Excerpt from space.com
The moon has long been viewed as a crucial component in creating an
environment suitable for the evolution of complex life on Earth, but a
number of scientific results in recent years have shown that perhaps our
planet doesn’t need the moon as much as we have thought.
In 1993, French astronomer Jacques Laskar ran a series of calculations indicating that the gravity of the moon is vital
to stabilizing the tilt of our planet. Earth’s obliquity, as this tilt
is technically known as, has huge repercussions for climate. Laskar
argued that should Earth’s obliquity wander
over hundreds of thousands of years, it would cause environmental chaos
by creating a climate too variable for complex life to develop in
relative peace.
So his argument goes, we should feel remarkably lucky to have such a
large moon on our doorstep, as no other terrestrial planet in our solar
system has such a moon. Mars’ two satellites, Phobos and Deimos,
are tiny, captured asteroids that have little known effect on the Red
Planet. Consequently, Mars’ tilt wobbles chaotically over timescales of
millions of years, with evidence for swings in its rotational axis at
least as large as 45 degrees.
The stroke of good fortune that led to Earth possessing an unlikely moon,
specifically the collision 4.5 billion years ago between Earth and a
Mars-sized proto-planet that produced the debris from which our Moon
formed, has become one of the central tenets of the ‘Rare Earth’
hypothesis. Famously promoted by Peter Ward and Don Brownlee, it argues
that planets where everything is just right for complex life are
exceedingly rare.
New findings, however, are tearing up the old rule book. In 2011, a
trio of scientists — Jack Lissauer of NASA Ames Research Center, Jason
Barnes of the University of Idaho and John Chambers of the Carnegie
Institution for Science — published results from new simulations
describing what Earth’s obliquity would be like without the moon. What
they found was surprising.
“We were looking into how obliquity might vary for all sorts of
planetary systems,” says Lissauer. “To test our code we began with
integrations following the obliquity of Mars and found similar results
to other people. But when we did the obliquity of Earth we found the
variations were much smaller than expected — nowhere near as extreme as
previous calculations suggested they would be.”
Lissauer’s team found that without the moon, Earth’s rotational axis
would only wobble by 10 degrees more than its present day angle of 23.5
degrees. The reason for such vastly different results to those attained
by Jacques Laskar is pure computing power. Today’s computers are much faster and capable of more accurate modeling with far more data than computers of the 1990s.
Lissauer and his colleagues also found that if Earth were spinning
fast, with one day lasting less than 10 hours, or rotating retrograde
(i.e. backwards so that the sun rose in the West and set in the East),
then Earth stabilized itself thanks to the gravitational resonances with
other planets, most notably giant Jupiter. There would be no need for a
large moon.
Earth’s rotation has not always been as leisurely as the current 24 hour spin-rate. Following the impact that formed the moon,
Earth was spinning once every four or five hours, but it has since
gradually slowed by the moon’s presence. As for the length of Earth’s
day prior to the moon-forming impact, nobody really knows, but some
models of the impact developed by Robin Canup of the Southwest Research
Institute, in Boulder, Colorado, suggest that Earth could have been
rotating fast, or even retrograde, prior to the collision.
“Collisions in the epoch during which Earth was formed determined its
initial rotation,” says Lissauer. “For rocky planets, some of the models
say most of them will be prograde, but others say comparable numbers of
planets will be prograde and retrograde. Certainly, retrograde worlds
are not expected to be rare.”
The upshot of Lissauer’s findings is that the presence of a moon is not
the be all and end all as once thought, and a terrestrial planet can
exist without a large moon and still retain its habitability. Indeed, it
is possible to imagine some circumstances where having a large moon
would actually be pretty bad for life.
Rory Barnes, of the University of Washington, has also tackled the problem of obliquity,
but from a different perspective. Planets on the edge of habitable
zones exist in a precarious position, far enough away from their star
that, without a thick, insulating atmosphere, they freeze over, just
like Mars. Barnes and his colleagues including John Armstrong of Weber
State University, realized that torques from other nearby worlds could
cause a planet’s inclination to the ecliptic plane
to vary. This in turn would result in a change of obliquity; the
greater the inclination, the greater the obliquity to the Sun. Barnes
and Armstrong saw that this could be a good thing for planets on the
edges of habitable zones, allowing heat to be distributed evenly over
geological timescales and preventing “Snowball Earth”
scenarios. They called these worlds “tilt-a-worlds,” but the presence
of a large moon would counteract this beneficial obliquity change.
“I think one of the most important points from our tilt-a-world paper
is that at the outer edge of the habitable zone, having a large moon is
bad, there’s no other way to look at it,” says Barnes. “If you have a
large moon that stabilizes the obliquity then you have a tendency to
completely freeze over.”
Barnes is impressed with the work of Lissauer’s team.
“I think it is a well done study,” he says. “It suggests that Earth
does not need the moon to have a relatively stable climate. I don’t
think there would be any dire consequences to not having a moon.”
Of course, the moon does have a hand in other factors important to life
besides planetary obliquity. Tidal pools may have been the point of
origin of life on Earth. Although the moon produces the largest tides,
the sun also influences tides, so the lack of a large moon is not
necessarily a stumbling block. Some animals have also evolved a life
cycle based on the cycle of the moon, but that’s more happenstance than
an essential component for life.
“Those are just minor things,” says Lissauer.
Without the absolute need for a moon, astrobiologists seeking life and habitable worlds
elsewhere face new opportunities. Maybe Earth, with its giant moon, is
actually the oddball amongst habitable planets. Rory Barnes certainly
doesn’t think we need it.
“It will be a step forward to see the myth that a habitable planet
needs a large moon dispelled,” he says, to which Lissauer agrees.
Earth without its moon might therefore remain habitable, but we should
still cherish its friendly presence. After all, would Beethoven have
written the Moonlight Sonata without it?
Source Article from http://feedproxy.google.com/~r/AscensionEarth2012/~3/Ismqo3GwuZ4/earths-moon-may-not-be-critical-to-life.html
Earth's Moon May Not Be Critical to Life Afterall
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