[meteorite-list] Life Could Have Hitched a Ride to the Moons of Jupiter and Saturn

[Ron Baalke]

http://www.astrobio.net/exclusive/5860/life-could-have-hitched-a-ride-to-the-moons-of-jupiter-and-saturn

Life Could Have Hitched a Ride to the Moons of Jupiter and Saturn
Charles Q. Choi
Astrobiology Magazine
December 9, 2013

Summary: A new study suggests the possibility of transfer of life from 
the inner solar system to the moons orbiting Jupiter and Saturn, although 
very rare, cannot be ruled out.

Life on Earth or Mars could have been brought to the moons of Jupiter 
or Saturn on rocks blasted off those planets, researchers say.

These findings suggest if scientists ever detect life on those moons, 
they might have to contemplate the possibility that it came from elsewhere 
rather than originating there on its own.

The idea that life can spread through space is known as panspermia. One 
class of panspermia is lithopanspermia - the notion that life might travel 
on rocks knocked off a world's surface. If these meteoroids encase hardy 
enough organisms, they could seed life on another planet or moon.

Although lithopanspermia might seem farfetched, a number of meteorite 
discoveries suggest it might at least be possible. For instance, more 
than 100 meteorites originating from Mars have been discovered on Earth, 
blasted off the red planet by meteor strikes and eventually crashing here.

Some researchers have even suggested that life on Earth may have originally 
been seeded by meteors from Mars. A great deal of research has explored 
whether the red planet once harbored life and whether life might still 
exist there today, based on findings that Mars might once have been significantly 
more hospitable to life than it is now, and that refuges for life could 
remain hidden under its surface. One Martian meteorite, Allan Hills 84001 
(ALH84001), was even initially claimed to contain evidence of life. However, 
research since has revealed that every item on this meteorite that was 
potentially suggestive of life could be generated inorganically.

Past computer simulations also have suggested that matter blasted off 
Earth by cosmic impacts could have escaped the pull of Earth's gravity 
and landed on the Moon. Billions of years of Earth dust may have accumulated 
on the lunar surface - as much as 22 tons (20 metric tons) of Earth material 
is spread over every 38 square miles (100 sq. km) of the Moon. If true, 
the Moon could hold fossils of some of the earliest microbial life on 
Earth.

The discovery of organisms on Earth that can survive in environments once 
thought too harsh for life has piqued interest over whether the moons 
in the outer reaches of the solar system, such as Jupiter's moon Europa 
or Saturn's moon Titan, could host life.

"There have been previous simulations looking at transfer between Earth 
and Mars, but we wanted to scale the simulations up in the hopes of seeing 
transfer to Jupiter and Saturn," said study lead author Rachel Worth, 
an astrophysicist at Pennsylvania State University.

Worth and her colleagues analyzed where batches of several thousand rocks 
traveled once ejected off both Earth and Mars. "We ended up simulating 
over 100,000 individual fragments," Worth said.

Most of these meteoroids slammed back into their home planet. A great 
many rocks also were either swallowed by the Sun or left the solar system 
entirely. In addition, large numbers hit planets more inward in the solar 
system from their home planet - for Earth, that means Venus and Mercury, 
and for Mars, that means Earth, Venus and Mercury. However, a small fraction 
of meteoroids did hit planets outward from their origin.

The researchers calculated that over the course of 3.5 billion years - 
roughly the amount of time Earth is known to have possessed life - about 
200 million meteoroids large enough to potentially shield life from the 
rigors of space were blasted off Earth. They also estimated roughly 800 
million such rocks were ejected off Mars during the same period. More 
rocks escape from Mars because Martian gravity is a little more than a 
third that of Earth's.

Past research suggested moderately-sized rocks ejected from impacts could 
protect organisms from the dangers of outer space for up to 10 million 
years. The scientist calculated about 83,000 meteoroids from Earth and 
320,000 from Mars could have struck Jupiter after traveling 10 million 
years or less. Also, roughly 14,000 from Earth should have hit Saturn 
in that time, and no more than 20,000 from Mars.

Since the moons of those giant worlds are relatively close to their planets, 
many of them might get peppered by these meteoroids as well. The researchers 
calculated that Saturn's moons Titan and Enceladus and Jupiter's moons 
Io, Europa, Ganymede and Callisto should each have received between one 
and 10 impacts both from Earth and from Mars.

These findings suggest the possibility of transfer of life from the inner 
solar system to the outer moons, although very rare, currently cannot 
be ruled out. "When planning missions to search for life on Europa or 
other moons, scientists will have to think about whether they can distinguish 
between life that is or is not related to that on Earth," Worth said.

The researchers caution they are not saying "that life has made it to 
any of these moons, just that it could," Worth said. "To know for certain 
that this kind of transfer has happened, we would need to actually identify 
an Earth or Mars rock on one of the moons in question. We tried to make 
our estimates as realistic as we could, but they are still estimates, 
and we can never know for sure what will be discovered in the future that 
might change our assumptions."

For instance, "we don't really know the probability that an ejected rock 
fragment would have microbes in it, or that they would be the type of 
microbes that might survive all the trauma of ejection and space travel," 
Worth said. "There's also the question of just how habitable they might 
find the moons if they did make it there."

Still, the researchers note the icy moons of Jupiter and Saturn were all 
once warmer and likely had little to no icy shell to prevent meteorites 
from reaching their liquid interiors as they do now. In addition, Europa 
currently has the thinnest ice crust of the six moons the researchers 
examined, and roughly 40 percent of its crust appears to be covered with 
"chaos regions," uneven terrain hinting that it often breaks into large 
chunks separated by liquid water that later refreezes. Any meteorites 
on top of such regions therefore might have a chance of falling down into 
the underground oceans that moon is suspected to have.

"I think the possibility of any life in Europa's oceans is exciting, whether 
it is descended from Earth life - showing us a novel evolutionary path 
in a very interesting environment - or life that comes from an independent 
origin, which would point towards life being fairly common in the universe."

Worth noted one factor not included in their simulations that could be 
significant was the Yarkovsky effect, where rotating objects about 4 inches 
(10 centimeters) to 6 miles (10 kilometers) large will radiate heat that 
can help propel them through space.

"We expect that this effect would basically spread the ejected rocks out 
faster, as some would be propelled outward and others inward, so we might 
see slightly faster transfer times," Worth said.

The scientists added that rocks crashing back onto their home planet could 
help reseed life on that world after the cosmic impact that created them 
partially or completely sterilized the planet in question, serving as 
refuges for life in space while the world's surface cooled enough to permit 
survival. This could help explain how life on Earth survived the era known 
as the Late Heavy Bombardment about 4.1 billion to 3.8 billion years ago, 
when untold numbers of asteroids and comets pummeled Earth, the Moon and 
the inner planets.

Worth and her colleagues Steinn Sigurdsson and Christopher House detailed 
their findings online Dec. 6 in the journal Astrobiology.