by John Walker
from FourMilab Website

Let's start with a copy of the image which I've color coded to point out the various components.

The Numbers

The first thing we need to discover in order to interpret the message is how numbers are encoded. The topmost segment of the message is intended to convey this. The fact that this portion of the message is the first to be transmitted should help identify it as the starting point when decoding it.

What we find in this portion of the message are the numbers from one through ten, written in the same binary code in which the message was sent. The pixels marked in red are the binary digits of the numbers, which start with one at the left of the image and count to ten at the right. Examining the encoded numbers 8, 9, and 10 reveals that when more significant bits are required than fit in a given dimension in the image, they continue in another row or column, starting next to the least significant bit of the previous row or column.

The position of the least significant digit is always marked by a one bit, which I will color pink for every number in the message.

 

The Atoms of Life

Directly below the numbers, which provide the key for interpreting its meaning, are a list of four numbers which give the atomic numbers of the four chemical elements of which DNA is composed, and are the principal components of all of our biological molecules.

From left to right (the same order as we found in the numbers above) we have:
 

I have colored the one bits representing these elements differently to provide a key to the section immediately below in the message.

The Chemistry of DNA

This portion of the message (which somewhat overlaps the section below in order to save space; I have separated them for clarity, as I will continue to do for subsequent overlapping areas without further mention) assumes you've figured out the section above giving the elements of which DNA is composed.

Here, groups of numbers of the same size as the list of elements give the number of atoms of each in the molecular components of DNA, arranged in a representation of the ladder-like structure of backbone and complementary base pairs.

The chemical formula for the molecular components of DNA are then given as shown in the table to the left. To aid in interpreting each encoded formula, the column giving the number of atoms for each element is colored the same as the corresponding column in the list of elements above.

After figuring out the molecules from the encoded formula (which may be difficult, as only the number of atoms of each element is given with no indication of the structure), we can learn more about the overall structure of DNA from where the groups appear in image.

First of all, we observe that the deoxyribose-phosphate sequence is repeated twice on both the left and right sides of the image. To one acquainted with polymers, this suggests the chain may be of arbitrary length, with the basic unit repeating over and over.

The appearance of the identical backbone sequence at both the left and right suggests a ladder-like structure.

The complementary base pairs are shown attached to the deoxyribose groups of the backbone, and the fact that Adenine pairs with Thymine and Cytosine with Guanine is indicated by the examples shown. If the recipient of the message managed to figure out the structural formula for each of the groups, further investigation of their chemistry would reveal that these were the only pairings possible, and thus suggest DNA was a reliable data storage medium capable of being copied.

Whether all of this could be figured out by a form of life completely unlike terrestrial biology (for example, sentient, self-reproducing computers made of silicon microchips, originally designed by a long-extinct carbon-based life form, of which all memory has been lost) is an open question.

 

The Structure of DNA and Size of the Human Genome

Below the chemistry of DNA is a representation of its double helical structure. This would confirm the interpretation of the molecular components which appear above and slightly overlap this section.

Knowing the molecule forms a helical structure would provide constraints useful in reducing ambiguity in the structure of the component groups, and experimental observation that DNA synthesized from the compounds indicated did indeed form a double helix would form provide strong confirmation its structure had been correctly decoded.

In the middle of the helix is a 32-bit number, oriented along the axis of the helix to suggest it gives the length of the overall molecule.

The number, three billion, gives the approximate number of nucleotides in the human genome.

 

Human Shape, Size, and Population

Directly below and slightly overlapping the helix, suggesting a relationship between them, is a cartoon depicting a human being. To an extraterrestrial who resembled a human being, this would immediately be read as "They look like us".

Certainly, to a human, this is one of the most striking components of the message. What an amorphous, jellyfish-like creature living in a sea of liquid nitrogen would make of it is less evident.

To the left of the figure representing the human are the binary digits 1110, representing the number 14, centered near the middle of a bar the same height as the human. (Note that this number is written with the least significant digit at the left, not the bottom.) The bar suggests (to humans, at least) that the number gives the size of the figure, but then one must decide in what units the size is given.

Since the recipient knows the wavelength on which the message was received, 12.6 centimeters, this provides a convenient scale of measurement. Fourteen of these units is 176 cm, which is just about Frank Drake's own height. At the right of the figure is another 32-bit number, this one about four billion, intended to give the approximate human population.

Aliens who correctly interpret this will know how large an army to send.

 

The Solar System

Below the information about human beings is a schematic representation of the Sun and planets, which I've colored to make clearer which is which. The gas giants are all larger than the terrestrial planets, with Jupiter and Saturn indicated as larger than Uranus and Neptune to their right.

Earth appears directly below the figure of the human and is offset in that direction, identifying it as the planet from which the signal originated.
 

The Antenna

The final item is a representation of Arecibo antenna used to transmit the message, with dimension bars (as used with the human figure) and a number at the very bottom giving its approximate: binary 100101111110 or decimal 2430 units of the 12.6 centimeter wavelength of the message.

This gives the size of the antenna as 30,618 centimeters, or about 306 meters.