[BLANK_AUDIO] We've looked a little bit at the history
search for life on Mars. It's worth now looking at it in a little More detail about Mars as a location for
life. We've seen what makes a planet habitable. It includes a source of liquid water, a
source of energy, a Source of elements and other nutrients,
and physical conditions suitable for life. Let's look at each one of these and see
how Mars matches up to those requirements. First of all water on Mars, well today
Mars has a desiccated surface. In fact there are many images such as this
one Sent back by Mars rovers showing this dry
desiccated surface. There's no liquid water on the surface of
Mars. The atmospheric pressure is too low for
standing bodies Of liquid water to persist for any length
of time. But the evidence is, that this wasn't the
case in the Martian past. Indeed as early as the Viking orbiters
images were sent back of Valley networks and catastrophic outflow
channels Dating back many billions of years. The early history of Mars that suggests
that early Mars had a lot more liquid water than
today. This is a rather beautiful image of Jezero
crater in the northern hemisphere Of Mars showing what looks like a river
flowing into an asteroid crater. It's a false color image some Of these colors correspond to minerals
such as Olivine and pyroxene found in volcanic
rocks, and Some of these colors correspond to clays,
alteration Products caused by volcanic rock reacting
with liquid water. We can also see here a feature that looks Very much like a delta, all of these
features found In different places on Mars, suggest that
in the Early history of Mars there was much more
liquid water Than there is today. We think that Mars essentially had three
epochs. The early epoch in the Noachian was when
there was
Abundant liquid water and this would've
been reacting with volcanic rock. Creating environments with essentially a
neutral PH in which clays were being produced. And then as Mars began to dry up later in Its history there was interactions between
sulfur dioxide produced in Volcanoes and liquid water producing
acidic waters. And Mars went through a phase where many
of its water Bodies may have been more acidic than in
its very early history. And then in the third epoch very broadly
Mars transitions Into what we observe today, a very dry
environment, no Standing bodies of liquid water, and
probably liquid water interactions Confined to the deep subsurface if it even
occurs there. There is some evidence, though, for liquid Water in the near surface environment,
even today. These are images of what seem to be brine
channels seeping out from the Subsurface of Mars reported only two years Ago that suggest that underneath the
surface. Conditions may be suitable for the
formation of brines that might seep out Around the edges of impact craters and
other features onto the surface of Mars. So it's possible That even in the subsurface of Mars
there's the liquid water. But if we think more broadly about Habitability of the planet we can
certainly Say that in the early history of Mars
there was more liquid water than today. So at least in terms of the requirement
for liquid water. In order to sustain life, early Mars would
have been habitable. What about elements and nutrients for
life? In an early lecture, we saw how life
needed six key elements. Let's have a look at those in turn. First of all, carbon, what we know is the
source of carbon There's carbon dioxide in the atmosphere,
there's just over 95% carbon dioxide. In the Martian atmosphere, and we know
that there are microbes that Can use Carbon Dioxide as a source of
Carbon to fix Carbon. So that doesn't seem to be a problem.
There's hydrogen available, either in
liquid water, maybe even present today, But certainly in ice. And some of that ice may have melted in
the past. If we think about the early history of
Mars, there Was certainly liquid water to provide a
source of hydrogen. What about nitrogen? Well, nitrogen is a big unknown, and we
don't know whether there are sources Of fixed nitrogen on the surface of Mars
that could supply nitrogen for biology. Indeed the Mars science Laboratory might
give us Greater insight into whether there's
nitrogen and where It might be distributed. Nitrogen could be one of the big problems
for Martian life. There's oxygen available on Mars a very
little bit in the atmosphere 0.14% In the atmosphere, but they're our oxygen
atoms in water and in other compounds. So there seemed to be an accessible supply
of oxygen Atoms at least to supply the oxygen
requirement for life. What about phosphorus? Well we know that there's phosphorus in
apatite, These are phosphate containing minerals
found commonly in volcanic rocks On the Earth but also known to exist on
Mars. So we know there's a source of phosphorus
and we also know there's sulfur. Sulfates have been found on Mars such as
magnesium sulfate. And these salts could plausibly supply
sulfur to life on Mars. So the six elements for life C, H, N, O, P, S, seem to be available apart from
possibly nitrogen and Finding the nitrogen on Mars is one Of the key challenges for astrobiologists
if we're To show that mars is habitable with Respect to the basic elements required for
life. Of course, there are many other elements
on mars as Well, magnesium, calcium, iron, potassium
and sodium that are found in Volcanic rocks and these could be used by
life for Different chemical reactions and in
different pathways for by biochemistry as They are on the earth.
One problem for Mars is geo-chemical
turnover. How do we recreate these energy supplies
that life needs? Unlike the Earth, Mars does not have plate
tectonics today. There's some evidence for ancient magnetic Fields that have essentially been trapped
in Rocks when they were liquid, in the early
history of Mars, and then solidified. Preserving a reminance of the Martian
magnetic field. But those plate tectonics have long Since stopped, and so one wonders how you
would get geochemical turnover, how you Would get new energy and nutrient supplies Turned over to provide these requirements
for life. Well one way could be volcanoes. Mars does seem to have active volcanism in
it's recent history. Volcanoes may have melted rock, stirred up
energy and nutrients and provided the Disequilibria, the chemical conditions on
the surface That would of provided energy for life. So, there are ways in which we could get
geochemical Turnover required to provide new energy
and nutrient supplies for life. What about the physical conditions on
Mars? Well, we've already seen how the surface
is very extreme. High levels of ultraviolet radiation,
there's no ozone shield on Mars. If you went sun bathing on Mars, you would
get sunburned about a Thousand times faster than you would be on
the surface of the Earth. This is a very damaging environment for
life. It's very desiccating, no liquid water on
the surface of Mars. High ionizing radiation as well. These are high energy particles from the
sun and the Galaxy that bombard the surface of Mars
and its near surface Environment, and make conditions very
challenging particularly as there is no Liquid water for any potential biology to
repair its biochemical processes. And as we saw in the Viking biology
experiments, There may be oxidants in the soil, there's
certainly perchlorate. And these compounds could provide a
challenge for life living on the surface. But this may not be the same in the
subsurface of Mars.
The ultraviolet radiation is cut out. The ionizing radiation is reduced from the
rocks and there might be liquid Water maybe even brines in the subsurface
of Mars providing habitats for life. That's the story of present Day Mars, but as we've already seen, the
physical conditions for life On the early history of Mars were very
different, and certainly, given that There was liquid water on the surface of
the planet, it seems that The physical conditions on the surface may
have been more conducive to life. Where would we look for this life on Mars? We'll today we would probably have to go
into the deep subsurface. The surface of Mars does Not look like a very good place for life
to be thriving. But in the subsurface maybe we could drill
beneath the surface. Beneath that layer of oxidants in the
soil, perhaps reach pockets on the Subsurface of Mars where there might be
liquid water and habitats for life. So it seems the search for present day
life on Mars Might best be accomplished by looking in
the deep sub surface. What about past life? Well if liquid Water was abundant on the surface of Mars
in the Past, all we have to do is go to ancient
sentiments. Laid down on the ancient surface of Mars And see whether there are remnants of life
there. This is an image of Mount Sharp taken by
the Mars Science Laboratory. And this rover will explore some of these
minerals. You can see an image here and these white
dots show what's called an unconformity. Above the white dots, you can see a lava
layer in there has been laid Down on top of ancient sediments and below Those white dots, sediments that may
contain clays. Sulfates and other minerals associated
with the interaction of Water with volcanic rocks in the
production of different minerals. The Mars science laboratory will explore
these ancient minerals to see Whether they might contain ancient organic
compounds, whether they might be places That were habitable for life, one of the
instruments with which it will do this.
Is the sample analysis Mars package, the
SAM instrument shown here. It will sample these materials in the
future and see whether they contain any Organic materials that might be of
interest For assessing Mars as a habitable world. What might we look for on Mars, well we
might look For the same things we saw in an earlier
lecture that we Looked for to show evidence for life on
early Earth. We might look for shapes of fossils that
resemble life. We might look for the organic remains of
living Things that have long since been buried
and died. We might also look for chemical signatures Of life or chemical changes caused by
life, Such as isotopic fractionation, or maybe
even Changes to minerals that are distinctive
for biology. And all of these lines of evidence Might help us to determine whether early
Mars ever had life. What would be the implications if we found
no life on Mars? Many people think that we're going to Mars
to Search for life, and we really want to
find it. If astrobiologists didn't find life on
Mars, we would be deeply disappointed. But in fact a lack of life on Mars would
be a very, very significant finding. What would it mean? Well it might mean that something was
missing On the surface of Mars. We've seen how we don't know about the
distribution Of nitrogen on the surface or subsurface
of Mars. Maybe that's missing. Maybe that prevents life from having taken
hold on Mars. It might be something to do with the
origin of life. If the origin of life is extremely rare,
and requires very specific Conditions, maybe it occurred on the
Earth, and did not occur on Mars. It might also be that there are habitats
on Mars that Are uninhabited. Perhaps they are uninhabited because there
was no origin of life on Mars,
Or these habitats are not connected with
environments on Mars that do contain life. Uninhabited habitats are habitats that
contain all the ingredients for Life, all the requirements for
habitability but there's no life there. It might be that we will find that we are
exploring uninhabited Habitats on Mars and yet it could be that
other regions of Mars are inhabited. So all of these problems may lead to a
lack of a discovery of life on Mars. And in order to find out why Mars did not Have life we would have to know more about
the Origin of life, and more about the way in
which Life can persist on planetary surfaces
over billions of years. It is intriguing though that when Earth
had Life in its early history, Mars had liquid
water. And so the question is, if Mars had No life, what was different about Mars
compared to The Earth, despite the fact that both
planets seemed To have been quite similar in their early
history? The search for life of Mars also has
implications for policy. This is an area of policy called planetary
protection That's been developed by the committee on
space research. And planetary protection's concerned
really with two problems. First of all, back contamination, that's
where We bring materials back to the Earth and There's a possible if somewhat unlikely
threat Of contaminating the Earth with some
extraterrestrial organism. This is actually an image of the Apollo 11 Astronauts being quarantined on their
return from the moon. The likelihood of contaminating the Earth
with some extraterrestrial organism is Very low. But out of prudence, planetary Protection regulations address this
potential problem. Another challenge addressed by planetary
protection is forward contamination. And that's the contamination of planets That have life with Earth's
micro-organisms.
And this much more of a concern for Mars,
if Mars has life, should we contaminate The planet and how can we go about Preventing the contamination of the planet
by cleaning Up spacecraft, for example, before we send
them there. This is an image of the Viking landers and
their heat shields. They were literally cooked like turkeys in
a giant oven before they Were sent to Mars to kill Microorganisms on their surface,
essentially sterilize them. These are the source of protocols that can
be Used to clean up spacecraft before they're
sent to Mars. Nowadays, different protocols are used
because of the sensitivity Of modern electronics, but planetary Protection's become a very serious
concern. In the exploration of Mars and the
possibility That it may have habitable environments
for life. Planetary protection essentially splits
missions into different categories. For example, category one is a mission
where there's no direct interest for Understanding the origin of life, and this
might be a completely desacated asteroid. Where there's really no chance of
contaminating Some sort of indigenous biosphere on that
asteroid. And it goes right away through to category
five, Which is returning samples to Earth, where
there may Be a concern about back contamination,
bringing biological entities From a planet such as Mars to the Earth. And the precautions that should be taken
to ensure those samples Are properly handled and properly
investigated Before they're released for wider
investigation. So what have we learned in this lecture? Well hopefully we've learned that Mars had
much more liquid water in it's past. It may even have liquid water today in the
form of brines. It seems to have many of the elements and
nutrients required for life to Exists, but we don't know the global Distribution of these particularly crucial
elements like nitrogen. Mars lacks plate tectonics and so the
regeneration of energy sources
And nutrients in the crust could be a
problem for life, Although geochemical turnover might be
caused by volcanoes. And we've also learned that search for
life on Mars has policy implications. Particularly for planetary protection,
which provides guidelines, Prudent safeguards for exploring Mars and
preventing contamination. [BLANK_AUDIO]
