landing on venus: past and future

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Planetary and Space Science 55 (2007) 2097–2112

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Landing on Venus: Past and future

A.T. Basilevskya,b,�, M.A. Ivanova,b, J.W. Headb, M. Aittolac, J. Raitalac

aVernadsky Institute of Geochemistry and Analytical Chemistry, RAN, Moscow 119991, RussiabDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA

cDepartment of Astronomy, Oulu University, Oulu 90014, Finland

Received 20 February 2007; received in revised form 19 July 2007; accepted 12 September 2007

Available online 25 September 2007

Abstract

We briefly describe the history of landings on Venus, the acquired geochemical data and their potential petrologic interpretations. We

suggest a new approach to Venus landing site selection that would avoid the potential contamination by ejecta from upwind impact

craters. We also describe candidate units to be sampled in both in situ measurement and sample return missions. For the in situ

measurements, the ‘‘true’’ tessera terrain (tt) material is considered as the highest priority goal with the second priority given to

transitional tessera terrain (ttt), shield plains (psh) and lobate plains (pl) materials. For the sample return mission, the material of

regional plains with wrinkle ridges (pwr) is considered as the highest priority goal with the second priority given to tessera terrain (tt)

material. Combining the desire to study materials of specific geologic units with the problem of avoiding potential contamination by

ejecta from upwind impact craters, we have suggested several candidate landing sites for each of the geologic units. Although spacecraft

ballistics and other constraints of specific mission profiles (VEP or others) may lead to the selection of different candidate sites, we believe

that the approaches outlined in this paper can be helpful approach in optimizing mission science return.

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Keywords: Venus; Landing sites; Venera lander; Vega lander

1. Introduction

Venus is a planet very similar to Earth in its mass, sizeand thus bulk density, but very different in surfaceenvironment and recent general geodynamic style (e.g.,Barsukov et al., 1992; Bougher et al., 1997). To understandbetter the cause of these differences more data aboutVenus geology, geochemistry and geophysics are needed.A crucial part of these data can be obtained only by in situmeasurements on the surface of Venus. This is why theplanetary science community and space institutions fromtime to time consider missions designed to land on Venusand even to do sample return from this planet (e.g., Surkovet al., 1993; Rodgers et al., 2000; Crisp et al., 2002;National Research Council, 2003; Boddy et al., 2004;Korablev et al., 2006). Recently, ESA began to consider the

e front matter r 2007 Elsevier Ltd. All rights reserved.

s.2007.09.005

ing author. Vernadsky Institute of Geochemistry and

istry, RAN, Moscow 119991, Russia.

4995; fax: +7 95 938 2054.

ess: [email protected] (A.T. Basilevsky).

possibility of sending landers to Venus as one of theversions of the Venus Entry Probe mission (Van den Bergand Falkner, 2006; Chassefiere, 2006; Leitner et al., 2007).Independently of this specific discussion, it is obvious thatin the not too distant future, mission(s) that involvelanding on Venus will be realized and eventually samplereturn from Venus will also be done. This paper brieflyreviews what was accomplished in previous landings,discusses a new approach of selection of landing sites,and considers several candidate sites for future landings.Preliminary results of this study have been published inAittola et al. (2006, 2007).

2. Previous landings on Venus

The first successful landing on Venus was achieved bythe Venera 7 mission in 1970 (Moroz and Basilevsky,2003). However, it was only a partial success: althoughseveral instruments were on board, only data on the surfacetemperature and pressure were transmitted back to Earth.In 1972, the Venera 8 lander reached the surface and using

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Table 1

Contents of K, U, and Th in Venus surface material measured by the

gamma spectrometry technique (Surkov, 1997)

Lander Venera 8 Venera 9 Venera 10 Vega 1 Vega 2

K (mass%) 4.071.2 0.570.1 0.370.2 0.4570.22 0.4070.20

U (ppm) 2.270.7 0.670.2 0.570.3 0.6470.47 0.6870.38

Th (ppm) 6.570.2 3.770.4 0.770.3 1.571.2 2.071.0

A.T. Basilevsky et al. / Planetary and Space Science 55 (2007) 2097–21122098

gamma-ray spectrometer (GRS) made measurements of thecontents of K, U and Th in the surface material (Table 1).The GRS instrument measured gamma radiation pene-trated through the lander structure, sensing the area �1macross beneath the lander and a few decimeters in depth.The Venera 8 probe also confirmed the Venera 7 data onthe high Venus surface temperature and pressure andmeasured the sunlight level, determining it to be suitablefor surface photography. The next landings occurred in1975; the Venera 9 and 10 probes measured the contents ofK, U and Th in the surface material by the GRS technique(Table 1) and took TV panoramas of the close vicinity ofthe landing sites. The panoramas revealed the presence oftwo types of the surface materials: soil and finely beddedrock (Fig. 1). In 1978, the Venera 11 and 12 landersreached the surface. During the descent and partly on thesurface, compositions of the atmosphere and cloudaerosols were measured by several instruments. The landershad TV cameras, but the camera port covers failed to openand no panoramas were taken.

In 1981, the Venera 13 and 14 landers measured thecomposition of the surface material using X-ray fluores-cence spectrometers (XRFS) (Table 2) and took two TVpanoramas each (fore and aft). A few cm3 sample wastaken by the drilling device on each lander from the top fewcentimeters of the surface material, delivered inside thelander capsule, and then analyzed. The panoramas againshowed the presence of two types of the surface materials:soil and finely bedded rock (Fig. 1). The next, and the last,landings in this series of lander missions were part of theVega 1 and Vega 2 probes in 1984. Vega 1 used GRS tomeasure the contents of K, U and Th in the surfacematerial (Table 1) and Vega 2 measured surface composi-tion by two techniques: K, U and Th by gamma-rayspectrometer, and petrogenic elements by X-ray fluores-cence spectrometer (Tables 1 and 2). Difference in thecontents of K determined by the two techniques at theVega 2 site could be partly due to analytical errors butpartly because the XRFS had analyzed material of the topfew centimeters while the GRS, as in the cases of Venera 9and 10, analyzed a layer of about a few decimeters thick.TV cameras were not among the instruments of the Vega 1and Vega 2 lander. Positions of the Venera–Vega landingsites on a global SAR map of Venus as well as new landingsites suggested in this paper are shown in Fig. 2. Fig. 3shows positions of the Venera–Vega landing sites on theregional topographic and geologic maps.

It is seen in Fig. 2 that the Venera landing sites areconcentrated within the region of Beta Regio and PhoebeRegio while Vega 1 and Vega 2 landed in a different regionwest of Atla Regio. These positions were controlled by thecelestial mechanics considerations. When Venus and Earthare at inferior conjunction (at the closest distance to eachother), which is the time favorable for missions to Venus,then due to orbital resonance the Beta-Phoebe region isdirectly seen from Earth, thus this makes communicationand mission control easier. Locations of the Vega 1 andVega 2 landing sites were determined by the trajectory ofthe spacecraft flyby on their way to comet Halley. For theVenera 8 through Venera 12 missions the targeting wasbased purely on ballistic considerations. Starting with theVenera 13 mission, maps of the radar properties of Venussurface, acquired by the Pioneer Venus orbiter, becameavailable, so within the ballistically achievable zones, areaswith low surface roughness were selected and used fortargeting.

3. Geochemical interpretation of the Venera–Vega analyses

The Venera–Vega geochemical measurements have beeninterpreted as evidence that the surface materials analyzedare generally mafic and compositionally close to tholeiiticbasalts (Venera 9, 10, 14, Vega 1, 2) and alkaline basalts(Venera 8 and 13). Different authors found similarities withdifferent varieties of terrestrial mafic, mostly basaltic rocks(Vinogradov et al., 1973; Surkov et al., 1976; Barsukovet al, 1982, 1986; Surkov et al., 1987; Nikolaeva, 1990;Kargel et al., 1993). The most recent and comprehensivecomparisons of the Venera–Vega compositions with thoseof terrestrial mafic rocks were done by Nikolaeva andAbdrakhimov. Their major concern was to avoid compar-isons with terrestrial samples which could be influenced byadmixture of the Earth’s continental (granitic) crustmaterial and affected by water-involved weathering andhydrothermal alterations. So they have compiled from theliterature the data bases of chemical analyses of mafic rocksof three petrologic associations of the terrestrial oceaniccrust: (1) Normal Mid-Oceanic Ridge Basalts (NMORBs),(2) Oceanic Island Arcs association, and (3) Oceanic HotSpots association (Nikolaeva, 1995, 1997; Abdrakhimov,2005) paying special attention that the locations fromwhich samples were accepted into the data bases are notinfluenced by continental crust. They then sorted out theanalyses of the samples affected by even weak hydro-thermal alterations and, following that, statistical compar-isons between the nonaltered terrestrial samples and theVenera–Vega analyses were done. The most likely analogsof the Venera–Vega compositions among the terrestrialmagmatic environments found by these authors are givenin Table 3. It is obvious that these comparisons weremade based on very incomplete sets of geochemical data(Tables 1 and 2) and thus suggest only some similarities.These cannot be considered as full analogs, however,especially in the sense of analogous petrogenic and

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Fig. 1. TV panoramas taken by the Venera 9, 10, 13, and 14 landers and showing the presence of soil and finely bedded rock. The optical axis of each

camera was inclined 501 from the vertical so that the middle part of each panorama is a close-up view while at the right and left ends the camera looked at

the horizon. The T-shaped structure seen on the right of the panoramas of Venera 9 and 10 is the gamma densitometer. Its most distant (transverse) part is

40 cm long. The bright linear and segmented structure near the center of the Venera 10 panorama is the 40 cm long and 10 cm wide view-port cover. The

Venera 13 and 14 panoramas show view-port covers of arcuate design (20 cm in diameter). The photometric standard on the right of the Venera 13 and 14

panoramas is 40 cm long. The trellis girder (center left of the Venera 13B and Venera 14B panoramas) is 60 cm long.

A.T. Basilevsky et al. / Planetary and Space Science 55 (2007) 2097–2112 2099

geodynamic environments. For example, as Table 3 shows,the Vega 2 chemistry suggests a similarity with the Islandarc tholeite series, but this does not imply that island arcsexist on Venus.

It is necessary to take in mind that volcanic materialsemplaced on the surface of Venus could be involved inchemical interaction with atmosphere gases, and thusmagmatic mineral assemblages could be replaced by‘‘weathering’’ products. Physico-chemical calculations be-

ginning with early studies (e.g., Barsukov et al., 1982) upuntil recent ones (e.g., Fegley, 2003; Zolotov, 2007) suggestseveral effects of chemical weathering on Venus, fromwhich we mention the following: (1) oxidation andsulfurization of surface rocks through gas–solid-typereactions; (2) isochemical weathering of individualsolid phases with respect to elements being nonvolatileat Venus’ surface temperature (e.g., Al, Si, Mg, Fe, Ca,Na); (3) unlikely current hydration and a possibility of

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dehydration of early formed phases; and (4) a strongaltitudinal effect for the chemistry and physics of gas–sur-face interactions. So the above discussion describing whichmaterials could be analyzed by the Venera–Vega landersand which petrologic associations they could represent,taken in the magmatic rock terminology, probably relatesnot to materials technically sampled by these landers but totheir unweathered precursors.

4. Geologic units sampled and analyzed by the Venera/Vega

landers

In 1990–1992, the Magellan mission obtained side-looking radar images of Venus with 100–200m/px resolu-tion, that is high enough to understand the nature ofgeologic formations in the vicinity of the Venera–Vegalanding sites (Saunders et al., 1992). This regional geologywas considered in a number of papers (e.g., Basilevskyet al., 1992; Weitz and Basilevsky, 1993; Kargel et al., 1993;

Table 2

Contents of major elements in Venus surface material measured by the

X-ray fluorescence spectrometry technique (Surkov, 1997)

Lander Venera 13 Venera 14 Vega 2

SiO2 45.173.0 48.773.6 45.673.2

TiO2 1.5970.45 1.2570.41 0.270.1

Al2O3 15.873.0 17.972.6 16.071.8

FeO 9.372.2 8.871.8 7.7471.1

MnO 0.270.1 0.1670.08 0.1470.12

MgO 11.476.2 8.173.3 11.573.7

CaO 7.170.96 10.371.2 7.570.7

K2O 4.070.63 0.270.07 0.170.08

S 0.6570.4 0.3570.31 1.970.6

Cl o0.3 o0.4 o0.3

Fig. 2. Positions of the Venera–Vega landing sites (only those which made g

global SAR map of Venus. Legend: V, Venera sites, Vg, Vega sites; new sites

transitional terrain; psh, shield plains; pwr, regional plains with wrinkle ridge

Basilevsky, 1997; Basilevsky and Head, 1998, 2000) alongwith the general analysis of Venus geology (e.g., Headet al., 1992; Solomon et al., 1992; Stofan et al. 1997; Guestand Stofan, 1999; Ivanov and Head, 2001a, b, 2004a, b;Basilevsky and Head, 2002, Bridges and McGill, 2002;Campbell and Campbell, 2002; Bleamaster and Hansen,2005; Brian et al., 2005). The most recent mapping byAbdrakhimov (2001a–g, 2005) showed that within thelanding ellipses of the Venera 9, 10, 13, 14 and Vega 1, 2sites (circles with 100 km radius), the most widespread unitsare extensive plains whose morphology suggests formationby high-yield eruptions of fluid lavas (Head et al, 1992;Crumpler et al., 1997; Basilevsky and Head, 2002). In theregional and global stratigraphy of Basilevsky and Head(1998, 2000), these are mostly plains with wrinkle ridges(pwr) and locally lobate plains (pl) (Fig. 4).Within the Venera 8 landing ellipse, the most widespread

unit is a variety of plains whose morphology suggests low-yield eruptions of fluid lavas, forming small gently slopingshields. In the stratigraphy of Basilevsky and Head (1998,2000), this is shield plains (psh). Although other units arealso present within the landing ellipse it was suggested thatthe most probable unit on which the landing took place isthe most widespread unit, psh (Basilevsky and Head, 1998,2000; Abdrakhimov, 2005) (Table 4, option ‘‘Seen inMagellan’’).Recently, however, Basilevsky et al. (2004) suggested

that the finely bedded rocks seen on Venera TV panoramascould be partly indurated airfall sediment consisting of thefine fraction of ejecta of upwind (located east of a givensite) impact craters forming so-called radar dark parabolas(see Section 6). This hypothesis is supported by the verylow mechanical strength of these rocks (see summaries inFlorensky et al., 1983; Basilevsky et al., 1985, 2004) and

eochemical observations) and the sites suggested for future landings on a

are marked according to the sampled unit: tt, tessera terrain; ttt, tessera

s; and pl, lobate plains.

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Fig. 3. Topographic (upper left) and geologic (upper right) maps of the region of the Venera 8–14 landing sites and topographic (lower left) and geologic

(lower right) maps of the region of the Vega 1 and Vega 2 landing sites. Positions of the Venera and Vega sites are shown by white circles. Topography is

shown in relation to the mean planetary radius, that is 6052 km. Information on several map units shown in this figure but not described in this paper can

be found in Basilevsky and Head (2000) and Ivanov and Head (2001a).


implies that the source of the sampled material at theVenera sites could be rocks from the kilometers-deepsubsurface excavated by those craters. Basilevsky et al.(2004) explored this possibility and suggested alternativeinterpretations of what material would be analyzed bythe landers in this case (Table 4, option ‘‘Airfall ejectasupply’’).

In summary, even considering these two options we maystill be sure that the material of the most widespread uniton Venus (pwr) was certainly analyzed by the Venera–Vegalanders (most probably by Venera 13 and Vega 1). Thesituation is less certain concerning other units. Venera 8appears to have landed on psh, but because there was nosurface TV panorama for this site we do not know if the

finely bedded surface rocks, suggested to be the partlyindurated airfall deposits, are present there. If they arepresent, then the discussion by Basilevsky et al. (2004)suggests that the material analyzed could be a mixture oftessera terrain (tt) materials and pwr materials provided byimpact excavation of the upwind craters Amalasthuna,Virve and Cynthia. Venera 14 appears to have landed onthe pl. Panoramas taken by this lander show the presenceof the finely bedded rocks, which could be airfall depositsof ejecta from the craters Ingrid, Cline and Bender. If thisinterpretation is correct then the material analyzed mostprobably represents material of pwr. No spacecraft landedon the surface of tessera terrain, but as noted above,material of this unit could be present at the Venera 8 and 9

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Fig. 4. Geological material and structural units of Venus according to

global and regional stratigraphy by Basilevsky and Head (1998, 2000) with

addition of structural unit of tessera transitional terrain (Ivanov and

Head, 2001a). Legend: tt, tessera terrain; pdf, densely fractured plains; pfr,

fractured and ridged plains; pwr, plains with wrinkle ridges; pl, lobate

plains; ps, smooth plains; ttt, tessera transitional terrain; fb, fracture belts;

and rt, rifted terrain. Information on several units shown in this figure but

not described in this paper can be found in Basilevsky and Head (2000)

and Ivanov and Head (2001a).

Table 3

Terrestrial compositional analogs of the Venera–Vega surface materials

Lander Options of geodynamic environments and magmatic series

considered as analogs

Venera

8

Hot spot alkaline series

Venera

9


Venera

10

Hot spot tholeite series (Island arc tholeite series)

Venera

13


Venera

14

NMORB tholeite series

Vega 1 Hot spot alkaline series, hot spot tholeite series (Island arc

tholeite series)

Vega 2 Island arc tholeite series

Less likely options are given in parentheses (Abdrakhimov, 2005).

Table 4

Two options of what material units have been analyzed by the

Venera–Vega landers (Basilevsky et al., 2004)

Lander Analyzed material

Seen in Magellan Airfall ejecta supply

Venera 8 psh tt, pwr

Venera 9 pwr tt

Venera 10 pwr psh

Venera 13 pwr pwr

Venera 14 pl pwr

Vega 1 pwr pwr

Vega 2 pwr Minor presence of tt,

pdf, pfr, psh, pwr


sites and have been analyzed there. This, however, is onlyspeculation.

5. Units to be sampled and geochemically analyzed in future

studies

We consider below what material units have to bestudied in future landing missions on the surface of Venus.There could be two types of such missions: making in situanalysis, and sample return. In our considerations weassume that the material of plains with wrinkle ridges hadalready been geochemically analyzed by the Venera–Vegalanders so we do not consider it as target unit for the in situanalysis but will consider it as high-priority target for asample return mission.The first candidate unit for in situ analysis, whose

composition is important to determine in future studies istessera terrain (unit tt). The surface of tessera displays awide variety of morphologies due to superposition ofnumerous sets of contractional (ridges) and extensional(grooves and graben) tectonic structures criss-crossing in atleast two directions (Barsukov et al., 1986). Typically, thesestructures completely erase the morphologic characteristicsof the tessera precursor terrain. Tessera is seen as ‘‘islands’’and ‘‘continents’’ amidst the widespread plains and otherunits and is always embayed by their materials. Thus,tessera is composed of the oldest material unit recognizedon Venus (Fig. 4) (Ivanov and Head, 1996, 2001a;Basilevsky and Head, 1998, 2000), representing the timewhen the geologic environment on Venus could be differentfrom what we see in the post-tessera morphologic record.In some regions of Venus, the so-called tessera transi-

tional terrain (ttt) is observed, first identified by Ivanov andHead (2001a, b). It partly resembles ‘‘true’’ tessera, beingcharacterized by two and more sets of intersecting tectonicstructures. Within tessera transitional terrain, however,small fragments of the precursor materials are recogniz-able. This material is typically represented by material offractured and ridged plains (pfr) and locally by material ofdensely fractured plains (pdf) (Fig. 3). We consider tessera

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transitional terrain as a structural unit (Ivanov and Head,2001a; Basilevsky and Head, 2005, 2007).

Tessera and tessera transitional terrain occupy alto-gether about 8% of the surface of Venus but what is seenon the surface is obviously only protrusions of a moreareally widespread (Ivanov and Head, 1996), perhaps evenalmost global, basement. Formation of true tessera couldhave occurred, for example, in the late stage of a plate-tectonic cycle such as envisioned by Turcotte (1995,1996), who considered the episodically occurring globalsubduction as a mechanism which could be responsible forglobal resurfacing event implied from the close-to-randomareal distribution of impact craters on Venus (Schaber etal., 1992; Phillips et al., 1992). Such a setting might providethe opportunity for reprocessing basaltic crustal materialand compositional differentiation towards geochemicallymore evolved (silicic?, alkaline?) materials. Joint analysis ofthe gravity field and topography of Ishtar Terra ledKuchinskas et al. (1996) to conclude that some parts ofthis structure (Maxwell Montes, consisting of materialsomewhat similar to tessera), could be composed ofmaterial less dense than basalt (silicic). Ivanov (2001),who described relics of possibly volcanic plains beingprecursor terrain in the localities of ttt, suggested that thematerial composing this unit is basaltic. No singleVenera–Vega probe landed on true tessera terrain (tt) oron ttt and the presence of their materials within the sitesstudied is still speculative.

In summary, concerning the composition of tesseraterrain and ttt materials, there are only speculations basedon indirect data. As stated above, tessera material, andpartly ttt, may be the basement of the volcanic plains thatdominate Venus and thus could be a significant, if notpredominant, component of the upper crust. The VenusEntry Probe Landing Sites workshop held in Vienna on14–15 November 2006 (http://www.univie.ac.at/EPH/venus)concluded that tessera terrain was the highest priority forstudy.

The second candidate unit, for the mission with in situanalysis, whose composition is important to determine infuture studies, is shield plains (unit psh). The presence ofnumerous volcanic shields, each a few kilometers across,that pepper the surface of this unit, implies that it wasproduced by eruptions from shallow and widespreadsources of magma (Head et al., 1992). These sources couldresult from partial melting within the basaltic crust, forexample, due to an increase of the greenhouse effect, assuggested by the hypothesis of Solomon et al. (1999).Partial melting of basaltic crust material could also be frombelow and could produce geochemically more evolved(silicic?, alkaline?) materials (Hess and Head, 1990; Nelsonand Phinney, 1990; Lukanin, 1993) such as the largeviscous domes that appear to be stratigraphically concen-trated in this part of the geological column (e.g., Ivanovand Head, 1999). This seems to be in agreement with theGRS analysis of Venera 8 (Table 1), which probably landedwithin the psh unit. But as described above, we cannot

exclude the possibility that this lander analyzed a mixture(tessera and pwr materials) delivered to the site by nearbycrater-forming impacts and subsequent downwind trans-portation in the atmosphere. To avoid this uncertainty infuture missions it is necessary to analyze undoubted pshmaterial. Shield plains occupy about 10–15% of Venussurface, but they are often embayed and buried by themore widespread plains with wrinkle ridges (Ivanov andHead, 2004b). So their contribution to the composition ofthe upper crust of Venus may be larger than it appearsfrom their surface area alone.The third candidate unit, for the mission with in situ

analysis, whose composition is important to determine infuture studies is lobate plains (unit pl). Compared to theplains with wrinkle ridges and shield plains, they representa different style of eruption related to formation of suitesof numerous lobate flows either associated with relativelyyoung rift zones or with possible hot-spot structures. Theyform either areas of subhorizontal plains or gently slopingvolcanic constructs with aprons of lobate flows merginginto subhorizontal plains. Both these varieties of the pl unitare typically superposed on pwr and psh, so in the localand probably global time sequences they are relativelyyoung. This different style of eruption may reflect adifferent petrogenic environment and thus some composi-tional differences. Material of lobate plains may have beenanalyzed by the Venera 14 lander, but as discussed above,if the finely bedded rocks seen on TV panorama taken bythe Venera 14 lander are airfall deposits of ejecta fromupwind impact craters, the material analyzed may compo-sitionally represent unit pwr. For a reliable understandingof the composition of the lobate plains lavas in futuremissions, it is necessary to analyze undoubted representa-tives of this unity.In summary, these three material units appear to be good

candidates for in situ studies by future landing mission(s).But if in some future, a Venus sample return mission isconsidered, the units of interest may be partly different.The first candidate in this case would be material of themost widespread unit on Venus, which is volcanic pwr.Samples of this material, if analyzed on Earth with the fullrange of geochemical tools, including trace elements, rareearth elements, isotopic analyses, microprobes, wouldprovide geochemical information crucially important forunderstanding the dominant petrogenetic processes in theupper mantle and crust of Venus. A special interest wouldrepresent determinations of the absolute age of the samplesand thus the age of the volcanism of the landing site. Thisinformation is crucial as ground truth for the existingestimates of surface ages on Venus based on impact craterstatistics (e.g., McKinnon et al., 1997). If the sample returnmission will be able to bring back to Earth samples fromtwo sites, then the second candidate to be delivered toEarth should be material of tt. Of course, while planningthe future missions an issue of how chemical weathering ofVenusian materials can influence the results of analysesshould be considered in detail.

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6. General approach to selection of landing site

Although the suggestion of Basilevsky et al. (2004) thatfine-bedded mechanically weak rocks observed on panor-amas of Venera 9, 10, 13 and 14 are airfall deposits ofejecta from the upwind craters is still a hypothesis, itshould be seriously considered in the selection of futurelanding sites. Thus, if one desires to land on a pristine unitof a specific type, one should search for places where theunit is present and has no relatively nearby upwind impactcraters whose parabola deposit (being materials ejectedfrom this crater) might overlay the unit under considera-tion. In order to estimate whether or not upwind craters(which are always present) could pollute the potentiallanding site with their ejecta, we recommend using the‘‘model parabolas’’ approach, as done by Basilevsky et al.(2004). Dark parabolas deposits are observed only inassociation with craters larger than 11 km in diameter(Campbell et al., 1992; Schultz, 1992). Formation ofsmaller craters probably does not lead to ejection ofexcavated material to high enough altitudes (450–70 km)to be carried by the zonal winds to form a parabola.

Basilevsky et al. (2004) found that area of the crater-associated parabola, A, depends on the crater diameter, D,as: A (km2) ¼ 29,846D (km)+59,000. It was also foundthat the parabola shape is close enough to the right half ofthe ellipse with a 2:1 semiaxis ratio with the crater to apexdistance equal to 0.15 of the model parabola width. Theapex is always facing to the east. Using this approach onecan draw the model parabolas around craters withD411 km in the region where one is considering selectingthe landing site and in this manner see if the potentialsite is covered by any parabola. All craters should beconsidered in this approach, those with currently observeddark parabolas, those with a dark non-parabolic halo andthose without any halo, because even if the parabolacannot be observed in the image, its material may still bepresent (see e.g., Bondarenko and Head, 2004). If anymodel parabola covers the potential landing site the sitecan be rejected from continued consideration. The landingsite region can be considered as a circle of 200 km diameter(the landing ellipse assumed in the Venera–Vega missions;Basilevsky et al., 1992), although in future missions theellipse may be significantly smaller. Model parabolas arenot drawn around craters Do11 km, but the presence ofsuch craters in the close vicinity of potential landing sites isundesirable.

For potential landing missions whose goal is to analyzethe true tessera (tt) and ttt materials, we use a variation onthis approach: the guideline is that parabolas of upwindcraters should not cover the potential site unless they areassociated with craters excavating correspondingly theunits tt and ttt. The site itself should be within the radar-dark halo associated with a crater sitting on tessera terrainor ttt. The crater-associated halo is considered to be thefine-grained fraction of the material ejected from the crater.In addition, the radar-dark surface is supposed to be

morphologically smooth and this decreases the risk ofspacecraft damage or overturning on landing. Although wehave no direct data on the thickness of the tessera material,the 1–3 km range of altitudes observed within large blocksof tessera and tessera transitional terrains (Ivanov andHead, 1996) suggests that the thicknesses of these materialsare not less that a few kilometers. Thus, using crateringguidelines (e.g., Melosh, 1989) craters with diameters notlarger than a few tens of kilometers, sitting on units tt andttt, should not penetrate through them (their depth ofexcavation is close to 1/10 of their diameter), and theirejecta are expected to be a source of these materialsuncontaminated by other materials. This approach cannotbe used, however, for other units of interest (pwr, psh, pl)whose estimated thickness is probably less than 1–3 km.The global map of model parabolas with candidate

landing sites is shown in Fig. 5.

7. Selection of potential landing sites

We examined Magellan images and found candidatesites for tessera (tt and ttt units), psh, pl and pwr, whichthen were tested using the model parabola approach. Thelist of suggested candidate sites is given in Table 5.

7.1. Tessera terrain (tt and ttt units)

The preliminary search for tessera landing sites led toselection of four (Aittola et al., 2006), then five (Aittolaet al., 2007) areas, all centered on impact craters. At thisnext stage of the study we recommend using nine candidatesites. Six of them are sites designed to sample the truetessera material and three sites are designed to sample tttmaterial (Fig. 5). The true tessera (tt) sites are: (1) TesseraTellus, crater Khatun (40.31N, 87.21E, 44.1 km), (2) TellusTessera, crater Tseraskaya (28.61N, 79.21E, 30.3 km),(3) Tellus Tessera, crater Merian (34.51N, 76.31E,22.2 km), (4) Fortuna Tessera, crater Frida (68.21N,55.61E, 21.6 km), (5) Fortuna Tessera, crater Roptyna(62.21N, 28.91E, 11.5 km), (6) Clotho Tessera, craterMagnani (58.61N, 337.21E, 26.4 km). The ttt sites are:(1) Ovda Regio, crater Carter (5.31N, 67.31E, 17 km),(2) Ovda Regio, crater deBeausoleil (51S, 102.81E,28.2 km), (3) Thetis Regio, crater Whiting (6.11S,128.01E, 35.7 km). Figs. 6 and 7 show the Magellan imagesand geologic maps of examples of the candidate landingsites on tt and ttt, correspondingly.Except for the Khatun landing site, all other tessera sites

are within the upwind crater parabolas. These parabolas,however, are related to craters situated on outcropsof tessera materials and, thus, are not the sources ofcontamination by non-tessera material. Within each land-ing site (either on tt or on the ttt) three types of materialunits typically occur. The most abundant unit (90–95% ofthe landing ellipse area) represents the complexly deformedsurface of either true tessera (unit tt) or tessera transitionalterrain (unit ttt). The second unit is related to the crater

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Table 5

Characteristics of proposed landing sites

Landing site Latitude Longitude Crater diameter (km) Covered by parabola Parabola source

True tessera (tt)

Tellus

Khatun 40.3 87.2 44.1 No

Tseraskaya 34.5 76.3 30.3 Yes Crater Bernhardt, Tellus, tt

Merian 28.6 79.2 22.2 Yes Craters Bernhardt, Khatun: Tellus, tt

Fortuna

Frida 68.2 55.6 21.6 Yes Crater Klenova, pwr

Roptyna 62.2 28.9 11.5 Yes Crater Baker, Fortuna, tt

Clotho

Magnani 58.6 337.2 26.4 Yes Crater Rossetti, Fortuna, pwr

Tessera transitional terrain (ttt)

Ovda Regio

Carter 5.3 67.3 17 Yes Crater Naomi, Ovda, ttt

deBeausoleil �5 102.8 28.2 Yes Craters Makola, Sullivan: Ovda, ttt

Thetis Regio

Whiting �6.1 128 35.7 Yes Crater Gilmore, Thetis Regio, ttt

Shield plains (psh)

Vellamo Planitia 43 131 – Partly Crater Cochran, pwr

Sedna Planitia 43.5 333.5 – No

Lobate plains (pl)

Sekmet Mons 47 242.5 – No

Sapas Mons 8 187 – Yes Crater Zamudio, pl; Crater Melba, pwr

Mylitta Fluctus �55 353.5 – Partly Crater Alcott, pwr (?)

Plains with wrinkle ridges (pwr)

Zhibek Planitia �33 170 – No

Sedna Planitia 42.5 342.5 – No

Fig. 5. Map of model parabolas showing positions of candidate landing sites. Projection of the map is simple cylindrical. Due to this, toward the polar

areas, the modal parabolas appear to be significantly larger and extended in an E–W direction.


materials and includes materials on the floor, walls, and inthe continuous ejecta of the crater, and small patchesof dark plains that form a halo around the crater. As a

whole, these materials constitute 5–7% of the landing sitearea and it should be tessera material in the compositionalsense. The third unit occurs as small areas of relatively

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Fig. 6. Candidate landing site to sample true tessera terrain material in Tessera Tellus, crater Khatun (40.31N, 87.21E, D ¼ 44.1 km).


dark plains that occur within valleys between tesseraridges. The plains embay tectonic structures and thuspostdate both the tessera-precursor material and the maintectonic phases of deformation of the tessera surface. Weinterpret this material as intratessera volcanic plains thatare related to late episodes of volcanism within the tesseraregions. These plains may potentially be the source ofcontamination of the landing sites by non-tessera materialbut the total area of such plains within the landing ellipse issmall, only a few percent of the landing site area, andprecise navigation on landing may hopefully avoid them.For example, in the case of landing sites shown in Figs. 6and 7, improvement of landing accuracy to 720 km willallow one to encounter on landing only tt and ttt materialscorrespondingly. In planning a mission to land on tesseraor ttt, one should take in mind that these are areas ofrelatively high surface roughness and that the lander designshould be adjusted for that.

7.2. Shield plains (psh unit)

Two areas of shield plains were selected in thepreliminary analysis. The first is at the easternmost edgeof Vellamo Planitia (centered at 431N, 1311E) and thesecond is on the western side of Sedna Planitia near theNW edge of Zorile Dorsa (centered at 43.51N, 333.51E)(Fig. 5). The site in Vellamo Planitia is the classicalexample of shield plains (Aubele, 1995). This arearepresents a broad field of shields plains �100 km acrosswith unambiguous stratigraphic position: on the west, thepsh embay massifs of tessera (the southern extension ofAnanke Tessera) and are embayed by regional plains (unitpwr) along their western edges. Thus, psh in this localityare sandwiched between tessera (tt) and regional plains(pwr). The northern side of the landing ellipse is near theedge of a model parabola related to crater Cochran that iswithin regional plains near Ananke tessera. Thus, some

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Fig. 7. Candidate landing site to sample tessera transitional terrain material (ttt) in Ovda Regio, crater deBeausoleil (51S, 102.81E, D ¼ 28.2 km).


material from that parabola representing units tt and pwrmay be on the surface within the Vellamo Planitia landingsite. Another complication is that shield plains in this siteare in close contact with a relatively large outcrop oftessera. Due to this, the psh material in this area potentiallymay also be contaminated by tessera material if it wasremelted during emplacement of psh.

The second region of shield plains represents a large andcontiguous area of the unit psh hundreds of km across thatoccurs on the western side of Sedna Planitia in theparabola-free region. psh here are superposed on smalloutcrops of densely fractured plains (unit pdf) and areembayed by two subunits of regional pwr. The lack ofeither observed or now-invisible parabolas and outcrops oftessera suggested that psh at this site are likely to beuncontaminated by remote materials or by material oftessera. The potential contribution of material of the unitpdf is thought to be insignificant because outcrops of thisunit are small and seldom within the area of psh. Fig. 8

shows the Magellan image and geologic map of thiscandidate-landing site. Small ‘‘islands’’ of densely fracturedplains material are seen on the western part of the targetedfield of psh. But improvement of landing accuracy to750 km will allow one to encounter only the psh materialon landing.

7.3. Lobate plains (pl unit)

We have selected three candidate regions for sampling ofthis type of plains (Fig. 5). The first is on the northern flankof Sekmet Mons (centered at 471N, 242.51E). This arearepresents a large long complex of interfingering radar-dark (morphologically smooth) and radar-bright (rough)lava flows several hundreds of kilometers across that havesources from both the summit area and flanks of the largevolcano Sekmet Mons. The complex is clearly superposedon the surrounding regional plains (pwr). Neither actualnor model parabolas cover this site and lobate plains in this

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Fig. 8. Candidate landing site to sample shield plains material (psh) in Sedna Planitia near the NW edge of Zorile Dorsa (centered at 43.51N, 333.51E).


area appear to be uncontaminated by remote materials.Within the landing site, bright flows are more abundant,which may be potentially dangerous for the landingbecause these flows are likely of the aa type and may bevery rough (Ford et al., 1993). However, this problem isshared with all sites selected to sample lobate plains. Fig. 9shows the Magellan image and geologic map of thiscandidate landing site. In this area among fields of lobateplains are a few ‘‘windows’’ of pwr plains, which are not atargeted unit in this place. But improvement of landingaccuracy to735 km will allow one to encounter only the plmaterial on landing.

The second site is on the western slope of Sapas Mons(centered at 81N, 1871E). It is a large area of mostly brightand some dark lava flows, which clearly postdate thesurrounding regional plains. In contrast to the previoussite, Sapas may represent different environments offormation of lobate plains. At the very summit of SapasMons there are two steep-sided domes that may suggest the

presence of more evolved magma (Keddie and Head,1994). On the flanks of the volcano there are numeroussmall shield- and cone-like edifices several km across thatpossibly indicate parasitic volcanic centers. These featuresare absent at Sekmet Mons. On the eastern side of SapasMons there is a small impact crater superposed on lobateplains. A model parabola related to this crater completelycovers the proposed landing site but materials that thisparabola may potentially deliver to the site appear to be ofthe same type and source as within the landing site. Onemore parabola that partly covers the site is related to thecrater Melba that is to the north of Maat Mons withinregional plains. Thus, materials within the Sapas Monslanding site potentially may be contaminated by materialof regional plains (pwr).The third pl-sampling site is within Mylitta Fluctus

(centered at 551S, 353.51E), which is a huge complex ofradar-bright and radar-dark lava flows many hundreds ofkm across (Magee et al., 1992). In contrast to the previous

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Fig. 9. Candidate landing site to sample lobate plains material (pl) on the northern flank of Sekmet Mons (centered at 471N, 242.51E).


selected sites, Mylitta Fluctus is not related to a volcanicconstruct similar to Sekmet Mons and Sapas Mons. Thelack of such a feature may suggest that the Mylitta flowswere emplaced during a relatively short period of timewithout a large magma reservoir, and may thus have less ofa chance evolve and more likely represent a more primitivemagma type. The southern portion of the lava complex ofMylitta Fluctus is within a model parabola of Alcottecrater that is embayed by lobate plains and probablyimpacted into regional plains. The influence of theparabola, however, can be minimized by the shift of thelanding site to the north, outside of the parabola.

7.4. Plains with wrinkle ridges

Two sites were selected for sampling the regional plainsunit (Fig. 5). The first is in Zhibek Planitia, 300 km NE ofMahea Tholus (centered at 331S, 1701E) and the second isin Sedna Planitia, 300 km east of Zorile Dorsa (centered at

42.51N, 342.51E). Both sites are within large expanses ofregional plains, the surface of which is characterized asrelatively topographically low and in general by a uniformalbedo. Some subtle albedo variations do occur, however,and in the Zhibek Planitia landing site (Fig. 10) two unitsof regional plains, darker and brighter, were defined.Boundaries between them are not easily distinguishableand are diffuse in places, and the age relationships of theseunits are unclear. It is possible that the units are related tothe late redistribution of otherwise homogeneous materialby wind. A characteristic feature of regional plains isnarrow and sinuous channels that occur on the surface ofthe plains from place to place. The channels were carved bysustained flow of a very fluid material; the range of possiblecompositions may vary from komatiites to carbonatites(e.g., Baker et al., 1997). A segment of one of thesechannels is seen near the center of the Zhibek Planitialanding site (Fig. 10), thus providing the opportunity tosample one of these unusual features. No other units are

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Fig. 10. Candidate landing site to sample material of plains with wrinkle ridges (pwr) in Zhibek Planitia (centered at 331S, 1701E).


present in the suggested pwr-targeted sites so the landingaccuracy in these cases may be on the level of that for theVenera–Vega landers (7100 km).

8. Summary

In the above discussions we bring to the attention ofthe planetary science community a new approach to theselecting of landing sites on Venus that would avoid thepotential unwanted contribution of ejecta from upwindimpact craters. We also described candidate units to besampled in both in-situ measurement and sample returnmissions. For the in-situ measurements, the ‘‘true’’ tesseraterrain (tt) material is considered as the highest prioritygoal with the second priority given to transitional tesseraterrain (ttt), shield plains (psh) and lobate plains (pl)materials. For the sample return mission, the material ofregional plains with wrinkle ridges (pwr) is considered asthe highest priority goal with the second priority given to

tessera terrain (tt) material. Combining the intent to studythe materials of specific units of interest with the issue ofpotential unwanted contamination by ejecta from upwindimpact craters, we have suggested several candidate-land-ing sites for each of the materials of interest. Of course,spacecraft ballistics and other constraints of specificmission profiles (VEP or others) may lead to the selectionof different candidate sites, but we believe that this papercan be used even then as a helpful approach.

Acknowledgments

Discussions held at the 1st Venus Entry Probe Landing-Sites Workshop, 14–15 November 2006, Vienna, Austria,motivated us to refine and complete this work. Thanks areextended to the co-organizers of this workshop JohannesLeitner and Maria Firneis and to two anonymousreviewers whose comment helped to improve the paper.

ARTICLE IN PRESSA.T. Basilevsky et al. / Planetary and Space Science 55 (2007) 2097–2112 2111

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