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46th Lunar and Planetary Science Conference (2015)
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SEDIMENTARY PROCESSES ON VENUS IMPLY RAPID LITHIFICATION IN THE PLAINS. R. C.
Ghail1, 1Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, United Kingdom.
[email protected]
Introduction: Venera lander images most closely
resemble sedimentary material, with layered strata
(which may be pyroclastic in origin) sometimes broken
into cobbles and fine grained sediment. The Magellan
SAR reveal a range of weathering processes, particularly at higher elevations, and mass wasting of steep
slopes. Mean wind speeds are strongly altitude dependant and are able to erode and transport material
throughout the highland regions. This material is deposited on plains where, under the extreme Venus surface conditions, lithification is an apparently rapid process so that the largely featureless plains may not be
igneous at all but sedimentary in origin. The settling
out and lithification of sedimentary material is consistent with observed crater degradation, in which lowlying crater floors are infilled first. These detailed observations reveal a much more complex and active geological picture, with evidence for a complete sedimentary cycle and significant exchange of materials, particularly volatiles, between the interior and atmosphere,
as evidenced by Venus Express.
A Sedimentary Landscape: Ten spacecraft, all
Soviet, have successfully operated on the Venus surface, including the first ever successful landing on another planet. With the possible exception of Venera 8,
the X-ray fluorescence and γ-ray spectrometry data
obtained by a number of these landers were all interpreted to indicate weathered basalts [1], consistent with
the extensive volcanism evident in Magellan imagery.
However, the four landers to successfully return surface images, Veneras 9, 10, 13 and 14, show little in
common with basaltic flows on other planets.
The bedrock recorded at the Venera 10, 13 and 14
sites consists of laminated or thinly bedded sheets with
varying degrees of coarse sediment or regolith. The 5
cm distance between notches on the lander rings of
Venera 13 and 14 give an indication of scale [2]. Although chemically similar to basalts, there is no evidence for any features typical of basaltic lava flows
such as pillows, rubble, and levées or other channel
features. The Venera 14 image, which recorded the
lowest sediment fraction, shows that the bedrock consists of a series of interlocked subangular to subrounded plates, possibly jointed. [3] describe the Venera 14
layers as sedimentary bedding, or possibly of basaltic
volcanic tuffs, formed by cycles of air fall or ground
flow.
These same features are apparent in the bedrock
component Venera 10 and 13 images. Venera 9 rec-
orded rock material similar in appearance to the bedrock elsewhere but in the form of subangular boulders
up to 60 cm wide and 20 cm tall, within a coarse sediment or gravel on a ~30 slope interpreted as talus [2].
The gravels are subangular to subrounded and apparently well graded down to the resolution limits, with
a distinct anisotropy arising from the layering. Materials finer than coarse sand are not discernable; their
absence may be a result of removal by wind. Even the
sluggish 1 and 3.5 m s-1 surface winds recorded by the
landers are readily capable of transport sand-sized particles and smaller [4].
Based on load carrying capacities measured from
the penetrometer and dynamic loads during lander impact [2, 5], the unconfined compressive strength (UCS)
of the sediment at the Venera 13 site is 1.3 to 5 MPa,
similar to a dense sand or weak rock, with UCS rock
strengths of 31 to 126 MPa at the Venera 14 site and
58 MPa at the Vega 2 site. These latter values are less
than half that of an average basalt UCS and are closer
to that of sedimentary sandstones.
The simplest interpretation is that while geochemically the surface materials are ultimately basaltic in
origin, the primary igneous rocks were weathered,
transported and deposited as wind-blown sands that
lithified into sandstones. From their appearance, chemistry and mechanical properties the rocks might best be
described as lithic arenites. Some time after lithification, these rock were jointed, perhaps by tectonic processes, weathered and disaggregated into well graded
gravels, with any fines removed by wind, presumably
to be deposited elsewhere in a repeat of the cycle.
Weathering and Erosion: Further evidence for a
sedimentary cycle is apparent in Magellan imagery. As
noted earlier, Venera 9 is inferred to have landed on a
talus slope. Small-scale steep slopes are very common
on Venus, most often associated with normal faulting
and graben, and mass wasting is an equally common
and probably frequent occurrence [6]. At higher elevations, these materials appear to be strongly affected by
surface winds (Figure 1) but note that these features
have reflectivity characteristics consistent with lithified
sediments, not loose accumulations. The low resolution
of the altimeter (from which the density of surface materials is derived) of course implies a mixed signal—
the red areas may well be a mixture of high dielectric
(highland) material and sediments, while the green
areas to the east may likewise be a mixture of talus and
volcanic rock—but even relatively large areas of
46th Lunar and Planetary Science Conference (2015)
streamlined material have the signature of lithified sedimentary rock rather than loose regolith.
Figure 1. Windblown sediments in south east Thetis Regio shown on derived surface materials. Mercator
projection.
This area is tectonically active or has been in the
recent past [7], so that fresh sedimentary material, particularly talus from over-steepened slopes, is likely
continually generated. The fines, less than ~1 mm in
diameter, are transported across the region apparently
to be deposited across the plains as the surface winds
slows. Material coarser than this is apparently mobilised only locally by the wind to form streamlines and
then lithified.
The styles, and probably rates, of modification appear more similar to submarine processes than subaerial. Thinking about the dense supercritical fluid atmosphere as an ocean may be more helpful when trying to
understand its behaviour. For example, [8] plausibly
suggest that canali might originate as particulate gravity currents, similar to oceanic turbidity currents, transporting slope materials great distances across the
plains. While slow, the degradation of flow boundaries
in the plains is likely a consequence of the transport of
slope materials by the atmosphere.
An estimate of the rate of this surface modification
may be made from the modification of impact craters.
[9] suggest that bright floor craters are modified by
volcanic processes to dark floored craters. If instead
this modification is a result of sedimentary processes,
inspection of their data show that the rim-floor depth is
~300 m less for dark floored than bright floored craters
and that rim heights are similarly ~150 m lower, although there is a considerable scatter in both datasets.
Again based on their data, the mean surface age of Venus based on bright floored craters alone is ~150 Ma,
implying that, on average, ~1 m of material is deposit-
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ed per million years, i.e. ~1 mm a-1, roughly 10 times
the accumulation rate of marine sediment.
A minimum estimate may be made by assuming
that the absence of flow boundaries in the plains is a
result of burial by sediments. Eliminating these boundaries requires the deposition of perhaps 20 m over a
period of about half the mean planetary age, implying
an accumulation rate of ~0.01 mm a-1, or about one
tenth the accumulation rate of marine sediment, but
still ten times the intrinsic oceanic sedimentation rate
(i.e. sediment derived from within the ocean system
rather than from terrestrial or biogenic sources).
Conclusions: The Venus landscape consists mainly of sedimentary materials, with little evidence of volcanic rocks, in stark contrast to the global picture of a
planet dominated by volcanism. Geochemical data imply that the sedimentary materials were of volcanic
origin and subsequently weathered and transported.
Rather than an absence of sediment supply, the lack of
extensive regolith may be a consequence of relatively
rapid lithification in the supercritical fluid atmosphere,
with a sulphate (anhydrite) cement a likely candidate.
While less obvious, tectonic processes are evident
at both the local and global scale: Venera 9 apparently
landed on a talus slope. As generators of steep slopes,
tectonic activity may be a major source of sedimentary
material, and talus is a very common feature of fault
scarps. At higher elevations, talus is apparently reworked by surface currents (winds) into streamlines,
with finer material transported long distances, forming
wind streaks across lower-lying plains. Despite the
ambiguities, altimeter-derived density data indicate that
many of these sedimentary features are lithified, supporting the hypothesis of relatively rapid lithification.
However, evidence that at least some landslides contain unlithified loose materials implies recent slope
failures in response to ongoing tectonic activity.
To an order of magnitude, sedimentation rates in
the plains appear similar to ocean floor sedimentation
rates. Refined estimates of these processes and rates
will be possible using interferometric radar such as the
proposed ESA EnVision mission.
References:
[1] Barsukov V.L. et al. (1982) JGR 87, A3-A9. [2]
Marov M.I.A. & Grinspoon D.H. (1998) The Planet
Venus Yale Univ. Press. [3] Florensky C.P. et al.
(1983) Science 221, 57-59. [4] Warner J.L. (1983)
JGR 88, A495-A500. [5] Surkov Y.A. et al. (1984)
JGR 89, B393-B402. [6] Malin M.C. (1992) JGR 97,
16337-16352. [7] Ghail R.C. (2002) JGR 107, E8
5060. [8] Waltham D. et al. (2008) JGR 113 E02012.
[9] Herrick R.R. & Rumpf M.E. (2011) JGR 116
E02004.