11.2.1 - The Challenge of Gravity

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

The strength of biological building materials sets an upper limit on size. Galileo once calculated that a tree taller than about 100 meters (on one-gee Earth) must buckle under its own weight. This is because its cross-sectional strength would be insufficient to stabilize such a tall mass against collapse. (Some sequoias, which slightly exceed this theoretical limit, are nevertheless close to the maximum height attainable using woody materials.)

The typical loads sustainable by animal bone are about: 600 atm laterally (shear strength), 1000 atm in longitudinal tension, and as much as 1700 atm in compression.^204,1730 These should be compared to the following: 340 atm for Douglas fir, 540 atm for hard-burned brick, and 5400 atm for cold-rolled steel.^48,924 Clearly, bone is an excellent building material.

The villain of this story is, of course, gravity. This force at the surface of any world determines to some extent the maximum mass and size of the animal life.

One way to meet this challenge is to take to the water. In the sea, the force of gravity is partly cancelled out by the opposing natural buoyancy of immersed bodies. There are no obstacles to large structures per se.

But subtle problems arise when sea creatures grow too large. It has been suggested that inertial mass, rather than weight mass, may be the limiting factor. That is, the larger a body in motion is, the more it wants to remain in motion. An extraterrestrial leviathan larger than a whale would experience severe steering, braking, and turning difficulties. Cornering too fast could easily exceed the strength of the building materials, and snap the behemoth in two.⁴⁰⁰

There are also ecological considerations. Larger aliens will generally eat more than smaller ones, all else being equal. Yet at the same time it is getting huger, the organism is also getting bulkier and less maneuverable. The animal needs to spend more and more of its day feeding. Indeed, the largest whales must drive incessantly through food-laden waters in order to meet the severe metabolic demands of their ponderous bodies.

Further, note that the mass which must be fed increases as the cube of the linear dimension, the nutrients must be absorbed through the surface area of a gut which increases only as the square of the linear dimension. The Square-Cube Law thus predicts that at the same time it is becoming harder to ingest food fast enough, it’s also becoming harder to actually utilize what is eaten.*

What is the largest skeletonless creature that can exist? We really don’t know, but in the ocean the answer is -- reasonably large.

Life without a rigid frame offers advantages difficult for humans to fully appreciate. Many molluscs such as the squid and octopus have pretty much lost their ancient shells and have become essentially skeletonless lifeforms. Octopuses can stretch themselves quite thin, passing rubberlike through small holes and narrow crevasses. Arms, eyes, and even head can alter shape and elongate when necessary. The octopus has been called "the supreme escape artist," and is known to be able to walk across desktops and the decks of ships.

But a creature of the land must be a creature of gravity. Whether resting on the surface or traveling across it, alien organisms must find some means of support or be reduced to a groveling mass on the ground.

As long ago as 1917, the well-known zoologist D’Arcy Wentworth Thompson speculated on the effects on evolution of altering the planetary gravity. "Were the force of gravity to be doubled," Thompson declared,

The concept of the small, squat, muscular high-gravity beasts and the tall, wiry, frail low-gravity beasts has been tediously reiterated by generations of writers.

First, we may properly assume that the strengths and densities of biological building materials are roughly the same on any terrestrial planet in our Galaxy. And it is certainly true that the maximum mass of a living organism cannot exceed the crushing strength of its bones. But it is very important to bear in mind that this observation is applicable only to maximum size. Indeed, few Earthly animals exist at or even near the theoretical maximum.

This is because animals, extraterrestrial or otherwise, are designed for motion. They must be able to withstand the peak pressures and accelerations encountered during running and jumping. Standing at rest, for example, a horse seems greatly overbuilt. But on the racetrack, where it may pull to a halt in as little as 0.3 second (near the breaking point of its bones), its design limits are more fully exploited.⁴⁰⁰

Second, it is relatively straightforward mathematically to demonstrate, from the simple laws of Newtonian mechanics, that the maximum height of animals on a planet is inversely proportional to gravity. That is, height ~ 1/g.^214,1309 Similarly, we can show that the cross-sectional area of supportive bone must increase directly with ~g, the bone radius as ~g^1/2, and the maximum mass as ~1/g³.

We recall from an earlier chapter that the most massive of all terrestrial worlds should not have a surface gravity in excess of 2.2 Earth-gees. (Although self-heating starless planets or superjovians could have tremendous forces, even monstrous Jupiter only musters 2.64 gees.) Luna, whose gravity is too feeble to hold sufficient oceans or atmosphere for life, checks in at 0.16 gees. Apparently the range of plausible terrestrial habitats for life spans a single order of magnitude of gravitational force.

With this in mind, let’s look at the maximum size of land animals here on Earth -- a typically exotic, standard one-gee planet.

The largest land creature alive today is the African elephant, weighing in at an impressive 6600 kg. But larger animals have trod the soil of our world. The Baluchitherium, an extinct land mammal, had a total mass of well over 12,000 kg. Tyrannosaurus rex, the largest land carnivore, was about 13,500 kg.²⁴⁰⁹ The largest land animal ever was the Brachiosaurus, weighing an estimated 45,000-78,000 kg, but we’ll ignore this majestic brute because it is believed that he had to spend a great deal of time sitting in swamps resting his tired bulk.

We shall hazard a crude guess, from these data alone, that the heaviest viable exclusively land-dwelling creature that can plausibly be designed on an average one-gee world is about 20,000 kg. (The precise value selected doesn’t affect the conclusions very much.)

Applying the aforementioned maximum mass ~ 1/g³ relation, we discover the following: The weightiest alien animal that can inhabit a 2.2-gee world might be about 1900 kg. On a tiny 0.16-gee world like Luna, the largest creature could be nearly five million kilograms (though I’d hate to try to keep it fed).

So on the heaviest of all reasonable terrestrial worlds, animals such as walruses, small elephants, and even 70 kg humanoids are not excluded. On massive, 2.2-gee planets, all animals the size of hippos or smaller will certainly be possible with a modicum of redesign -- no need to call for "powerfully built, squat creatures, perhaps rather like an armoured pancake on multiple legs... limited to slow, creeping motions across the surface."^41,45 There is no reason why such relatively minor alterations in surface gravity should drastically affect the allowable sizes of typical alien animals.

This is not to suggest that gravity won’t control the construction of large ETs to some extent. It is true that, in any given mass category, the members of an animal species evolving on a high-gee world will have shorter, stockier bones than those evolving in low-gee environments. We can estimate how large this effect might be.

Consider the form of man. A typical human femur -- the most perfectly cylindrical and the largest single bone in our bodies (found in the thigh) -- is perhaps 3.5 cm in diameter. From the bone radius ~ g^1/2 relation noted above, we find that the femur should increase to 5.2 cm on a 2.2-gee world, or fall to 1.4 cm on a 0.16-gee world, to provide equivalent support for a 70 kg human body mass.

Such changes would probably necessitate major alterations in bone distribution, structural stress loading, and internal organ design. Experiments have shown that animals reared in high gravity environments tend to grow slightly thicker than normal bones, stronger hearts, and to lose fat. (See especially Kelly et al.,¹³⁰⁹ Oyama and Platt,²⁴¹⁸ Smith and Kelly,²⁴¹⁹ Steel,²⁴¹⁷ and Wunder.²⁴¹⁶)

But by and large, alien creatures should not appear grossly over- or under-built as compared with Earthly lifeforms of comparable mass.

* There is another still more subtle twist to the story of pelagic lifeforms. On a high pressure world (which may or may not correlate with higher gravity), more gaseous oxidant (e.g., oxygen) will dissolve in the water in the oceans. The amount of O₂ dissolved at any given temperature is directly proportional to the atmospheric surface pressure (Henry’s Law). So on a world with a hundred times the Earthly partial pressure of oxygen, a hundred times more can dissolve in the sea. Warm-blooded fish should be common. Cold-blooded fish without gills could also exist, breathing directly through their skin like the earth worm and the salamander of Earth. On a cold oceanic planet these effects would be even more pronounced, since oxygen dissolves more readily in cold water than in hot.^86,2513 And, of course, more oxygen means larger bodies.