Encyclopedia of the Solar System 2nd ed

854 Encyclopedia of the Solar System

while simultaneously causing changes to that environment.
All of these records are palimpsests in that they have
been overwritten—often repeatedly—over time. Labora-
tory simulations of prebiotic chemistry—the chemistry as-
sumed present before life—can provide clues to the condi-
tions and chemical solutions leading up to the origin of life.
Experiments of DNA/RNA replication sequences can pro-
vide clues to the selection process that optimize mutations
as well as provide a basic understanding of reproduction.
Perhaps one day the process that initiates life will be studied
in the laboratory or discovered on another planet.
The major events in the history of life are shown in Fig-
ure 4. As the Earth was forming about 4.5 Gyr ago, its sur-
face would have been inhospitable to life. The gravitational
energy released by the formation of the planet would have
kept surface temperatures too high for liquid water to exist.

Eventually, as the heat flow subsided, rain would have fallen

for the first time and life could be sustained in liquid wa-

ter. However, it is possible that subsequent impacts could

have been large enough to sterilize the Earth by melting,

excavating, and vaporizing the planetary surface, removing

all liquid water. Thus, life may have been frustrated in its

early starts. Following a sufficiently large impact, the en-

tire upper crust of the Earth would be ejected into outer

space and any remnant left as a magma ocean. Barring these

catastrophic events, however, sterilizing the Earth is a dif-

ficult task because it is not sufficient to merely heat the

surface to high temperatures. At present, microorganisms

survive at the bottom of the ocean and even kilometers be-

low the surface of the planet. An Earth-sterilizing impact

must not only completely evaporate the oceans but must

then heat the surface and subsurface of the Earth such that

FIGURE 4 Major events in the history of the Earth and Mars. The period of moist surface
conditions on Mars may have corresponded to the time during which life originated on Earth. The
similarities between the two planets at this time raise the possibility of the origin of life on Mars.