About 1630, Giambattista Baliani, a contemporary and acquaintance of Galileo, was perhaps the first to state that we are living at the bottom of an ocean of air. Galileo himself had noted that a vacuum pump couldn’t raise a water column more than about 10 meters, but remained unconvinced about the phenomenon of air pressure itself. Evangelista Torricelli, the amanuensis of Galileo in his later years, invented the mercury barometer to measure air pressure; and, somewhat later, Blaise Pascal used this instrument to measure the reduced air pressure at a mountain top with a known height, and was then able to estimate the depth of Earth’s atmosphere.

By the late 18th Century, Joseph Priestly discovered that plants took up "fixed air" (carbon dioxide) and exhaled "dephlogisticated air" (oxygen), and later Antoine Lavoisier’s experiments found that there were some 30 basic elements, a few of which were gases existing in the atmosphere. He completely discredited the phlogiston theory that had been put forward in the late 17th Century by Germans J. J. Becher and G. E. Stahl to explain oxidation. John Dalton, one of the early pioneers of meteorology, studied the problem of gaseous mixtures in the atmosphere – particularly nitrogen and oxygen – and why they didn’t stratify because of their ascertained differences in density, as determined by earlier chemists, such as Robert Boyle. It wasn't until the late 18th Century when lighter-than-air balloons were invented that the atmosphere itself was found to be indeed stratified into distinct layers.

The composition of Earth’s primordial atmosphere is wide open for speculation. There is a consensus that Earth’s early atmosphere contained considerable carbon dioxide (CO2) from the amounts locked up in crustal limestone and other mineral deposits. This chemically combined CO2, if released into our present air mass, has been estimated to be equivalent to some 120 atmospheres pressure, or some 30% more than that found on the planet Venus.

The coexistent sizeable amount of nitrogen in our atmosphere may have come from primordial ammonia gas (NH3) as is presently found in the atmospheres of the outer gas giant planets. However, ammonia and CO2 react chemically to form carbamates, which in turn are decomposed by heat and radiation to release free molecular nitrogen (N2), carbon monoxide (CO), CO2, and possibly some simple oligomeric hydrocarbons such as methane (CH4).

Our present moiety of oxygen is thought to be due to the runaway proliferation of plantlife during subsequent epochs in Earth’s history, through the respiratory uptake of CO2 and release of free molecular oxygen (O2). (Plants use chlorophyll as the active respiratory mechanism, which has magnesium as the central atom of the molecular structure. This molecular structure is similar in many respects to hemoglobin in red blood cells, which has iron as the central atom.)

The sizeable bathysphere (oceans) that covers some 70% of Earth’s surface has been conjectured as coming from spaceborne cometary ice infalls, although the thermal release of water from hydrated mineral deposits by vulcanism is considered a secondary source. The uptake, distribution, and condensation of water vapor by the atmosphere varies from region to region, but still comprises a small fraction of the lower troposphere, and ranges in saturated air from about 5 ppm at 0 degrees C (32 degrees F) to almost 50 ppm at 38 degrees C (100 degrees F).

There have been reasons to think that Earth harbored a much more massive atmosphere in its geologic past. Two Canadians, palaeontologist Dale Russell and aeronautics engineer Parvez Kumar, have conjectured that the large Mesozoic Age pterodactyls could have flown only in an atmosphere at least 50% denser than our present air mass. Such an atmosphere could support considerably more water vapor and other gaseous constituents for an enhanced greenhouse effect.

It has been suggested that cometary or asteroidal infalls have bereft Earth of a substantial portion of its primordial atmosphere, such as the Chicxulub event closing the Mesozoic era 65 megayears ago that wiped out the dinosaurs. There have been at least five such identified major upheavals in Earth’s history that resulted in wholesale extinctions, as well as several minor disturbances. Not all of these are attributed to meteoric impacts, however the speculation is that major bolide impacts could have resulted in atmospheric loss to space due to impact shock waves and thermal expansion. This raises the question: Did Earth once have a more massive atmosphere in its preterprimordial past? Palaeontological evidence seems to indicate that throughout much of Earth’s history the climate was at least semitropical over a substantial spread of latitudes (+/- 70 degrees).

The bottom layer of Earth’s presentday atmosphere, the troposphere, extends from 10 kilometers in the frigid zones to about 20 kilometers in the tropics. The well-advertised ozone layer forms from photochemical processes at an altitude range of some 12-40 kilometers. The stratosphere extends beyond the variable 10-20 km tropopause (the top of the troposphere) to about 55 kilometers, a layer in which temperature rises with altitude. It has been known since 1983 that there are at least 192 chemical reactions and some 48 photochemical processes at work in Earth’s atmosphere. Beyond this, the mesosphere comprises a layer that may range from 45 to 95 kilometers, wherein temperature falls again with altitude. The ionosphere begins at some 70-80 kilometers and extends to the edge of space. The E layer, sometimes referred to as the Heaviside, or Kennelly-Heaviside layer, is a reflecting layer of the ionosphere for electromagnetic (radio) waves occurring at 100-120 kilometers. The anomalous so-called sodium layer exists at approximately 145 kilometers.

The current composition of Earth’s atmosphere is nominally 78% nitrogen, 21% oxygen, and 0.9% argon, with other gases to taste. Carbon dioxide is currently at about 360 parts per million (0.036%), and has been rising steadily since at least the beginning of the Industrial Age. This is substantially higher than estimates during the last several ice ages. However, the amount of CO2 in Earth’s early atmosphere during the Palaeozoic era (620-225 megayears BP) has been estimated as between 4-12%, which gave rise to the prolific plant and sea life during the Carboniferous (360-280 megayears BP) and other such periods in geologic history. Massive deposits of chalk and limestone from fossilized foraminifera were also laid down during these epochs through concomitant uptake of CO2.

On a speculative note, storms and tempests in Earth’s troposphere seem to fall between that of the winds on Mars which, because of its sparse atmosphere (about 1% of Earth’s), have velocities in excess of 300 kph (200 mph), while the surface winds in Venus’s massive atmosphere have been measured by spaceprobes at 10 kph or less. We see some extraordinarily violent storms on Jupiter, but these are in the turbulent upper atmosphere – not dissimilar in some respects to the outer visible atmosphere of Venus, or in our own jet stream. Could this mean that if we lose more atmosphere by some mechanism or other, our storms would become increasingly violent?

Alvarez, Walter, T. rex and the Crater of Doom, Princeton U. Press, Princeton, 1997.

Baker, Robert T., The Dinosaur Heresies, Zebra Books, New York, 1996.

Cagin, Seth, & Dray, Philip, Between Earth and Sky, Pantheon, New York, 1993.

Cardwell, Donald, The Norton History of Technology, Norton, New York, 1995.

Chapman, Clark R., & Morrison, David, Cosmic Catastrophes, Plenum: New York, 1989.

Gribbin, John, Hothouse Earth, Grove, New York, 1990.

Jueneman, Frederic, "Mammoths in the Myst", R&D, April 1999

-- "More Mammoths in the Myst", R&D, September 1999

-- "Pleiongaea: A Myth for All Seasons", AEON III:3, 1990

-- "Pterodactyls in the Mesozoic", AEON V:2, 1997

-- "The Deccan Traps", R&D, February 1999

-- "The Hole Truth About Ozone", R&D, July 1992

-- "The Ozone Layer Demystified", R&D, January 1997

-- Raptures of the Deep, Cahners, Chicago, 1995

Nance, John J., What Goes Up, Morrow, New York, 1991.

Norman, David, Prehistoric Life, Macmillan, New York, 1994.

Stillman, John M., The Story of Early Chemistry, Appleton, New York, 1924.

Verschuur, Gerrit L., Impact!, Oxford U. Press, New York, 1996

Weiner, Jonathan, The Next One Hundred Years, Bantam, New York, 1990.

A Millennial Review Chaired by Nancy Cox

NCHALADA, Saturday Afternoon, August 5, 2000

We've been summing up lots of things with the beginning of year 2000. In astronomy (and physics) there have been numerous discoveries in the past 100 years, from spectral classification of stars, to the realization of external galaxies ("island universes"), the velocity/distance relation (expanding universe), the magnetic nature of sunspots, white dwarfs, pulsars, quasars, the nucleosynthesis of elements in stars and in the Big Bang, the Cosmic Background Radiation, the Big Bang, gamma-ray bursts, early galaxies, and much more.

As a beginning point, the Astrophysical Journal's new book "100 Years of the AAS" (American Astronomical Society) contains 53 pathbreaking articles. Helmut Abt, ApJ editor, gave a recent Dean Lecture at the Academy of Sciences outlining some of the papers (the list will be available at NCHALADA).

Other wonderful references are the three Source Books in Astronomy:

• 1929, edited by Harlow Shapley and Helen Howarth, 412 pages, McGraw-Hill;
• 1960, edited by Harlow Shapley, Harvard U Press; and
• 1979, edited by Kenneth R. Lang and Owen Gingerich, 922 pages, Harvard U Press.

Participants are requested to rummage through copies before the session.

The 1960 volume includes these original papers:

George Ellery Hale on sunspots

Clyde Tombaugh on discovering Pluto

Annie Jump Cannon on spectral classification

Henrietta Leavitt on the Cepheid Period-Luminosity relation

Hertzsprung and Russell on their diagram

Bart Bok on dark nebulae

Predicting hydrogen's 21cm radio line

Hubble and Humason on the Velocity/Distance relation

Einstein saying E=mc²

Lemaitre on the primareview atom

We should also include:

• one of the all-time classics, on nucleosynthesis of elements by Burbidge, Burbidge, Fowler and Hoyle, Reviews of Modern Physics, 1957
• The first paper on the Big Bang model: Alpher, Bethe and Gamow (a cosmic joke), Physical Review, April 1, 1948
• 2 papers side-by-side in the Astrophysical Journal, vol 142, pp 414 and 419, on discovering the cosmic background radiation: Robert Dicke of Princeton, on the theory, followed by Penzias and Wilson on the discovery, with the inauspicious title "A Measurement of Excess Antenna Temperature at 4,080 Mc/s". It gives one sentence to a possible explanation by Dicke and Peebles. This one got the Nobel Prize.

Participants are encouraged to bring up other notable papers and references, whether or not they are in these books.