mjimih wrote:
Don't most rocky planets around our size or larger, produce "a" magnetic field bc of their hot liquid cores? How important is it that a magnetic field be present for surface life?
As stephen63 pointed out, Mars, which, is much smaller than the Earth, doesn't have a global magnetic field. This may have led to the gradual loss of its atmosphere.
More interestingly, Venus, which is almost the same size as the Earth, has a very weak magnetic field driven by an interaction between its upper atmosphere and the solar wind.
http://en.wikipedia.org/wiki/Venus#Magn ... d_and_core wrote:
In 1967, Venera 4 found the Venusian magnetic field to be much weaker than that of Earth. This magnetic field is induced by an interaction between the ionosphere and the solar wind,[64][65] rather than by an internal dynamo in the core like the one inside the Earth. Venus's small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation. This radiation may result in cloud-to-cloud lightning discharges.[66]
Unlike Earth, Venus doesn't have an intrinsic magnetic field at all. Venus' internal makeup must therefore be quite different than the interior of the Earth.
The lack of an intrinsic magnetic field at Venus was surprising given it is similar to Earth in size, and was expected also to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, while its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo.[67][68] This implies the dynamo is missing because of a lack of convection in the Venusian core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much hotter than the top.
Venus does not have plate tectonics. The same event that shut down its plate tectonics may have destroyed Venus' internal magnetic field and heated its surface.
On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This caused the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, no internal geodynamo is available to drive a magnetic field. Instead, the heat energy from the core is being used to reheat the crust.[69]
There may be other reasons why Venus does not have an internal magnetic field:
One possibility is Venus has no solid inner core,[70] or its core is not currently cooling, so the entire liquid part of the core is at approximately the same temperature. Another possibility is its core has already completely solidified. The state of the core is highly dependent on the concentration of sulfur, which is unknown at present.[69]
The lack of a protective magnetic field means that the solar wind has been able to interact with and erode the atmosphere of Venus in a way that it hasn't been able to do with the atmosphere of the Earth. This erosion has probably led to the loss of water on Venus.
The weak magnetosphere around Venus means the solar wind is interacting directly with the outer atmosphere of the planet. Here, ions of hydrogen and oxygen are being created by the dissociation of neutral molecules from ultraviolet radiation. The solar wind then supplies energy that gives some of these ions sufficient velocity to escape the planet's gravity field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, while higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind probably led to the loss of most of the planet's water during the first billion years after it formed. The erosion has increased the ratio of higher-mass deuterium to lower-mass hydrogen in the upper atmosphere by 150 times compared to the ratio in the lower atmosphere.
What I find particularly interesting about Venus is that it illustrates our human tendency to think that the Earth is a "typical" planet and that other planets "ought to be" like Earth.
In my opinion, many people are too eager to think that other planets can be expected to be like the Earth. So far we haven't found a planet that is like the Earth at all.
mjimih, you wrote:
Don't most rocky planets around our size or larger, produce "a" magnetic field bc of their hot liquid cores?
In our own solar system, the Earth is the only rocky body that has a global magnetic field, or so I believe anyway. The Earth is the largest and most massive rocky body in the solar system, so we can't use our solar system as a model for what may be "typical" for rocky planets larger than the Earth.
Personally I can't help thinking that the Earth may be a highly unusual case for a lot of reasons. Many astronomers believe that the Earth collided with a Mars-sized body when the solar system was very young, and this event is believed to have created the Moon. But consider, too, what such a collision must have done to the Earth. The titanic collision must have caused much of the heavier material of both planets to merge, and much of the lighter material was flung into orbit around the newly-created beefed-up Earth.
Most of the core of the colliding Mars-sized body must have ended up in the Earth. And the Earth has largest core and the highest density of the rocky bodies in the solar system. To me it seems very likely that the increased size of the Earth's core, coupled with the energy that was pumped into the Earth by the titanic collision in itself, has something to do with our own protective magnetic field. I really don't think that a magnetic field like our own can be taken for granted when it comes to Earth-sized planets in other solar systems.
Ann
. For some reason, that particular mammalian feature is indispensable.
