Thanks for the link. If that theory has legs - and on first and second read-throughs I have some doubts - then my suggestion that Mars might not have been massive enough to sustain the dynamo might also have some merit. But, whether it's correct or not, I'm very happy to learn that some have come up with possible answers to this.
Found a link which may (or may not) address your question to:
Planetary Science
Interesting points (Magnetic Dynamo theory): [snipped quotes, read the link for full description]
1. Venus has no magnetic field (or one so weak, it hasn't been detected yet). It probably has a liquid conducting interior for a couple of reasons:
(a) Since it is almost the size of the Earth, its interior should still be very warm. Larger planets lose their heat from formation and radioactive decay more slowly than small planets. A planet with a larger volume than another planet of the same composition will start off with a larger supply of heat energy. In addition, the heat in a large planet's interior has a great distance to travel to reach the planet's surface and the cold outer space.
The
rate of heat loss increases with the surface area. A planet with a larger surface area than another planet with the same internal temperature will have a larger rate of heat loss. The time it takes for a planet to cool off depends on the total amount of heat stored/rate of heat loss or (its volume)/(its surface area). Recall from the
planet volume section that the volume increases as the diameter3. The surface area increases as only the diameter2, so the planet's cooling time increases as diameter3/diameter2 = diameter. Even though its heat loss rate is greater, a larger planet has a much larger amount of energy stored in it and, thus, it will take longer to cool off than a smaller planet. Venus should have a iron-nickel core that is still liquid like the Earth's.
(b) High resolution radar imaging of Venus' surface by the Magellan spacecraft shows several places where volcanos have erupted recently and produced large lava flows. Recent
infrared imaging of Venus' surface by the Venus Express spacecraft shows material that has just come out of some shield volcanoes, so Venus is still active.
The reasons why Venus does not have a global magnetic field are that it spins very s-l-o-w-l-y (about once every 243 Earth days!) and the absence of convection in the liquid core (probably because of the lack of plate tectonics for the past half billion years).
2. Mars has an extremely weak magnetic field but for a different reason than Venus. Mars is about half the diameter of the Earth and has about 1/10th the Earth's mass, so its internal heat should have disappeared to space long ago. So even though Mars spins quickly (once every 24.6 hours), its metallic core is mostly solid---the charges are not able to swirl about. A
recent gravity field map shows that Mars has a liquid outer core of molten rock. Mars' crust is also probably too thick for plate tectonics to occur even if the core had not cooled.
3. Earth has a strong magnetic field because it spins fast (once every 23.93 hours), it has a liquid conducting core made of liquid iron-nickel, and it has plate tectonics.
4. Jupiter has a HUGE magnetic field. Jupiter has a large amount of hydrogen that is super-compressed to form the strange liquid called liquid metallic hydrogen. This material cannot be produced on the Earth because the super-high pressures needed to squeeze some of the electrons out of liquid hydrogen cannot be produced. Jupiter also spins very quickly---one rotation in under 10 hours!
5. Mercury is a bit surprising because it has a weak magnetic field. Mercury is the smallest of the terrestrial planets, so its interior should have cooled off long ago. Also, Mercury spins slowly---once every 58.8 days. Mercury's high density tells us that it has a proportionally large iron-nickel core. Its magnetic field implies that Mercury's interior is probably partially molten. In mid-2007 astronomers announced independent evidence in favor of a molten core for Mercury. Using very careful observations of Mercury's rotation, they found that Mercury's core could not be solid (
see the next section for more on this technique).
PS--Thanks for raising this question! Found the information about Venus fascinating.
