[BLANK_AUDIO] In this lecture we'll address a somewhat
intriguing question, could we be martians? And this is slightly flippant way of
asking, are planets isolated islands? Or can they transfer life? In other words, could life have been Transferred from Earth to Mars in
meteorites? Is it even possible there could be an
origin of Life on Mars, and then life transferred
from Mars to Earth? In which case, we'd all be Martians. We know that life travels Around on the Earth. Birds for example can fly from one country
to another, even one continent to another. And seeds can also drift through the wind
from one continent to another. This is an area that's been of interest to
ecologists for a very long time. It's called island biogeography. How life can be transferred from one
island to another. The question, can life be transferred from
one planet to another, is just an Extension of this old question in ecology. It sometimes been called panspermia, the
idea Of transfer of life through interplanetary
space. It was first considered by Svante
Arrhenius, Who thought about the idea of microbes, Other small lifeforms, being pushed
through space By the pressure of the sun's radiation. And Lord Calvin considered the possibility
of Rocks being transferred through space
containing life, Allowing other planets to be inoculated. It's a reasonable idea because we know
that Rocks can be transferred from one planet
to another. We collect for example Martian meteorites
in Antarctica, and we know These meteorites are from Mars because the
rocks correspond to the rock Composition on Mars and they also contain
bubbles of gas that Correspond to the composition, the
chemical And isotopic composition of gases in
The Martian atmosphere. So if Martian meteorites are on Earth,
then it's Probably true there are some Earth
meteorites on Mars. And so the transfer of material has
occurred between Earth and Mars. And the question is, could life have been
transferred between planets? Well, for life to be transferred between Two planets it has to survive three
processes. First of all it's going to have to survive
being launched From a planetary surface, in other words
being ejected, from the surface of a Planet in an asteroid or comet impact, and
the intense pressures that that involves. It's then going to have to survive the
journey through Space, many years traveling through The extreme conditions of interplanetary
space. And then finally, it's going to have to
survive atmospheric entry through the Atmosphere of the host planet to land on
the new planet, and then take Root on the new planet in which it has
arrived. Let's have a look at these three processes
and see what We know about the survival of life in
these three different stages. First of all, launch. We can simulate the launch of microbes
from a Planet in an asteroid comet impact with
this facility. This is a light gas gun and it accelerates Pieces of rock at kilometers a second and
slams them Into other pieces of rock, much like an
asteroid or comet impact. And we can ask the question, can microbes
survive that process? On the right you can see a graph, and on The x-axis is shock pressures up to
gigapascals of pressures. And, on the left-hand side, you can see An axis showing the survival fraction of
microbes. What this graph shows you is that microbes
can Survive tens of gigapascals, the pressures
required to survive Being launched from a planetary surface at
fast enough speeds to Escape the gravity of a planet and be
launched into space. So on the face of it, life can survive the
pressures
Required to be launched in an asteroid
impact into interplanetary space. What about survival in space? Well experiments have been done by the
European Space Agency where microbes Have been launched into space and left
there for a long time, Up to six years. This is a facility that you can see here
called the exposed Facility, that was bolted onto the outside
of the International Space Station. And my lab, and some other European
investigators As well, had microbes that they put into This facility to ask the question, can
they Survive in space for a year and a half? And indeed they can. In our own samples, a single microorganism
managed to survive, the Sign of bacterium, for a year and a half
in space. Well, that's rather a short space of time,
it may take many decades. At least the shortest length of time may
be Many decades to transfer between planets,
but what these Experiments do show us is that microbes
can survive In space for long periods of time,
possibly many years. Once they've traveled through space, what
about atmospheric entry? Well one of the problems with coming
through the atmosphere in a rock Is the intense temperatures of those rocks As they hurtle through a planetary
atmosphere. But the good news is that that temperature
occurs for a very brief period of time. In fact, the rock on the outside of a Meteorite melts and forms what's called a
fusion crust. But deep inside the rock, the temperatures
may be Low enough for life to be able to survive. And in fact, there's some evidence in
meteorites that the Interior of the meteorite remains at well
below 60 degrees. Which is low enough for life to be able to
persist inside the rock. So the outside of the rock is melted. Life would certainly not survive that, but
the rock is big enough, the interior Re, remains cool enough for life to be
able to survive that atmospheric transit.
So, when we look at all Three of those stages, launch, survival in
space, And atmospheric entry, it seems that taken
individually microorganisms Can in fact survive each one of those
three Processes required to travel from one
planet to another. Of course we have have no evidence that
life Has been transferred for example between
Earth and Mars. But these experiments give some sort of
perspective On the possibility of life to be
transferred From one planet to another, and suggests
that it might be possible. Of course it goes without saying that it's
important that The conditions on the host planet are
actually conducive to life. If a Earth rock landed on the surface of
Mars Today for example, it wouldn't have much
luck in perpetuating life. The surface is very dry, high Levels of ultraviolet radiation, and
ionizing radiation. But that might have been different in the
early history of Mars when There was more liquid water available.
So what we have learned in this lecture? Well, we've learned that the idea that
life can Be transferred between planets is a very
old one. It addresses the question of whether
planets are biological islands. Microbes can apparently survive the three
phases Of transfer from one planet to another. They can survive the high pressures
associated with launch. They seem to be able to survive in space For quite some reasonable periods of time. And it looks like they would also survive
atmospheric Entry if they were buried deep within a
rock. We don't know what the maximum time for
survival of microbes Is in space and this is an on going
question of astrobiology. How long can microbes really survive in
the severe, Desiccating conditions of outer space with
high levels of radiation? This idea Of life being transferred from one planet
to another,
Panspermia, has not yet been shown to have
occurred. But whether it can occur remains an Important question in island biogeography
and in astrobiology. And panspermia of course also has
implications for the origin of life. The idea that life could originate on one Planet and then be transferred to another
planet. [BLANK_AUDIO]
