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

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

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


5.4.1 Climate and Weather

We’ve already hinted at the effects of evolutionary history on a planet’s surface temperature. What else can be said about the overall climate? First of all, the thinner the atmosphere the greater will be the diurnal variations in temperature. This is because a dense, massive atmosphere has more ”thermal inertia.” Since huge amounts of heat are stored, a brief nighttime cooling-off period has very little effect. But if the air is thin and lightweight (as on Mars), very little heat is reposited. Thus, on the night side the surface and the air above it cool rapidly, leading to large swings in temperature between the two sides of the planet. This results in faster-moving winds (Table 5.9), but because the air is less dense the energy available is actually less.


Table 5.9. Wind Speed and Planetary Surface Conditions for Terrestrial Planets1566,2066,2087
Temp. Differential
Wind Velocity
Driving Energy
5 x 10-3
< 2 x 10-6


Perhaps one of the most decisive factors in planetary meteorology is the rotation rate of the planet. On a planet such as Venus, where a single “day” lasts months, surface winds are believed to be no more than a few kilometers per hour, maximum.1257,2041 On worlds with intermediate rotation rates like Earth and Mars, typical wind speeds range around 50-70 km/hour.1257,2067 Fast-spinning bodies like Jupiter are known to have winds averaging 140-290 km/hour and higher near the equator.1141,1257,2045 Naturally, faster rotation and stronger winds means larger Coriolis forces, along with more violent cyclonic disturbances such as tornadoes, hurricanes, typhoons and water-spouts. Also, slow worlds tend to have greater day/night thermal differentials than faster ones because the air is not as well stirred, Surface temperatures are less uniform as a result.214

The heat capacity of the molecules in the atmosphere is also important. This may be thought of as the amount of energy which must be added to a unit of air to raise its temperature a fixed amount. It can also be conceptualized in terms of energy loss: How much heat must be lost to drop the atmospheric temperature one degree?

An atmosphere like Earth’s in every respect but comprised of hydrogen would have nearly fifteen times the heat capacity of normal air. It would thus take fifteen tithes longer to heat up or cool down, so surface temperatures on a hydrogen-atmosphere planet should be pretty much the same every where.1257 There would be little if any “climate” as we know it on such a terrestrial.1257

The presence of oceans affects the climate in many ways. Largely pelagic worlds should experience smaller variations in surface temperature because the water acts as a giant thermal buffer.286 On dry worlds, the climate is likely to be more “continental,” or desert-like.214 With no seas, meteorology becomes more volatile -- weather changes more rapidly.

Many other factors are important too. The winds are driven by the energy supplied fun a planet’s star. Worlds near the inside edge of the habitable zone should therefore have more violent weather, because more energy is available. Unfortunately, life is more complicated than this because of the vagaries of atmospheric evolution, albedo differences, and the problem of self-heating planets (like Jupiter and Saturn).

Another factor which is extremely complicated is the effect of planetary mass and surface gravity on wind and air pressure patterns. If Dole’s empirical relation between mass and angular momentum holds up,* then it is a fair guess that worlds with high mass will have higher velocity winds, in general. And there are other, more subtle problems. For instance, the winds on Mars often blow at more than half the local speed of sound. One wonders what a “transsonic meteorology” might be like.2037

Some insight into comparative meteorology can perhaps be gained by looking at the peculiar manifestations of weather on other planets in the solar system. Mars has global-scale storms the likes of which have never been seen on Earth. Most every Martian year, dust storms enshroud the entire world in a dull-ochre blanket for months on end. Winds exceed 320 km/hr during this time -- far in excess of most Earthly hurricanes. Yet Mars has roughly the same rotation rate as our planet, is colder and farther from Sol, and has a thinner and less massive atmosphere. How can such a magnificent storm develop?

A small, natural cyclonic disturbance is where it all begins (Figure 5.8). Airborne particles absorb more sunlight and heat up the surrounding gas; outside of this local turbulence the air is cooler. The temperature differential causes major winds to begin to circulate. While hurricanes on Earth are caused by water vapor condensation near the eye, Martian hurricanes get their energy directly from the sun.2044


Figure 5.8 Different Patterns of Cyclonic Meteorology

Baroclinic Flow:
Climate powered by large temperature differential between equator and poles (DT > 10-100C). Vertical pressure gradient minimal.
Characteristic of:
  1.  Planets with low pressures.
  2.  Planets with slower rotation.
  3.  Planets with negligible internal heating, or which are heated from above (e.g. an optically thick atmosphere).
  4.  Planets whose atmospheric constituents have relatively low heat capacity (e.g. O2, N2). 
  5.  Planets having a solid surface.

CALMS are regions of “Coriolis pileup.” unstable with little wind, source of cyclonic disturbances (hurricanes). Cold, dry air falls, removes low altitude moisture, creating most of world’s deserts.

DOLDRUMS -- moist, warm, rising air causes cloud cover “zone” at Equator 10o latitude.

Low and high PRESSURE REGIONS form into localized eddies and whorls.

FEATURES persist for weeks (Earth) or for months (Mars).

Typical examples in our solar system:
   EARTH, MARS (especially in Martian autumn and spring),

   VENUS (single Hadley cell, “symmetric” regime circulation)


Barotropic Flow:

Climate powered by vertical pressure gradient forces. Temperature differential between equator and pole minimal (DT < 5C).
Characteristic of: 
  1. Planets with high pressures.
  2. Planets with fast rotation.
  3. Planets with significant internal heating.
  4. Planets whose atmospheric constituents have relatively large heat (e.g. capacity (e.g. H2, He).
  5. Planets with no solid surface.
ZONES contain moist, warm, rising air.

BELTS contain dry, cool, falling air.

WINDS flow around planet at zone/belt boundaries.

Low and high PRESSURE REGIONS girdle planet in a series of concentric zonal systems.

Atmospheric FEATURES can persist for centuries because there is no solid surface below the weather, and therefore an real frictional drag.

Typical example in our solar system: JUPITER, SATURN



Earth has a relatively massive atmosphere with large thermal inertia, so temperature changes occur only very slowly. Our planet thus has a long “response time” to change. Not so on Mars. The Martian air responds to changes in temperature in a matter of hours, because its thermal inertia is low. Winds can build up much faster.

The cyclonic disturbance grows larger and the winds go higher still. One planetologist has estimated that once the turbulence extends about ten kilometers vertically and perhaps 50-90 kilometers horizontally, the storm cannot be stopped,1313 A kind of “runaway weather,” the Martian hurricane continues to grow until it virtually covers the globe. At this point, the thermal gradient which drives the winds lessens and finally disappears, and the storm soon begins to taper off.**

Science fiction writer Arthur C. Clarke has considered an unusual form of weather that might exist on cold terrestrials (like Titan), which are thought to possess large amounts of solid ammonia and gaseous methane. We know that the smaller the liquidity range of a thalassogen, the more volatile will be the meteorology. Sudden weather changes should be commonplace. As an example, liquid methane may be present in small pools on Titan in local cold spots on the surface. Because it has such a narrow liquidity range, the methane could abruptly flash into steam at the first gust of warmer air or if there is a momentary break in the clouds. The high winds thus generated, Clarke suggests, might be called “methane monsoons.”1947

Another hard science fictioneer, Hal Clement, has written of the peculiar behavior of weather on planets with very high surface pressures. Gases -- and air -- are generally at least a thousand times less dense than liquids. But what if we have an atmosphere with a base pressure from 100-1000 times Earth-normal? The air will take on liquid-like densities, becoming thick and viscous.1936

What can we say about the presence of frozen thalassogen on the planetary surface? It is well-known that for the greater part of its history, Earth was without polar icecaps. We have them now only because we are in the middle of an Ice Age. Ice Ages are believed by some to occur cyclically every 200 million years or so, triggered by small changes in Sol’s output or by orbital and rotational resonances.2068,3678

(Of course, icecaps need not form only at the poles. A tidally-locked, one-face planet might have a single icecap on the night side only. Or, peculiar resonances between planetary rotation rate and orbital eccentricity could give rise to icecaps located on either side of the equator -- although this remains a strictly speculative possibility.2070)

Will all planets with open oceans have icebergs? The answer to this deceptively simple question actually has deep climatic significance. We know that the present climate of our world is in a state of very delicate balance. Surface conditions are largely dictated by the overall energy balance. The greenhouse effect acts to hold heat in and trap energy; Earth’s shiny polar caps tend toward the opposite extreme, reflecting energy back into space and cooling the planet.

Icebergs are floating chunks of frozen thalassogen. This proves to be a destabilizing factor in Earth’s climate, because ice reflects energy away far better than the liquid water of the oceans. If there is a prolonged, unusually cold spell planetwide and abnormally great: amounts of ice are produced, more of Sol’s life-giving warmth is cast away by the highly reflective ice floating on the surface. Our planet cools because less heat is available. The icecaps spread, and Earth cools still further. The effect is the exact opposite of the runaway greenhouse discussed earlier, and might properly be termed “runaway icecaps” -- an Ice Age.

On the other hand, if the solid form of the thalassogen is less reflective (i.e. darker) than the liquid, the climate should be relatively stable. Any ice formed during a sudden cold snap must subsequently absorb more energy than the surrounding liquid -- and soon melt. Icecaps would be unlikely, Ice Ages practically impossible.

Similarly, if a thalassogen cannot form floating icebergs, then even if the ice is highly reflective it still will submerge below the surface of the liquid before it can give rise to thermal instability and runaway icecaps. That is, it moves itself out of the way before it can do much damage. Of course, one man’s bread is another’s poison. The lack of icebergs may promote a more stable climate, but it will also make biology much less likely.

If there are no icebergs, and frozen thalassogen sinks to the ocean bottom because it’s denser, then the sea may freeze from the bottom up and thaw only from the top down. Over the normal range of temperature variations, it is entirely possible that the whole body of liquid could freeze solid for various lengths of time. This is xenologically significant, as the viability of life in such an inimical environment must necessarily be greatly decreased.47,1551

Water is virtually unique in this respect: The frozen form, water-ice, floats atop the liquid form. Water expands slightly when it freezes, so the ice is less dense than the fluid. (Only elemental bismuth metal and a very few other rare substances display this behavior.) Hence, where water is the thalassogen, bergs will float and life is not precluded by the threat of a planetwide oceanic freezeup during cold spells.974 (The price paid for this advantage is climatic instability -- it would appear that Ice Ages are possible only if water is the thalassogen.)

Not so with all other thalassogens of interest. As we see from Table 5.10 below, no other single thalassogen has the unique property of floating iceberg production. Even if we allow for a dual thalassogen system, say of ammonia and methane,1947 it is rather difficult to arrange for icebergs or floes of solid ice. Ammonia-ice not only sinks in liquid ammonia, but in liquid methane as well.***

But there are a few possibilities. Water icebergs should float on oceans of liquid oxygen, as should methane and ammonia bergs. Water-ice will also float on carbon dioxide seas at the right pressures. But sulfur, hydrogen, carbon dioxide and oxygen floes are probably out of the question on any kind of reasonable planet.


Table 5.10. Densities of Some Thalassogens of Interest2062,2063,2069
Liquid Density
Ice Density
Hydrogen   14.0   20.6   0.0708    0.0807
Methane   90.7 111.7   0.415 ~0.5
Ammonia 195.4 239.8   0.683   0.81
Hydrogen Chloride* 158.3 188.2   0.95   1.51
Water 273.1 373.1   1.00   0.917
Hydrogen Fluoride* 190.0 292.7 ~1.1 ~1.3
Carbon Dioxide 216.6 304.3   1.101   1.56
Chlorine* 172.2 239.1   1.11   2.06
Oxygen   54.8 90.2   1.14   1.426
Carbon Disulfide 162.4 319.5   1.26   1.49
Sulfur Dioxide 200.5 263.2   1.434 >1.6
Fluorine*   50.1   86.1 ~1.6 ~1.8**
Sulfur 386.0 717.8   1.7   2.0
Hydrogen Bromide* 184.6 206.1   1.91   2.76
Hydrogen Iodide 222.3 237.8   2.82   3.36
Bromine 265.8 331.9   3.10   4.11
Iodine* 386.7 457.5   3.85   5.02
* Unlikely to occur in oceanic quantities, but may be present in small pools or lakes.
** Fluorine ice is colorless, although the liquid and gas are yellowish.


Many other specific meteorological phenomena are also of major interest to xenologists. For instance, clouds and fogs should be common in any atmosphere with reasonable pressures. Condensation nuclei will always be plentiful, and most thalassogens can condense to tiny droplets around them at moderate temperatures. Rain should likewise be a regular occurrence at the surface of worlds possessing large open bodies of liquid thalassogen. (Of course, other things may rain down -- such as the periodic volcanic ash “rains” in Iceland.)

The height at which clouds form is a function of humidity, thalassogen vapor pressure, atmospheric thermal lapse rate, and a score of other interrelated factors. The suggestion that more massive worlds with higher gravity must have lower-hanging clouds2075 is simply too facile to be of much use to us.

Any planet which has clouds, rain, and sunlight reaching the surface will also have rainbows from time to time. These beautiful spectral arcs are the result of thalassogen droplets suspended in the air, acting as tiny prisms in concert to separate the incoming light into its constituent colors. The larger the droplets, the more intensely vivid the bow will appear.2149

Ignoring for the moment many other important factors, a larger planet with higher surface gravity will pull raindrops down before they have a chance to grow very large. Rainbows on larger worlds should tend to be rather dim, unimpressive affairs. On smaller worlds, where droplets can grow to larger sizes because they fall more slowly, rainbows should be impressive riots of color.2059 Furthermore, if there happens to be a very bright moon overhead or more than one sun; bows might appear in several parts of the sky at the same time.2059

How about lightning discharges? Electrical storms occur because molecules are split apart in the upper atmosphere to ions, which are then carried to the ground by dust and rain. This charges up the planet to at least half a million volts from ground to top of atmosphere -- a process likely on any world, save for the exact details of scale height and voltage. Planets with regular and intense sand or dust storms may generate intense electrical fields that could lead to more severe or more frequent discharges.1232

Another important factor is the breakdown voltage of the air -- the voltage at which a spark will jump a gap of unit distance. A charged cloud may be 100 million volts higher than the surface below, which is high enough for the “spark” to leap to earth. The spark gap voltage for dry air (at 1 atm) is usually listed as 11,000 volts/cm, and can be corrected for variations in temperature, pressure, and humidity. Now, if the atmosphere was comprised of a more conductive gas, such as neon, the spark gap voltage would only be 800 volts/cm (at 1 atm). This means that lightning should occur more frequently in neon (hydrogen, helium, etc.) than in oxygen (nitrogen, halogens, etc.).

This prediction may perhaps claim some support from the radio observations of Jupiter in the last decade or so. Decameter radio wave outbursts lasting from seconds to hours have been detected, with an equivalent energy of trillions of terrestrial lightning strokes per event.609 Similar outbursts have been observed on Saturn.2097

Will alien worlds have auroras too? Probably. These displays appear at the north and south planetary magnetic poles, and are caused by the funneling of solar wind ions in the converging magnetic field of the planet. Rapidly rotating, massive worlds should tend to have stronger magnetic fields. Also, hotter stars most likely have more vigorous solar winds. We would guess that a 4 Mearth planet with a ten-hour day circling an F5 sun will probably have far more striking auroral displays than a tidally-locked 0.4 Mearth planet orbiting a K2 star.

Mirage physics is also rather interesting. On Earth, mirages often result when there is a layer of warm air lying close to the ground. This air, being hotter, is less dense. It acts as a giant lens. Light coming from the sky near the horizon swoops down close to the ground and is refracted back up.2073 The mirage of water on an open highway is just a smeared-out image of blue sky.

Mirages on Earth generally appear about 100 meters away from the observer at ground level. On Mars, where the atmosphere is so thin the air is hardly heated by the ground at all, the refraction layer is thinner.950 The mirage backs away, out to about one kilometer. (To date, no Martian mirages have been photographed by the Viking landers, possibly due to the extreme roughness of the terrain and because the camera horizon is too close.2094)

On planets with very high density air, as on Venus, the mirage concept literally takes on new meaning. The transfer of heat from ground to near-surface air is complete, and it is believed by many that the extreme refraction near the ground will cause a kind of “fishbowl effect.”15,2060 The horizon would appear above the observer at all times,**** appearing to bend upward at the sides.2034 (The idea has already been used in science fiction.2071)

Dr. Conway Snyder at the Jet Propulsion Laboratory in Pasadena, California has performed a numerical simulation of the light-bending phenomenon at the Cytherian surface.2066 Let us imagine with him, for a moment, that we are aliens on the surface of Venus. Our eyeballs can see into the microwave region of the spectrum as well as the visible. What do we see?

The horizon appears to be elevated upward, all around us, at 9.40 from the horizontal. (Only 5°, if visibility drops to 200 kilometers.) Since Venus rotates backwards, the sun rises in the west and sets in the east, creeping across the sky at an imperceptible eight minutes of arc per hour. We are standing at the equator at the time of the equinox, so Sol lies directly over head at noon, Cytherian daylight time.

As the sun slowly falls toward the horizon, its shape begins to change. Its vertical dimension commences to shrink, while the horizontal component remains unchanged. At 6 PM Cytherian time, Sol should just be setting -- but it isn’t. Instead, it lies 10.4° up, but is squashed down to a quarter of its normal size. By 7 PM the squashing has become 250:1 compared with the horizontal dimension, and by 8 PM, 30,000:1.

Sometime close to 12 PM, the tiny solar sliver suddenly increases in length dramatically, and at the stroke of midnight wraps itself around the horizon in a pencil-thin ring of light. The line then breaks in the east, the sun begins to reassemble itself in the west, and sunrise begins.

If we are more than 3/8° away from the solar latitude, however, the ring of light will not appear. Instead, we see the compressed sun-image “crawl like a worm across the horizon during the night, from the point where it has set to the place where it is planning to rise.”2086


* Using our own solar system as his source of data, Dole finds that angular velocity is directly proportional to the square root of planetary mass for planets which are not tidally braked or locked.214

** Because the Martian atmosphere is only 1% as dense as that of Earth, the wind packs only about 10% as much punch, An astronaut standing in a 320 km/hour gale on the surface of the red planet would feel the equivalent of a 32 km/hour wind on Earth.1313

*** It should be noted that there are some six different allotropic forms of water-ice which form at various temperatures and pressures. Only one of these -- ”natural ice” or ice I as the chemists call it -- is lighter than water. Ice II through ice VII all sink if placed on the liquid.

**** Calculations indicate the effect would be rather small, though, perhaps a few degrees inclination at most.2068 The first pictures back from the two Russian Venera spacecraft that landed on Venus in 1975 showed no evidence of the fish bowl,2034,2079 but since the maximum range in the photos was only a few hundred meters the issue remains unresolved.


Last updated on 6 July 2013