Transcript Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Announcements
• Extra credit papers due today, Tuesday December 5th.
• Warning about this class.
• Pairs exercise: try to define ‘life’ in the general sense what are the minimum requirements for something to be
‘living’.
• You may want to consider: plants, bacteria, viruses (both
organic and computer!), prions, and organic molecules
such as amino acids, DNA and RNA.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 27:
Life In the Solar System
Picture credit: (i) Levin, The Earth Through Time (ii) NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Life In The Solar System
• In this class we will consider how and where life could have arisen in
the solar system, and how we might go about looking for it.
• Topics are:
• Scientific theories for the origins of life on Earth.
• Conditions necessary for life.
• Mars and Venus, compared to Earth, as possible habitats.
• Catastrophic impediments to life.
• Searching for life on Mars.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
From There to Here
• The puzzle of life for scientists is to imagine how the inanimate
chemicals available on the early Earth could have wound up creating
complex life forms.
• The approach to the problem to was break it down into steps. The first
step in the chain is the creation of complex organic molecules - amino
acids - from more simple inorganic compounds.
• The term ‘organic’ does not necessarily imply life - organic molecules
are any carbon molecules other than a few very simple ones.
• What chemicals would have been available on the early Earth? From
our earlier classes we know that a variety of volatile ice species
containing C, H, N and O in various combinations would be likely to be
delivered to Earth via planetesimals.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Miller-Urey Experiment
• One of the most famous chemistry experiments of all time is the MillerUrey experiment, named after two chemists at the University of Chicago.
• In 1953 (same year as Watson and Crick: DNA helix structure unraveled)
these scientists mixed NH3, CH4 and H20 in a vessel, intended to simulate
a possible composition of the early Earth’s oceans.
• The mixture was heated
to vaporize the gases,
which then passed
through a spark chamber,
simulating lightening
discharges.
• Finally, any reaction
products were condensed
and collected for analysis.
Picture credit: John D Simon, Duke University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Amino Acids
• At the end of 1 week, the
resulting condensate showed a
huge variety of compounds. 1015% of the carbon had formed
organic compounds, and 2% had
formed amino acids.
• What are amino acids, and why
are they so important?
• Chemically, an amino acid is a carbon molecule which has three types
of bonding other than simple C-H bonds: C=O, C-OH, and C-NH2.
• Amino acids are the building blocks of proteins, and essential to cellular
life as we know it. They are sometimes called prebiological molecules.
Picture credit: John D Simon, Duke University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Miller-Urey Experiment: Issues
• The original Miller-Urey experiment has been criticized on a number of
technical grounds:
• The early atmosphere of the Earth was probably not primarily
chemically reduced, as originally thought, but rather may have been
largely CO, CO2, N2 and H2O.
• The experiment used continuous lightening discharge for 1 week:
an improbable concentration of energy.
• However, other experiments using different starting ingredients have
found similar results: it is relatively easy to make amino acids.
• Even a sample Titan atmosphere (N2 and CH4) has been used in a
Miller-Urey type apparatus, and again amino acids could be made (with a
little acid for the oxygen!)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Amino Acids In The Solar System
• The Miller-Urey experiment has somewhat lost its impact over time, as it
has been shown that amino acids are actually present in space.
• This was dramatically demonstrated by analysis of the Murchison
Meteorite, a 1969 carbonaceous chondrite fall in Australia.
• More recently, in 2002 the discovery of glycine in interstellar giant
molecular clouds (GMCs - the sorts of nebula where stars and planets
form) was announced.
• Sugars and ethanol had already been previously found in GMCs,
thought to be formed by the action of UV light on ices.
• Hence, these molecules can form under a wider range of conditions
than shown by Miller and Urey, and indeed life on Earth may have been
‘seeded’ from space!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
What Is Life?
• You think you know what life is, but try defining it!
• Is a bacteria alive? What about a virus? A computer virus?
• Life can be distinguished by two principal properties:
1. Metabolism: the ability to utilize energy from the environment.
2. Reproduction: the ability to code and transmit information
through DNA.
• Which ability came first: reproduction or metabolism? At the current
time, the steps taken from inanimate chemicals to simple forms of life
are still largely unknown.
• All we know is, that it did happen!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Earliest Lifeforms on Earth
• It would be wonderful if the fossil record preserved the history of life on
Earth for us to look at. However, the Earth’s geology does not seem to have
preserved rocks older than 3.8 to 4.0 Gyr old: 600 million years after the
Earth had solidified.
• Why might this be?
• The earliest fossilized traces of life that we can recognize are of
cyanobacteria, or ‘blue-green’ algae.
• We see definite evidence of these bacteria in rocks as old as 3.5 Gyr. By
this time, the bacteria were already forming organized, layered structures
called stromatolites (if dome-shaped) or oncolites (if round).
• These structures form as mats in aquatic environments, and can trap
sediments, and sometimes secrete calcium carbonate (limestone), thereby
leaving a solid imprint.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Stromatolites
• Stromatolites (and oncolites etc) are still found today!!
• (Left) stromatolites at Lake Thetis,
Western Australia.
• (Right) stromatolites at Shark Bay,
Western Australia.
Picture credit: wikipedia.org
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Archean Microfossils
• The image (below left) shows a present-day, living cyanobacterium. The
right-side image shows a 1-billion year old fossil of a cyanobacterium,
from northern Australia. When archean stromatolites are sliced open, we
can see similar fossilized shapes to this.
• Aside from the cyanobacteria other, rarer types of fossilized bacteria can
be found. For example, the unusual magnetobacteria form tiny crystals of
magnetite inside their cells, which are left behind when the organic part of
the cell is gone. These tiny remnants, just a few nanometers (10-9 m) in
size, hold the record as the smallest fossils.
Picture credit: UCMP Berkeley
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Genomic Record
• A second way of tracing the origins of life is to hunt inside living
organisms today! But how?
• Inside every cell in the human body (except red blood cells) is the DNA
sequence that we call the human genome: the blueprint for creating us.
• Most of the DNA is ‘junk’: genes that are no longer used, probably
remnants of our long evolutionary history.
• We may compare the genetic code of humans with other animals and
plants, and record correlations or similarities.
• In this way, we can figure out mathematically which organisms are more
or less closely ‘related’ to us as a species.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Domains of Life
• By comparing the ribosomal RNA (an
ingredient of all known life forms) of
different organisms, a branched pattern of
relationships has been deduced.
• The more closely related the two species
are, the more recently in time that their
two genomes diverged.
• By studying the ribosomal sequences,
we now know life exists in 3 main types.
• Scientists also try to reconstruct the probable characteristics of the ‘last
common ancestor’: the very primitive life-form which later evolved down
the separate pathways of bacteria, archaea and eukaryota.
• Our best guess is that our earliest common ancestor was an anaerobic
(oxygen-avoiding), thermophilic (heat-loving) bacterium.
Picture credit: fossilmuseum.net
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Life and Oxygen
• Ironically, we now believe that life must have originated in an oxygenfree environment! Why?
• Free oxygen (O2) is a highly reactive gas, which will tend to combine with
reduced gases such CH4 and NH3, converting them to oxides.
• Most of the compounds necessary for the formation of life are suboxidized (e.g. CO is sub-oxidized compared to CO2), such as the amino
acids, proteins and so on. If oxygen was plentiful, these biological
precursors would have quickly been oxidized.
• When we examine rocks older than 2.2 Gyr on Earth, we find that they
are strongly sub-oxidized. These rocks must have formed in the absence
of free oxygen.
• Stromatolites became widespread at around 2.2 Gyr ago, and as green
plants produce O2 as a by-product of photosynthesis, the amount of
atmospheric O2 increased.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
CO2 on Mars, Venus and Earth
• When we look at our planetary neighbors (Mars and Venus), we see
atmospheres dominated by carbon dioxide. Why not on Earth?
• We have marine shells to thank for that! The activity of countless billions
of shell-forming creatures (mostly microscopic) over billions of years has
removed most of the CO2 from the Earth’s atmosphere. How?
• Shells are manufactured from calcium and carbon dioxide, which
dissolves from the atmosphere into the oceans. Eventually, these shells
are incorporated into the sediment as limestone.
• How much CO2 was removed in this way?
• If the all the current carbonate deposits on Earth were put back into the
atmosphere, we would have 70 bars of CO2 pressure, close to the 90 bar
atmosphere of Venus, which causes such a massive greenhouse effect.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheric Evolution
• Even without life, the Earth’s atmosphere would slowly evolve over time.
• CO2 is removed by non-biological processes, a reaction with silicate
rocks, which can lead to carbonate formation.
• Another process driving atmospheric change is photo-dissociation,
which we have mentioned in the context of the outer planets.
• On Earth, molecules such as CH4 and NH3 would be broken apart by
solar UV light in the upper atmosphere, and the hydrogen is then able to
escape to space.
• Over time, CH4 would convert to CO2 (taking oxygen from water) and
NH3 to N2, which we see today. CO would also convert to CO2, and finally
much of the CO2 would became carbonate rocks.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Energy and Life
• Why have we just been discussing atmospheric change, when the
subject is life? The problem for life is that oxidization removes the highly
reduced molecules which are the ‘food’ for living organisms.
• E.g. the sugars used by the human body for energy are highly reduced
molecules: we ‘burn’ them in our cells in the same way that we burn
hydrocarbon gasoline in our cars.
• If all the food becomes oxidized, then the ‘free’ chemical energy source
for early life is ended. What’s a bacterium to do?
• The answer is photosynthesis. This process connected life with solar
power, enabling plants to manufacture food so long as the Sun shines.
• From that time on, life had evolved past being dependent on a reducing
environment, and could weather the gradual transition to an oxidized one.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Late Heavy Bombardment
•
We know from the geological record preserved on the Moon that a
very heavy bombardment of the inner solar system was in progress
between 3.9 and 4.2 billion years ago. At earlier times, the
bombardment may have been even heavier - we do not know.
•
The Earth would have been pelted even more intensely than the
Moon: why would this be?
•
The reasons are:
1. Greater cross-section of the Earth
2. Greater gravitational influence (‘focusing’) of the Earth.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Early Impacts and Life
• It is quite likely therefore that impactors as large as several hundred
kilometers across were striking the Earth during this period: large enough
to boil off all the oceans - for a time.
• After a few decades, the planet would have cooled enough to regain a
hydrosphere.
• This would clearly have been a very
inhospitable place for lifeforms to make
their home!
• Without water, any lifeforms would
probably have been wiped out: a full
planet sterilization.
• This could have happened several
times during the Earth’s early history.
Picture credit: Karen Carr.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Later Impacts
• At later times, life on Earth still had to contend with massive impacts
making conditions difficult, but not impossible for life to continue.
• These were the mass extinction events: epochs when many species
became extinct in a short time. Until the 1980s it was not realized that
these events were due to impacts.
• That all changed when the K/T
extinction 65 Myrs ago between the
cretaceous and tertiary periods was
positively linked to a giant impact causing the Chicxulub crater in Mexico.
• The first evidence for an asteroid
impact was the high abundance of
iridium in the boundary clays: rare on
Earth, but more common in asteroids.
Picture credit: Levin, The Earth Through Time
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
K/T Extinction
• In the K/T extinction, more than half of all marine species at the time
became extinct, along with many land animals, including the dinosaurs.
• 99% of the species which have lived on the planet are now extinct. We
should be grateful: these impacts gave our ancestors a chance to take
over!
Picture credit: Levin, The Earth Through Time
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mass Extinctions
(and possible causes)
1. End Ordovician (~445 Ma); ~12% of
families, ~ 65% species; large glaciation/sea
level fall??
2. Late Devonian (~365 Ma); ~ 14% of
families, ~ 72% species; impact (Siljan
Crater)?
3. End Permian (~250 Ma); ~ 52 % families,
>90% species; impact (Bedout Crater)?;
flood basalts (Siberia); one continent; global
warming; low oxygen conditions.
4. End Triassic (~210 Ma); ~ 12% families, ~
65% species; impact (Manicouagan Crater);
flood basalts (Central Atlantic)
5. End Cretaceous (65 Ma); ~11% families, ~
62% species; impact (Chixculub Crater);
flood basalts (Deccan, India)
Picture credit: Norm McLeod: Firstscience.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary So Far
• Let’s summarize the conditions which were occurring in the first few
hundred million years of the Earth’s history:
• Impacts: somehow life was able to take a foothold after the very
largest (sterilizing impacts) ceased.
• Secondary atmosphere formation: from impacts and outgassing
from the interior. Enough atmospheric pressure to allow liquid water
to exist at the prevailing temperature.
• Chemical removal of CO2: enough removed to stop the runaway
greenhouse effect from occurring, but enough remained to keep the
planet warm enough for liquid water.
• Escape of hydrogen: the gradual transformation of the atmosphere
from reduced to oxidized: late enough for life to have already arisen,
and developed photosynthesis.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comparing Earth and Venus
• Now that we have some information on the conditions of the early Earth
which made it a suitable habitat for life, we can ask the question: were
Mars and/or Venus also habitable at some point in the distant past?
• Venus and Earth probably began with similar volatile inventories.
However, Venus was close enough to the Sun for the runaway
greenhouse effect to occur, destroying water.
• The best evidence for the destruction of water on Venus comes from
the D/H ratio: the ratio of the heavy hydrogen isotope to the lighter
(normal) isotope.
• D and H may both be found in the water molecule. In an earlier class
we saw how the higher temperature of Venus could allow water to push
through the tropopause (where water is trapped and condensed on the
Earth) and thereby reach the upper atmosphere. At these levels, water is
exposed to break-up by solar radiation.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Venus, Water and Life
• As water was photodissociated by sunlight in the upper atmosphere of
Venus, H, D and O atoms would be produced.
• Hydrogen, being light, could then escape to space. Since H escapes
much more readily than D, we expect to see an abnormally high D/H ratio
on Venus if water has been destroyed this way.
• In fact, we do see a D/H ratio on Venus about 100 times higher than on
the Earth.
• Without water, life could not have arisen on Venus.
• In the absence of water, extreme amounts of carbon dioxide would have
remained in the atmosphere,ensuring the runaway greenhouse effect,
and the high surface pressure and temperature.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Detecting Life on other Planets
•
We can turn the question of how we could detect life on other planets
around, and ask how aliens could detect life on Earth, from a nearby
planet.
•
There are three ways:
1. Radio or city-light emissions: only in the last hundred years would
this be detectable.
2. Physical changes, I.e. vegetation. This would require extremely
good telescopes, even for a nearby planet.
3. Atmosphere. Life on Earth as modified the atmosphere: green
plants are responsible for the free oxygen in the Earth’s
atmosphere, which would otherwise combine with rocks.
Spectroscopy could have revealed life several billion years ago.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Search For Life On Mars
• The search for life on Mars was one of the major objectives of the Viking
landers. Four experiments were carried.
• The first experiment was a GCMS: an instrument designed to look for
organic molecules. None at all were found.
• The other three experiments
attempted to look for the effects of
biological metabolism: changes in
gases in reaction vessels.
• You can read the details in the
textbook. The upshot is that all the
effects seen could be attributed to
the unique properties of Martian soil.
Control (sterilized) samples showed
the same activity as unsterilized.
Picture credit: Wayne County RESA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Martian Meteorites
• Since the late 1970s, the biggest debate about life on Mars was
generated by the announcement in 1996 of possible microfossils inside a
Martian meteorite.
• You will recall from our earlier
class on meteorites that such
pieces of Mars have definitely
been identified amongst Antarctic
meteorite finds, due to analysis of
trapped gases.
• The meteorite in question,
ALH84001 is shown (left). This
meteorite is 4 billion years old,
dating from a time when liquid
water was present, causing some
sedimentary deposits.
Picture credit: NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Martian Microfossils
• The most spectacular, and controversial evidence of life, were
photographs of purported ‘microfossils’ - minute structures 1/100 the
width of a human hair.
• The main problem with these structures is the size: about the size of a
virus on Earth, which cannot survive without access to the DNA of a
bacterium.
• The other main strand of evidence
which has not been adequately
explained is the existence of chains
of magnetite crystals: similar to the
remains of magnetobacteria on
Earth. There are possible nonbiological explanations for this, so
the case is still open.
Picture credit: NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Water on Mars
• We believe that water was a likely requirement for life to evolve. How
much water could once have been flowing on Mars?
• If we try to re-construct the ancient atmosphere of Mars, based on the
ratios of noble gases to reactive gases today on Earth compared to Mars,
we suspect that much of Mars’ volatiles have been lost, and in the ancient
atmosphere was at least 0.07 bar.
• This is ten times the current atmosphere, but still 1000 times less than
the volatile inventories of Venus and Earth.
• A CO2 atmosphere of 0.07 bar should be accompanied by enough water
to cover the planet to a depth of 9 m.
• However, this is much less than geologic evidence suggests: the outflow
channels must have been carved by mighty flows, and equivalent depth of
1000 m! How to clear up this mystery?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Impacts on Mars
• The answer seem to lie in impact erosion.
• Mars is smaller than the Earth, and closer to the asteroid belt. Hence,
impacts were both more likely to occur, and more likely to have enough
energy to blast the atmosphere off into space.
• Calculations suggest that Mars could have had an original atmosphere
100-1000 times the current amount, most of which has been eroded by
impacts large and small over geologic time.
• A test of this theory is to look for carbonate deposits: if the entire
atmosphere was removed by impacts, then very little would be bound up
in carbonate rocks (as on Earth).
• The search for carbonate rocks is currently a major quest for the Mars
Exploration Rovers.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary: Venus
• Venus is too hot! Being 41 million km closer to the Sun was fatal to this
world.
• The sunlight intensity is a factor of 2 higher than at the Earth, leading
inevitably to the runaway greenhouse effect and the destruction of water.
• The main evidence for this scenario is the very high D/H ratio on Venus:
showing that water was indeed destroyed.
• If we compare the volatile inventories of Venus and Earth, we see that
they started with similar amounts of each gas.
• However, on Earth, most of the CO2 was later bound up into rocks, and
on Venus the water was destroyed.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary: Mars
• The problem with Mars is not that it is too far from the Sun, the main
problem is that it is too small to resist impact erosion of atmospheric
gases.
• In the past, Mars may have had a peak atmospheric density of 1-2
bars, of a CO2-N2 atmosphere, before 99% was lost to erosive impacts.
• This could have permitted a 0.9 km deep ocean, enough to cause the
outflow channels we observe.
• The main challenge is to see if live arose briefly at some point in the
past.
• About 3.5 billion years ago on Mars, conditions would have been
favorable. At this time, life was certainly producing fossils on Earth.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. What types of ‘raw materials’ were present on the early Earth for life to
build with? What oxidation states were these chemicals in?
2. What was the Miller-Urey experiment, and what did it find?
3. What is an amino acid, and why are they significant?
4. Where have scientists found amino acids, other than on the Earth?
What other organics have been found?
5. What are the two major traits of life?
6. What are the earliest records on life on Earth?
7. What other methods can we use, other than fossils, to trace the
evolution of life on Earth?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
8. What are (I) the three domains of living things (ii) the ‘last common
ancestor’. What was the LCA probably like?
9. What was the oxidation state of the early Earth’s atmosphere, and why
was that important? How did it change over time?
10. How was (i) CO2 (ii) hydrogen lost from the Earth’s atmosphere?
11. What is photosynthesis?
12. What was the role of impacts on life 4 billion years ago? What about 65
million years ago?
13. Were all mass extinctions due to impacts?
14. What happened to water on Venus, and what evidence do we have for
this?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
15. Why did the greenhouse effect on Venus runaway?
16. What ways could we detect life on Earth, from the distance of a nearby
planet?
17. What did Viking find on Mars, as regards traces of life?
18. What rock caused a controversy in 1996? Why?
19. What happened to the atmosphere of Mars?
20. What geologic evidence do we have for large amounts of water in the
past on Mars?
21. Could life have arisen on Mars in the past? When? Is it likely to be still
there, dormant?
Dr Conor Nixon Fall 2006