Some of the earliest proposals for making contact with extraterrestrial intelligence (ETI) imagined communication with inhabitants of our Moon. But by the 1870s, scientists knew that the Earth's satellite is too small to sustain an atmosphere. Because the Moon is considerably smaller than Earth, it has a weaker gravitational field, and thus all gases escape from its surface.

With no atmosphere, it was argued, there could be no life on the Moon. The planet Mars, however, is considerably bigger, and even in the late 1800s it seemed likely that our planetary neighbor might have at least a thin atmosphere.

When people started thinking about how to communicate with potential Martians, a new challenge arose. Since the Moon is relatively close to Earth, earlier proposals to signal possible inhabitants of the Moon relied on huge diagrams "drawn" on Earth's surface.

But a similar message, if at all visible from Mars, would be much harder to see, simply because of the greater distance. Therefore, an alternative approach was suggested for interplanetary communication.

Though scientists today no longer believe theres any evidence for intelligent life on Mars, the same methods work using radio signals for communication at interstellar distances.

One example of this method -- proposed in 1920 -- would rely on a variation of telegraph code, using flashes of light to attempt communication with ETI. How would this work?

Imagine you were an early 20th-century astronomer, peering through your telescope at Mars, and suddenly you saw a series of flashes from a point on Mars surface. As you watched more closely, suppose you noticed that the flashes are of two different durations -- short and long like the "dits" and "dahs" of Morse code. Imagine, in fact, that you noticed the following sequence. What would it mean?

dit dit dit dit dit dit dah dah dah dit dit dah dah dah dit dit dah dah dah dit dit dit dit dit dit

As a hint, you might count the total number of "dits" and "dahs" and notice there are 25 in all. As a further hint, imagine you then noticed a flashing sequence of 36 pulses. And finally, you were able to make out a separate series of 49 pulses.

For starters, you might ask, "What do the numbers 25, 36 and 49 have in common?"

While it might take some ingenuity and a few false starts, the hope of the message-makers is that eventually you would come to recognize that each of these numbers is a perfect square, or a number multiplied by itself. If you multiply five by five, you get 25; six squared is 36 and seven times itself is 49. At least part of the message has been understood.

The next interpretive leap involves turning these strings of numbers into two-dimensional pictures. Starting with the sequence of 25 "dits" and "dahs," you might rearrange all of these pulses into five rows, each with five pulses. If you did, you would see the same 25 pulses in a new format:

dit dit dit dit dit
dit dah dah dah dit
dit dah dah dah dit
dit dah dah dah dit
dit dit dit dit dit

To make the pattern a bit more obvious, suppose we replace the "dits" with "ones" and the "dahs" with "zeros." The message would then appear as follows:

1 1 1 1 1
1 0 0 0 1
1 0 0 0 1
1 0 0 0 1
1 1 1 1 1

Now lets do the same for the longer sequence of 36 "dits" and "dahs." When reconstructed in the same manner, we would discover the following message:

1 1 1 1 1 1
1 0 0 0 0 1
1 0 0 0 0 1
1 0 0 0 0 1
1 0 0 0 0 1
1 1 1 1 1 1

Finally, imagine that the message of 49 pulses yielded the following two-dimensional array:

1 1 1 1 1 1 1
1 0 0 0 0 0 1
1 0 0 0 0 0 1
1 0 0 0 0 0 1
1 0 0 0 0 0 1
1 0 0 0 0 0 1
1 1 1 1 1 1 1

After seeing enough messages in this format, we might recognize a recurring pattern. All of the pulses at the outside of the message are "dits" and the remainder of the pulses in the interior are "dahs." In effect, we see a pattern reminiscent of a picture frame, the same for each message.

But what good is a picture frame without a picture inside it?

Even these early proposals for sending messages by interplanetary Morse code suggested sending something more interesting than pictures of empty squares. In next months column well look at some of the sorts of pictures that can be communicated using this decades-old scheme.