by Michael Paine, The Planetary Society Australian Volunteers


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

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

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


Bortman H. (2000) 'Life Under Bombardment', NASA Astrobiology Institute,
November 2000.

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

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

Steel D. (1995) 'Rogue Asteroids and Doomsday Comets', John Wiley & Sons.

For more information and links see


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".

CCCMENU CCC for 2001