Tuesday Jan. 20, 2015

A nice selection of music from Sergio Mendoza y La Orkesta recorded in Austin, TX, this past summer during the SXSW festival.  They're a local group.

Experiment #1 materials checkout
About 40  sets of Experiment #1 materials were checked out before class today.  Those of you that have materials will eventually find your name on the Expt. #1 signup list.  Even though your report isn't due until Feb. 10, the experiment can take several days, maybe even a week, to run to completion so don't wait too long to get it started; this weekend would be a perfect time.  You may need to check the experiment fairly frequently at the beginning (every hour or two).  It slows down somewhat as it progresses and eventually you will only need to look at it once or twice a day.  You'll find more information about the experiment here.

Once you have collected your data, return your materials and pick up the supplementary information handout.  Try to do this before the experiment report is due because the handout will help with the analysis portion of your report.  It will also make materials available for someone else that wants to do the experiment.  Your name should turn this rust color on the signup list when you have returned your materials. 

Signup sheets for the remaining experiments were circulated in class.  If you didn't get a chance to signup don't worry, I'll bring the lists to class again next week.  Remember you only need to do one of the experiments.  You can check on the appropriate list to see if your name is there (it will take a few days to enter all the names)


Here's the list of the 5 most abundant gases in our atmosphere again together with what we learned about them in class on Thursday.


We'll be learning more about some of these gases today and will be adding some new information.

The earth's original atmosphere and the origin(s) of our present atmosphere

Our present day atmosphere is very different from the earth's original atmosphere which was mostly hydrogen and helium with lesser amounts of ammonia and methane. 



This early atmosphere either escaped (the earth was hot and light weight gases like hydrogen and helium were moving around with enough speed that they could overcome the pull of the earth's gravity) or was swept into space by the solar wind (click on the link if you are interested in learning more about the solar wind, otherwise don't worry about it). 

With the important exception of oxygen (and argon), most of our present atmosphere is though to have come from volcanic eruptions.  In addition to ash, volcanic eruptions send a lot of water vapor, carbon dioxide, and sulfur dioxide into the atmosphere.  Carbon dioxide and water vapor are two of the 5 main gases in our present atmosphere.


Volcanoes also emit lots of other gases, many of them are poisonous.  Some of them are shown on the right side of the figure (I found the gases in the "also" list mentioned in a lot of online sources, the gases in the "perhaps" list were mentioned less frequently).  

As the earth began to cool the water vapor condensed and began to create and fill oceans.  Carbon dioxide dissolved in the oceans and was slowly turned into rock.  Smaller amounts of nitrogen ( N2 ) are also emitted by volcanoes..  Because nitrogen is relatively nonreactive it remained in the air and its concentration was able to built up over time. 




The photo above shows the Eyjafjallajokull volcano in Iceland photographed on Apr. 17, 2010 (image source)
Here are some additional pictures of the Eyjafjallajökull volcano.  It caused severe disruption of airline travel between the US and Europe.  Here's another set of photos also from the Boston Globe.




Even more amazing than the photographs of the Icelandic volcano is this mosaic of 4 photographs of Comet 67P/Churyumov-Gerasimenko taken by the Rosetta spacecraft on Jan. 10, 2015 from an altitude of 27.5 km.  The image is 4.2 x 3.8 km.  The Rosetta spacecraft deployed the Philae lander which successfully touched down on the surface of the comet and operated for a brief time.  The lander is not receiving enough sunlight to power its systems at the present time and has gone into "sleep mode."  There is hope that it may reawaken later
this year once the comet has moved closer to the sun. 

I've included this photograph of a comet because some researchers don't believe that volcanic activity alone would have been able to account for all the water that is on the earth (oceans cover about 2/3rds of the earth's surface.  They believe that comets and asteroids colliding with the earth may have brought significant amounts of water.  The Rosetta spacecraft has determined that the water on this particular comet differs from the composition of the water in the earth's oceans.  This suggests that comets might not have been an important source of the earth's water. (source of the image and information about the Rosetta mission)

I may come back and add a short section explaining how the water on the comet is different from that in the oceans.

Where did the oxygen in our atmosphere come from? 


Volcanoes didn't add any of the oxygen that is in the atmosphere.  Where did that come from?  There are a couple of answers to that question.

1st source of atmospheric oxygen


The oxygen is thought to have come from photo-dissociation of water vapor and carbon dioxide by ultraviolet (UV) light (the high energy UV light is able to split the H20 and CO2 molecules into pieces).  Two of the pieces, O and OH,  then react to form O2 and H.

By the way I don't expect you to remember the chemical formulas in the example above.  It's often easier and clearer to show what is happening in a chemical formula than to write it out in words.  If I were to write the equations down, however, you should be able to interpret them.  Ultraviolet is a dangerous, high energy, potentially deadly form of light and it's probably also good to remember that ultraviolet light is capable of breaking molecules apart.



Once molecular oxygen (O2) begins to accumulate in the air UV light can split it apart to make atomic oxygen (O).  The atoms of oxygen can react with molecular oxygen to form ozone (O3). 

Ozone in the atmosphere began to absorb the dangerous and deadly forms ultraviolet light and life forms could then begin to safely move from the oceans onto land (prior to the buildup of ozone, the ocean water offered protection from UV light.  A molecule of O3 absorbs some UV  preventing it from reaching the ground.

O3 + UV light --->  O2   + O

You might think the O2 and O would recombine.  But if you picture hitting something with a hammer and breaking it, the pieces usually fly off in different directions.  That what happens with the O and O2 .

2nd and most important source of atmospheric oxygen.
Once plant life had developed sufficiently and once plants moved from the oceans onto land, photosynthesis became the main source of atmospheric oxygen. 



Photosynthesis in its most basic form is shown in the chemical equation above.  Plants need water, carbon dioxide, and sunlight in order to grow.  They can turn can turn H20 and CO2 into plant material.  Photosynthesis releases oxygen as a by product. 

Combustion is really just the opposite of photosynthesis and is shown below.


We burn fossil fuels (dead but undecayed plant material) to generate energy.  Water vapor and carbon dioxide are by products.  Combustion is a source of CO2 (photosynthesis is a "sink" for atmospheric CO2 , it removes CO2 from the air). We'll see these two equations again when we study the greenhouse effect and global warming.

And a detail that I didn't mentioned in class (and something you probably don't need to remember).  The argon we have in the atmosphere apparently comes from the radioactive decay of potassium in the ground.  Three isotopes of potassium occur naturally: potassium-39 and potassium-41 are stable, potassium-40 is radioactive and is the source of the argon in the atmosphere.  


Stromatolites, banded iron, red beds - geological evidence of oxygen on earth

The following figure is the first page in the packet of photocopied ClassNotes.


This somewhat confusing figure shows some of the important events in the history of the earth and evolution of the atmosphere.  There were 5 main points I wanted you to take from this figure, and really 1-3 are the most important.

First, Point 1: the earth is thought to be between 4.5 and 4.6 billion years old.  If you want to remember the earth is a few billion years old that is probably close enough.  Something I might not have mentioned in class, it's in small type above.  The formation of a molten iron core was important because it gave the earth a magnetic field.  The magnetic field deflects the solar wind and keeps the solar wind from blowing away our present day atmosphere.

Stromatolites (Point 2) are geological features, column-shaped structures made up of layers of sedimentary rock, that are created by microorganisms living at the top of the stromatolite (I've never actually seen a stromatolite, so this is all based on photographs and written descriptions).  Fossils of the very small microbes (cyanobacteria = blue green algae) have been found in stromatolites as old as 2.7 B years and are some of the earliest records of life on earth.  Much older (3.5 to 3.8 B years old) stromatolites presumably also produced by microbes, but without microbe fossils, have also been found. 




Blue green algae grows at the top of the column, under water but near the ocean surface where it can absorb sunlight.  As sediments begin to settle and accumulate on top of the algae they start to block the sunlight.  The cyanobacteria would then move to the top of this sediment layer and the process would repeat itself.  In this way the stromatolite column would grow layer by layer over time.  Now, this isn't a geology class; we're learning about stromatolites because the cyanobacteria on them were a very early form of life on the earth and were able to produce oxygen using photosynthesis.

Living stromatolites are found in a few locations today.The two pictures above are from Lake Thetis (left) and Shark Bay (right) in Western Australia (the two photos above and the photograph below come from this source).  The picture was probably taken at low tide, the stromatolites would normally be covered with ocean water.  It doesn't look like a good place to go swimming, I would expect the top surfaces of these stromatolites to be slimy.


Living stromatolites at Highborne Cay in the Bahamas.


Point 3 refers to the banded iron formation, a type of rock formation.  These rocks are 2 - 3 billion years old (maybe older) and are evidence of oxygen being produced in the earth's oceans.  Here are a couple of pictures of samples of banded iron formation rock that I passed around in class (thanks for being careful with them and not stealing them).  Thanks also for being careful with the glass graduated cylinders.  I don't believe any were broken in either of the sections this morning.


The main thing to notice are the alternating bands of red and black.  The next paragraph and figure explain how these formed.

Rain would first of all wash iron ions from the earth's land surface into the ocean (at a time before there was any oxygen in the atmosphere).  Oxygen from the cyanobacteria living in the ocean water reacted with the dissolved iron (the iron ions) to form hematite or magnetite.  These two minerals precipitated out of the water to form a layer on the sea bed.  This is what produced the black layers.


Periodically the oxygen production would decrease or stop (rising oxygen levels might have killed the cyanobacteria or seasonal changes in incoming sunlight might have slowed the photosynthesis).  During these times of low oxygen concentration, red layers of jasper would form on the ocean bottom.  The jasper doesn't contain as much iron. 

Eventually the cyanobacteria would recover, begin producing oxygen again, and a new layer of hematite or magnetite would form.  The rocks that resulted, containing alternating layers of black hematite or magnetite and red layers of jasper are known as the banded iron formation.  In addition to the red and black layers, you see yellow layers made of fibers of quartz in the samples passed around class.   The rocks are fairly heavy because they contain a lot of iron, but the most impressive thing about them in my opinion is their age - they are a few billion years old! 

Eventually the oxygen in the oceans reacted with and used up all of the iron ions.  Oxygen was then free to move from the ocean into the atmosphere.  Once in the air, the oxygen could react with iron in sediments on the earth's surface.  This produced red colored (rust colored) sedimentary rock.  These are called "Red Beds" (Point 4).  None of these so-called red beds are older than about 2 B years old.  Thus it appears that a real buildup up of oxygen in the atmosphere began around 2 B years ago.





Red State Park near Sedona Arizona.  An example of "red beds" that formed during the Permian period 250-300 million years ago.

Oxygen concentrations reached levels that are about the same as today around 500 to 600 million years ago (Point 5 in the figure).

Trace gases in air - pollutants and greenhouse gases

We add to the list of the 5 main gases in the atmosphere in the figure below.



Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants.  We'll cover 3 of these in more detail later this week and next week.

Water vapor, carbon dioxide, methane, nitrous oxide (N2O = laughing gas), chlorofluorocarbons, and ozone are all greenhouse gases.  Increasing atmospheric concentrations of these gases are responsible for the current concern over climate change and global warming.  We'll discuss this topic and learn more about how the greenhouse effect actually works later in the course.

Ozone has sort of a Dr. Jeckyl and Mr. Hyde personality
(i)  Ozone in the stratosphere (a layer of the atmosphere between about 10 and 50 km altitude) is beneficial because it absorbs dangerous high energy ultraviolet (UV) light coming from the sun.  Without the protection of this ozone layer, life as we know it would not exist on the surface of the earth.  It was only after ozone started to buildup in the atmosphere that life could move from the oceans onto land.  Chlorofluorocarbons are of concern in the atmosphere because they destroy stratospheric ozone.

(ii)  In the troposphere (the bottom 10 kilometers or so of the atmosphere and where we live) ozone is a pollutant and is one of the main ingredients in photochemical smog.

(iii)  Ozone is also a greenhouse gas.



Finally, I wasn't being entirely honest when I said that gases are invisible.  Some gases can be seen, here are some examples.  I would like to bring some actual samples to class, but some are very toxic and require careful handling. 





Bromine in both liquid and gaseous phases. Bromine and mercury are the only two elements that exist as liquids at room temperature.  The bromine is in a sealed glass ampoule inside an acrylic cube.  Bromine could be safely brought to class in a container like this.

Webelements.com states: " It is a serious health hazard, and maximum safety precautions should be taken when handling it."  I'm not sure what maximum safety precautions are, that's why I don't bring it to class.

This photo was taken by Alchemist-hp and was Picture of the Day on the English Wikipedia on Oct. 29, 2010.
 Chlorine (Cl2)

I found this image here
 Iodine
Also an element that is normally found in solid form.  The solid sublimates, i.e. it changes directly from solid to gas (you would probably need to heat the solid iodine to produce gas as dense as seen in the picture above).  source of this image

I think we can probably handle iodine safely and might well bring some to class.
 Nitrogen dioxide  (NO2)
An important pollutant.  I used to make this in class but I've read that you can inhale a fatal dose of  NO2 before showing any symptoms.  NO2 also has an anesthetic effect - it can deadens your sense of smell.

source of this image