Today’s APODAPOD  Homework & Quiz 10 on Friday  Reading on Oncourse – “Essay 3” The Sun Today A100 Life in the Universe

 This week’s reading is available from the Resource Tool on Oncourse – Essay 3

Final Exam…  Friday, 2:45 PM, here.  2 hours  100 multiple choice questions  Comprehensive  50% on chapters 10, 11 & Essay 3  50% on material covered in the first three exams  Review sheet on Oncourse

Today’s Topics  Origin and History of Life on Earth  What about elsewhere in the Solar System?

Life may be common in the Universe  Life arose quickly on Earth  Life utilizes naturally occurring chemistry  Some microorganisms can live in extreme conditions

Beginnings of Life on Earth  For its first 500 million years, Earth was inhospitable to life  The era of heavy bombardment - oceans vaporized, killing early life  Heavy bombardment ended about 4 billion years ago  Life arose quickly once conditions became hospitable

Fossils and Geologic Time Scale  Fossils tell us about the history of life on Earth  Based of the layering of the rocks and fossils Earth’s history can be divided into several distinct intervals or geological time scales.

Fossil Evidence of Early Life on Earth  Ancient bacteria left fossil rocks called stromatolites  The amount of carbon-13 in rocks with fossils suggests life was present 3.85 billion years ago

Mechanisms of Life on Earth  DNA (deoxyribonucleic acid) is the genetic material of all life on Earth  DNA can reproduce itself - the key to heredity

The First Living Organism  DNA has the same basic chemical nature in all Earth life  All Earth organisms build proteins from the same set of amino acids  All Earth life had a common ancestor which arose some 3.85 billion years ago

Where Did the First Life Come From?  A mixture of early-Earth organic molecules plus lightning can produce all the major molecules of life including amino acids and DNA bases  Strands of RNA (ribonucleic acid) which resemble single strands of DNA have been reproduced in the laboratory

The Panspermia Hypothesis  The idea that life started elsewhere and then came to the Earth (via meteor impacts) is called panspermia  Organic molecules are found in meteorites  Some microbes can survive space for years

Cyanobacteria and Oxygen  Photosynthesis from single-celled cyanobacteria produced oxygen some 3.5 billion years ago  For more than 1 billion years, this oxygen reacted with surface rocks and little stayed in the atmosphere  Eventually, some 2 billion years ago, the oxygen began to accumulate, but would not be “breathable” until just a few hundred million years ago

The first plants on land were probably algae  After the heavy bombardment ended, the common ancestor of life formed  Life rapidly grew and diversified, but remained single cell organisms for 1 billion years.  The land was still inhospitable until the ozone layer formed – this required atmospheric oxygen, and the first land plants emerged

The Cambrian Explosion  About 540 million years ago, tiny plants and animal organisms changed dramatically in about 40 million years and formed into all the basic plans (phyla) that we find on Earth today.  The dramatic change in the diversity of life is called the Cambrian explosion.  The earliest humans formed only a few million years ago (after 99.9% of Earth’s history).

Requirements for Life  Three basic requirements for life:  A source of nutrients  Energy to fuel the activities of life  Liquid water (the biggest constraint)

Life in the Solar System Where in the Solar System can we find the basic requirements for life?

Life on other Terrestrial Planets?  MERCURY  MOON  VENUS  MARS X Extreme Temperatures, No Atmosphere, UV, Cosmic Rays, No Liquids Extreme Temperatures No Atmosphere, UV, Cosmic Rays No Liquids X High Temperatures No or Little Water Young Surface  No Fossil Record X ? Once had Liquid Water Ice Present Now, Temperatures OK

The Habitable Zone The planet needs to be the right distance from the star. WHY? The star needs to have the right mass. WHY? Too hot! Too cold!

Life on Mars?  Rovers studied Martian conditions to see if life might have existed  No evidence for life now  Maybe life in the past

The Martian Meteorites  Meteorite found in Antarctica is of Martian origin  Inside the meteorite were complex organic materials and structures which looked like nanobacteria  Similar structures can be made by chemical and geological means  Contamination from Earth may also explain the presence of organic materials

Life on the Gas Giants?  Ingredients for organic chemistry  Atmospheric layers with temperatures like Earth  But no solid surfaces or liquid water  Winds and turbulence mix gas quickly over extremes of T & P  Sunlight is very weak, but some internal heat  Hard to explore!

What about the Moons of Jupiter? Io Europa Ganymede Callisto

Io – The most volcanic object in the Solar System Europa – A “cracked ice crust” over a water ocean

Life on Jupiter’s Moon Europa?  Europa may have enough tidal heating to form a subsurface ocean underneath its icy crust  Life there could form like the deep sea vents on Earth  Larger life forms could exist in the vast oceans, but energy sources are limited and this would tend to limit the size of any life there

Jupiter’s Moons are Heated by Tides Tidal heating occurs because Io and Europa’s orbits are eccentric

Enceladus  Small icy moon (500 km diameter) of Saturn  Young, crater-free surface regions with like those on Europa  Orbit resonance with Dione  South polar hot spot and ice plumes  Thin “atmosphere” of water vapor  Subsurface ocean!?

Titan  Moon of Saturn  The atmosphere is denser than Earth’s but very cold (100K) and mostly CH 4 and N 2  Enshrouded in smog-like clouds  Methane acts like water  Few craters on the surface  Surface eroded by liquids but no oceans

Ganymede, Callisto, and Titan  Ganymede and Callisto might have subsurface oceans, but their internal heat is small and liquid water would not be terribly abundant  Titan has no native liquid water, but an abundance of organic materials.  Could life evolve from the lakes of methane?  Water might be brought in from comets, but this would eventually freeze

Triton  Extremely cold (<40K) moon of Neptune  Moons made from volatile materials produce icy volcanism  Huge geysers of nitrogen!  Pluto and the Kuiper Belt Objects may look and act similarly.

Environments for Life  Surfaces of Planets (or Moons)  Location, location, location…  Size matters (for retaining an atmosphere)  The star matters  Overall, However  There is an incredible diversity of worlds!!!  Warm pockets or oceans of liquids plus organics may exist in a variety of environments outside the classic Habitable Zone  Even on Earth not all life requires starlight for an energy source. Sources of potentially life-giving energy may exist even in the cold outer reaches of our own and other planetary systems

Is Life Rare or Common?  Some feel that an Earth type planet (with its complex type of life) is rare:  Galactic constraints  Too close to the galaxy’s center and the rate of supernovae are too great.  Too far from the center and “metal” content is too low.  A stellar system needs a Jupiter-like planet to sweep-out and deflect meteors that might wipe out life on Earth.  Climate stability  Plate tectonics and the carbon dioxide cycle.  Earth’s large Moon keeps axial tilt relatively stable.  Counterarguments to the Rare Earth Hypothesis  The above conditions may not affect the creation and advancement of complex life as much as we think  There may be other overlooked conditions and processes that could assist the creation and advancement of complex life

Or is the Universe Life-Friendly?  Water and carbon chemistry are everywhere!  Hydrogen, Oxygen, Carbon, Nitrogen  1st, 3rd, 4th, and 6th most abundant elements  Organic chemistry is found in  Interstellar gas clouds, comets, meteorites, outer planet and moon atmospheres  Physical Laws  Copasetic Time Scales  Expansion rate of the Universe & stellar lifetimes compatible with time to evolve complex life  Abundant Materials  Even small changes in physical constants would cause little hydrogen or carbon to exist  Huge Diversity of Environments

 Reading, homework, quiz next week  Last Homework, Quiz on Friday  Final on Dec. 19 at 2:45 PM Dates to Remember