4

key concepts

4.1

Nucleic Acid Structures Reflect Their Functions

4.2

The Small Molecules of Life Originated on Primitive Earth

4.3

The Large Molecules of Life Originated from Small Molecules

4.4

Cells Originated from Their Molecular Building Blocks

Looking for Life

On Earth you can find organisms and the chemical signatures of life everywhere—even deep under the sea, inside volcanoes, buried in ice, and floating in the air. Determining life’s origin is difficult because (with few exceptions) the simple organisms that first existed left no fossils. On Mars, however, things might be different.

Mars has fascinated scientists (and science-fiction writers) since its discovery by astronomers more than 3,000 years ago. A cold planet, Mars has a geology that has changed little over several billion years, and so any evidence of life may be preserved. By the late nineteenth century, distinguished professors of astronomy firmly believed there was life on Mars, based on observations through telescopes that showed polar ice caps that changed with the seasons, darker areas thought to be liquid water, and even markings that resembled canals.

Better telescopes, chemical analyses based on light emission (spectroscopy), orbiting satellites, and landers controlled from Earth have laid to rest many of the initial ideas about life on Mars. But the challenge of finding present or past life on Mars remains. The search revolves around finding organisms, evidence of a current or past environment where life could exist, and chemical signatures of present or past life.

Unlike Earth, Mars now lacks a magnetic field to protect against cosmic radiation arriving from space. The surface of Mars is assaulted with very high levels of radiation, making it inhospitable to life. So any current life would have to be below the surface, where radiation is much lower.

Since life as we know it requires water, determining whether Mars now has or ever had water has been a central question. It is now established that water exists on the surface of Mars—at the poles, frozen, and in water vapor in its atmosphere. There is even evidence that trickles of water may exist on the surface, kept liquid in the bitter cold by a high salt concentration. The discovery of methane (CH₄) in the atmosphere of Mars has generated additional excitement and speculation. Might this methane be a by-product of an organic reaction? The search for evidence of the chemical elements (C, H, O, P, N, and S) of life introduced in Chapter 2 continues.

Can we find evidence of life on Mars?