Are the Planets Inhabited? Part 3

"Confronting the observer are lines and spots that but impress him the more, as his study goes on, with their non-natural look. So uncommonly regular are they, and on such a scale as to raise suspicions whether they can be by nature regularly produced" (p. 188).

"... Unnatural regularity, the observations showed, betrays itself in everything to do with the lines: in their surprising straightness, their amazing uniformity throughout, their exceeding tenuity, and their immense length" (p. 189).

"As a planet ages, its surface water grows scarce. Its oceans in time dry up, its rivers cease to flow, its lakes evaporate (p. 203)....

Now, in the struggle for existence, water must be got.... Its procuring depends on the intelligence of the organisms that stand in need of it.... As a planet ages, any organisms upon it will share in its development. They must evolve with it, indeed, or perish. At first they change only, as environment offers opportunity, in a lowly, unconscious way. But, as brain develops, they rise superior to such occasioning.... The last stage in the expression of life upon a planet's surface must be that just antecedent to its dying of thirst.... With an intelligent population this inevitable end would be long foreseen.... Both polar caps would be pressed into service in order to utilize the whole available supply and also to accommodate most easily the inhabitants of each hemisphere" (pp. 204-11).

"That intelligence should thus mutely communicate its existence to us across the far reaches of s.p.a.ce, itself remaining hid, appeals to all that is highest and most far-reaching in man himself. More satisfactory than strange this; for in no other way could the habitation of the planet have been revealed. It simply shows again the supremacy of mind.... Thus, not only do the observations we have scanned lead us to the conclusion that Mars at this moment is inhabited, but they land us at the further one that these denizens are of an order whose acquaintance was worth the making" (p. 215).

For the moment, let us leave Prof. Lowell's argument as he puts it.

Whether we accept it or not, it remains that it is a marvellous achievement of the optician's skill and the observer's devotion that from a planet so small and so distant as Mars any evidence should be forthcoming at all that could bear upon the question of the existence of intelligent organisms upon its surface. But it is of the utmost significance to note that the whole question turns upon the presence of water--of water in the liquid state, of water in a sufficient quant.i.ty; and the final decision, for Mr. Lowell's contention, or against it, must turn on that one point. The search for Life on Mars is essentially a search for Water; a search for water, not only in the present state of Mars, but in its past as well. For, without water in sufficient quant.i.ties in the past, life on Mars could not have pa.s.sed through the evolutionary development necessary to its attaining its highest expression,--that where the material living organism has become the tabernacle and instrument of the conscious intelligent spirit.

CHAPTER VII

THE CONDITION OF MARS

The planet Mars is the debatable ground between two opinions. Here, the two opposing views join issue; the controversy comes to a focus. The point in debate is whether certain markings--some linear, some circular--are natural or artificial. If, it is argued, some are truly like a line, without curve or break, as if drawn with pen, ink, and ruler; or others, so truly circular, without deviation or break, as if drawn with pen, ink, and compa.s.s; if, moreover, when we obtain more powerful telescopes, erected in better climates for observing, these markings become more truly lines and circles the better we see them; then they are _artificial_, not natural structures.

But artificial structures imply artificers. And if the structures are so designed as to meet the needs of a living organism, it implies that the living organism that designed them must have a reasonable mind lodged in a natural body. If, then, the "lines" and "circles" that Prof. Lowell and his disciples a.s.sert to be artificial ca.n.a.ls and oases are really such, they premise the order of being that we call Man. But these ca.n.a.ls and oases also premise the liquid that we call Water--water that flows and water utilized in cultivation. In this chapter we will leave out of count the first premiss--Man--and only deal with what concerns the second premiss--Water; with water that flows and is utilized in vegetation.

PLANETARY STATISTICS

+--------------------------------------------++--------++---------------- | || Minor || Inner | ||Planets.|| +--------------------------------------------++--------++-------+-------+ | || Ceres || Moon |Mercury| +--------------------------------------------++--------++-------+-------+ |PROPORTIONS OF THE PLANETS:-- || || | | | Diameter in miles || 477 || 2163 | 3030 | | " [Symbol] = 1 || 006 || 0273 | 0383 | | Surface, [Symbol] = 1 || 0004 || 0075 | 0147 | | Volume, [Symbol] = 1 || 00002|| 002 | 006 | | Density, Water = 1 || 28 ? || 339 | 472 | | " [Symbol] = 1 || 05 ? || 061 | 085 | | Ma.s.s, [Symbol] = 1 || 00001|| 0012 | 0048 | | Gravity at surface, [Symbol] = 1 || 0028 || 017 | 033 | | Rate of Fall, Feet in the First Second || 045 || 273 | 530 | | Albedo || 014 || 017 | 014 | | || || | | |DETAILS OF ORBIT:-- || || | | | Mean Distance from Sun in millions of miles||2571 ||929 |360 | | " " Earth's distance = 1 || 2767 || 1000 | 0387 | | Period of Revolution, in years || 460 || 100 | 024 | | Velocity, in miles per second || 111 ||185 | 97 | | Eccentricity || 00763|| 00168| 02056| | Aphelion Distance, Perihelion = 1 || 1157 || 1034 | 1517 | | Inclination of Equator to Orbit || (?) || 132'| (?) | | || || d h m | d | | Rotation period || (?) ||27743| 88(?) | | || || | | |ATMOSPHERE, a.s.suming the total ma.s.s of the || || | | | atmosphere to be proportional to the ma.s.s || || | | | of the planet:-- || || | | | Pressure at the surface in lb. per sq. in. || 0014 || 040 | 16 | | " " " in "atmospheres" || 00009|| 0027 | 0108 | | Level of half surface pressure in miles ||1190 ||196 |101 | | Boiling point of water at the surface || || 22C | 53C | | || || | | |TEMPERATURE:-- || || | | | Light and heat received from Sun, || || | | | [Symbol] = 1 || 013 || 100 | 667 | | Reciprocal of square-root of distance, || || | | | [Symbol] = 1 || 060 || 100 | 161 | | Equatorial temp. of ideal planet, Absolute || 188 || 312 | 502 | | " " " " Centigrade|| -65 || +39 | +229 | | Average temp. of ideal planet, Absolute || 174 || 290 | 467 | | " " " " Centigrade || -99 || +17 | +194 | | Upper limit under zenith sun, Absolute || 248 || 412 | 664 | | " " " " Centigrade || -25 || +139 | +391 | | Average temp. of equivalent disc, Absolute || 223 || 371 | 598 | | " " " " Centigrade|| -50 || +98 | +325 | | || || | | +--------------------------------------------++--------++-------+-------+

------------------++--------++--------------------------------------+ Planets. || || Outer Planets. | || || | +--------+--------++--------++---------+---------+--------+---------+ | Mars | Venus || Earth || Ura.n.u.s | Neptune | Saturn | Jupiter | +--------+--------++--------++---------+---------+--------+---------+ | | || || | | | | | 4230 | 7700 || 7918 || 31900 | 34800 | 73000 | 86500 | | 0534 | 0972 || 1000 || 4029 | 4395 | 9219 | 10924 | | 0285 | 0945 || 1000 || 162 | 193 | 850 | 1193 | | 015 | 092 || 100 || 65 | 85 |760 |1304 | | 392 | 494 || 555 || 122 | 111 | 072 | 132 | | 071 | 089 || 100 || 022 | 020 | 013 | 024 | | 0107 | 0820 || 1000 || 146 | 170 | 948 | 3177 | | 038 | 087 || 100 || 090 | 089 | 118 | 265 | | 611 | 1399 || 1608 || 1447 | 1431 | 1897 | 4261 | | 022 | 076 || 050? || 060 | 052 | 072 | 062 | | | || || | | | | | | || || | | | | |1415 | 672 || 929 ||17819 |27916 |8860 | 4833 | | 1524 | 0723 || 1000 || 19183 | 30055 | 9539 | 5203 | | 188 | 062 || 100 || 8402 | 16478 | 2946 | 1186 | | 150 | 219 || 185 || 42 | 34 | 60 | 81 | | 00933| 00068|| 00168|| 00463| 00090| 00561| 00483| | 1207 | 1013 || 1034 || 1097 | 1018 | 1107 | 1101 | |240' | (?) || 2327'|| (?) | (?) | 2649'| 35' | |h m s | || h m s|| h m | | h m | h m | |243723| (?) || 23564|| 930(?) | (?) | 1014| 955 | | | || || | | | | | | || || | | | | | | || || | | | | | | || || | | | | | 21 | 111 || 147 || 119 | 115 | 194 | 1038 | | 0143 | 0754 || 1000 || 081 | 078 | 132 | 706 | | 88 | 38 || 33 || 37 | 38 | 28 | 13 | | 53C | 92C || 100C || 94C | 93C | 108C | 166C | | | || || | | | | | | || || | | | | | 043 | 191 || 100 || 0003 | 0001 | 0011 | 0037 | | 081 | 118 || 100 || 023 | 018 | 032 | 044 | | 253 | 368 || 312 || 71 | 56 | 101 | 137 | | -20 | +95 || +39 || -202 | -217 | -172 | -136 | | 235 | 342 || 290 || 66 | 52 | 94 | 127 | | -38 | +69 || +17 || -207 | -221 | -179 | -146 | | 337 | 486 || 412 || 94 | 74 | 133 | 180 | | +64 | +213 || +139 || -179 | -199 | -140 | -93 | | 300 | 438 || 371 || 84 | 67 | 120 | 162 | | +27 | +165 || +98 || -189 | -206 | -153 | -111 | | | || || | | | | +--------+--------++--------++---------+---------+--------+---------+

For in regard to this particular premiss we can do away with hypothesis, and deal only with certain physical facts that are not controversial and are not in dispute.

The first of this series of facts concerning Mars about which there can be no controversy or dispute relates to its size and ma.s.s. As the foregoing Table shows, it comes between the Moon and the Earth in these respects.

The figures show at a glance that Mars ranks in its dimensions between the Moon and the Earth, and that, on the whole, it is more like to the Moon than it is to the Earth.

But in what way would this affect Mars as a suitable home for life? In many ways; and amongst these the distribution of its atmosphere and the sluggishness of its atmospheric circulation are not the least important.

It was mentioned in Chapter III that at a height of about three and a third miles the barometer will stand at 15 inches, or half its mean height at sea level, showing that one half the atmosphere has been pa.s.sed through. Mont Blanc, the highest mountain in Europe, is under 3 miles in height, so that it is not possible, in Europe, to climb to the level of half-pressure; Mt. Everest, the highest mountain in the world, is not quite six miles high, so that no part of the solid substance of our planet reaches up to the level of the quarter pressure. On a very few occasions daring aeronauts have soared into the empyrean higher than the summits of even our loftiest mountains, but the excursion has been a dangerous one, and they have with difficulty brought their life back from so rare and cold, so inhospitable a region. When Gay-Lussac, in 1804, attained a height of 23,000 feet above sea level, the thermometer, which on the ground read 31 C., sank to 9 below zero, and the rare atmosphere was so dry that paper crumpled up as if it had been placed near the fire, and his pulse rose to 120 pulsations a minute instead of his normal 66. When Mr.

Glaisher and Mr. c.o.xwell made their celebrated ascent between 1 and 2 o'clock on the afternoon of September 5, 1861, they found that at a height of 21,000 feet the temperature sank to -104; at 26,000 feet to -152; and at 39,000 feet the temperature was down to -160 C. At this height the rarefaction of the air was so great and the cold so intense that Mr.

Glaisher fainted, and Mr. c.o.xwell's hands being rendered numb and useless by the cold, he was only able to bring about their descent in time by pulling the string of the safety valve with his teeth. Yet when they attained this height they were far above all cloud or mist, and the Sun's rays fell full upon them. The Sun's rays had all the force that they had at the surface of the Earth, but in the rare atmosphere of seven miles above the Earth, the radiation from every particle not in direct sunlight was so great that while the right hand, exposed to the Sun, might burn, the left hand, protected from his direct rays, might freeze.

But gravity at the surface of Mars is much feebler than at the surface of the Earth, and in order to reach the level of half-pressure a Martian mountaineer would have to climb, not three and a third miles, but eight and three-quarter miles; that is to say, the distance to be ascended is in the inverse proportion of the force of gravity at the surface of the planet. The atmosphere of Mars, therefore, is much deeper than that of the Earth, and one great cause of precipitation here is much weakened there. A current of air heavily laden with moisture, if it encounters a range of mountains, is forced upwards, and consequently expands, owing to the diminished pressure. The expansion brings about a cooling, and from both causes the atmosphere is unable to retain as much water-vapour as it carried before. On Mars, the same relative expansion and cooling would only follow if the ascent were nearly three times as great, and the feeble force of gravity has its effect in another way; for just as a weight on Mars will only fall six feet in the first second as against sixteen on the Earth, so a dense and heavy column of air will fall with proportionate slowness and a light column ascend in the same languid manner. An ascending current on Mars would therefore take 1/038 1/038 = 1/0145, or seven times as long to attain the same relative expansion as on the Earth.

The winds of Mars are therefore sluggish, and precipitation is slight. So far at least it resembles

"The island valley of Avilion; Where falls not hail, or rain, or any snow, Nor ever wind blows loudly;"

and R. A. Proctor, acute and accurate writer on planetary physics as he was, fell into a mistake when he referred to Mars as being "hurricane-swept." There are no hurricanes on Mars; its fiercest winds can never exceed in violence what a sailor would call a "capful."

This holds good for Mars, but it also holds good for every planet where the force of gravity at the surface is relatively feeble. The greater the force of gravity the more active the atmospheric circulation, and more violent its disturbances; the feebler the action of gravity the more languid the circulation, and the slighter the disturbances.

The atmosphere of Mars is relatively deeper than that of the Earth, so that we, in observing the details of its surface, are looking down through an immense thickness of an obscuring medium. And yet the details of the surface are seen with remarkable distinctness; not as clearly indeed as we can see those of the Moon, but nearly so. For instance, the "ca.n.a.ls"

appear to have a breadth of from 15 to 20 miles, corresponding to 1/16th, and 1/12th, of a second of arc, at an average opposition. The oases, as a rule, are about 120 miles in diameter, that is to say about half a second of arc. These are extraordinarily fine details to be perceived and held, even if Mars had no atmosphere at all; it would certainly be impossible to detect them unless the atmosphere were exceedingly thin and transparent.

For we must remember that, though our own atmosphere is a hindrance to our observing, yet the atmosphere of the planet into which we are looking is a greater hindrance still. Like the lace curtains of the window of a house, it is a much greater obstacle to looking inward than to looking outward, and as the perfect distinctness with which we see the Moon is a proof that it is practically without an atmosphere, so the great detail visible on Mars bears unmistakable testimony to the slightness of the atmospheric veil around that planet.

And when we turn again to the statistics of Mars, we see that this must inevitably be the case. Of two planets, one heavier than the other, it is not possible to suppose that the lighter should secure the greater proportional amount of atmosphere. With planets, as with persons, it is the most powerful that gets the lion's share: "to him that hath it is given, and from him that hath not is taken away even that which he seemeth to have." But if we a.s.sume that Mars has acquired an atmosphere proportional to its ma.s.s, then we see from the Table that this must be a little less than 1/9th of that of the Earth; exactly 0107. It is distributed over a smaller surface, 0285. Consequently the amount of air above each square inch of Martian surface is 0107 0285 = 038. But since the force of gravity at the surface of Mars is less than on the Earth, this column of air will only weigh 038 038 = 0145; or one-seventh of the column of air resting on a square inch of the Earth's surface. The pressure at the surface of Mars will therefore be 21 lb.; and the aneroid barometer would read 43 inches. (In order to express the diminished pressure of the Martian atmosphere, it is necessary to refer it to the aneroid barometer. The mercury in a mercurial barometer, or the water in a water barometer would lose in weight in consequence of the diminished force of gravity in the same proportion as the air would, and the mercurial barometer would read 114 inches.)

But a pressure of 21 lb. on the square inch is far less than that experienced by c.o.xwell and Glaisher in their great ascent; it is about one-half the pressure that is experienced on the top of the very highest terrestrial mountains. But the habitable regions of the Earth do not extend even so far upward as to the level of a pressure of 73 lb. on the square inch; that is, of half the terrestrial surface pressure. Plant life dies out before we reach that point, and though birds or men may occasionally attain greater heights, they cannot domicile there, and are, indeed, only able thus to ascend in virtue of nourishment which they have procured in more favoured regions. If we could suppose the conditions of the whole Earth changed to correspond with those prevailing at the summit of Mt. Everest, or even at the summit of Mont Blanc, it is clear that the life now present on this planet would be extinguished, and that speedily.

Much more would this be the case if the atmosphere were diminished to one half the pressure on the summit of the highest earthly mountain.

The tenuity of the atmosphere on Mars has another consequence. Here water freezes at 0 C. and boils at 100 C.; so that for one hundred degrees it remains in a liquid condition. On Mars, under the a.s.sumed conditions, water would boil at 53 C., and the range of temperature within which it would be liquid would be much curtailed. But it is only water in the liquid state that is useful for sustaining life.

The above estimate of the density of the atmosphere of Mars is an outside limit, for it a.s.sumes that Mars has retained an atmosphere to the full proportion of its ma.s.s. But as the molecules of a gas are in continual motion, and in every direction, the lighter, most swiftly moving molecules must occasionally be moving directly outwards from the planet at the top of their speed, and in this case, if the speed of recession should exceed that which the gravity of the planet can control, the particle is lost to the planet for ever. A small planet therefore is subject to a continual drain upon its atmosphere, a drain of the lightest const.i.tuents. Hence it is, no doubt, that free hydrogen is not a const.i.tuent of the atmosphere of the Earth.

To what extent, then, has the atmosphere of Mars fallen below its full proportion? Mr. Lowell has adopted an ingenious method of obtaining some light on this question, by comparing the relative albedoes of the Earth and Mars; that is to say the relative power of reflection possessed by the two planets. Of course the method is rough; we have first of all no satisfactory means of determining the albedo of the Earth itself, and Mr.

Lowell puts it higher than most astronomers would do; then there is the difficulty of determining what portion of the total albedo is to be referred to the atmosphere and what to the actual soil or surface of the planet. But, on the whole, Mr. Lowell concludes that the amount of atmosphere above the unit of surface of Mars is 0222 of that above the unit of surface of the Earth. This would bring down the pressure on each square inch of Mars to 12 lb., and the aneroid barometer would read 25 inches; and water would boil at 44 C. The range of temperature from day to night, from summer to winter, at any place on the planet would be increased, while the range within which water could retain its liquid form would be diminished.

These statistics may seem rather dull and tiresome, but if we are to deal with the problem before us at all, it is important to understand that one factor in the condition of a planet cannot be altered and all the other factors retained unchanged. It will be seen that in computing the density of the atmosphere of Mars, we had to take into consideration not only the diameter of the planet, but the surface, which varies as the square of the diameter; the volume, which varies as the cube; the ma.s.s, which varies in a higher power still; and various combinations of these numbers. Novelists who write tales of journeys to other worlds or of the inhabitants of other worlds visiting this one, usually a.s.sume that the atmosphere is of the same density on all planets, and the action of gravity unchanged. In their view it is only that men would have a little less ground to walk upon on Mars, and a good deal more on Jupiter. Dean Swift, in _Gulliver's Travels_, made the Lilliputians take a truer view of the effect of the alteration of one dimension, for, finding that Gulliver was twelve times as tall as the average Lilliputian, they did not appoint him the rations of twelve Lilliputians, which would have been rather poor feeding for that veracious mariner, but allotted him the cube of twelve, viz.

seventeen hundred and twenty-eight rations. Mr. J. Holt Schooling, in one of his ingenious and interesting statistical papers, tried to bring home the vast extent of the British Empire by supposing that it seceded, and taking the portion of Earth that has fallen to it, set up a world of its own--the planet "Victoria." He allots to the British Empire 21 per cent of the land surface of the world. If the Earth were divided so as to form two globes with surfaces in proportion of 21 to 79, the smaller globe, which would correspond to Mr. Schooling's new planet "Victoria," would be less than half the present Earth in diameter; it would be considerably smaller than Mars. But "the rest of the world" would be 096 of the present Earth in diameter, or very nearly the size of Venus, and it would contain just eight-ninths of the substance of the Earth, leaving only one-ninth for "Victoria." The statistics given above will suggest to the reader that, could such a secession be carried out, the inhabitants of the British Empire would not be happier for the change during the very short continued existence that remained to them. The "rest of the world" could spare our fraction of the planet much better than we could spare theirs.

This is a principle which applies to worlds anywhere; not merely within the limits of the solar system but wherever they exist. Everywhere the surface must vary with the square of the diameter; the volume with the cube; everywhere the smaller planet must have the rarer atmosphere, and with a rare atmosphere the extreme range of temperature must be great, while the range of temperature within which water will flow will be restricted. Our Earth stands as the model of a world of the right size for the maintenance of life; much smaller than our Earth would be too small; much larger, as we shall see later, would be too large.

So far we have dealt with Mars as if it received the same amount of light and heat from the Sun that the Earth does. But, as the Table shows, from its greater distance from the Sun, Mars receives per unit of surface only about three-sevenths of the light and heat of that received by the Earth.

The inclination of the axis of Mars is almost the same as that of the Earth, so that the general character of the seasons is not very different on the two planets, and the torrid, temperate, and frigid zones have almost the same proportions. The length of the day is also nearly the same for both, the Martian day being slightly longer; but the most serious factor is the greater distance of Mars, and the consequent diminution in the light and heat received from the Sun. The light and heat received by the Earth are not so excessive that we could be content to see them diminished, even by 5 per cent, but for Mars they are diminished by 57 per cent. How can we judge the effect of so important a difference?

The mean temperature of our Earth is supposed to be about 60F., or 16C.

Three-sevenths of this would give us 7C. as the mean temperature of Mars, which would signify a planet not impossible for life. But the zero of the Centigrade scale is not the absolute zero; it only marks the freezing-point of water. The absolute zero is computed to be -273 on the Centigrade scale; the temperature of the Earth on the absolute scale therefore should be taken as 289, and three-sevenths of this would give 124 of absolute temperature. But this is 149 below freezing-point, and no life could exist on a planet under such conditions.

But the mean temperature of Mars cannot be computed quite so easily. The hotter a body is the more rapidly it radiates heat; the cooler it is the slower its radiation. According to Stefan's Law, the radiation varies for a perfect radiator with the 4th power of the absolute temperature; so that if Mars were at 124 abs., while the Earth were at 289 abs., the Earth would be radiating its heat nearly 30 times faster than Mars. The heat income of Mars would therefore be in a much higher proportion than its expenditure; and necessarily its heat capital would increase until income and expenditure balanced. Prof. Poynting has made the temperature of the planets under the 4th power law of radiation the subject of an interesting enquiry, and the figures which he has obtained for Mars and other planets are included in the Table.

The equatorial and average temperatures are given under the a.s.sumption that Mars possesses an atmosphere as efficient as our own in equalizing the temperature of the whole planet. If, on the other hand, its atmosphere has no such regulating power, then under the zenith Sun the upper limit of the temperature of a portion of its surface reflecting one-eighth would be, as shown in the Table, 64C. This would imply that the temperature on the dark side of the planet was very nearly at the absolute zero. "If we regard Mars as resembling our Moon, and take the Moon's effective average temperature as 297 abs., the corresponding temperature for Mars is 240 abs., and the highest temperature is four-fifths of 337 = 270 abs. But the surface of Mars has probably a higher coefficient of absorption than the surface of the Moon--it certainly has for light--so that we may put his effective average temperature, on this supposition, some few degrees above 240 abs., and his equatorial temperature some degrees higher still.

It appears as exceedingly probable, then, that whether we regard Mars as like the Earth or, going to the other extreme, as like the Moon, the temperature of his surface is everywhere below the freezing-point of water."[14] As the atmospheric circulation on Mars must be languid, and the atmosphere itself is very rare, the general condition of the planet will approximate rather to the lunar type than to the terrestrial, and the extremes, both of heat and cold, will approach those which would prevail on a planet without a regulating atmosphere.

There is another way of considering the effect on the climate of Mars and its great distance from the Sun, which, though only rough and crude, may be helpful to some readers. If we take the Earth at noonday at the time of the equinox, then a square yard at the equator has the Sun in its zenith, and is fully presented to its light and heat. But, as we move away from the equator, we find that each higher lat.i.tude is less fully presented to the Sun, until, when we reach lat.i.tude 64-1/2--in other words just outside the Arctic Circle--7 square yards are presented to the Sun so as to receive only as much of the solar radiation as 3 square yards receive at the equator. We may take, then, lat.i.tude 64-1/2 as representing Mars, while the equator represents the Earth. Or, we may take it that we should compare the climate of Archangel with the climate of Singapore.

Now the mean temperature of lat.i.tude 64-1/2, say the lat.i.tude of Archangel, is just about freezing-point (0C.), while that of the equator is about 28C. We should therefore expect from this a difference between the mean temperatures of the Earth and Mars of 28; that is to say, as the Earth stands at 16C, Mars would be at -12C. But, on the Earth, the evaporation and precipitation is great, and the atmospheric circulation vigorous. Evaporation is always going on in equatorial regions, and the moisture-laden winds are continually moving polewards, carrying with them vast stores of heat to be liberated as the rain falls. The oceanic currents have the same effect, and how great the modification which they introduce may be seen by comparing the climates of Labrador and Scotland.

There appear to be no great oceans on Mars. The difference of 28 which we find on the Earth between the equator and the edge of the Arctic Circle is a difference which remains after the convection currents of air and sea have done much to reduce the temperature of the equator and to raise that of high lat.i.tudes. If we suppose that their effect has been to reduce this difference to one half of what it would have been were each lat.i.tude isolated from the rest, we shall not be far wrong, and we should get a range of 56 as the true equivalent difference between the mean temperatures of Singapore and Archangel; i.e. of the Earth and Mars; and Mars would stand at -40C. The closeness with which this figure agrees with that reached by Prof. Poynting suggests that it is a fair approximation to the correct figure.

The size of Mars taught us that we have in it a planet with an atmosphere of but one half the density of that prevailing on the top of our highest mountain; the distance of Mars from the Sun showed us that it must have a mean temperature close to that of freezing mercury. What chance would there be for life on a world the average condition of which would correspond to that of a terrestrial mountain top, ten miles high and in the heart of the polar regions? But Mars in the telescope does not look like a cold planet. As we look at it, and note its bright colour, the small extent of the white caps presumed to be snow, and the high lat.i.tudes in which the dark markings--presumed to be water or vegetation--are seen, it seems difficult to suppose that the mean temperature of the planet is lower than that of the Earth. Thus on the wonderful photographs taken by Prof. Barnard in 1909, the Nilosyrtis with the Protonilus is seen as a dark ca.n.a.l. Now the Protonilus is in North Lat. 42, and on the date of observation--September 28, 1909--the winter solstice of the northern hemisphere of Mars was just past. There would be nothing unusual for the ground to be covered with snow and the water to be frozen in a corresponding lat.i.tude if in a continental situation on the Earth. Then, again, in the summer, the white polar caps of Mars diminish to a far greater extent than the snow and ice caps of the Earth; indeed, one of the Martian caps has been known to disappear completely.

Yet, as the accompanying diagram will show, something of this kind is precisely what we ought to expect to see. The diagram has been constructed in the following manner: A curve of mean temperatures has been laid down for every 10 of lat.i.tude on the Earth, derived as far as possible from accepted isothermals in continental countries in the northern hemisphere.

From this curve ordinates have been drawn at each 10, upward to show average deviation from the mean temperature for the hottest part of the day in summer, downward for the deviation for the coldest part of the night in winter. Obviously, on the average, the range from maximum to minimum will increase from the equator to the poles. The mean temperature of the Earth has been taken as 16C, and as representing that prevailing in about 42 lat. The diagram shows that the maximum temperature of no place upon the Earth's surface approaches the boiling-point of water, and that it is only within the polar circle that the mean temperature is below freezing-point. Water, therefore, on the Earth must be normally in the liquid state.

In constructing a similar diagram for Mars, three modifications have to be made. First of all, the mean temperature of the planet must be considerably lower than that of the Earth. Next, since the atmospheric circulation is languid and there are no great oceans, the temperatures of different lat.i.tudes cannot be equalized to the same extent as on the Earth. It follows, therefore, that the range in mean temperature from equator to pole must be considerably greater on Mars than on the Earth.

Thirdly, the range in temperature in any lat.i.tude, from the hottest part of the day in summer to the coldest part of the night in winter, must be much greater than with us; partly on account of the very slight density of the atmosphere, and partly on account of the length of the Martian year.

[Ill.u.s.tration: THERMOGRAPHS OF THE EARTH AND MARS]