The rotation period, direction of the North Pole, and geodetic control network of Venus

Abstract

Before the Magellan mission, the best estimates of the rotation rate and pole direction of Venus were derived from Earth‐based radar measurements. A new determination of these rotational parameters has now been made from an analysis of Magellan radar images. Control points were selected from the north polar region and measured on full‐resolution radar strips. The measurements were entered into a least squares adjustment to solve for the pole direction and rotation rate of Venus, as well as the coordinates of the control points themselves. The current data set contains 3893 measurements of 571 points on 560 radar strips. Spacecraft ephemeris errors dominate the measurement errors. One technique used to remove ephemeris errors is to adjust the averaged orbital inclination and argument of periapsis for each orbit. In a more precise technique used for selected blocks of orbits, an improved spacecraft ephemeris is computed by optimally fitting measurements of additional points at all latitudes of the radar strips together with Earth‐based spacecraft radiometric tracking measurements. In a separate experiment, measurements were made of a few points common to both Venera 15/16 and Magellan images. The long time baseline between the images should have led to an accurate determination of the rotation period and pole direction of Venus. However, the measurement residuals were unexpectedly large, and these solutions are not currently considered reliable. A rotation period estimate of 243.0185 ± 0.0001 days was determined via the ephemeris improvement technique applied simultaneously to two overlapping orbit blocks with many common points and separated by two full Venus rotations. Using this period value, the control network computations estimated the north pole direction as right ascenson = 272.76° ± 0.02° and declination = 67.16° ± 0.01° in the J2000 frame.

The four final rotation states of Venus

“…Assuming that the observed rotation state of Venus corresponds to a stable retrograde rotation 11,19,21 , the only possibilities for this observed ®nal state are e 08 and q obs n 2 q s , or e 1808 and q obs q s 2 n. In both cases, q s n jq obs j, and we can determine all four ®nal states for Venus (Fig. 1).…”

Insolation patterns on synchronous exoplanets with obliquity

“…For example, the planet Mercury is also in Cassini state 1, despite being in the 3:2 spin-orbit resonance (Peale, 1969(Peale, , 1974; while Venus is near Cassini state 3, although it is in no spin-orbit resonance at all (Shapiro et al, 1979;Slade et al, 1990;Davies et al, 1992).…”

Rotation of the Terrestrial Planets

“…This period is close to the 243.16-day period for the synodic resonance with the orbital motion of the Earth, in which Venus would show exactly the same face to the Earth at each inferior conjunction and would rotate exactly five times as seen from the Sun between consecutive inferior conjunctions, suggesting that Venus might occupy that resonance (Gold and Soter, 1969;Goldreich and Peale, 1966a,b). Further observations with the Goldstone, Arecibo, and Haystack radar systems yielded an improved period of 243.026 AE 0.006 days (Carpenter, 1966(Carpenter, , 1970Dyce et al, 1967;Shapiro et al, 1990), and the analyses of Magellan data have led to an averaged rotation period of 243.0185 AE 0.0001 days based on radar images (Davies et al, 1992) and of 243.0200 AE 0.0006 days based on X-band radio-tracking data (Konopliv et al, 1993(Konopliv et al, , 1999, all three very close to but not exactly equal to the resonance period of 243.16 days. The analyses of Magellan radar data show that the current obliquity of Venus is about 2.6 (or 177.4 ) (Davies et al, 1992;Yoder, 1995).…”

“…Therefore, with planetary perturbations included, the final states are no longer steady states. The current obliquity of Venus is 2.6 ( Davies et al, 1992;Yoder, 1995). Planetary-induced changes in the eccentricity of the orbit of Venus can also lead to long periodic changes in the rotation rate.…”