphere is in the form of the tightly bound two-atom molecule, N2. To be biologically useful, the nitrogen must somehow be “fixed” (altered) into nitrate or ammonia, forms of nitrogen that organisms can use.

  For the first 2 billion years or so of life’s existence on Earth, the atmosphere consisted principally of nitrogen and carbon dioxide. Living organisms depended on lightning bolts to split the N2 into biologically usable compounds. In the intense heat of a lightning strike, both nitrogen and carbon dioxide are dissociated and then recombined to form NO, nitric oxide. Thus, the frequency of lightning strikes set a strict limit on life’s ability to increase and multiply.

  Moreover, as studies by a team of researchers from the National University of Mexico and Ames Research Center have shown, about 2 billion years ago the amount of carbon dioxide in Earth’s atmosphere began to dwindle, reducing the amount of NO that lightning could produce. The nitrogen crisis deepened.

  Within roughly 100 million years, though, certain species of bacteria developed a mutation that allowed them to take in atmospheric nitrogen and transform it into ammonium, NH4 (not ammonia, NH3), which organisms can use in their metabolism.

  Of the tens of thousands of bacterial species living in the world’s soils and seas today, only a few hundred have this ability to “fix” nitrogen. The existence of all the other life-forms on Earth has depended on them for billions of years.

  Early in the twentieth century, human scientists, industrialists, and politicians became aware of a second nitrogen crisis. The nitrogen-based fertilizers then being used for farm crops were running low. Supplies of manure and nitrates on which the organic fertilizers are based were not keeping pace with the world’s growing human population. The British physicist Sir William Crookes painted a gloomy picture of mass starvation at a meeting of the Royal Society in London.

  That was bad enough, but nitrates were also needed for munitions. Politicians might wring their hands over mass starvations, but their armies must not be allowed to run short of ammunition! Just in time for World War I, the German che
mist Fritz Haber (1868–1934) invented a process for “fixing” gaseous nitrogen to produce ammonium. He received the Nobel Prize in 1919.

  In England, the Russian-born chemist Chaim Weizmann (1874–1952) developed, in 1911, a different process for producing acetone, a vital ingredient in munitions manufacturing, for the British Admiralty. In gratitude to Weizmann, who was an ardent Zionist, the first lord of the Admiralty, Arthur Balfour, issued the Balfour Declaration in 1917, which promised British support for establishing a Jewish homeland in Palestine. In 1949 Weizmann became the first president of the new nation of Israel.

  Artificial fertilizers eventually helped to swell the Earth’s human population past the 6 billion mark. And thanks to nitrogen-based explosives, the world’s armies—and terrorists—have continued to blast people to smithereens.

  Appendix 11

  Charles Darwin

  Darwin’s concept of evolution through natural selection is as central to biology as Newton’s laws of motion and gravity are to physics.

  Darwin’s basic idea is quite simple: Organisms are constantly struggling to survive against an environment that is often hostile and always changing. An organism that might be ideally suited for its environment can be driven into extinction when that environment changes. Why hasn’t all life been destroyed by the floods, droughts, earthquakes, meteor impacts, volcanic eruptions, ice ages, and all the other natural catastrophes that constantly rack our world?

  Life survives because it adapts, Darwin realized. How does life adapt? Through mutations. Organisms are constantly mutating; most mutations are so minuscule they have no effect on a creature’s ability to survive, although some are so damaging they result in the creature’s death. A few mutations, though, happen to be favorable and help the creature to survive in its environment better than it would have fared without the mutation.