Chapter 23
BACKSCATTERING OF LIGHT BY SNOW
Field measurements
J. Piironen (1), K. Muinonen (1, 2), S. Keranen (1), H. Karttunen (3) and J.
Peltoniemi (4)
(1) Observatory, University of Helsinki, Finland, (2) Astronomical Observatory, Uppsala,
Sweden, (3) Tuorla Observatory, University of Turku, Finland and (4) Finnish Geodetic
Institute, Masala, Finland.
1.
INTRODUCTION
We measured the scattering of light by snow in order to compare the
small phase angle characteristics of snow and planetary objects. The phase
angle range in our measurements is within the range of the opposition effect
and opposition spike of Saturn’s rings, asteroids, and planetary satellites
without atmospheres.
The opposition effect is a gradual brightening of an object with
decreasing phase angle toward the backscattering geometry, while the
opposition spike is a sharp increase of brightness usually at sub-degree phase
angles. The phase angle is the angle between the direction of illumination
(light source) and the direction of observation (detector), as seen from the
target. The opposition effect has been studied in the laboratory by, e.g.,
Hapke and van Horn (1963), Egan (1969), and Capaccioni et al. (1990),
while the opposition spike has been measured by Oetking (1966), Simonelli
and Veverka (1978), Pleskott (1981), Smythe et.al (1986), and Buratti et. al
(1988).
One of the most interesting studies was made by Oetking (1966), who
measured several types of materials at very small phase angles. These
measurements showed that bright materials, e.g., MgO,
and sugar
crystals, exhibit well-defined opposition spikes. All these measurements
were usually made without measuring the size, shape, or optical properties of
219
M.M. Verstraete et al. (eds.), Observing Land from Space: Science, Customers and Technology, 219–228.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
BACKSCATTERING OF LIGHT BY SNOW
Field measurements
J. Piironen (1), K. Muinonen (1, 2), S. Keranen (1), H. Karttunen (3) and J.
Peltoniemi (4)
(1) Observatory, University of Helsinki, Finland, (2) Astronomical Observatory, Uppsala,
Sweden, (3) Tuorla Observatory, University of Turku, Finland and (4) Finnish Geodetic
Institute, Masala, Finland.
1.
INTRODUCTION
We measured the scattering of light by snow in order to compare the
small phase angle characteristics of snow and planetary objects. The phase
angle range in our measurements is within the range of the opposition effect
and opposition spike of Saturn’s rings, asteroids, and planetary satellites
without atmospheres.
The opposition effect is a gradual brightening of an object with
decreasing phase angle toward the backscattering geometry, while the
opposition spike is a sharp increase of brightness usually at sub-degree phase
angles. The phase angle is the angle between the direction of illumination
(light source) and the direction of observation (detector), as seen from the
target. The opposition effect has been studied in the laboratory by, e.g.,
Hapke and van Horn (1963), Egan (1969), and Capaccioni et al. (1990),
while the opposition spike has been measured by Oetking (1966), Simonelli
and Veverka (1978), Pleskott (1981), Smythe et.al (1986), and Buratti et. al
(1988).
One of the most interesting studies was made by Oetking (1966), who
measured several types of materials at very small phase angles. These
measurements showed that bright materials, e.g., MgO,
and sugar
crystals, exhibit well-defined opposition spikes. All these measurements
were usually made without measuring the size, shape, or optical properties of
219
M.M. Verstraete et al. (eds.), Observing Land from Space: Science, Customers and Technology, 219–228.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.