PLEASE NOTE:
*
CCNet ESSSAY: ESTIMATED FLUX OF ROCKS BEARING VIABLE LIFEFORMS
EXCHANGED
BETWEEN EARTH AND MARS
by Michael Paine, The Planetary Society Australian Volunteers
<mpaine@tpgi.com.au>
Introduction
A possible mechanism for transfer of life between planets is via
rocks
ejected by major asteroid or comet impacts. The term
"transpermia" was
coined by Oliver Morton to describe the transfer of lifeforms by
this method
and to distinguish it from the more general concept of
panspermia. Davies
(1998a-c) discusses several possibilities for transpermia
including
hypothetical Mars-life reaching Earth; Earth-life reaching Mars,
the Earth's
Moon and moons of the outer solar system and interstellar
transfers via
meteoroids. Melosh (1994) outlines the mechanisms by which such
transfers
can take place. Mileikowsky and others (2000) build on Melosh's
work and
provide estimates of transfer rates between Mars and Earth over
the past 500
million years.
The transfer mechanism
The analysis by Mileikowsky considers the ejection of surface
rocks from
Mars during impacts by large asteroids, the proportion of ejected
rocks that
reach the escape velocity of the planet and go into orbit around
the Sun and
the proportion (they estimate about 5%) that eventually collide
with Earth
and reach its surface.
Mileikowsky's Table 2 provides calculations for one scenario.
They select
conditions that optimise the chances of lifeforms surviving the
journey.
These "hospitable" conditions are:
The radius of ejected rock is between 0.67 and 1 metre (mainly to
provide protection from radiation in deep space). [note 1]
The core temperature within the rock during ejection or re-entry
did
not exceed 100 C (two of the dozen or so Martian meteorites that
have been
found on Earth meet this criterion)
The journey time between planets was 100,000 years or less
These criteria are likely to be very conservative and therefore
serve to set
a lower limit to the exchange of hospitable rocks between Mars
and Earth.
Under this scenario the quantity of "hospitable" ejecta
reaching the Earth
from Mars averages out at 150 kg per year. This represents
roughly 15% of
the total estimated quantity of Martian material falling to Earth
each year.
[note 2]
There is a trap in considering average (annual) values because
the transfer
of rocks occurs in spikes. It is assumed that impacts by
asteroids 1km in
diameter or larger are needed to launch ejecta into
interplanetary flight.
Such impacts produce craters 20km or more in diameter. They occur
on Mars
and Earth (land impacts only) over typical timescales of one to
ten million
years.
By definition, viable transfers only take place within 100,000
years of the
impact so there are long periods between impacts when Mars rocks
that fall
to Earth have remained in space for too long and any hitchhiking
microbes
are assumed to have died. There do not appear to have been large
impacts on
Mars (or the Earth for that matter) over the past 100,000 years
so it is
unlikely that "hospitable" Mars rocks are reaching the
Earth at present, or
vice versa. [Note 3]
Survival rates
Likely survival rates of any viable micro-organisms within the
rocks are
influenced by numerous hazards during the journey. Mileikowsky
estimates
that 7% of the micro-organisms will survive. This is based partly
on a range
of tests involving (Earthly) B.subtilis bacteria that included
shooting
specimens out of a cannon (Mastrapa 2000). Again, this may be
conservative
because there are likely to be tougher micro-organisms on Earth
(Davies
1998b).
The long term average transfer rate of 150kg of hospitable rocks
per year,
with 7% of resident microbes surviving (if any were present in
the rocks at
the time of launch), is equivalent to a series of space missions
that return
samples of about 10 kg of Martian rocks each year under protected
conditions
that are favourable to the survival of any life within the rocks.
Of course there is no firm evidence of life on Mars at this stage
so the
above numbers are speculative. The same cannot be said for the
reverse -
transfer of Earth-life to Mars.
Earth-life reaching Mars
There are differences between Earth and Mars but the number of
hospitable
rocks reaching Mars from Earth is similar to that considered
above.
Therefore, based on Mileikowsky's conservative estimates, roughly
150 kg of
hospitable Earth rocks reach Mars each year, on average, and some
7% of
hitchhiking microbes can be expected to survive the journey.
Colonisation of
present day Mars by these microbes appears to be formidable. The
microbes
would tend to be trapped in fragments of the original boulder
scattered over
the dry, cold surface of Mars. Under these conditions they would
probably
remain dormant after a freezing journey through space. Indeed
some frozen,
dormant Earth-life might be found by geologists when they
eventually explore
Mars and find Earth meteorites on its surface.
If any hitchhiking microbes were lucky enough to land in a warm
moist spot
on Mars then the chances of colonisation could be expected to be
much
higher. Conditions were probably more favourable to such
colonisation on
ancient Mars, when volcanoes were active and the planet was
thought to be
warmer and wetter.
Beyond Mars
Although the chances of "hospitable" rock transfers are
substantially less,
the same mechanisms may have delivered microbe-bearing Earth
rocks to
Jupiter's moon Europa. It is thought that Europa has a thick
water ocean
covered by a crust of ice. Therefore, if a life-bearing Earth
rock reached
the surface of Europa intact the impediments to colonisation
might be less
than those on present day Mars. A major difficulty, however, is
the lack of
an atmosphere on Europa. Collisions with the icy crust would
usually take
place at interplanetary speeds and the impacting rock could be
expected to
be vaporised in an impact explosion. [note 4]
About one fifth of the ejected rocks eventually return to planet
from which
they were launched. Davies (1998a) points out the possibility
that microbes
in these rocks might reseed a planet after its biosphere had been
sterilised
by huge impacts. This is a possible mechanism for life becoming
re-established on Earth after the Late Heavy Bombardmant (Bortman
2000- note
that Bortman does not consider this mechanism in his report).
Melosh recently estimated that, over the lifetime of the Earth, a
few dozen
Earth rocks might have made it to planets in nearby star systems
(Melosh
2001, Hecht 2001). With journey times of millions of years the
chances of
any viable lifeforms reaching an Earth-like planet by this
mechanism appear
to be extremely slim [note 5]. As noted by Davies (1998a) this
could not be
expected to be a mechanism by which life spread widely throughout
the
galaxy.
References
Bortman H. (2000) 'Life Under Bombardment', NASA Astrobiology
Institute,
November 2000. http://nai.arc.nasa.gov/index.cfm?page=lifebombard
Davies P. (1998a) 'The Fifth Miracle: The Search for the Origin
and Meaning
of Life', Penguin Press.
Davies P. (1998b) 'Planetary Infestations', Sky & Telescope,
September 1999.
Davies P. (1998c) 'Survivors from Mars', New Scientist, 12
September 1998.
Hecht J. (2001) 'Galactic Hitchhikers', New Scientist, 14 March
2001.
Hills J.G. (1981) 'Comet Showers and the Steady Infall of Comets
from the
Oort Cloud', The Astronomical Journal, Vol.86, No. 11 1730-1740.
November
1981.
Mastrapa, R. M. E.; Glanzberg, H.; Head, J. N.; Melosh, H. J.;
Nicholson, W.
L. (2000) 'Survival of Bacillus Subtilis Spores and Deinococcus
Radiodurans
Cells Exposed to the Extreme Acceleration and Shock Predicted
During
Planetary Ejection', 31st Annual Lunar and Planetary Science
Conference,
abstract no. 2045
Mileikowsky C., Cucinotta F.A., Wilson J.W., Gladman B., Horneck
G.,
Lindegren L., Melosh H.J., Rickman H., Valtonen M. and Zheng J.Q.
(200)
'Risks threatening viable transfer of microbes between bodies in
our solar
system', Planetary and Space Science 48 (2000) 1107-1115.
Melosh H.J. (1994) 'Swapping Rocks: Exchange of Surface
Material Among the
Planets', The Planetary Report, The Planetary Society, July 1994.
Melosh H.J. (2001) 'Exchange of Meteoritic Material Between
Stellar
Systems', 32nd Annual Lunar and Planetary Science Conference,
abstract
no.2022.
Steel D. (1995) 'Rogue Asteroids and Doomsday Comets', John Wiley
& Sons.
For more information and links see
http://www4.tpg.com.au/users/tps-seti/swaprock.html
Notes
1. Rocks in the size range of interest have an average mass of 7
tonnes but
they tend to fragment during re-entry so that smaller pieces
usually reach
the surface of the destination planet.
2. The estimate of 150kg is based on Mileikowsky's estimate that
7.9x1013
grams is transferred over 500 million years. Melosh (2001) refers
to
estimates which suggest that, at present, about 500kg of Martian
rocks
larger than 100mm fall to Earth each year. Averaged over millions
of years,
the value would be higher - perhaps one tonne per year - so
"hospitable"
rocks make up roughly 15%. Two-thirds of these fall in the
oceans. Steel
(1995) indicates that at present, about 40,000 tonnes of
extraterrestrial
material collides with the Earth each year but when the effects
of larger
impacts are taken into account the average over long periods
becomes 160,000
tonnes per year. The estimated Mars flux is therefore a very
small
proportion of all of the material colliding with the Earth.
3. It has been estimated that the average transit time between
Mars and
Earth is about one million years but the distribution is skewed
to shorter
transit times.
4. Although very thin compared to the Earth, Mar's atmosphere is
dense
enough to slow meteorites sufficiently so that they do not
explode on impact
with the surface. As with Europa, a lack of atmosphere also
appears to make
it unlikely that Earth-life would colonise the Moon by
transpermia.
5. It has been estimated that every 100 million years or so
another star
system passes within 3000 AU of the Sun - well within the Oort
Cloud (Hills
1981). I have suggested that such close approaches might increase
the
chances of transpermia between planetary systems. Although this
would not
make a difference to the overall statistics calculated by Melosh
(that is,
only a few dozen rocks would reach extra-solar planets over the
lifetime of
the Earth) the transit times might be reduced by this mechanism
so that
survival chances might be slightly better. I also suggested that
close
approaches by stars might increase the rate of bombardment of the
Earth by
comets disturbed from the Oort Cloud by the passing star. This
could
possibly increase the transpermia launch rate. However, in
personal
correspondence Melosh points out that close approaches by the
other star
systems would typically last no more than 10,000 years but the
infall of
comets from the Oort Cloud would take hundreds of thousands of
years. Also
ejection of rocks from our solar system, usually through
encounters with
Jupiter, typically takes tens of millions of years so the
planetary system
"will be long gone before the harvest from the increased
cratering rate can
be reaped".