User blog:Cerne/How density works, and what I am aiming for.

Another early blog entry, but I am only typing this one because a few things came to my mind and I wanted to get them in before I move on.

In my last two entries I mentioned tweaking my planet's density...thinking more about this, I do not believe it is possible if I want to keep the gravity my planet has right now. I should have realized this sooner, but my planet's mass should be somewhere around the same as Earth's mass is. I increased my planet's density because I wanted the same mass - and hence the same gravity - that Earth has. Doing anything with my planet's density alone will change my planet's gravity. The good thing is that if I did want a planet that was lighter than Earth is in terms of gravity, I could merely change my planet's density and my problem will be solved. The bad thing is that this could consequently change a bunch of other things as well. I would need to recalculate mass, gravity, escape velocity (not so much a problem here - there is a site that will do this for you), and rotational velocity. I still want a fairly small planet, and a radius of 4,100km seems about right to me. Changing the radius would change too many other things, including many orbital characteristics, and I don't think I want to go through that again. I am almost tempted to leave my planet the way it is and see if I can get away with it. This is actually more of an understatement; I am more than tempted to leave my planet the way it is.

There are lots of things to think about that could affect whether my planet will still be Earthlike or not. Namely, will my planet even have Iron in it or something different? Another thing to think about is how radioactive the planet's core will be and consequently how dynamic it will be. Basically the planet needs to rotate; it needs to sustain some form of tectonic activity which will allow it to vent heat as it rotates; it needs a magnetic field that will do many things like protecting the planet's atmosphere from solar radiation and driving its tectonic activity; it needs a gravity that will hold on to an atmosphere, and jointly (for lack of a better word) a type of atmosphere that will stay with the planet. I am not really sure why having an exceedingly dense core would necessarily increase the core radius. Most of my problems concerning this particular scenario seem to have something to do with the distance seperating the asthenosphere from the lithosphere. Apparently, heat convection cannot occur if the asthenosphere and the lithosphere are in direct contact with each other. This would consequently cool the planet's mantle and make it tectonically dead. My solution, then, is clear: I need to decrease the core's radius. There are several ways I can go about doing this.

First, I can make the planet less dense. When I designed my planet, I used Iron for my planet's core because I wanted an Earth-like planet and an Iron core happened to be Earth-like.. Iron has a density of 8.3 relative to water, which has a density of 1. Meaning, If I decrease the core's radius from where it is, I decrease the core's density. And vice versa. This gives me an upper limit as to how much density I can have. Too much density means not enough room for heat convection within the mantle and not a thick enough crust to keep it in for very long. Not to mention a much higher gravity to work with.

The second thing I could do is to change the density of the element making up the core, which should in turn affect the inter-relationship between core radius and core mass, and which will consequently affect surface gravity. The general rule here is that a core with a certain size will be able to hold in more molecules of a less dense element because those molecules will be smaller. Likewise, the same core will not be able to hold in as many molecules of a more dense element because those molecules will be bigger. Molecules of a certain quantity with a lower density require more space while molecules with a higher density require less space.

Using elements that are less dense is out of the question because I would either need a larger overall radius to accommodate the larger size the core would need to be in order to have the same gravitational pull Earth has, or I would need to put up with a much thicker crust and probably a much lower gravity. Depending on the element, my planet's core might not be as dynamic when surrounded by a rock mantle and crust, and the tectonic activity might not be very substantial either.

I had always opted for an element that was more dense than Iron is to make up my planet's core. Understandably, I would be able to have a smaller core radius if I wanted the same mass and gravity. If the element in question was considerably more dense, my planet's radius would need to be that much greater to accommodate a core that was as big as Earth's core is. Which is not an option for me because I want a planet that is smaller than Earth is, not larger. I could also shrink the core, but once the rock mantle and crust of a terrestrial planet go beyond a certain thickness, it becomes more difficult for heat from the core to reach the surface of the planet. This could mean less volcanism, less tectonic activity, a slower rotation, and maybe more.

Of course there are other things to consider along with density. Each element also has an atomic weight which decides how heavy a given element is. Density and atomic weight do not directly correlate with each other so it is possible to increase the molecular density of something without actually increasing the object's overall weight. Now obviously, with something as big as a planetary core, the weight of the core will probably increase with its molecular density. But if, say, we wanted to replace the core's Iron with an element that was more dense but not as heavy, the core will not increase its weight as much as it would if the element replacing the Iron was also heavier. This may have an effect - no matter how small - on surface gravity. It may make surface gravity more difficult to predict, which may make it more difficult to work with when recreating surface conditions.

Another thing to think about is the element's melting point. Ideally, you want an element in your planet's core that will go from solid to liquid at a temperature range coinciding with the temperature that is achieved upon reaching a rotational velocity rapid enough to generate a magnetic field, which will in turn generate tectonic activity. In the past, I had opted for my planet's core to be made of Lead. At room temperature, Lead has a density of 11.34 g·cm−3, which is more dense than Iron, but its melting point is 1210 degrees lower (in Censius) than that of Iron. This means it will exist as a liquid longer under lower temperatures and will solidify too readily, making it a bad choice for convection once a rotational velocity is achieved that will accommodate it in its solid state. A planet would need to rotate much more slowly to support both a molten and solid state within its core. The silicate rock portion of the mantle would not heat as readily, and I am not certain whether the energy achieved from the core would be enough for any tectonic activity. I am not necessarily saying that Lead is a bad idea; I am only saying - for something that has a higher molecular density - that the transitional properties of Lead do not allow it to work as well as Iron does. In which case I should probably look for a better candidate if I want my planet's core to be made with something more dense than Iron.

For reference sake, Iron's atomic weight is 55.845; its atomic radius is 126 pm (picometres) and its density at melting point is 6.98 g·cm−3. Again, its density relative to water is 8. The element I plan to use should ideally exceed this if I want to increase my planet's core density, hopefully without increasing its core radius.

I am going to end this entry now. I feel I have explained and elaborated well enough on what I am trying to do with my conworld right now to give readers an idea of what I need to do and therefore hope to accomplish before proceeding into other areas of its creation as a background for my main conworld ideas. It should be worth the trouble, after all, for what is a story without its setting?

Thanks for reading.