VENUS

VENUS: EARTH'S TWIN SISTER

Mass is 0.815 that of Earth (via artificial satelites)

Radius is 6052 km or 0.95 of Earth (via angular diameter at conjunction)

Density is 5.2 g/cm^3 or 0.94 of Earth.

As seen from Earth:

Venus's surface is invisible in the visible band---completely cloud shrouded.

Always within 47 degress of the Sun --- greatest elongation, so
NEVER seen more than about 3 hours after dusk or 3 hours before dawn.

Brightest object in the sky after the moon; as a crescent (about 38 degrees) from Sun.

UV markings on clouds show fast rotation (around planet in ~4 days).

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Radar measurements from Earth showed very slow, retrograde rotation:

rotational period (or sidereal day), R = -243.0 days compared to

orbital period, P = 224.7 days, combine for

solar day, D = -116.7 days.

If we define angular speeds in degrees/day, the rotation speed is:
360/R and the revolution speed is 360/P.

A solar day passes when the spin ``laps'' the orbit --- goes another 360 degrees.
This immediately leads to:

1/D = 1/R - 1/P

Note if P >> R (as for Earth) then D ~= R
(solar day just a little longer than sidereal day)
But, if P = R (as for Moon), D = infinity, implying synchronicity.

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VENUS' SURFACE

Radar from Earth could also reveal crude topography:

the relative elevation of large portions of the surface via reflection times;

relative roughness of the surface via reflection strengths.

SPACE MISSIONS HAVE TOLD US MUCH MORE, via DETAILED RADAR MAPING

Venera (USSR), Pioneer Venus & Magellan (USA)

Two ``continents'': Ishtar Terra, Aphrodite Terra (8%)

lowlands have modest elevation changes compared to Earth
but Maxwell Montes rises 14 km above lowest depressions.

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VENUS' ATMOSPHERE

Data from Pioneer Venus and Veneras imply it is
MUCH HOTTER & DENSER THAN EARTH'S

P_V = 92 P_Earth = 92 atm;

P drops to 0.1P_V around 30 km up from the surface.

T_V = 730 K --- runaway greenhouse effect (hotter than Mercury);
the thick atm. means this temperature is nearly uniform around
Venus and there's little difference between day- and night-side T's.

Fast winds (300--400 km/h) above 70 km;

slow surface winds: < 10 km/h
Even those slow winds would FEEL like a hurricane, since the atmospheric density is so high.

ATMOSPHERIC COMPOSITION

96.5% CO_2; most of rest N_2.

Above troposphere (to 30 km) there exists a H_2 SO_4 haze

Clouds, mostly sufuric acid drops, between 50--70 km.

More H_2 SO_4 clouds seen even higher.

On Venus, these thick clouds reflect most of the intense sunlight back,
but the fraction (about 1/2) that reaches the surface heats it;
The reradiated IR from the surface is MOSTLY TRAPPED BY THE CO_2 ATM.
This is the RUNAWAY GREENHOUSE EFFECT.

SO, you wouldn't want to live there: it's too hot, the pressure will
crush you and your rain is sulfuric acid!

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ACTIVITY ON VENUS

Many shield Volcanos are seen via radar images.

Evidence for current volcanism:

Fluctuations in SO_2 concentrations

radio energy bursts -- lightning

No evidence for tectonic motions.

Many craters, many the calderas of dead volcanos.

Impact craters too, but lots fewer than Mercury or Moon
and none smaller than 3 km across --
pretty big meteoroids burn up in Venus's atm. and sulfuric acid rain
and heavy winds produce lots of erosion.

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INTERNAL STRUCTURE

Probably very similar to Earth, with partially molten Fe/Ni core.

BUT NO MEASUREABLE MAGNETIC FIELD:
because of much slower rotation, despite likely liquid magnetic material.

ALSO, less internal heat and a hotter surface --> less convection.

So no current tectonic motions, despite active volcanos.

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SPACECRAFT LANDED ON VENUS

Measured atmospheric composition, wind speeds, etc.

6 VENERA LANDERS came close and 3 landed and sent back data.

They took photographs in (the very low) visible light on the surface of Venus
and analyzed rocks.

most basalt (volcanic); some ancient granite

some sharp edged, ---> relatively young;

many small & rounded, ---> erosion

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GREENHOUSE EFFECT ON VENUS

On Venus, the thick clouds reflect most of the intense sunlight back,
but the fraction (about 30%) that reaches the surface heats it;
The reradiated IR from the surface is MOSTLY TRAPPED BY THE CO_2 ATM.
This is the RUNAWAY GREENHOUSE EFFECT.

The temperature rises until the IR radiation from the surface and atmosphere
can balance the (mostly) VISIBLE radiation coming from the Sun.

On Venus this leads to the nearly constant surface T of 730 K.

On Earth, the CO_2 produced from burning fossil fuels is causing
our atmospheric T to rise.

HISTORY OF THE VENUTIAN ATMOSPHERE

Intially Venus and Earth had similar primary atms,
which were mostly H and He: lost quickly.

But should have had similar secondary atmospheres from
volcanic outgassing: CO, CO_2, H_2 O, H_2 S, etc.

Since Venus was closer to the Sun, it would be hotter.

More water vapor could be held in Venus's atm.

This water trapped IR radiation, rasing the temperature more.

Now it is so high that any liquid water evaporated, adding to
water vapor in the atm, increasing the T even more!

The CO_2 also added to the greenhouse effect and now dominates,
with most of the H_2 O having evaporated by now.

Where's the Carbon Dioxide?

For Venus: in the atmosphere.

For Earth: only a little in atm; most either dissolved in
the oceans or precipitated into sedimentary rocks (e.g. calcium carbonate).

SO, the total amounts of CO_2 are similar on Earth and Venus.

Where's the Water?

For Venus: a little bound in the atmosphere (H_2SO_4); almost all evaporated.

For Earth: most in the oceans, much trapped in rocks (hydrate molecules)