The Good, the Bad, and the Ozone

Earth and Moon
Image of the Earth and Moon taken by Galileo spacecraft.
Credit: NASA

A powerful new instrument heading to space this week is expected to send back long-sought answers about greenhouse gases, atmospheric cleansers and pollutants, and the destruction and recovery of the ozone layer. Only a cubic yard in size but laden with technical wizardry, the High-Resolution Dynamic Limb Sounder (HIRDLS) will measure a slew of atmospheric chemicals at a horizontal and vertical precision unprecedented in a multi-year space instrument.

Scientists at the National Center for Atmospheric Research (NCAR), University of Colorado, and University of Oxford developed HIRDLS (pronounced "hurdles") with funding from NASA and United Kingdom sources. The U.S. space agency plans to launch the 21-channel radiometer along with three other instruments aboard its Aura satellite from Vandenberg Air Force Base in California.

Comparison of Mars, Venus and Earth in water and ozone bands, showing their clear presence on Earth uniquely. Credit: NASA Workshop, Pale Blue Dot

HIRDLS will capture the chemistry and dynamics of four layers of the atmosphere that together span a region 8 to 80 kilometers (5 to 50 miles) above Earth’s surface: the upper troposphere, the tropopause, the stratosphere, and the mesosphere.

Using infrared radiation as its yardstick, the radiometer will look through Earth’s atmosphere toward the planet’s limb, or edge. It will find and measure ten different chemical species, characterize airborne particles known as aerosols, and track thin cirrus clouds, all at a vertical resolution of half a kilometer (a third of a mile) and a horizontal resolution of 50 kilometers (30 miles). The signal-to noise ratio is one tenth that of previous detectors.

"The angular resolution of the instrument’s mirror position is equivalent to seeing a dime eight miles away," says principal investigator John Gille, of NCAR and the University of Colorado.

A few questions HIRDLS data will answer

What are the concentrations of the primary greenhouse gases and their height in the atmosphere? The answer should reveal where Earth will warm or cool as the global climate changes.

Why does the tropopause exist and what is its role in conveying gases from the troposphere into the stratosphere, especially in the tropics? Convection was once thought to be the vehicle, but scientists now know warm, rising air normally stops at the frigid, dry tropopause.

Why is the stratosphere, historically dry, now getting wetter? The answer could shed light on how a changing climate is modifying the atmosphere and how those modifications could in turn feed back into our climate and weather near the ground.

Earth today in proportions of chemical elements that are biologically significant, as measured by the departing Mars Express spacecraft, which now orbits the fourth planet from the sun Credit: ESA/Mars Express

How much ozone is sinking from the stratosphere into the upper troposphere? The answer will help scientists separate natural ozone pollution from human-made sources and give new information on how the gas is affecting chemistry closer to the ground.

Scientists also expect to see clearly for the first time the dynamic processes that cause water vapor filaments and tendrils to break off and mix with other gases in the troposphere.

Good and bad ozone at different altitudes

At 50 kilometers (30 miles) above the ground, ozone is good: it blocks dangerous ultraviolet radiation and prevents it from harming life and materials at ground level. At 10 kilometers (6 miles), ozone is a greenhouse gas, which is good because the natural greenhouse effect is necessary to warm the planet, but bad if the warming continues to increase at too rapid a rate. At 5 kilometers (3 miles), ozone is a source of the hydroxyl radical, which cleanses the atmosphere of pollutants. But at ground level, ozone is a primary pollutant in smog, causing respiratory problems and damaging trees and crops.

Unless molecular oxygen in the atmosphere is constantly replenished by photosynthesis, it is quickly consumed in chemical reactions, in the atmosphere, on land and in seawater. So the presence of a large amount of oxygen, or its ozone proxy, in an extrasolar planet’s atmosphere would be a sign that it might host an ecosystem like present-day Earth’s.

A spectroscope that might detect infrared or visible light looking back on Earth or outwards to other planets might focus mainly on four gases that are found in Earth’s atmosphere and linked to life:

  • Water vapor A baseline sign, indicating the presence of liquid water, a requirement of known life.
  • Carbon dioxide Can be created by biological and non-biological processes. Because it is necessary for photosynthesis, it would indicate the possible presence of green plants.
  • Methane Considered suggestive of life, it also can be made both by biological and non-biological processes.
  • Molecular oxygen (O2) – or its proxy, ozone (O3). The most reliable indicator of the presence of life, but still not conclusive.


Related Web Pages

The Unexplored Cosmos
Lonely Planets and Rare Earths
Carl Sagan: The Lonely Pale Blue Dots?
Terrafirma Now
The Earth as Seen by Galileo