Introduction to the Solar System

How do scientists know how old a surface is (whether it's been reworked by volcanism or tectonism)?
We think that the number of objects flying around the solar system has been decreasing exponentially since the solar system began.
We know that parts of the moon are very old, because it has collected impacts for a long time without wiping them out.
We know the surface of Venus is very young, because it doesn't have very many craters.
There are more small ones craters than big ones, because there are many more smaller rocks in space than large ones.
Size-frequency plot is a created by counting the number of craters in several size ranges, and plotting the number of craters in each bin vs. crater size.
The vertical axis is the number of craters/km2 in a size bin.
The horizontal axis is crater diameter.
The size frequency is a log-log plot, and usually is a straight line.
Where the line falls tells you how old the surface is.
The tilt of the line tells you that small craters may have been wiped out.
So what planetary scientists do when they first see a planetary surface is to figure out how long ago a planet has been geologically active. Count up all the craters, and put them in bins, and then make a plot of the size-frequency distribution.
They compare the height of the line with the line made by the craters counted around where Apollo sampled rocks -- ones we know the age of.
So you see fewer craters on the maria -- how much younger are they?

We are immersed in and part of a biosphere.
Therefore, we should recognize that we have some biases and perspectives about what life, essentially hard-wired in.
Analogy -- Earth centered Universe, our cosmology reflected our biases
Our view of life may be reflected by biases inherent in human consciousness.
What distinguished life from a rock?
Commonly agreed -- replicating molecules are required for life.
But rocks replicate -- crystal structure remains intact after they break apart. Rocks also 'grow', because they are often crystalline, and grow as crystals.
Then how are they different?
In life, replicating molecules contain information, and that information is passed on with replication.
The information is contained in the sequence of nucleotide bases making up DNA -- the genes
The information tells how to build the organism from the available materials.
The expression of the genes in the form and function of the organism is its phenotype.
The genetic information is subject to changes and errors as copies are made of copies, of copies, etc.
Random changes in the genetic information cause random changes in the organism's phenotype -- mutations.
These changes may cause the organism to be either less or more competitive with other replicating organism, in a given environment.
If the organism becomes more competitive, that random genetic change, or mutation, survives.
Organisms with that advantage proliferate.
This is called Natural selection or Darwinian Evolution
So life is: a self sustaining chemical system capable of undergoing Darwinian evolution.
Requirements for participating in natural selection:
Systems that reproduce
Random variations in their offspring
Variations can be inherited
Variations might incur some survival advantage
Competition between reproducing entities
Overproduction so that not all will survive to produce offspring
Changing environmental selection pressures
But with this definition, is life always recognizable?
There is only one kind of life on the Earth -- the DNA and protein based life form that is common to all known terrestrial biology.
All the molecules in our body are built of carbon chains, with a variety of other elements attached.
We need the chains to give us structural strength, and to give organic molecules the complex shapes required for specific chemical tasks.
But what about life made from chains of different elements, Si, S, Ge (they also form chains)?
What about life that uses a completely different information carrying system, say, information-carrying genes made from sound or plasma waves?

Earth is 4.5 By old, from the radioactive decay of elements
Radioactive dating -- Start with 1 million atoms of 235U
Half-Life -- 700 my -- 500,000 atoms of U left after 700 million years.
If rock formed with pure 235U grains at the start of the solar system, it would have 1% of its 235U atoms left, or 10,000.
We can date any rock we find on the surface.
Oldest rocks are found in old sediments of the centers of the continents -- cratons.
They date to about 4 By.
Fossils are the main source of information about past life.
Animals die and get contained in the sediments, hard parts preserved with the sediments, creating fossils.
Almost all our fossil record is from the Cambrian, 600 my or less.
But there are some fossils from very long ago.
Photosynthetic bacteria, called cyanobacteria -- 3.5 By old.
Cyanobacteria are also called blue-green algae.
Stromatolites -- layered deposits of calcium carbonates and sand, created by ancient cyanobacteria.
Also, cyanobacteria form the blue-green scum that grows in a water trough.
They use the sun for photosynthesis: They build their carbon chains from CO2 in the atmosphere, and use the energy of sunlight to do it.
They are DNA based life, which means they have the same system of genetic information as we do.
Even older evidence -- isotope record of carbon.
Life alters the isotopes of carbon, favoring one isotope over another when it gets incorporated into the organism -- creates a geochemical signature -- 3.8 Billion years old (sediments in Greenland).
But we know from the bombardment record of the Moon that the Earth-Moon system had heavy bombardment until about 3.8 By ago.
Impact frustration of life -- impacts would have heated huge areas, cut off the sun, melt things.
But life must have started very quickly after the number of impacts began to die down.

The information in DNA is carried by a code of 4 letters, (molecules), G, T, A, C
Nucleotides -- guanine, thymine, adenine and cytosine
This is the alphabet -- Sections of the code mean -- 'make this protein' This is called the genetic code.
Proteins are the building blocks of all life.
They also act as catalysts, controlling and speeding up reactions -- enzymes
RNA is a kind of molecule like DNA (helix), but has a code with A, C, G,U Uracil
RNA copies portions of the genetic information from the DNA -- messenger RNA -- transcription.
Takes it to a ribosome -- work bench that takes the RNA-transcribed information, and assembles proteins.
Proteins have a 20 letter alphabet. The letters are called amino acids.
Process of taking the messenger RNA code and translating it into the 20 letter amino acid alphabet is called translation.
Millions of different proteins, all have a unique function.
Their shapes define their function.
Complex chemical reactions happen by the fitting of one molecule into another.
Proteins control all reactions, including the replication of DNA.
Proteins codes determine how the nucleotides are sequenced.
Fundamental problem -- DNA is needed to make proteins, but proteins are needed to make DNA!

Must have been some vastly simpler scheme for carrying and replicating information from early organisms.
RNA world -- RNA instead of DNA as prime information carrier.
Thomas Cech, CU, demonstrated that RNA was capable of enzymatic activity.
That is it could work as genes and as proteins.
But RNA is very, very complex, and couldn't just come about by random chemical reaction.
Amino acids, that make up proteins, are easy to make by natural processes, but nucleotides are not.
Life is too 'high tech'.
Maybe there was an even simpler scheme -- clays act as catalysts for organic reactions, they have crystal structure, can be replicated, mutations.
Pictures of filamentary clays that grow and replicate in the spaces of rock.
Simple systems to more complex ones RNA --> DNA -- genetic takeover

CO2 and H2O atmosphere
Frequent impacts
Lots of volcanism, heat from core formation and radioactive decay
Wet and hot
Chemicals mix in the ocean
Water is a solvent, so it mobilized all kinds of atoms and gets them in contact with each other.
Thermal energy -- molecules are vibrating and rotating, and bumping into each other, so that they can eventually fit together and react.
Both these things are very important for reactions leading to life on Earth.
Most origins of life researchers believe life demands an aqueous environment, so that molecules are free to interact and reactions can occur.
Organic molecules dissolved in sea (remember that atmosphere was CO2)
Urey Miller experiment:
Took primordial Earth atmosphere (CH4,NH3, H2, CO2, H2O) and zapped it with lightening.
Found many complex organic molecules, including amino acids.
Gas mixture was not quite right, but the process still happens with CO2 + H2O, CH4.
So lightening makes complex organics, they dissolve and react in the sea.
The two most basic organic molecules that were produced (and seen in space) are:
H2CO -- formaldehyde, which is the building block for sugars
HCN -- hydrogen cyanide, which is the building block for amino acids and nucleotides
At tide pools, water evaporates, and the organic molecules get more concentrated--lots of reactions.
Information molecules may have eventually resulted.
Process called 'chemical evolution'
Darwinism takes off, and there you go
The origin of life may be characterized by chemical evolution, punctuated by genetic takeovers.