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 compass; 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 assert to be artificial canals and oases are really such,
they premise the order of being that we call Man. But these canals 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 || 0.06 || 0.273 | 0.383 |
| Surface, [Symbol] = 1 || 0.004 || 0.075 | 0.147 |
| Volume, [Symbol] = 1 || 0.0002|| 0.02 | 0.06 |
| Density, Water = 1 || 2.8 ? || 3.39 | 4.72 |
| " [Symbol] = 1 || 0.5 ? || 0.61 | 0.85 |
| Mass, [Symbol] = 1 || 0.0001|| 0.012 | 0.048 |
| Gravity at surface, [Symbol] = 1 || 0.028 || 0.17 | 0.33 |
| Rate of Fall, Feet in the First Second || 0.45 || 2.73 | 5.30 |
| Albedo || 0.14 || 0.17 | 0.14 |
| || || | |
|DETAILS OF ORBIT:-- || || | |
| Mean Distance from Sun in millions of miles||257.1 ||92.9 |36.0 |
| " " Earth's distance = 1 || 2.767 || 1.000 | 0.387 |
| Period of Revolution, in years || 4.60 || 1.00 | 0.24 |
| Velocity, in miles per second || 11.1 ||18.5 | 9.7 |
| Eccentricity || 0.0763|| 0.0168| 0.2056|
| Aphelion Distance, Perihelion = 1 || 1.157 || 1.034 | 1.517 |
| Inclination of Equator to Orbit || (?) || 1 deg..32'| (?) |
| || || d h m | d |
| Rotation period || (?) ||27.7.43| 88(?) |
| || || | |
|ATMOSPHERE, assuming the total mass of the || || | |
| atmosphere to be proportional to the mass || || | |
| of the planet:-- || || | |
| Pressure at the surface in lb. per sq. in. || 0.014 || 0.40 | 1.6 |
| " " " in "atmospheres" || 0.0009|| 0.027 | 0.108 |
| Level of half surface pressure in miles ||119.0 ||19.6 |10.1 |
| Boiling point of water at the surface || || 22 deg.C | 53 deg.C |
| || || | |
|TEMPERATURE:-- || || | |
| Light and heat received from Sun, || || | |
| [Symbol] = 1 || 0.13 || 1.00 | 6.67 |
| Reciprocal of square-root of distance, || || | |
| [Symbol] = 1 || 0.60 || 1.00 | 1.61 |
| Equatorial temp. of ideal planet, Absolute || 188 || 312 deg. | 502 deg. |
| " " " " 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 || Uranus | Neptune | Saturn | Jupiter |
+--------+--------++--------++---------+---------+--------+---------+
| | || || | | | |
| 4230 | 7700 || 7918 || 31900 | 34800 | 73000 | 86500 |
| 0.534 | 0.972 || 1.000 || 4.029 | 4.395 | 9.219 | 10.924 |
| 0.285 | 0.945 || 1.000 || 16.2 | 19.3 | 85.0 | 119.3 |
| 0.15 | 0.92 || 1.00 || 65. | 85. |760. |1304. |
| 3.92 | 4.94 || 5.55 || 1.22 | 1.11 | 0.72 | 1.32 |
| 0.71 | 0.89 || 1.00 || 0.22 | 0.20 | 0.13 | 0.24 |
| 0.107 | 0.820 || 1.000 || 14.6 | 17.0 | 94.8 | 317.7 |
| 0.38 | 0.87 || 1.00 || 0.90 | 0.89 | 1.18 | 2.65 |
| 6.11 | 13.99 || 16.08 || 14.47 | 14.31 | 18.97 | 42.61 |
| 0.22 | 0.76 || 0.50? || 0.60 | 0.52 | 0.72 | 0.62 |
| | || || | | | |
| | || || | | | |
|141.5 | 67.2 || 92.9 ||1781.9 |2791.6 |886.0 | 483.3 |
| 1.524 | 0.723 || 1.000 || 19.183 | 30.055 | 9.539 | 5.203 |
| 1.88 | 0.62 || 1.00 || 84.02 | 164.78 | 29.46 | 11.86 |
| 15.0 | 21.9 || 18.5 || 4.2 | 3.4 | 6.0 | 8.1 |
| 0.0933| 0.0068|| 0.0168|| 0.0463| 0.0090| 0.0561| 0.0483|
| 1.207 | 1.013 || 1.034 || 1.097 | 1.018 | 1.107 | 1.101 |
|24 deg..0' | (?) || 23 deg..27'|| (?) | (?) | 26 deg..49'| 3 deg..5' |
|h m s | || h m s|| h m | | h m | h m |
|24.37.23| (?) || 23.56.4|| 9.30(?) | (?) | 10.14+-| 9.55+- |
| | || || | | | |
| | || || | | | |
| | || || | | | |
| | || || | | | |
| 2.1 | 11.1 || 14.7 || 11.9 | 11.5 | 19.4 | 103.8 |
| 0.143 | 0.754 || 1.000 || 0.81 | 0.78 | 1.32 | 7.06 |
| 8.8 | 3.8 || 3.3 || 3.7 | 3.8 | 2.8 | 1.3 |
| 53 deg.C | 92 deg.C || 100 deg.C || 94 deg.C | 93 deg.C | 108 deg.C | 166 deg.C |
| | || || | | | |
| | || || | | | |
| 0.43 | 1.91 || 1.00 || 0.003 | 0.001 | 0.011 | 0.037 |
| 0.81 | 1.18 || 1.00 || 0.23 | 0.18 | 0.32 | 0.44 |
| 253 deg. | 368 deg. || 312 deg. || 71 deg. | 56 deg. | 101 deg. | 137 deg. |
| -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 mass. 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 passed
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 deg. C., sank to 9 deg. 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. Coxwell 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 -10.4 deg.; at 26,000 feet to -15.2 deg.;
and at 39,000 feet the temperature was down to -16.0 deg. C. At this height
the rarefaction of the air was so great and the cold so intense that Mr.
Glaisher fainted, and Mr. Coxwell'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 1/0.38 x 1/0.38 = 1/0.145,
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 "canals"
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 assume that Mars has acquired an atmosphere
proportional to its mass, then we see from the Table that this must be a
little less than 1/9th of that of the Earth; exactly 0.107. It is
distributed over a smaller surface, 0.285. Consequently the amount of air
above each square inch of Martian surface is 0.107 / 0.285 = 0.38. But
since the force of gravity at the surface of Mars is less than on the
Earth, this column of air will only weigh 0.38 x 0.38 = 0.145; 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 2.1 lb.;
and the aneroid barometer would read 4.3 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 11.4 inches.)
But a pressure of 2.1 lb. on the square inch is far less than that
experienced by Coxwell 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 7.3 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 deg. C. and boils at 100 deg. C.; so that for one hundred degrees it
remains in a liquid condition. On Mars, under the assumed conditions,
water would boil at 53 deg. 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 assumes that Mars has retained an atmosphere to the full
proportion of its mass. 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 constituents. Hence it
is, no doubt, that free hydrogen is not a constituent 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 0.222 of that above the
unit of surface of the Earth. This would bring down the pressure on each
square inch of Mars to 1.2 lb., and the aneroid barometer would read 2.5
inches; and water would boil at 44 deg. 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 mass, 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 assume 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 0.96 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 60 deg.F., or 16 deg.C.
Three-sevenths of this would give us 7 deg.C. 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 deg. on the
Centigrade scale; the temperature of the Earth on the absolute scale
therefore should be taken as 289 deg., and three-sevenths of this would give
124 deg. of absolute temperature. But this is 149 deg. 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 deg. abs., while the Earth were at 289 deg. 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 assumption
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, 64 deg.C. 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 deg. abs., the corresponding temperature for Mars is 240 deg.
abs., and the highest temperature is four-fifths of 337 deg. = 270 deg. 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 deg. 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 latitude is less fully presented to
the Sun, until, when we reach latitude 64-1/2 deg.--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, latitude 64-1/2 deg. 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 latitude 64-1/2 deg., say the latitude of
Archangel, is just about freezing-point (0 deg.C.), while that of the equator
is about 28 deg.C. We should therefore expect from this a difference between
the mean temperatures of the Earth and Mars of 28 deg.; that is to say, as the
Earth stands at 16 deg.C, Mars would be at -12 deg.C. 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 deg. 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 latitudes. If we suppose that their effect has been to reduce this
difference to one half of what it would have been were each latitude
isolated from the rest, we shall not be far wrong, and we should get a
range of 56 deg. 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 -40 deg.C. 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 latitudes
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 canal. Now the Protonilus is in North Lat. 42 deg., 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 latitude 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 deg. of latitude 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 deg., 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 16 deg.C, and as representing that prevailing
in about 42 deg. 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 latitudes 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 latitude, 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.
[Illustration: THERMOGRAPHS OF THE EARTH AND MARS]
We cannot know the exact figures to adopt, but the general type of the
thermograph for Mars as compared with that of the Earth will remain. The
mean temperature of Mars will be lower, the range of temperature from
equator to pole will be greater, and the extremes of temperature in any
given latitude more pronounced than upon the Earth. And the general lesson
of the diagram may be summed up in a sentence. The maximum temperature on
the planet is well above freezing-point, and the part of the planet at
maximum temperature is precisely the part that we see the best. But while
this is so, it is clear that water on Mars must normally be in the state
of ice; Mars is essentially a frozen planet; and the extremes of cold
experienced there, not only every year but every night, far transcend the
bitterest extremes of our own polar regions.
The above considerations do not appear to render it likely that there is
any vegetation on Mars. A planet ice-bound every night and with its mean
temperature considerably below freezing-point does not seem promising for
vegetation. If vegetation exists, it must be of a kind that can pass
through all the stages of its life-history during the few bright hours of
the Martian day. Every night will be for it a winter, a winter of
undescribable frost, which it could only endure in the form of spores. So
if there be vegetation it must be confined to some hardy forms of a low
type. At a distance of forty millions of miles it is not easy to
discriminate between the darkness of sheets of water and the darkness of
stretches of vegetation. Some of the so-called "seas" may possibly be
really of the latter class, but that there must be expanses of water on
the planet is clear, for if there were no water surfaces there would be no
evaporation; and if there were no evaporation from whence could come the
supply of moisture that builds up the winter pole cap?
The great American astronomer, Prof. Newcomb, gave in _Harper's Weekly_
for July 25, 1908, an admirable summary of the verdict of science as to
the character of the meteorology of Mars. "The most careful calculation
shows that if there are any considerable bodies of water on our
neighbouring planet they exist in the form of ice, and can never be
liquid to a depth of more than one or two inches, and that only within the
torrid zone and during a few hours each day.... There is no evidence that
snow like ours ever forms around the poles of Mars. It does not seem
possible that any considerable fall of such snow could ever take place,
nor is there any necessity of supposing actual snow or ice to account for
the white caps. At a temperature vastly below any ever felt in Siberia,
the smallest particles of moisture will be condensed into what we call
hoar frost, and will glisten with as much whiteness as actual snow....
Thus we have a kind of Martian meteorological changes, very slight indeed
and seemingly very different from those of our earth, but yet following
similar lines on their small scale. For snowfall substitute frostfall;
instead of feet or inches say fractions of a millimetre, and instead of
storms or wind substitute little motions of an air thinner than that on
the top of the Himalayas, and we shall have a general description of
Martian meteorology."
What we know of Mars, then, shows us a planet, icebound every night, but
with a day temperature somewhat above freezing-point. As we see it, we
look upon its warmest regions, and the rapidity with which it is cleared
of ice, snow, and cloud shows the atmosphere to be rare and the moisture
little in amount and readily evaporated. The seas are probably shallow
depressions, filled with ice to the bottom, but melted as to their
surfaces by day. From the variety of tints noted in the seas, and the
recurrent changes in their outlines, they are composed of congeries of
shallow pools, fed by small sluggish streams; great ocean basins into
which great rivers discharge themselves are quite unknown.