VENUS AFTER MAGELLAN:
Where Do We Go From Here?
Walter S. Kiefer, Lunar and Planetary InstituteLunar and Planetary Information Bulletin, vol. 74, February 1995
A perspective view of the Guor Linea rift zone in Eistla Regio. The shield volcanos Gula Mons and Sif Mons are in the background. NASA image.
The Magellan spacecraft's exploration of Venus spanned just over four years, from orbit insertion on August 10, 1990, to atmospheric entry on October 12, 1994. Its mission was divided into two parts. The first two years were dedicated to radar mapping of the planet, with 98% of the surface imaged at resolutions between 100 and 250 m. In addition, an altimetry map of the surface was developed at a resolution of about 10-20 km. Stereo imaging was obtained for about 20% of the planet, allowing higher- resolution local topography maps to be generated. Such stereo studies are quite time-consuming but are proving useful in some instances, such as assessing the degree to which some impact craters have been modified by later volcanic activity. This early phase of the mission was both exciting and often nerve- wracking for many of the participants, as documented in Henry Cooper's excellent book, The Evening Star: Venus Observed.
The last two years of the Magellan mission were dedicated to measuring regional variations in the gravity field of Venus. Initially, these measurements were made from Magellan's highly elliptical orbit. An aerobraking maneuver in mid 1993 transformed the orbit to near-circular, allowing much-improved resolution of the gravity field at high latitudes. These data provide our only means at present of "seeing," however nonuniquely, into the interior of Venus.
Because so much of the Earth's surface is covered by water, our knowledge of the features on the surface of Venus actually exceeds our knowledge of the Earth's surface. However, knowing what the surface looks like is not the same thing as understanding how it got that way, and many arguments still rage about the geological and geophysical processes that have been important in the evolution of Venus.
As a way of taking stock of the current state of affairs, the Venus II Conference was organized by the University of Arizona and held in Tucson, January 4-7, 1995. Over the next several years, many of the studies reported at that conference will appear in the journal literature and in a volume of the University of Arizona's Space Science Series. The thoughts in this essay are not so much a formal conference summary as a personal reflection on the current state and future prospects of Venus geoscience.
Prior to Magellan, many of the basic tectonic and volcanic landforms on Venus were known based on 1- 2-km-resolution radar images obtained by the Arecibo Radio Observatory and by the Soviet Union's Venera 15 and 16 spacecraft. In addition, there was also a broad range of models for these features waiting to be tested with Magellan data. Magellan's much sharper view allowed some of these early models to be quickly rejected, for example, the proposal that Aphrodite Terra was analogous to a terrestrial oceanic spreading center/transform fault system.
In turn, Magellan data has posed many new mysteries. For example, there are numerous long lava channels on Venus; one is more than 6000 km long! Although clearly formed magmatically, these channels remain poorly understood. Thermal erosion (melting of the underlying rock by magmatic heat) of such long channels seems implausible. On the other hand, mechanical erosion (as in terrestrial rivers) requires large volumes of magma over long periods of time. What type of magma was involved, and where did it disappear to at the end of the channel? Detailed analysis of the landforms imaged by Magellan is likely to continue for years to come.
In addition to these landform and process-oriented studies, Magellan has also left us with two big-picture questions about Venus: (1) How has the tectonic and volcanic activity in the planet varied with time? Was there a "catastrophe" 300-500 million years ago? If so, how rapidly did activity subside after this event, and what processes in the mantle caused such an event? (2) How thick and strong is the present- day elastic layer on the outermost surface of Venus, and what does this imply for the amount of heat flowing out of the planet?
The 37-km-diameter crater Somerville in Beta Regio, which has been rifted by Devana Chasma.
Based on the random distribution of impact craters and the limited number of tectonized or volcanically flooded craters on Venus, it has been proposed that Venus experienced a catastrophic resurfacing event 300-500 million years ago. In this a model, there has been little subsequent volcanic and tectonic activity, with the surface acting primarily as a passive accumulator of impact craters. In the most extreme model of this sort, the transition between rapid resurfacing and the near-cessation of activity occurs in less than 10 million years.
Other investigators have considered how crater density varies among geologically defined terrain units and concluded that a much longer period of activity is recorded on the presently visible surface. Further clues about the evolutionary history of Venus are likely to come from systematic, quadrangle-scale mapping. Regrettably, funding for such studies in the Venus Mapping Program has recently been terminated as part of the early shut-down of the Venus Data Analysis Program.
Understanding the origin of tessera units is likely to be one of the keys to understanding the global evolution of Venus. Tessera are the most tectonically deformed units on Venus and typically are the stratigraphically oldest layer in any given region. How did they form, and why is the deformation in these regions so much higher than in most other parts of the planet? Do they represent a single, global event, or have the various tessera units formed over a range of times? At present, tessera occupy about 10% of the surface, although it has been suggested that the distribution of small tessera within the lowland plains represents just the "tip of the iceberg" of a much more extensive, now mostly buried distribution of tessera. If correct, then this argues strongly for a rather marked change in the tectonic style of Venus. On the other hand, if buried tessera units are not significant, then the concentration of strain in geographically limited regions of the planet must be explained. One possibility is that tessera are intrinsically weaker than other regions on Venus (perhaps because of differing rock type) and hence more easily deformed.
Tessera in Tellus Regio. Image width is 600 km. Magellan F-MIDR40N088;201.
How quickly tectonic and volcanic activity declined is an important constraint on venusian mantle dynamics. Although a number of mechanisms exist for introducing time-dependent variations in mantle activity, the cessation of volcanism and tectonism ultimately depends on diffusive cooling and thickening of the thermal boundary layer at the top of the mantle. The 10-million-year cessation interval proposed by some investigators is far too short for significant boundary layer cooling. From this perspective, a much longer evolutionary timescale is more plausible.
Doppler tracking of the Magellan spacecraft has recently allowed determination of the Venus gravity field up to spherical harmonic degree 75, corresponding to a surface resolution of 500 km, a factor of 4 better than the best pre-Magellan models. At least in some locations, it may be possible to extract still- higher-resolution gravity models from the tracking data. Such high-resolution models are useful in assessing flexure of a planet's elastic lithosphere, whose manifestations are most pronounced in short- wavelength gravity anomalies. Magellan made such studies possible for Venus for the first time, and initial analysis suggests an elastic lithosphere of 30 km or more in a number of locations. Such a thick lithosphere was unexpected and implies a very low surface heat flow. However, recent simulations of very vigorous mantle convection produce short-wavelength gravity anomalies and topography that resemble those produced by flexure models. As a result, the elastic layer thickness on Venus remains uncertain. Further modeling that simultaneously incorporates both elastic flexure and mantle convection, as well as further work on determining the short-wavelength gravity field, will hopefully lead to better constraints on the elastic lithosphere thickness and indirectly on the near-surface thermal gradient and heat flow.
Magellan has completed our first-order reconnaissance of Venus, much as the Viking spacecraft did for Mars. So where do we go from here? From the standpoint of solid-body studies (atmospheric studies are another matter, and a problem for some other group of scientists), the most valuable follow-up missions involve landing spacecraft on the surface. Particularly useful would be one or more(!) sample return missions, which would allow detailed sample petrology and geochemistry measurements, including radiometric age dating, to be performed. Unfortunately, this is likely to be a long time in coming given the current fiscal climate.
From a geophysics perspective, a network mission to deploy seismometers and measure heat flow at various locations would be of great value. Such a mission would require a significant technology breakthrough to allow long mission durations at the very high Venus surface temperature, which again seems unlikely for the foreseeable future.
A financially more feasible mission would involve in situ chemical analysis of material at selected locations, similar to those performed by seven Venera and Vega landers. Those landers revealed a generally basaltic surface composition, although there were hints of a more felsic composition at two sites. A high-priority target for future probes of this sort should be a tessera unit, although the rough surface texture of these regions implies a relatively high risk of mission failure. Another high-priority target should be Ishtar Terra, where Lakshmi Planum provides a much safer landing target. For both the tessera and Ishtar, there are plausible reasons for thinking that these regions might be compositionally distinct from the basaltic plains. Testing this possibility would both narrow the allowed range of models for these particular features as well as place constraints on the overall degree and range of planetary differentiation.
Although landers provide important ground truth on crustal composition, trying to characterize global variations in composition using landers alone on a point-by-point basis is a slow and frustrating activity. For planets with little or no atmosphere, orbital remote sensing admirably solves this problem. Unfortunately, the thick clouds of Venus prevent spectral studies from orbit in the visible and infrared, and radar wavelengths are poor discriminants among most rock types. What is needed to solve this problem is a mobile platform such as a balloon or propeller-driven drone aircraft, flying below the clouds but at a sufficiently high altitude (and low temperature) to allow reasonable mission lifetime. If atmospheric absorption bands do not obscure all of the interesting mineral absorptions in the visible and near-infrared, such a mission could characterize variations in crustal composition on at least a regional scale, while at the same time serving as a platform for atmospheric chemistry and dynamics studies.
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