Biology Reference
In-Depth Information
From early Greek times through to modern space exploration it has been believed
that water is essential for life as we know it. The Greek natural philosopher Thales of
Miletus (ca. 640
548 B.C.) is reputedly the first to have emphasized the fundamental
importance of water, assigning it as the first principle of the cosmos, being 'the origin
of all things'. Hippo of Samos (450 B.C.) perceived life as water. NASA now bases its
search for life in the universe on the assumption that without liquid water, life is not
possible. Therefore the search for extra-terrestrial life is predicated on the search for
water.
Although not yet proven, it is likely that water is abundant in all corners of the
universe
[2,3]
. Since H and O are among the most abundant elements in the universe,
it is reasonable to assume that water will be found everywhere as well. In our solar
system, evidence of water has been found on Earth and its moon, Mercury, Mars,
Neptune, Pluto, Jupiter's moons Triton and Europa, and Saturn's moon Enceladus
[4]
.
Of particular interest to the question of extra-terrestrial life are Mars, Europa, and
Enceladus. Speculation about the existence of water and even life on Mars has existed
since the first reports of 'canals' on the red planet by Angelo Secchi in 1858 and Giovanni
Schiaparelli in 1877. Percival Lowell took the questionable descriptions of long straight
channels and the unquestioned existence of Martian seasons to another level in the
1890s when he adopted the view they were 'canals' built by intelligent beings. Intelligent
life on Mars soon became ingrained in the public conscience after publication in 1897 of
H.G. Wells' TheWaroftheWorlds. More recently, NASA's Martian exploration has shown
the planet to be dry, with no pooled surface water. However, the Mars Global Surveyor
spacecraft found evidence for mostly sub-surface water and that water clearly flowed
on Mars in the distant past, as evidenced by erosion channels. Also, strong evidence
for past water on the Martian surface was provided by the robotic rovers, Spirit and
Opportunity. More recently, (July, 2008), water has been physically found in Martian
soil by the Phoenix Lander.
Perhaps of even more importance than Mars is Jupiter's moon Europa and Saturn's moon
Enceladus. Europa is unlike other planetary moons. Its surface is smooth and un-cratered,
indicating the existence of a giant subterranean ocean made of water that is covered by
a frozen ice crust. The frozen ice surface (
e
260
o
F) cracks as a result of Jupiter's enormous
gravitational pull. The Jovian moon's surface is therefore covered with stress fracture lines
that are colored, indicating the presence of slushy water that is extruded from the moon's
interior, refreezing on the surface. The extrudate contains organic solutes, accounting for its
color. The underlying water ocean has been estimated to be perhaps as deep as 100 Km.
Warmed by Jupiter's gravitational field, Europa's interior is full of liquid water and may in
locations be of sufficient temperature to support life. Even if life did not have enough time
to evolve on Europa, the moon may be a museum of a chemical evolution 'laboratory' frozen
in time.
Enceladus is a small moon revolving just outside Saturn's rings. Like Europa, Enceladus
is home to a vast underground ocean. The ocean appears to be 'fizzy like a soft drink and
could be friendly to microbial life'
[5]
.
Figure 3.1
shows a Cassini spacecraft image of
vaporous, icy jets emerging from fissures on Enceladus' surface
[5]
. If water is the most
important biochemical for the development of life, Europa and Enceladus are the places
to explore.
