Geology Reference
In-Depth Information
mantle. Like Earth, it has an atmosphere and a lot of water. Its relatively weak gravity can-
not easily hold speeding gas molecules in the upper atmosphere, so billions of years have
eaten away at both air and water, and yet Mars still holds warm, wet underground reser-
voirswherelifemightmaintainatenuousrefuge.Nowondermostplanetarymissionshave
targeted the red planet.
Earth itself, the “third rock from the Sun,” is smack in the middle of the habitable
“Goldilocks” zone. It's close enough to the Sun, and hot enough, to have relinquished sig-
nificant amounts of hydrogen and helium to the outer realms of the Solar System, but it's
far enough from the Sun, and cool enough, to have held on to most of its water in liquid
form. Like the other planets in our Solar System, it formed about 4.5 billion years ago, es-
sentially fromcolliding chondrites andtheirsubsequentgravitational clumpings intolarger
and larger planetesimals, over a span of a few million years.
Deep Time
Layered into all the evidence for how the Sun, the Earth, and the rest of our Solar System
were born is the concept of immense time spans—4.5 billion years and counting. Americ-
ans love to quote the dates of famous events in human history. We celebrate great accom-
plishments and discoveries, like the Wright brothers' first flight on December 17, 1903,
and the first manned Moon landing on July 20, 1969. We recount days of national loss and
tragedylikeDecember7,1941,andSeptember11,2001.Andwerememberbirthdays:July
4,1776,and,ofcourse,February12,1809(thecoincidentbirthdaysofCharlesDarwinand
Abraham Lincoln). We trust the validity of these historic moments because an unbroken
written and oral record links us to that not-so-distant past.
Geologists also love to quote historic time markers: about 12,500 years ago, when the
last great glaciation ended and humans began to settle North America; 65 million years
ago,whenthedinosaursandmanyothercreaturesbecameextinct; theCambrianboundary,
at 530 million years ago, when diverse animals with hard shells suddenly appeared; and
more than 4.5 billion years ago, when Earth became a planet in orbit about the Sun. But
how can we be sure those age estimates are correct? There's no written record of Earth's
ancient chronology past a few thousand years, or any informing oral tradition.
Four and a half billion is a number almost beyond reckoning. The current Guinness
world record for longevity is held by a French woman who lived to celebrate her 122nd
birthday—so humans fall far short of living even for 4.5 billion seconds (about 144 years).
All of recorded human history is much less than 4.5 billion minutes. And yet geologists
claim that Earth has been around for more than 4.5 billion years.
There's no easy way to comprehend this
deep time
, but I sometimes try by taking
long walks. Just south of Annapolis, Maryland, twenty miles of stately, undulating, fossil-
