What Happened to our Twin?

Venus has often been called Terra's twin (yes, I go with the original Latin name for our planet) because of the similarity in size to our own homeworld. Its diameter is 12,103 kilometres, as opposed to Terra's 12,756 waist line. Its mass is a noteworthy 82% of our planet, giving it a surface gravity of 0.91 G. Though the point could, and has been, heavily debated in the past, it's believed to orbit just inside Sol's habitable zone. (Yes, that's the Latin name for our local star.)

But that's where the similarities end. If Venus is our twin, it gives evidence to the whole idea of one good and one evil twin. The planet's atmosphere is about 96.5% carbon dioxide, as opposed to 0.039% in our atmosphere. The surface temperature on Venus is about four and a half times hotter than the boiling point of water. And let's not forget the atmosphere weighs down at 92 times heavier than our own. So, what is it that caused such a fundamental shift away from Terra?

The major difference is the geomagnetic field of Venus. It's different, in that it doesn't exist. In 1947, American physicist Walter Elsasser first proposed that our planet's geomagnetic field is created by electrical currents running through the planet's iron-rich molten outer core, otherwise known as a dynamo. His mathematical models showed that three things were required for this to happen: a solid inner core; a rotating, and electrically conductive, molten outer core; and, thermal convection from the core to the mantle.

Venus is believed, though the point is debated, to have a solid inner core. Evidence of lava flows on the surface shows it has a liquid mantle, and therefore likely to have a molten outer core. But what about heat transfer away from the planet's centre? The answer comes from a slightly surprising direction: plate tectonics.

The driving force of plate tectonics is convection currents in the mantle. Magma pushed up by these currents from the core slams into the underside of the crust, what is called the lithosphere, and spreads out, pushing the plates along with the current. The same physics that drives a summer thunderstorm is also responsible for earthquakes. But earthquakes are another thing Venus is missing - there's no active plate tectonics.

Because there's no great moving rafts of rock on the planet, it's fairly safe to assume there's no thermal convection in the mantle. Thermal convection currents, like wind, requires areas of both hot and cold for heat transfer to actually happen. If the heat on both sides is the same, no convection occurs. In 1995, a family trip to Pennsylvania led us to the Laurel Caverns. While there, I learned that the caves stay a constant temperature, even in winter, because of heat transferred up from several kilometres below.

But with Venus under a greenhouse effect gone psychotic, the transfer of heat stops cold. (No pun intended.) If no heat can be transferred from the mantle, then no heat can be transferred to the mantle, from the core itself. The field itself protects our atmosphere from the solar winds. Without it, the atmosphere would be stripped away, starting with hydrogen, which itself is required for that most basic substance, water.

So, the next obvious question is, could anything theoretically be done to cool Venus down enough for convection to resume? To answer that, I give a quote from the movie, I, Robot.

"That, detective, is the right question."