18 February 2016

Mars
Mercury
Terrestrial
Planets
Venus
Earth
Moon

Hubble observes
Venus

Life is definitely possible in this atmosphere

But it is too hot at the surface!

Venus - Isolated continents, extensive volcanic plains.
• Plate tectonics? No. Explains Earth geology, but not Venus.
• Resurfaced ~600 million years ago

Volcanic
Resurfacing of
Venus

7000 km long lava river - Baltis Vallis longest river in the solar system
Extensive cracking from:
• loading by mass of lava
• shrinking on cooling
• stresses from underlying convection

Stresses from underlying plume pushing up crust from below

Arachnoid crater

Tick crater

Lava flows

Pancake domes

It does not look good for life: Too hot, poisonous gases

• Putting Earth habitability in context: Venus is like
Earth’s ‘twin’
• The possibility of extinct or extant life.
• The future of life on Earth.
• Planetary protection issues?

• Who kept its ocean longer, Venus or Mars? How was the habitable real estate distributed in the inner solar system?
• Understanding the longevity of oceans , and the range of physical parameters which facilitates plate tectonics, is key to defining stellar habitable zones.
• Further exploration of Venus will greatly advance this cause, and provide needed context for extrasolar terrestrial planet discoveries.

• ≈ 2 Gy Loss of surface water, subduction of hydrated sediments ceases.
• Lack of water makes lithosphere thicker & more difficult to break.
• Crustal recycling is inhibited: Carbon cycle does not provide a thermostat
• ≈ 1 Gy Plate tectonics ceases, Venus becomes a “ 1 plate planet ”
• 700 My to present: localized volcanism and tectonism
Venus may have been a habitable planet (with an oxygenated atmosphere) for much of Solar System history.

• Longevity of oceans remains highly uncertain. During an extended period of water loss, Venus probably enjoyed an oxygenated atmosphere.
• When young, the terrestrial planets were constantly exchanging material by impacts
• Favorable environmental conditions for origin or transplantation of life.
• As surface conditions became hostile, life could have adapted to an atmospheric niche under directional selection.

• In about 1 Gy, the Earth may experience a runaway
H
2
O greenhouse.

• Don’t confuse evidence for life with organisms brought from Earth
• Don’t pollute other planets with invasive Earth organisms
• Protect the Earth from life from other planets

• Can life survive on the target body?
• Might life have arisen there?
• Will the spacecraft land or just fly by?
• Will samples be returned to Earth?
• ‘Forward’ or ‘Back’ contamination?
• How to reduce the risk: we can’t be 100% certain…

• Sterilize spacecraft to reduce bioload: One sneeze contains a million microbes
• Don’t hit a possible life abode by chance
• Park the spacecraft in a high orbit
• Crash into some lifeless world or deadly location at mission end
• Quarantine samples returned to Earth

• Venus is a world like Earth, gone terribly bad from a greenhouse effect
• The planets were more similar when life arose on Earth: Venus likely had an ocean
• Early Venus life might still survive floating in the Venus clouds
• We must be responsible to not contaminate other worlds (UN treaty 1967)