by Carl Sagan
Oddly enough, the Cornell Astronomy Department’s books turned up no photograph of the sun’s spectrum that was exactly what we wanted. So NAIC staff scientist Valentin Boriakoff and NAIC staff engineer Dan Mitler photographed the sun, using a simple prism and film typical of that used in many of the other color pictures.
9, 10, and 11. Mercury, Mars and Jupiter
As we make our first tentative exploration of the solar system, we feel a sense of adventure exploring its distant worlds. Yet from an interstellar point of view, the solar system is a little family huddled around a little star, and if we wish to tell extraterrestrials that “this is home,” Jupiter and Mars are as much a part of home as New York or New Delhi. At the suggestion of astronomer A. G. W. Cameron, we included a few pictures of the other planets: Mercury, as seen by Mariner 10; Mars, as Viking saw it when it was approaching and preparing to land; and Jupiter from our first close-up look furnished by Pioneer 11. Thus we restate the implied message of Voyager: we are spacefaring and wish to explore—and contact—the universe around us.
The diameter and mass of Jupiter and Mars, shown on the pictures, provide a cross reference to the solar-system chart on pictures 5 and 6.
12. Earth
Photographed in color, this is a snapshot of home, the “meadow in the middle of the sky,” in Sagan’s words, from which these pictures were launched.
The pictures of planets should be objects that spacefaring races will recognize. Of course they won’t have seen these planets, but in other solar systems they should have seen gas giants like Jupiter, cratered and airless wastes like Mercury, and perhaps even lovely blue water/oxygen worlds like Earth. Looking at objects that are at least somewhat familiar should help the recipients calibrate their system for reproducing pictures and understand our concept of a picture—how we represent reality on a two-dimensional surface. Certainly it will help them to see something familiar before moving on to those pictures of things that are totally unfamiliar to them.
13. Egypt, the Red Sea, the Sinai Peninsula and the Nile, with the Composition of Earth’s Atmosphere
Descending to the surface of the blue planet, another color photograph establishes the idea that this is the planet we live on. The cloud patterns and the views of land and water may provide clues to our meteorology. The relative abundance of gases in the atmosphere is shown, in symbols defined in the following pictures. The five most important biological atoms have been given letter symbols; all others are indicated by showing their atomic number in a circle. (So, for example, argon becomes .) Pictures 9 through 13 were selected by Sagan.
14, 15, and 16. DNA Structure and Replication
One of the crucial and unanswered questions of biology is whether the chemical make-up of life on Earth is the only way matter can form the structures we call living, or whether there are other ways. Is carbon chemistry the only viable one? Must replication of life involve helical molecules? What is necessary about our biology, and what merely an accident of our evolution? One of the reasons biologists are so eager to find life elsewhere—say, on Mars—is to try to answer these questions. We felt that some hint of the chemical nature of our biology would be of great interest to the scientists of the race that finds Voyager.
On Earth, all life uses the DNA molecule to store and replicate the information that tells an organism how to grow itself from scratch. So the structure of DNA seemed a good thing to show. However, it is possible that DNA is the only way, in which case we will be telling them something they already know. At the same time, we will reveal our ignorance: “The Earthlings don’t know yet that everybody is made of DNA!”
Diagraming the DNA molecule proved difficult, since it is a complex molecule with many atoms and a twisting, helical structure, difficult to show clearly in a drawing. None of the ordinary diagrams or models—stick and ball or space filling—seemed clear enough, so with the help of Dr. Stuart Edelstein, a biochemist at Cornell, I devised a diagram.
The shaded portion on the left-hand side of picture 14 shows schematically the five atoms that compose DNA. The indicated size of the hydrogen atom (1 angstrom) should make it obvious that we are talking about atoms (nothing else is that small). Atoms are given letter symbols and symbols for their atomic number (the number inside a circle). Note that we had to put a little tail on the h of hydrogen, to eliminate the possibility of confusing it with the n of nitrogen.
The right-hand part of the picture shows the four bases that connect the two twisting spirals of the DNA backbone. The four bases are always found paired—thymine with adenine, and guanine with cytosine. These base pairs are the two letters of the genetic alphabet. The composition of the two possible pairs are shown. The bases are indicated by a letter in a square. Here we confronted a problem that cropped up all the time. We had to try to eliminate all possible sources of ambiguity, and it seemed ambiguous to use the same symbol for two different things: “cytosine” begins with a C, but we had already used a C to define carbon. I brought the problem up with Frank, and he shrugged and said, “Spell it with an S.” “Cytosine with an S?” I said dubiously. “The biochemists will have a fit.” “Do it anyway,” Frank said. “It’ll be a good object lesson in the special problems of communicating with extraterrestrials.” “Cytosine with an S?” Edelstein complained. I’m not sure our explanation convinced him.
The left-hand portion of picture 15 shows the composition of the backbone of the DNA molecule with two of the bases attached, and the right-hand portion is a diagram of the whole molecule, showing how the backbone and base pairs form a helical structure capable of self-replication. On the bottom of the picture appears the legend “4000000000 A–T G–S,” indicating that a molecule of DNA uses a large number of base pairs to code the information necessary to construct a living being (the number shown is the number of base pairs in a DNA strand from a human being).
In our working room at Cornell we had a small television camera and screen so we could look at the pictures in the resolution at which they would be recorded. The letters indicating the base pairs in the helix of picture 15 were close to the resolution limit, so for insurance we repeated a close-up of this portion of the diagram in picture 16.
17. Cell Division
This superb photograph shows DNA replication in action. The scale “1/300 cm.” indicates that we are showing something that is much larger than the molecules of the previous photographs but is still quite small. As a clue we show the same shape that appears in picture 16—the DNA double helix unwinding and replicating. The cells shown, from the blastula of a frog, exhibit several of the phases of cell division.
18 through 25. Human Anatomy
Frank Drake thought that the transparent overlays of human anatomy found in encyclopedias would be ideal for showing structural (if not functional) human anatomy, and he obtained a set of the acetate overlays used in The World Book Encyclopedia. These eight pictures show superficial and deep human anatomy, including skeletal structure, the nervous system, the circulatory system, and so forth. Picture 23 was recorded in color to emphasize the difference between veins (blue) and arteries (red). Unfortunately, the figure in the overlays is neuter—where its sex organs should be is a gaping void. Both to emphasize our sexuality (a nontrivial aspect of human anatomy) and to link the anatomy diagrams with the pictures to come, we added to picture 25 a rough sketch showing that in the area of the groin there existed either of two sets of organs—and introduced the male and female symbols to distinguish them. We used these symbols again in pictures 26, 28, and 32.
We’d noticed that the overlays were covered with hundreds of tiny black numbers, to relate the parts of the body to a list of names that appeared, in the encyclopedia, alongside the overlays. The World Book staff couldn’t provide us with a set that didn’t include these little numbers, so Linda Sagan, who is a painter, painted out all the numbers on the acetate, skillfully matching the colors. Many tedious hours later, she had finished. But after the paint had dried, it flaked off the acetate.
All Linda’s work was wasted, and there was no time to redo it. It is anybody’s guess what extraterrestrials will make of the myriad of black spots covering our ribs, spleen, and biceps. (Photographs © 1978 World Book– Childcraft International, Inc. Used with permission of the publisher. The artwork was adapted for the exclusive use of The World Book Encyclopedia from The Human, copyright 1955 by Harper & Row Publishers, Inc.)
26. Diagram of Human Sex Organs
The symbols for male and female should link this picture with pictures 25 and 32. This picture and the seven that follow form a sequence describing human reproduction. Not only did this sequence have to be clear, it also had to be cleared by a NASA approval board. It isn’t difficult to understand the reluctance of a government agency, which has to get its funds from Congress every year, to swim in the murky waters of human sexuality. That NASA approved this portion at all was something of a wonder, but they did veto the picture we submitted (shown in this article, but not on the Voyager record), of a man and a pregnant woman quite unerotically holding hands. A silhouette of this photograph, showing the position of the fetus, appears in picture 32. (From Life; Cells, Organisms, Populations, by E. O. Wilson et al. Copyright © by Sinauer Associates, Inc. Drawing by Sarah Landry.)
27 and 28. Conception with Silhouette
The Swedish physician/photographer Lennart Nilsson is renowned for his beautiful photographs of all stages of human prenatal development. We included a photograph of human conception taken by him, but we took a small liberty with it. To show the time it took to grow a human being in the womb, we wanted to start at the beginning, or “0 seconds.” Nilsson’s photograph, certainly the finest we could find, was of a sperm a fraction of a second before it reached the egg. It seemed vital to make clear that the moment of conception was the moment the sperm actually reached the egg. I decided to draw in a sperm just touching the egg so we could more accurately label this picture “0 seconds.”
This is the first time we used a silhouette before the photograph, and on it we indicated, besides “0 seconds,” the size of the sperm and the fact that the sperm was male and the egg female. (Photographs of conception and fertilized ovum from original edition of A Child Is Born by Lennart Nilsson. Copyright © 1965 by Albert Bonniers Förlag, Stockholm. English translation copyright © 1966 by Dell Publishing Company, Inc. Photographs by Lennart Nilsson for the book Ett Barn Blir Till by Albert Bonniers Förlag AB, Stockholm, 1976. Used by permission of Delacorte Press/Seymour Lawrence.)
29. Fertilized Ovum
These two photographs, also by Lennart Nilsson, show the fertilized egg at two stages. The left-hand picture, “1 second,” shows the thickening of the membrane around the egg immediately after conception. The right-hand picture shows the time until the first cell division, about “43,200 seconds.” The cell membranes have been enhanced to make the cell division more obvious. (Photographs of conception and fertilized ovum from original edition of A Child Is Born by Lennart Nilsson. Copyright © 1965 by Albert Bonniers Förlag, Stockholm. English translation copyright © 1966 by Dell Publishing Company, Inc. Photographs by Lennart Nilsson for the book Ett Barn Blir Till by Albert Bonniers Förlag AB, Stockholm, 1976. Used by permission of Delacorte Press/Seymour Lawrence.)
30 and 31. Fetus and Silhouette
The photograph, by Dr. Frank Allan of George Washington University, shows a fetus after about sixty days of development. The silhouette shows an embryo after about forty days and a silhouette of a fetus the same age as the fetus in the photograph. In the silhouette, the time from conception is given, and also the size of the developing child. Originally we planned to use a photograph of an embryo and a fetus that matched the silhouette exactly, but last-minute permissions problems forced us to substitute a different picture of a fetus.
32. Silhouette of Male and Female
This is the fetus of the previous picture shown in its position inside the mother. The male and female symbols distinguish the mother from the father, and the typical sizes of human beings are shown. At this point, the recipients should begin to have a good idea of the body shape of human beings.
33. Birth
This photograph is from the famous collection The Family of Man. We had hoped to find a photograph that showed a child actually emerging from its mother, but in all such photographs we looked at, the mother was so hidden by sheets that it wasn’t clear that it was a woman (or even a person) that was giving birth. We called Wayne Miller, who took this picture, to get permission for its use. He was out of town, so we spoke to his son, who sounded as if he were in his twenties. When we explained the nature of the project, he gasped and said, “You want to send that picture into space forever? That birth picture? That’s me being born! Of course you can use it! I guarantee my father’s permission!” Interestingly enough, it turned out that the physician delivering the younger Miller was the boy’s grandfather—and the photographer’s father.
Up to now the picture sequence has consisted largely of scientific information, mostly diagrammatic, to introduce ourselves. From this point on, most of the pictures are photographs, and while the major criterion for selection was still the information conveyed, the information is often cultural and many different kinds of information may appear in the same photograph. For example, the two pictures that follow the birth picture each show a parent and a child. We wanted to convey some sense of the parent/child relationship. We also wanted to show close-up views of the human face and hands.
34. Nursing Mother
This color photograph shows a woman from the Philippines nursing her child. A secondary kind of information is present in the design of the woman’s dress, showing perhaps that we enjoy making printed fabric and adorning ourselves with it. It is even possible that the recipients may make some association between the flowers on the dress and flowers appearing elsewhere in other pictures.
35. Father and Child
To humans, this picture, also sent in color, embodies the pride of parenthood. For us selecting it, it was a source of pleasure for additional reasons. You can see everything—the man’s ears (amazing how many pictures do not show ears) and teeth (ditto teeth); the child’s toes. The man’s eyes are directed toward the child (a girl, by the way), while the child looks at the camera—a clue that, along with other pictures, might suggest that eyes are organs of vision. Their possible motion is clearly shown here. The muscles in the man’s arm are well-defined. The skin tones show perfectly. And it is a beautiful, well-composed picture to boot.
36. Group of Children
Early on, when the picture package was going to consist of a half-dozen photographs, someone suggested showing a group of humans of various races and cultures doing something together. The idea resulted finally in this color photograph and numbers 72 and 74. Ruby Mera, a photographer working for UNICEF, took the picture at the UN International School, a school for the children of delegates in New York City. As well as including a spectrum of racial types and nations, the picture shows human beings sitting in a circle, that almost archetypal arrangement for a group of people. It illustrates various forms of sitting and kneeling, as well as various positionings of the hands and arms. Seeing human bodies rotated through 360 degrees should give the recipients a good sense of the volume of the human form in space.
37 and 38. Family Portrait and Silhouette
This photograph of a Midwestern American family is also from The Family of Man collection. Five generations of the family are present, with a sixth generation in the portraits on the walls (in fact, one of the men in the paintings is the father of the old woman). The silhouette gives the estimated weight and age of some members of the group, with the implication that the oldest member of the group is approximately the age of the oldest human being. We hope that recipients will reason that we would want to show how long people live, and if there was a person a thousand years old, we would show that.
While I was in New York to get permission to use this picture from Time/Life, Nina Leen, the photographer,
happened to be in the office. I told her we wanted to send her photograph to the stars, perhaps to be viewed by the beings of another world. She was willing and seemed pleased but somehow unimpressed. “You see,” she told me, “I’ve been in contact with Them for years anyway—but I know They’re glad you’re doing this.”
Pictures 32 through 38 introduce human beings. Before presenting any more information about our culture, we give a more detailed picture of our planet—its landscapes, oceans, and biology.
39. Diagram of Continental Drift
The upwelling and obliteration of large chunks of Earth’s surface by the process called plate tectonics is a relatively recent geological discovery. Its implications for meteorology, geology, and the evolution of life are not well understood, but it seems to have been an important factor in the history of our planet—and perhaps in the histories of other planets in the galaxy as well. It certainly reveals something about the structure of the Earth. This diagram shows Pangaea, the primordial continent, as it existed three billion years ago; the continents now; and the continents as they will appear ten million years from now. The time scale dates from the formation of the Earth, so the present time is four and a half billion years since the Earth was formed, and this gives a strong clue to what the phenomena are that we are talking about; if we are talking about billions of years, we must be talking about something either astronomical or geological, and pictures 12 and 74 also show the continents of Earth. The human hand marks the era of man. It says, “This is the era when the hand that launched the spacecraft existed.” The basic diagram comes from Sagan’s design of the LAGEOS plaque.