Anouncements

1
Anouncements

• Homework and EC due on Tuesday

2
Life in the Solar System
3
Life in the Solar System

• Today we will cover how and where life could have
  arisen in the solar system, and how we might try
  to look for it.
• Main Topics,
• Scientific theories on the origins of life on
  Earth,
• Conditions necessary for life,
• Mars and Venus as possible habitats compared to
  Earth,
• Catastrophic events impeding life,
• Searching for life on Mars.

4
life

• The puzzle for scientists is to imagine how
  inanimate chemicals on Earth could end up
  creating complex life forms.
• It is easiest to break this down into steps, the
  first being the creation of complex organic
  molecules, amino acids, from simple inorganic
  compounds.
• Organic does not imply life, organic
  molecules are any carbon molecules other than
  some very simple ones.
• We know most chemicals available at early Earth.
  There were plenty of ices containing, C, N, H,
  and O in various combinations.

5
Miller-Urey Experiment

• This is one of the most famous experiments of all
  time, named after the scientists at U Chicago who
  performed it, Stanley Miller and Harold Urey.

• In 1953 the scientist mixed NH3, CH4, and H2O in
  a vessel, simulating a possible early Earth
  ocean.
• The mixture was heated to vaporize the gases,
  which then passed through a spark chamber, which
  simulates lightening discharges.
• The products were then analyzed

www.physics.hku.hk
6
Amino Acids

• After one week there was a wide range of
  condensates. 10-15 of the carbon was in organic
  compounds, and 2 were in amino acids.

• Amino acids are carbon molecules with specific
  bonding,
• CO
• C-OH
• C-NH2
• Amino acids are the building blocks of proteins,
  and are essential to life as we know it. Also
  called, prebiological molecules.
• ? Phenylalanine is one of the standard amino
  acids.

Wikipedia.org
7
Problems with Miller-Urey

• This famous experiment has been criticized in a
  number of ways,
• The early atmosphere at Earth was not likely
  chemically reduced, it was more likely mostly CO,
  CO2, N2, and H2O.
• The experiment used continuous lightening
  discharge for 1 week. A lot of energy.
• Other experiments have been able to make amino
  acids with a variety of starting conditions.
• Even Titan-like compositions (N2 and CH4) have
  been used and successfully created amino acids.

8
Amino Acids in the Solar System

• An incredible result at the time, the Miller Urey
  experiment has lost some luster, now that we are
  finding Amino Acids in space.
• The Murchison Meteorite in 1969, a carbonaceous
  chondrite, was found to have them.
• In 2002 glycine was detected in giant molecular
  clouds.
• Sugars and Ethanols have also been found in GMCs,
  probably formed by UV radiation acting on ices.
• So we dont need Miller-Urey, many prebiotic
  compounds can be made in space, and dont need
  lightning charges and specific mixtures of
  chemicals.

9
What is Life?

• So, now we should try to define life.
• Bacteria? Viruses?
• The two properties we are looking for are,
• Metabolism the ability to utilize energy from
  the environment, and
• Reproduction the ability to code and transmit
  information through DNA.
• Which came first? Good question. We are still
  unsure about the steps from inanimate chemicals
  to simple life forms.
• But, it did happen. A pretty powerful constraint.

10
Earliest lifeforms on Earth

• We do not find rocks on Earth older than 3.8-4.0
  Gyr old. This is 600 million years after the
  Earth solidified.
• The first unambiguous record of life is of
  cyanobacteria (or blue-green algae) on Earth from
  about 3.5 Gyr ago.
• By this time the bacteria had already organized
  into complex colonies of micro-organisms called
  stromatolites (dome-shaped), or oncolites
  (round).
• These structures form in aquatic environments,
  and can trap sediments, sometime secreting
  calcium carbonate (limestone).
• It would be 2.7 billion more years before
  multicellular life appeared.

11
Stromatolites

• Stromatolites are not simple, and would have
  required hundreds of millions of years for life
  to have evolved to this state.

• These grow by the trapping of sedementary grains
  by cyanobacteria (blue-green algae).

wikipedia
12
Archean Microfossils

• This is a billion year old fossil of a
  cyanobacterium, from N. Australia. Similar things
  can be found inside of archean stromatolites.

• There are other rare types of fossilized bacteria
  which can be found. Magnetobacteria form with
  crystals of magnetite in their cells, which get
  left behind when the organic part of the cell is
  gone.

ucmp.berkeley.edu
13
The Genomic Record

• We can also trace the origin of life using
  DNA/RNA.
• Most of DNA is junk, meaning that it is genes
  which are no longer used, remnants from our
  history.
• So we can compare our genetic code with those of
  other animals and plants, watching for
  correlations or similarities.
• This will provide a mathematical method to match
  closely related species.

14
Domains of Life

• Comparing the Ribosomal RNA of different
  organisms, we can build a map of where life has
  branched out.

• Closely related species will have had their
  genomes diverge more recently.
• The ribosomal sequences point to 3 main types of
  life.
• It is also a goal to hunt down the last common
  ancestor, that life-form which later evolved
  into, bacteria, archaea and eukaryota.
• The best guess is some anaerobic thermophile.
  What?

fossilmuseum.net/
15
Life and Oxygen

• Currently, we believe that life originated in an
  oxygen-free environment.
• Free Oxygen gas (O2), is highly reactive and will
  convert reduced gases (CH4, and NH3) into oxides.
• Most components needed for life are sub-oxidized
  (meaning they contain compounds which have not
  combined with all the oxygen they could CO is
  sub-oxidized compared to CO2 ),
• Amino acids, proteins.
• If Oxygen was plentiful, these would have been
  quickly oxidized.
• Rocks older than about 2.2 Gyr old on Earth are
  strongly sub-oxidized. So they formed in the
  absence of Oxygen.
• Stromatolites became widespread around 2.2 Gyr
  ago

16
CO2 on Mars, Venus and Earth

• Our neighbors, Mars and Venus have atmospheres
  loaded with CO2.
• Our Marine shells have kindly removed a large
  portion of our CO2 from the atmosphere
• Shells are made from calcium and CO2, which is
  dissolved in the oceans from the atmosphere.
  These shells eventually end up in the sediments,
  like limestone.
• The total amount of CO2 removed this way would
  amount to a surface pressure of about 70 bars on
  the surface of Earth, pretty close to the 90 bars
  found at Venus.

17
Atmospheric Evolution

• Our atmosphere would have evolved regardless of
  life.
• CO2 is removed in non-biological reactions with
  silicate rocks, leading to carbonate formation.
• Photo-dissociation can also drive some
  atmospheric change.
• CH4 and NH3 would be broken up at the top of
  Earths atmosphere, allowing Hydrogen to escape.
• Over time, CH4, would convert to CO2, and NH3 to
  N2, which is what we see today. CO would be
  converted to CO2 and a lot of CO2 would end up in
  carbonate rocks.

18
Energy and Life

• The reason we are focusing on atmospheric change
  is that oxidization removes highly reduced
  molecules which are the food for living
  organisms.
• For example the sugars that we burn as humans
  are highly reduced molecules, we burn them in our
  cells the same way we burn hydrocarbons in cars.
• So, if all the food become oxidized then the
  easy chemical energy source for life is gone.
• Photosynthesis provides a way around all of this.
  Once photosynthesis provided a means of energy
  production without a reducing environment, life
  could survive the slow transition to an oxidized
  one.

19
Late Heavy Bombardment

• Cratering on the Moon suggests that there was a
  period of very heavy bombardment in the inner
  solar system between 3.9-4.2 Gyr ago (the LHB),
  including many impacts of 10-100 km bodies.
• When this happened, the Earth should be impacted
  much more than the moon
• Larger physical target,
• More gravity, more gravitational focusing.

20
Early Impacts

• During the LHB, it is likely that there were
  several hundred km impacts. Large enough to boil
  off oceans.
• Decades after one of these impacts, things would
  cool down, allowing the hydrosphere to return.

• During this time of the LHB, Earth would not have
  been a hospitable place.
• With water temporarily removed, any life would be
  wiped out, effectively sterilizing Earth.
• This could have happened many different times
  during this very early history on Earth.

21
Later Impacts

• Well after the LHB, impacts would still frustrate
  life on Earth, but not deny it entirely.
• There have been many mass extinction events,
  removing huge numbers of species in one event.

• 65 Myrs ago the K/T event between the cretaceous
  and tertiary periods has been positively linked
  to an impact in Mexico.

22
K/T extinction

• At the K/T extinction, more than half of all
  marine species at the time became extinct, along
  with many land mammals, including dinosaurs.
• 99 of all species that have ever lived on Earth
  are now extinct.

23
Mass extinctions

• End Ordovician (445 Myr) 12 of families, 65
  of species.
• Late Devonian (365 Myr) 14 of families, 72
  species, Impact (Siljan Crater?).
• End Permian (250 Myr) 52 families, 90
  species impact (Bedout Crater?). 96 of all
  marine species, the great dying
• End Triassic (210 Myr) 12 families, 65
  species, impact (Manicouagan Crater).
• End Cretaceous (65 Myr), 11 of families, 62 of
  species impact Chixculub crater.

www.firstscience.com
24
Summary .

• So here is what we are thinking for the first few
  hundred million years
• Impacts After the largest sterilizing impacts
  life somehow took hold.
• Secondary atmosphere formation from impacts and
  outgassing from interior. Always enough
  atmospheric pressure to allow liquid water at the
  surface temperature.
• Chemical removal of CO2 enough was removed to
  prevent the runaway greenhouse effect, enough
  remained though to keep the planet warm enough
  for liquid water to be present.
• Escape of hydrogen the atmosphere transforms
  from reducing to oxidizing, this happens late
  enough that life has already arisen and developed
  photosynthesis.

25
Earth v. Venus

• So, we have an idea of what made Earth habitable
  for Life, what about Mars or Venus in the past?
• Venus and Earth probably started with similar
  quantities of volatiles. However, Venus, being
  closer to the Sun, to allow the runaway
  greenhouse effect to occur, destroying water.
• The evidence from Venus is from the D/H ratio,
  this is the ratio of heavy Hydrogen (Deuterium)
  to the lighter isotope.
• Both D and H are found in water molecules. With
  the high temps at Venus, water is pushed high in
  the atmosphere and separated by solar radiation.

26
Venus v. Earth

• As the water is dissociated by sunlight at the
  top of the atmosphere, H, D, and O atoms are
  released.
• Hydrogen, H, is the lightest and most likely to
  escape the atmosphere. D, is heavier and more of
  it will be retained.
• So we should see an elevated D/H ratio on Venus
  from this effect.
• This is what we see, it is 100 times higher than
  what we have on Earth.
• So, without water, life would not arise in Venus.
• With no water extreme amounts of CO2 would remain
  in the atmosphere, ensuring the runaway
  greenhouse effect, and high pressure and temps.

27
Detecting life on other planets

• So in thinking about detecting life on other
  planets, we can consider how someone would detect
  life on Earth.
• Radiation emission (man-made) City lights, radio
  etc. only in the last hundred years or so would
  this be possible.
• Physical changes Vegetation changing from winter
  to summer. One would need a GOOD telescope.
• Atmosphere Life on Earth has modified the
  atmosphere plants free Oxygen in the Earths
  atmosphere, which would otherwise combine with
  rocks. Spectroscopy of the atmosphere would
  easily detect this. A long time ago.
• Which of these methods will be best suited for
  searching newly found planets?

28
Search for Life on Mars

• Searching ofr life on Mars was the major
  objective of the Viking Landers, which carried
  out 4 experiments.
• The first experiment was a GCMS, with an ability
  to search for organic molecules. None were found.

• The other experiements were looking for effects
  of metabolism changes in the chemistry of gases
  in a reaction vessel.
• There were exciting results. Which ended up all
  being negative.

www.resa.net
29
Martian Metabolism experiments

• GEX exposed martian soil to a mix of nutrients,
  looking for Oxygen produced by metabolism.
• LR again, mixed carbon-rich molecules with
  soil, looking for gases that would be created.
• PR Heated soil with a mixture of gases, looking
  for gases combining with soil compounds

30
Martian Meteorites

• The biggest debate regarding life on Mars came in
  1996 when possible fossils were found inside a
  martian meteorite.

• We covered Martian meteorites, and we found that
  these have been positively identified, due to
  analysis of trapped gases.
• The important meteorites was ALH84001, which is 4
  billion years old, dating from when liquid water
  was present on Mars.

www.racine.ra.it/
31
Martian Microfossils

• The big news was the photos of the controversial
  microfossils, things only 1/100th the width of
  a human hair.
• The problem is, these are very small, about the
  size of a virus, which itself cant survive
  without access to DNA from bacteria.

• The other evidence from the meteorite was the
  existence of magnetite crystals similar to the
  remains of magnetobacteria on Earth. There are
  still non-biological explanations for this.

serc.carleton.edu
32
Water on Mars

• We are pretty sure that water was a requirement
  for life so how much was ever flowing at Mars?
• Working backwards using noble gas concentrations
  compared to those of Earth, we can estimate that
  the atmosphere was once at least 0.07 bars.
• This is ten times its current size, but will 1000
  times less than Earth and Venus.
• A CO2 atmosphere of 0.07 bar should be
  accompanied by enough water to cover the surface
  to a depth of 9 m.
• However, we see signs of very deep channels,
  carved by very large flows, things up to 1000 m..

33
Impacts on Mars

• Impacts are the likely answer.
• Mars is smaller than the Earth, so it has less
  gravity to hold onto gases.
• Mars is also closer to the asteroid belt, and
  therefore more likely to be hit by impactors,
  which can remove atmosphere.
• We think that Mars could have had an atmosphere
  100-1000 times its current level, most of which
  has been removed over time via large impacts.
• A test is with carbonate rocks. If impacts
  removed most of the atmosphere, then very little
  would have ended up in carbonate rocks (as we
  find on Earth).
• Carbonate rocks are a big target for the Mars
  Rovers

34
Summary Venus

• Venus is too hot. The extra 41 million km closer
  to the Sun is the issue.
• The sunlight is twice as intense, leading to
  runaway greenhouse effect, and the destruction of
  water.
• The evidence for this scenario is the high D/H
  ratio on Venus, which shows that water was
  destroyed.
• Venus and Earth started with similar amounts of
  most volatiles.
• On Earth the CO2 ended up in rocks, and on Venus
  the water was destroyed.

35
Summary Mars

• For Mars the issue is not distance, but size. It
  is too small to hold its atmosphere after large
  impacts.
• In the past Mars may have had an atmosphere up to
  1-2 bars, with a CO2-N2 atmosphere, before 99 of
  it was lost from impacts.
• This would have allowed an ocean 0.9 km deep,
  enough to carve the outflows we observe.
• We still really want to know if any life arose
  during those times of habitability.
• The prime range was around 3.5 Gyr ago, when
  there could have been life, as there already was
  on Earth.

36
The habitable zone
wikipedia.org

• The habitable zone is a region around a star in
  which conditions for life as found on Earth are
  favorable.

• The habitable zone is a region where a planet is
  thought to be able to maintain liquid water.

37
Galactic Habitable Zone

• The location of a star within a Galaxy is also
  important.
• A solar system that forms closer to the center of
  a Galaxy is more likely to have plenty of heavy
  elements needed for the formation of rocky
  planets.
• It also must be far enough away to avoid hazards
  of high density regions, and dangerous radiation
  from the Super Massive Black Hole at the center
  of the Galaxy.
• Of course, both versions of the HZ, are based on
  the form of life that we know.. It in no way
  precludes life forming in a way we dont
  understand.

38
Binary Stars

• Can planets form in habitable zones around binary
  stars? About 50 of stars are in binaries.
• It appears that two scenarios are still possible.
• Having a habitable zone around each star, or
• Having a habitable zone around two closing
  orbiting binary stars.

39
Red Dwarfs

• Red Dwarfs make up 70-90 of all stars in the
  Galaxy.
• The discovery of Gliese 581b around a Red Dwarf,
  a star with only 1.3 the luminosity of the Sun.
• Potential problems,
• Close orbits are needed around Red Dwarfs, so
  close that planets may be tidally locked, cooking
  one side and freezing the other.
• Infrared light is the main radiation from Red
  Dwarfs, rather than visible light at the Sun, due
  to cooler temps at Red Dwarfs. This complicated
  photosynthesis as we know it.
• Variability and violent flares are potential
  problems, with dimming down to 40 due to star
  spots, and flares can double their brightness.
• One upside is their long life, they are small
  enough to burn their fuel very slowly, living for
  trillions of years. It took 4.5 billion years for
  intelligent life to form on Earth, so this is a
  big advantage.