The Solar System

CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 597

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5 rows of 7

An anticoded message

7 rows of 5

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

0 1 0 0 1 1 1

1 1 1 1 0 0 0

0 1 0 0 1 1 1

0 1 1 0 0 0 0

Binary numbers 1 to 0

Atomic numbers of

hydrogen, carbon,

nitrogen, oxygen,

and phosphorus

DNA double helix

Number of units

in DNA

Height of human

in wavelengths

Sun and planets with

Earth offset

Diameter of dish

in wavelengths

Formulas for sugars

and bases in DNA

Start of number markers

Start of number marker

Human figure

Population of Earth

Arecibo radio dish

transmitting signal

Start of number marker

Start of number markers

The Arecibo message

■ Figure 26-11

Radio noise from various astronomical sources and Earth’s atmospheric opacity
make it diffi cult to detect distant signals at wavelengths longer than 100 cm
or shorter than 1 cm. In this range, wavelengths of radio emission lines from
H atoms and from OH molecules mark a small wavelength range named the
water hole that may be a likely channel for interstellar communication.

■ Figure 26-10

(a) An anticoded message is designed for easy decoding. Here a string of 35 radio

pulses, represented as 1s and 0s, can be arranged in only two ways, as 5 rows of

7 or 7 rows of 5. The second way produces a friendly message. (b) The Arecibo

message describes life on Earth (color added for clarity). Binary numbers give

the height of the human fi gure (1110) and the diameter of the telescope dish

(100101111110) in units of the wavelength of the signal, 12.3 cm. (NASA)

Background

radiation

The

water

hole

Quantum

mechanical noise

Noise from Earth’s atmosphere

Noise from galaxy

H OH

Total noise

300 30 3

Wavelength (cm)

Signal strength

0.3 0.03

contains Earth’s orbit, and, arguably, Venus and Mars as well.

Europa and Enceladus in our solar system show that liquid water

can exist due to tidal heating outside the conventional habitable

zone. Th us, nHZ may be larger than had been originally thought.

Th e factor fL is the fraction of suitable planets on which life

begins, and fI is the fraction of those planets where life evolves to

intelligence.

Th e six factors on the right-hand side of the Drake equation

can be roughly estimated, with decreasing certainty as you pro-

ceed from left to right. Th e fi nal factor is extremely uncertain.

Th at factor fS is the fraction of a star’s life during which an intel-

ligent species is communicative. If a society survives at a techno-

logical level for only 100 years, the chances of communicating

with it are small. On the other hand, a society that stabilizes and

remains technologically capable for a long time is much more