Friday, 18 December 2009

"Anatomy of an alien": high gravity

A while ago, I showed a fragment of a 1997 BBC documentary called 'Anatomy of an Alien'. The fragment in question was about Epona, which was also what the post was about. The program contained discussions of more such extraterrestrial ideas, and contained short discussions with a variety of people involved in speculative biology, a term which I do not think had yet been introduced at the time.

Looking at the program makes you realise how fast computer graphics have evolved. Admittedly, the designers at the BBC probably did not have an enormous budget available, so they may not have been able to achieve the very best results technology could offer in 1997. In that period, 'Jurassic Park' was probably the yardstick you could get if you threw lots of money at the problem, and 'Jurassic park' dates from 1993. Still, the computer effects were certainly better than what an amateur could achieve.

I do not think that this means the program is no longer interesting to watch, so I decided to show some more fragments of the program. The one for today concerns life on high gravity planets. You will see an interview with Jack Cohen, a biologist with a strong interest in biology in science fiction. He has written a few books that all people who wish to design worlds should find interesting.  One I particularly recommend is 'Figments of reality'; a search on Amazon should result in several others.

In the video, Jack Cohen goes into the mechanics of legs for heavy worlds. He compares land-living crabs with sea-dwelling crabs to make the point that higher loads require stronger and more columnar legs, an effect encountered in this blog more than once. The resulting animation is quite nice. Another point that should be kept in mind is that high gravity is largely irrelevant under water. If your body mass is close to that of water, it will not take much effort to keep floating at the same height, and a relatively small swim bladder should allow you to change height at will. There's no risk of broken limbs from tripping under water.

On land, of course, things are quite different. In a truly high gravity the simplest trip could shatter your legs, so falling is something to be avoided at all cost. Even staying upright and walking requires legs that look different from those of animals of similar size on a low gravity planet (that's why I thought that Alex Ries' Birrin must live on a low-gravity world).

Click to enlarge (copyright BBC)

The video fragment shows a vaguely arthropod-looking animal, shown above, with a large number of vertically placed columnar legs. This makes excellent sense. The animal has large wings though, and that may seem surprising. Wouldn't high gravity make it more difficult to become airborne? It would, as a moment's thought reveals: staying aloft requires that weight, dragging an animal down, is exactly countered by the amount of lift pushing the animal upwards. If you double gravity and keep everything else the same, the situation is no longer in equilibrium, as weight is now twice as large as lift. Down you go.

But Jack Cohen makes the point in the video that high gravity may also make it easier to fly, by increasing the density of the air. It is indeed more easy to achieve lift in a soupy atmosphere than in a rarefied one, and vice versa. Disney's people knew that in 1957, as evidenced by the enormous wings of his Martian flying animals, designed to fly in the rare Martian atmosphere.

I checked some books and found that the amount of lift provided by wings is directly proportional to the density of the air. Here is the formula:

lift = 0.5 x density x wing area x velocity squared x lift coefficient

What that boils down to is that doubling the density of the air will double the amount of lift. That is nice: in the example above gravity was supposed to be twice as much as on Earth, so a doubling of lift is just what we need to keep the same animal in the air. Not that is at all likely that an animal living on a planet with twice the gravity and twice the air density could be the same as one living on a lighter world, but never mind that now.

So, to make things work, the only remaining question is whether doubling the gravity a terrestrial planet is compatible with doubling its air density. I have no idea. Comparing Venus and Earth suggests that similarly sized terrestrial planets can vary widely as far as their atmospheric density is concerned, so I guess a double air density is feasible. If anyone knows more about the relationships between gravity and likely atmospheric density of Earth-like planets, feel free to comment on this post.



Josh said...

I've often wondered about the relationship between an increase in gravity and atmosphere density and how that would affect flight. Nice to see that i'm not the only one.

j. w. bjerk said...

I can't provide the whole answer to the gravity/atmosphere question, but maybe i can provide some information.

The relationship can't be too simple. You have for instance, substantial bodies like the moon (1/6th G) that are too light to have any atmosphere. Then at the other end you have gas giants like Jupiter with less than 3 Gs of gravity, but unbelievable crushing atmospheric pressure.

Gravity and the amount of atmosphere determine the pressure.

The amount of atmosphere is determined by how much there was to start with, how much is being added (volcanoes, comets, etc.), and how much is leaking away (which again involves gravity, and also i think temperature).

To figure out how much atmosphere was there in the first place you can either make it up, or rely on IMHO highly speculative theories of planetary formation.

I realize this doesn't really answer the question. I too would be happy if someone can provide a better one.

Dominic said...

I don't suppose you have a link to a full version of the BBC programme? Been trying to find it for ages with no luck.

Sigmund Nastrazzurro said...

Josh and J.W.:
I've asked people and am waiting for a response. my guess is that we simple have too few data to draw firm conclusions about atmospheric density and gravity, so, for 1.0 Earth gravity, anything between 0.2 and 5.0 Earth density I would believe (and wilder geuesses too, I think).

Dominic: No; I have not find a complete entry either. I'm happy to own a personal copy...

Evan Black said...

I've read Stephen Gillett's World Building and, comprehensive as the book is, it doesn't give a nice tidy formula for calculating planetary density based on other factors. It talks about air density, and even includes it in other factors, but it doesn't have anything to determine exactly what the density is and how it got that way. For my own project I've just fudged the density numbers (with a little research) to suit my needs.