- Venus has nearly the same diameter and density as the Earth
- Its interior composition and structure is probably similar
to the Earth's (rocky mantle and iron core).
- Venus orbits the Sun at a distance of about 0.7 AU
- One might expect that the surface of Venus would be
slightly warmer than the Earth's surface.
- Venus is surrounded by an atmosphere that is 90 times
more massive than the Earth's atmosphere.
- The primary constituents are carbon dioxide (96%) and nitrogen (4%).
- Unlike Earth, apparently Venus never had widespread oceans to disolve the carbon dioxide.
- The carbon dioxide is responsible for a strong greenhouse effect.
- The surface temperature is 750 oK (900 oF).
- The Venusian clouds are composed largely of droplets of concentrated sulfuric acid.
- The Earth and Venus are similar in many other respects. They
must have originally outgassed similar atmospheres. Why are they so
- Water! -- Liquid water oceans on
the Earth removed CO2 from the atmosphere.
- Venus likely initially had water (since it is, in bulk,
a twin of the Earth) but its proximity to
the Sun may have prevented it from keeping the oceans needed
to remove the carbon dioxide from the atmosphere.
- Water vapor is also a strong greenhouse gas.
- Venus may have briefly had oceans but, being warmer,
the oceans evaporated more rapidly than Earth, contributing
to an even stronger greenhouse effect.
- The even warmer temperatures increased the evaporation
rate leading to more heating, and so on...
- The water molecules in the atmosphere were eventually broken apart by ultraviolet light from the Sun and the hydrogen escaped into space or
combined with atoms with other compounds.
- Since the heavy form of hydrogen, deuterium, is less easily lost, we might expect that Venus has an overabundance of deuterium today.
- Indeed, deuterium is ten times more abundant relative to hydrogen on Venus compared with Earth.
- Venus has forever lost an ocean of water due to the "runaway" greenhouse effect.
- In a fanciful scenario, if oceans could be introduced to Venus today, the planet
could be transformed (over the course of tens of thousands of years),
into a more temperate place.
- This gross modification of a planet to make it more Earth-like
is called teraforming.
- Impacting thousands of comets on Venus might yield the desired effects, but it would take a long time.
- The sulfuric acid clouds of Venus entirely hide the surface from view at visible wavelengths.
- Unlike Earth, Venus does not show a clear distinction between "continents" and "basins".
- The surface of Venus appears "young" and possibly geologically active
- Only about a thousand craters can be found on the surface (compared with about 200 known craters on Earth).
- The craters are all very distinct indicating that erosion is not nearly as
effective as on the Earth.
- The craters are all large, demonstrating that objects smaller that
about 1 kilometer in size are destroyed in the thick Venusian atmosphere.
- The uniformity of cratering across the surface indicates that the entire planet may have been
resurfaced a few hundred million years ago or is undergoing continuous resurfacing nearly everywhere.
- The radar images reveal a number of volcanoes, many with apparently young
- The crust is heavily crumpled and fractured suggesting that it is quite thin.
- The high surface temperature of Venus may contribute to its significant geological activity.
- Since the crust is heated strongly from above and below, it may be very thin and susceptible to fracture.
- Such a thin crust cannot sustain large "plates" as is seen on the Earth.
- Interestingly, an alternative view suggests that the lack of water has
made the upper layers of Venus so rigid that plate formation and movement
cannot occur. Venus resurfaced itself catastrophically 500 million years ago and has been relatively inactive since. Heat tends to build up inside the world between resurfacings, finally reaching the point of breaking through the rigid surface.
- Venus rotates very slowly (and BACKWARDS).
- The sidereal day (true rotation period) is 243 days long
- The synodic day ("sunrise to sunrise") is 117 days long.
Revised November 14, 2007