Atmospheric Opacity

FIELDS OF STUDY: Observational Astronomy; Theoretical Astrophysics

ABSTRACT: Atmospheric opacity is the degree to which electromagnetic radiation penetrates the layers of atmosphere surrounding a celestial body. Electromagnetic radiation in the form of light or radio waves reaches the surface in differing amounts, depending on the type of wave, its frequency and energy, and the level of opacity of the surrounding atmosphere. Atmospheric opacity affects surface conditions on Earth and provides information about the potential for life on other celestial bodies.

Properties of the Atmosphere

Earth’s atmosphere is a complex, layered field of gases, water vapor, and dust surrounding the planet. It provides and protects the conditions necessary for life. The atmosphere holds in the oxygen and other gases that make up breathable air and allows the light and heat of the sun to reach Earth’s surface. It also deflects or absorbs most harmful forms of electromagnetic waves, such as ultraviolet, gamma rays, and x-rays.

Earth is not the only celestial object with an atmosphere. While all atmospheres serve as filters for light, heat, and radiation, each differs in the amount of waves it allows to reach the surface. The degree to which an atmosphere does this is called atmospheric opacity. Opacity is affected by both the conditions of the atmosphere and the properties of the electromagnetic waves. Relevant atmospheric conditions include temperature, cloud cover, and amount of water vapor.

Early Atmospheric Study

Scientists have been learning about atmospheric opacity for as long as they have been studying Earth’s atmosphere. The earliest known experiments in the seventeenth and eighteenth centuries were limited to studying temperature and air pressure on Earth’s highest mountains. The invention of hot air balloons in the late eighteenth century allowed scientists to study the higher levels of the atmosphere, but the effects of extreme cold and air pressure on humans hampered these experiments. At very high altitudes, the scientists got frostbite and lost consciousness. French meteorologist Léon Teisserenc de Bort (1855–1913) became one of the first to launch an unmanned weather balloon. Instruments attached to the balloon allowed him to record information at altitudes higher than could be safely reached by humans.

In 1901, Italian physicist and inventor Guglielmo Marconi (1874–1937) proved that one layer of the atmosphere—the ionosphere—reflects radio waves. He used this property to bounce the first wireless radio signal across the Atlantic Ocean, from England to Canada. The next year, de Bort presented his research on the upper atmosphere, identifying the troposphere and stratosphere. In 1932, American physicist Karl Guthe Jansky (1905–1950) found that the static heard in radio signals is caused by radio waves from deep in the Milky Way galaxy. This was the first proof that radio waves could also pass through the atmosphere.

Improved technology in the twentieth century enabled closer study of atmospheric layers and their properties. The development of spaceflight in the late 1950s and 1960s represented a huge leap forward. Scientists began using satellites and special telescopes to study Earth’s atmosphere, both from space and from within its layers.

Several key research instruments, including the International Space Station (ISS), orbit in the part of Earth’s atmosphere known as the thermosphere. The outer boundary of this layer is 690 kilometers (429 miles) above Earth’s surface. Other satellites and telescopes have allowed scientists to study the atmospheres of other planets.

Properties of Electromagnetic Radiation

The nature of electromagnetic waves is determined by their frequency and wavelength. These properties are related through the following wave equation:

c = λf

In this equation, c represents the speed of light, λ is wavelength, and f is frequency. Wavelength is the distance between a point in one wave and the same point in the following wave, measured in meters. Frequency is the number of wave cycles per unit time. The International System of Units (SI) unit of frequency is the hertz (Hz). This is a derived unit, equal to one cycle per second.

The different types of electromagnetic waves, in order of decreasing wavelength and increasing frequency, are radio waves, microwaves, infrared, optical or visible light, ultraviolet, x-rays, and gamma rays. All electromagnetic waves travel at the speed of light in a vacuum. Radio and optical waves are the easiest to study because they can pass readily through the atmosphere. Studying other forms of electromagnetism requires specially calibrated filters, cameras, and telescopes.

Effects of Atmospheric Opacity

The opacity of an atmosphere affects the conditions on the surface below. Earth’s atmosphere filters the amount of ultraviolet radiation that reaches the planet and helps keep Earth at the right temperature to sustain life. First it allows the warming rays of the sun to reach the surface and then it absorbs the heat energy as it rises, reflecting it back to space in what is known as the greenhouse effect.

Opacity also impacts radio-wave transmission. One of the highest levels of Earth’s atmosphere, the ionosphere, is so named because it is made up of ions and free electrons. When atoms or molecules collide with ultraviolet and x-rays, they lose electrons, becoming positively charged ions. Together, the free electrons and positive ions form a plasma. A plasma is a state of matter that consists of unbound positively and negatively charged particles but is electrically neutral as a whole, because the positive and negative charges balance out. It was the ionosphere that bounced Marconi’s radio waves across the ocean. The ionosphere also has sublayers that change seasonally and even daily. When there is no sunlight, one of the layers disappears and another two merge. This allows radio waves to travel farther at night than in the daylight.

Importance of Atmospheric Opacity

The opacity of Earth’s atmosphere is important to sustain human life, and humans may be just as important to atmospheric opacity. Some researchers think that the gases produced by certain human activities, such as farming, burning fossil fuels, and maintaining landfills, cause changes in the atmosphere’s opacity that enhance the greenhouse effect. This could increase the surface temperature on all or part of Earth’s surface. Scientists differ on how drastic these changes could be, their overall effects, and how fast they might occur.

Scientists also study the atmospheric opacity of other planets. NASA’s Viking program, which sent two orbital probes to Mars in the 1970s, included studies of the depth and opacity of Mars’s atmosphere. NASA is preparing for future missions to the planet by studying the conditions that contribute to dust movement, which could have a significant impact on equipment such as life support systems. The rover Opportunity, for example, lost power permanently when its solar panels were rendered useless during a dust storm in 2018. Further studies have been done on other planets in an effort to determine if their atmospheric opacity allows for the conditions necessary to sustain life.

PRINCIPAL TERMS

• electromagnetic waves: the classical form of electromagnetic radiation, produced when electric and magnetic fields come together and interact; can be in the form of radio waves, microwaves, infrared, optical, ultraviolet, x-rays, or gamma rays, depending on their frequency, energy, and wavelength.
• opacity: the degree to which a substance or object lets various forms of electromagnetic radiation pass through it.

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