Engineering topics
Wenkert, D.
Publications and source records attributed to Wenkert, D..
The Earth Observing System
The idea that the climate of Earth has changed is not new. Much evidence has been collected from the sedimentary record indicating periodic as well as secular changes in climate parameters such as surface temperature, precipitation amount, and ice extent and abundance, on many time scales. But in the last quarter of the twentieth century, there are several new elements in the way climate change is perceived. One is the view that human activity is having an impact on climatic conditions, on a global scale. The conclusion of recent studies indicating that anthropogenic chlorofluorocarbons are the root cause of a measured secular decrease in atmospheric ozone column abundance over the Antarctic in spring, illustrates this point (e.g., Solomon 1990).
Visible phase curves of Uranus and Neptune and scattering in their atmospheres
The Voyager 1 and Voyager 2 narrow angle cameras have been imaging Uranus and Neptune (through several filters) from distances of several AU for the past four years. The justification for this is to determine the way in which the albedoes of the two planets vary with angle from the sun. The Voyager 1 spacecraft has already reached a phase angle of approx. 40 deg. for Neptune and approx. 70 deg. for Uranus. The albedo of a gaseous planet depends on the phase angle (angle from the sun through the planet to the observer), the color being observed, and the vertical distribution and nature of aerosols and clouds in the atmosphere of the planet. Since the colors are known and the phase angles, and phase curves will be used to constrain models of the aerosol and cloud structrures of Uranus and Neptune. A knowledge of the way albedo varies with phase angle allows one to compute the total amount of sunlight being absorbed by a planet. With knowledge of the solar flux incident on the planet and knowledge of how much infrared is being emitted by the planet (at least in the direction of the Earth and sun) one can determine a rough extimate of the internal heat source of the planet. Although Jupiter and Saturn have measured internal heat sources of magnitude comparable to their absorbed energy from the sun, preliminary results indicate that Uranus may not have an internal heat and that Neptune if it has one that heat source is small.
Estimates of the bolometric albedos and radiation balance of Uranus and Neptune
Models possessing an upper haze layer of finite optical depth and a lower cloud layer of infinite optical depth at discrete altitudes are used to bound the wavelength-averaged phase integrals and bolometric albedos of Uranus and Neptune. The models differ in the assumed value of the particles' single scattering phase function and the wavelength dependence of the haze optical depth. A range of phase functions, from the isotropic to those characterizing Titan, Jupiter, and Saturn atmosphere particles, are discussed. The results obtained imply that the meteorological regimes in the observable atmospheres of Uranus and Neptune may differ considerably; internal heat flux could play a much more important role for Neptune than for Uranus.
Hyperion - 13-day rotation from Voyager data
The rotation period of Hyperion is determined here on the basis of a light-curve derived from low-resolution Voyager images taken over a two-month span. The data indicate a coherent 13-day spin period over 61 days, with the spin axis nearly parallel to the orbital plane. Since Hyperion is not in synchronous rotation, it cannot be used to test the hypothesis that dark dust spiralling inward on retrograde orbits causes the pronounced leading side/trailing side albedo asymmetry observed on the surface of Iapetus.