Graphic by Tiany Farrant-Gonzalez February 2019, ScientificAmerican.com 61
1 billion years agoTodayPlanet formation4 billion years ago3 billion years ago2 billion years ago1 billion years agoSunHabitable ZoneMercur enus Eart MarsPeriod of
heavy crateringLow levels of
cratering continueAstronomical units: 1.0 1.5Sun and planets
not drawn to scale 0.5TodayPlanetary Comparisons
at ma e a anet a ita e This is one of the biggest
questions in astronomy. Earth and Venus started out quite
similar, but one is now a bastion of life and the other an inhos-
pitable wasteland. By studying the development of volcanism,plate tectonics and other conditions on Venus, scientists hope
to understand why the planet evolved as it did and whether it
can teach us about the prospects for life on the many Venus-
like exoplanets throughout the Milky Way.VOLCANIC/
TECTONIC ACTIVITY
Volcanism and the related process of
shifting planetary plates called plate
tectonics play a large role in the
habitability of Ear th. They help sus-
tain our planet’s atmosphere and
oceans and provide the con tinents
needed to support life. Venus is the
only other inner planet with signs
of both phenomena today.LIQUID SURFACE
WATER PRE SENT
Water, the key ingredient for life,
was present inside the building
blocks of planets and was re -
leased to the surface via volcan-
ism, with lesser contributions
from cometary impacts. Aside
from Earth, Venus most likely
held onto its oceans the longest.CONDITIONS
FAVORABLE TO
DNA-BASED LIFE
Scientists can only speculate on
when and how long each planet
had the necessary ingredients
to host life. But researchers have
reason to believe Venus became
a habitable world before Earth and
spent more than a billion years
with the conditions needed for life.HABITABLE ZONE
This region is the area around a star
where planets could sustain liquid
water and thus potential life.
Be cause the sun has grown more
luminous over time, the boundaries
of the habitable zone in the solar
system have shifted outward.© 2019 Scientific American