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CHAPTER 36

The Solar System at

Ultraviolet Wavelengths

Amanda R. Hendrix

Jet Propulsion Laboratory

California Institute of Technology

Pasadena California

Robert M. Nelson

Jet Propulsion Laboratory

California Institute of Technology

Pasadena California

Deborah L. Domingue

Applied Physics Laboratory

Johns Hopkins University

Laurel, Marylard

1. A Brief History of Ultraviolet Astronomy 4. Observations of Solid Surfaces


2. Nature of Solar System Astronomical Observations 5. Conclusions


3. Observations of Planetary Atmospheres Bibliography

U

ltraviolet imaging and spectroscopy are powerful tools
for probing planetary atmospheres and surfaces. In
this chapter, we review the significant contributions to our
understanding of the solar system that have been made
by ultraviolet observing methods. We cover results from
the near-ultraviolet (NUV, 2000–3500 A), far-ultraviolet ̊
(FUV, 1000–2000A), and extreme-ultraviolet (EUV, 500– ̊
1000 A) wavelength ranges. These are shorter than visi- ̊
ble and near-infrared wavelengths and involve photons of
increasingly higher energy. Ultraviolet observations there-
fore provide unique insight into planetary processes in-
volving more energetic processes that cannot be studied
using photons of longer wavelength. Many of the solar sys-
tem observations in the ultraviolet have been performed
by Earth-orbiting telescopes, such as theInternational Ul-
traviolet Explorer(IUE) and theHubble Space Telescope
(HST). We also review results from ultraviolet instruments
onGalileo,Cassini,Voyager, and other spacecraft. Each
planet in the solar system except Mercury has been ob-
served in the ultraviolet by Earth-orbiting telescopes. Many
of the larger planetary satellites, selected asteroids, and
comets have also been observed. This data set has provided
important information regarding the atmospheres and sur-
faces of solar system objects and the processes shaping their
compositions.

1. A Brief History of Ultraviolet Astronomy

The ultraviolet spectral region is important to the entire

community of astronomers, from those who study nearby

objects such as Earth’s Moon to those who study objects

at the edge of the observable universe. From the perspec-

tive of a planetary astronomer, the spectral information is

important for determining the composition of, and under-

standing the physical processes that are occurring on, the

surfaces and atmospheres of solar system objects.

Prior to the dawn of the space age, spectrophotometry of

solar system objects at wavelengths shorter than∼ 3000 A ̊

had long been desired in order to complement observations

made by ground-based telescopes at longer wavelengths.

However, the presence in Earth’s atmosphere of ozone, a

strong absorber of ultraviolet light between 2000 and 3000

A, and molecular oxygen (O ̊ 2 ), which is the dominant ul-

traviolet absorber below 2000A, prevented astronomers of ̊

the 1950s and earlier from observing the universe in this

important spectral region.

The ultraviolet wavelengths of astronomical sources be-

came observable midway through the last century when

instruments could be deployed above Earth’s atmosphere.

A rocket or spacecraft provides a platform from which as-

tronomical observations can be made where the light being

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