GEOCHEMICAL ASPECTS OF THE GEOLOGICAL

HISTORY OF VENUS:

GEOLOGICAL / GEOCHEMICAL DOMAINS

OF VENUS

A.T. Basilevsky1 and J.W. Head2

1Vernadsky Institute of Geochemistry and Analytical

Chemistry, RAS, Moscow 119991 Russia, atbas@geokhi.ru2Department of Geological Sciences, Brown University,

Providence, RI, 02912 USA, James_Head@Brown.edu

Venus Geochemistry: Progress, Prospects, and New Missions

February 26-27, 2009

Houston, Texas

GEOLOGICAL / GEOCHEMICAL DOMAINS OF

VENUS: Zooming to the thinnest

1. Whole planet

Emphasis on planet interior

2. Planet crust: Geologic bodies / units seen in the Venera 15-16 /

Magellan images

Tens kilometers – hundreds meters thick

3. Meters to centimeters thick surface layer: Eolian and “impact”

sediments seen in the Venera panoramas and implied from

the Magellan radiophysical properties

4. Millimeters to microns thick surface layer: Dust,

condensates, weathering products seen in the

Magellan images and implied from the IR data

Outer

core

Inner

coreCore

Lower

mantle

Lower

mantle

Upper

mantle

Upper

mantle

Whole planet

Earth-like model

of internal structure

of Venus based on

similarities in mass

and size of Venus

and EarthZharkov, 1992

It implies similarity

in bulk composition

of Venus and Earth.

Is this correct?

1

Cosmochemical

considerations

do not provide

certain answer

What can help:

• Seismic sounding

• More accurate

chemical analyses

of surface material

including trace

elements

Whole planet

From Kuskov, 2008

1

Geologic bodies seen in the Venera 15-16 / Magellan

images: Tens km to tens meters thick.

Basaltic plainsWhy basaltic:

Morphology suggest

fluid lavas, e.g.,Head et al.,

1992

Basaltic composition

of the surface layer

determined by the

Venera-Vega landers,e.g., Surkov, 1997

Basalts are eutectics for

manles of terrestrial

planets, Basaltic volcanism, 1981

• More chemical analyses with higher accuracy and including

trace elements // Very high resolution (1-2 m to 10-20 m) images

What we need:

2

Steep-sided domes

Why steep-sided:

=> Viscous lavas

Evolved composition

e.g. Dacites?

or Basalt with gas bubbles?

Tessera stands above basaltic plains Why stands above:

=> Less dence material

Evolved composition

Dacites? Granites?

or Thicker (and fractured)

bodies of basalts?

What we need:• Direct analyses of materials of steep-sided domes and tessera

• Very high resolution (1-2 m to 10-20 m) images

2

Meters to centimeters thick surface layer: Eolian and

“impact” sediments seen in the Venera panoramas

Magellan SAR images

Venera TV panoramasThinly layered material of “dark” parabolas

Basilevsky et al., 2004

3

Crater Stuart Crater Aurelia

Dark parabolas as a source of material for impact-

-eolian sediments seen in the Venera panoramas

Parabolas are formed by the meter-scale-thick flat-surface

mantle of loose material of fine-grained fraction of ejecta of the

crater which is in the parabola apex Campbell et al., 1992; Bondarenko and Head, 2004, Carter et al., 2004

3

Model dark parabolas, which could be a source of material

for impact-eolian deposits, and suggested landing sites

to study different geologic unitsBasilevsky et al., 2007

3

Landing site should be either outside of any parabola or within

the parabola whose apex crater ejected the material of interest

• In-situ mapping IR spectrometry to study mineralogy

• More chemical analyses with higher accuracy and including

trace elements

• Better knowledge of chemistry of lower atmosphere incl. P02

• In-situ high-resolution color TV images of the surface

What we need to better understand the nature of

the domain of meters to centimeters thick surface

layer:

3

Venera 13 image:

Mosaicing and

artistic rendering

by Don Mitchell,

http://mentallandscape.com

Maxwell Montes

Above some critical altitude, surface of very high mountains looks

very bright in SAR images and has very low radar emissivity

metal-like cover: condensate or chemical weathering product.

Pettengil et al., 1997; Wood ,1997; Starukhina nad Kreslavsky,

Millimeters to microns thick surface layer4

Slide provided by Gabriele Arnold, DLR, Germany, VEX VIRTIS Team

May be different

mineralogy /

chemistry of

micron-thick

surface layer?

Eolian sorting?

4

What we need to better understand the nature of

the domain of millimeters to microns thick surface

layer:• In-situ mapping IR spectrometry to study mineralogy

• TV microscope with color images

• Better knowledge of dynamics and chemistry of lower

atmosphere (including pO2)

4

Suggestions for future geochemical

measurements on Venus:

• Geochemical measurements should be supported by geologic

context (images) from regional to microscopic scale.

• To study composition of geologic units a presence of surface

layer, whose composition may be foreign to local bedrock,

should be taken into account.

• Chemical analyses should provide knowledge not only about

major petrogenic elements, but also about petrogenically

indicative trace elements such as Rb, Sr, Ni, Cr, Zr, Nb …

• As “cheap” solution (no sampling device) can be used gamma-

spectrometry, which provides knowledge about K, U, Th, and,

if combined with neutron generator, - about H, O Al, Si, ….

We acknowledge help of

G. Arnold, G.M. Kolesov, A.S. Kozyrev, O.L. Kuskov

and N.S. Muravieva

Thank you for your attention!