Space - Tullamore Astronomical Society

crust (read the article about
Apophis and a likely Earth impact
in 2036 in the Sept/Oct 2005 issue
of Réalta –Ed.).
Earth's Moon, on the other
hand, is pockmarked with millions
of craters because it lacks both
atmosphere and geologic activity.
Similarly, Mars has thin
atmosphere and relatively little
geologic activity.
On both the Moon and
Mars, teasing out the primary
impacts from the secondaries is
difficult because the craters are just
too numerous.
The researchers instead
turned to Europa and a world
covered in a thick crust of ice.
More importantly, Europa is
geologically active like Earth. Its
surface is constantly being repaved
with new ice and as a result,
Europa has very few craters.
Using high-resolution
images from NASA's Galileo
spacecraft, the researchers
measured the number, size and
distribution of craters on Europa.
They then ran a computer
simulation of meteors randomly
striking Europa but with the
condition that the number and size
of the craters had to match the real
number and size observed in the
images. After running the
simulations hundreds of times and
comparing the results to the
images, they found that the crater
distributions were not similar as
would be expected if most of the
craters were caused by primary
impacts.
The finding is important
because scientists typically use
crater counts to date the ages of
planet and moon surfaces. When
comparing two similar regions on a
moon, for example, scientists
generally assume that the region
with more impact craters is older.
Scientists can also use a region's
crater density to calculate it's
absolute age. They usually use our
own Moon as a reference because
scientists have reliably dated the
age of some its craters based on
rocks brought back by astronauts.
If it turns out that most of
these small caters are secondary
and not primary, then that means
the calibrated age from our Moon
is not right.
Most of the objects that
strike Jupiter and its moons come
from a region of the Solar System
known as the Kuiper Belt.
Therefore, another implication of
the finding may be that there are
fewer small asteroids in the Kuiper
Belt than previously thought. It
may be that small asteroids are
rarely made or perhaps some
process depletes them before they
can reach Jupiter and its moons.
Life's Building Blocks
'Abundant in Space'
The idea that comets and meteorites seeded an early
Earth with the tools to make life has gained
momentum from recent observations of some of these
building blocks floating throughout the cosmos.
Scientists scanning a galaxy 12 million light-years away
with NASA's Spitzer Space Telescope detected copious
amounts of nitrogen containing polycyclic aromatic
hydrocarbons (PANHs), molecules critical to all known
forms of life.
PANHs carry information for DNA and RNA
and are an important component of haemoglobin, the
molecule that transports oxygen through the body. They
also make chlorophyll, the main molecule responsible
for photosynthesis in plants, and perhaps most
importantly, they're the main ingredient in caffeine and
chocolate.
"There once was a time that the assumption was
that the origin of life, everything from building simple
compounds up to complex life, had to happen here on
Earth," said study leader Doug Hudgins of Ames
Research Center. "We've discovered that some very
biologically interesting molecules can be formed outside
our earthly environment and delivered here."
While organic compounds have been discovered
in meteorites that have landed on Earth, this is the first
direct evidence for the presence of complex, important
biogenic compounds in space. So far evidence suggests
that PANHs are formed in the winds of dying stars and
spread all over interstellar space.
"This stuff contains the building blocks of life,
and now we can say they're abundant in space," Hudgins
said. "And wherever there's a planet out there, we know
that these things are going to be raining down on it. It
did here and it does elsewhere." Using the Spitzer Space
Telescope, Hudgins and his colleagues detected the
familiar chemical signature of regular polycyclic
aromatic hydrocarbons (PAHs) in the spiral galaxy M81,
as well as a similar, but unknown signature.
"There were a few anomalies in the spectrum
that we couldn't explain," Hudgins says. The researchers
compared their readings to the infrared signatures of
similar molecules, finally settling on nitrogen containing
PANHs because their data showed there was nitrogen in
the regions they were investigating.
"When we did that, we found that by putting a
little nitrogen in these molecules explained the troubling
molecules," Hudgins said. "This discovery takes this
reservoir of molecules that we didn't think were
interesting and transforms all this stuff into something of
biologic interest."
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Réalta – Volume 7, Issue 2 – November/December 2005 – Tullamore Astronomical Society