BEHAVIOR OF THE POLAR CAPS
Assured by physical properties that our visual appearances are quite
capable of being what they seem we pass to the phenomena of the cap
itself. Like as are the polar caps of the two planets at first regard,
upon further study very notable differences soon disclose themselves
between the earthly and the Martian ones; and these serve to give us
our initial hint of a different state of things over there from that
with which we are conversant on Earth.
To begin with, the limits between which they fluctuate are out of all
proportion greater on Mars. It is not so much in their maxima that the
ice-sheets of the two planets vary. Our own polar caps are much larger
than we think; indeed, we live in them a good fraction of the time. Our
winter snows are in truth nothing but part and parcel of the polar cap
at that season. Now, in the northern hemisphere snow covers the ground
at sea-level more or less continuously down to 50° of latitude. It
stretches thus far even on the western flanks of the continents, while
in the middle of them and on their eastern sides it extends ten degrees
farther yet during the depth of winter. So that we have a polar cap
which is then ninety degrees across. In our southern hemisphere it is
much the same six months later, in the corresponding winter of its year.
On Mars at their winter maxima the polar caps extend over a similar
stretch of latitude. They do so, however, unequally. The southern one
is considerably the larger. In 1903, 136 days after the winter
solstice, in the Martian calendar February 27, it came down in
longitude 225° to 44° of latitude and may be taken to have then
measured ninety-three degrees across; in 1905, 121 days after the same
solstice, it stretched in longitude 235° to latitude 42°, and 158 days
later, in longitude 221° to latitude 41°; values which, supposing it to
have been round, imply for it a diameter on these occasions of
ninety-six and ninety-seven degrees. It was then February 20 and March
10 respectively of the Martian year. These determinations of its size
at the two oppositions agree sufficiently well considering the great
tilt away from us of the south pole at the time and the horizonward
foreshortening of the edge of the snow. It seems from a consensus of
the measures to have been some five degrees wider in 1903 than in 1905,
which may mean a colder winter preceding the former date. The cap was
still apparently without a dark contour in both years, showing that it
had not yet begun to melt.
[Illustration: South Polar Cap.
(Lowell Observatory, 1905.)]
Less has been learnt of the northern cap. In 1896-1897 when it was
similarly presented skirting the other rim of the disk, a gap occurred
in the observations corresponding to the time by Martian months between
February 24 and March 22. On the former date the cap came down only to
latitude 55° in longitude 352°; on the subsequent one and for several
days after the latitude of the southern limit of the snow was such as
to imply a breadth to it of about eighty degrees. The cap was now
bordered by a dark line, proving that melting had already set in. It
cannot, however, at its maximum have covered much more country than
this, in view of its lesser extent on February 24.
Fair as our knowledge now is of the dimensions of the Martian polar
caps at their maxima, we have much more accurate information with
regard to their minima, and this, too, was obtained much earlier. That
we should first have known their smallest rather than their greatest
extent with accuracy may appear surprising, exactly the opposite being
our knowledge of our own. It is not, however, so surprising as it
appears, inasmuch as it is an inevitable consequence of the planet’s
aspect with regard to the sun. When the tilt of the axis inclines one
hemisphere toward the sun, that hemisphere’s polar cap must melt and
dwindle, while at the same time it is the one best seen, the other
being turned away from the sun and therefore largely from us as well;
so that even such part of the latter as is illumined lies low down
toward the horizon of the disk where a slight change of angle means a
great difference in size.
It has thus come about that both the south and the north polar caps
have been repeatedly well seen and measured at their minimum; and the
measures for different Martian years agree well with one another. For
the northern cap six degrees in diameter is about the least value to
which it shrinks. The south one becomes even smaller, being usually not
more than five degrees across, while in 1894 it actually vanished, a
thing unprecedented. Its absence was detected by Douglass at Flagstaff
and shortly after the announcement of its disappearance the fact was
corroborated by Barnard at the Lick. The position the cap would have
occupied was at the time better placed for observation in America than
in Europe, inasmuch as the cap is eccentrically situated with regard to
the geographic pole and its centre was then well on the side of the
disk presented to us while in Europe it was turned away. This, together
with the fact that it undoubtedly came and went more than once about
this time, accounts for its disappearance not having been recognized
there, haze left by it having apparently been mistaken for the cap
itself.
[Illustration: North Polar Cap.
(Lowell Observatory, 1905.)]
On Earth the minima are much larger. In the northern hemisphere the
line of perpetual snow or pack-ice in longitude 50° east runs about on
the 80° parallel, including within it the southern end of Franz Joseph
Land. Opposite this, in longitude 120° west, above the North American
continent, it reaches down lower still to 75°. So that the cap is then
from twenty to thirty degrees in diameter. In the southern hemisphere
it is even larger. In longitude 170° west the land was found by Ross to
be under perpetual snow in latitude 72°. Cook had reached in longitude
107° east an impassable barrier of ice in latitude 70° 23′. The season
was then midsummer, January 30. So that we are perhaps justified in
considering 71° south as about the average limit of perpetual snow or
paleocrystic ice. This would make the southern cap at its minimum
thirty-eight degrees across. Pack-ice with open spots extends still
farther north. The Pagoda in 1845 was stopped by impenetrable pack-ice
in south latitude 68° and the Challenger in 1874 encountered the pack
in latitude 65° on the 19th of February, which corresponds about to our
19th of August, the time at which the sea should be most open. The
limit of perpetual snow is thus lower in the southern than in the
northern hemisphere. Here again, then, the two minima differ, but in
the reverse way from what they do on Mars.
From this we perceive that the variations in size of the caps are much
more striking on Mars than on the Earth and that these are due chiefly
to the difference in the minima, the maxima not varying greatly.
To explain these interesting diversities of behavior in the several
polar caps we shall have to go back a little in general physics in
order to get a proper take off. It is a curious concomitant of the law
of gravity that the amount of heat received by a planet in passing from
any point of its path to a point diametrically opposite is always the
same no matter what be the eccentricity of the orbit. Thus, a planet
has as many calories falling upon it in travelling from its vernal
equinox to its autumnal as from the autumnal to the vernal again,
although the time taken in the one journey be very different from that
of the other. This is due to the fact that the angle swept over by the
radius vector, that is, the imaginary bond between it and the sun, is
at all points proportional to the amount of heat received; just as it
is of the gravity undergone, the two forces radiating into space as the
inverse square of the distance. Thus the heat received by a point or a
hemisphere, through any orbital angle, is independent of the
eccentricity of the orbit.
But it is not independent of the axial tilt. For the force of the sun’s
rays is modified by their obliquity. The amount of heat received at any
point in consequence of the tilt turns upon the position of the point,
and for any hemisphere taken as a whole it depends upon the degree to
which the pole is tilted to the source of heat. In consequence of being
more squarely presented to its beams, the hemisphere which is directed
toward the sun and therefore is passing through its summer season gets
far more insolation than that which is at the same time in the depth of
its winter. For a tilt of twenty-four degrees, the present received
value for the axis of Mars, the two hemispheres so circumstanced get
amounts of heat respectively in the proportion of sixty-three to
thirty-seven.
But, though the summer and winter insolation thus differ, they are the
same for each hemisphere in turn. Consequently the mere amount of heat
received cannot be the cause of any differences detected between the
respective maxima and minima of the two polar caps. If heat were a
substance which could be stored up instead of being a mode of motion,
the effect produced would be in accordance with the quantity applied
and the two caps would behave alike. As it is the total amount has very
little to say in the matter.
Not the amount of heat but the manner in which this heat is made at
home is responsible for the difference we observe. Now, though the
total amount is the same in passing from the vernal to the autumnal
equinox as from the autumnal to the vernal, the time during which it is
received in either case varies from one hemisphere to the other. It is
summer in the former while it is winter in the latter and the
difference in the length of the two seasons due to the eccentricity of
the orbit makes a vast difference in the result. Winter affects the
maxima, summer the minima, attained. Of these opposite variations
presented to us by the two caps, the maxima, the one most difficult to
detect, is the easiest to explain, for the difference in the maxima
seems to be due to the surpassing length of the antarctic night.
Owing to the eccentricity of the orbital ellipse pursued by Mars and to
the present position of the planet’s solstices, the southern hemisphere
is farther away from the sun during its winter and is so for a longer
time. The seasons are in length, for the northern hemisphere: spring,
199 days; summer, 183 days; autumn, 147 days; and winter, 158 days;
while for the southern hemisphere they are: spring, 147 days; summer,
158 days; autumn, 199 days; and winter, 183 days. The arctic polar
night is thus 305 of our days long; the antarctic, 382. Thus for 77
more days than happens to its fellow the southern pole never sees the
sun. Now, since the total sunlight from equinox to equinox is the same
in both hemispheres, its distribution by days must be different. In the
southern hemisphere the same amount is crowded into a smaller compass
in the proportion of 305 to 382; that being that hemisphere’s relative
ratio of days. But since during winter the cap increases, there is a
daily excess of accumulation over dissipation of snow and each
twenty-four hours must on the average add its tithe to the sum total.
Since the northern days are the warmer each adds less than do the
southern ones; and furthermore there are fewer of them. On both these
scores the amount of the deposition about the northern pole should be
less than about the southern one. Consequently, the snow-sheet there
should be the less extensive and show a relatively smaller maximum,
which explains what we see.
With the minima the action is otherwise. Inasmuch as the greater heat
received during the daylight hours by the southern hemisphere is
exactly offset by the shortness of its season, it would seem at first
as if there could be no difference in the total effect upon the two
ice-caps.
But further consideration discloses a couple of factors which might,
and possibly do, come in to qualify the action and account for the
observed effect. One is that though the total amount of heat received
is the same, the manner of its distribution differs in the two
hemispheres. In the northern one the time from vernal to autumnal
equinox is 382 days against 305 in the southern. Consequently, the
average daily heat is then five fourths more intense in the southern
hemisphere. Indeed, it is even greater than this and nearer four
thirds, because the melting occurs chiefly in the spring and in the
first two months of summer when the contrast in length of season
between the two hemispheres is at its greatest. Now, a few hotter days
might well work more result than many colder ones. And this would be
particularly true of Mars where the mean temperature is probably none
too much above the freezing-point to start with. Ice consumes so much
caloric in the process of turning into any other state, laying it by in
the form of latent heat before it can turn into water and then so much
more before this water can be converted into steam that a good deal has
to be expended on it before getting any perceptible result. Once
obtained, however, the heat is retained with like tenacity. So that the
process works to double effect! If sufficient heat be received the ice
is first melted, then evaporated and finally formed into a layer of
humid air, the humidity of which keeps it warm. Dry air is unretentive
of heat, moist air the opposite. And for the melting of the ice-cap to
proceed most effectively the temperature that laps it about must be as
high as possible and kept so as continuously as may be. If between days
it be allowed to fall too low at night much caloric must needs be
wasted in simply raising the ice again to the melting-point. This a
blanket of warm air tends to prevent, and this again is brought about
by a few hot days rather than by many colder ones. It is not all the
heat received that becomes effective but the surplus heat above a
certain point. The gain in continuity of action thus brought about is
somewhat like that exhibited between the running of an express and an
accommodation train. To reach its destination in a given time the
former requires far less power because it does not have to get up speed
again after each arrest. Thus the whole effect in melting the snow
would be greater upon that hemisphere whose summer happens to be the
more intense.
The greater swing in size of the cap most exposed to the effects of the
eccentricity is, then, the necessary result of circumstances when the
precipitation is not too great to be nearly carried off by the
subsequent dissipation. This is the state of things on Mars and the
second of the factors above referred to. On the Earth as we have seen
the polar caps are somewhat larger at their maximum and very much so at
their minimum. Now, this is just what should happen were the
precipitation increased. Suppose, for example, that the amount of
precipitation were to increase while the amount of summer melting
remained the same, and this would be the case if the vapor in the air
augmented for one cause or another, and the result of each fresh
deposit was locked up in snow. After a certain point the cap would grow
in depth rather than in extension; the winter deposit would be thicker
but the summer evaporation would remain the same. Now, if this
occurred, it is evident that the minimum size of the cap would increase
relatively much faster than the maximum, and furthermore, that the
relative increase of the minimum in the two caps would be greatest for
that which had seasons of extremes. The result we see in the case of
the Earth. In the arctic cap, where in consequence of the eccentricity
of the orbit the winter is shorter, the maximum is less than in the
antarctic and this extra amount of precipitation cannot be wholly done
away with in its intenser summer, so that the minimum too is greater
there.
We reach, then, this interesting conclusion. We find that eccentricity
of orbit by itself not only causes no universal glaciation in the
hemisphere which we should incidentally suppose likely to show it, but
actually produces the opposite result, in more than offsetting by
summer proximity what winter distance brings about. To cause extensive
glaciation we must have, in addition to favorable eccentricity, a large
precipitation. With these two factors combined we get an ice age, but
not otherwise. The result has an important bearing on geologic glacial
periods and their explanation.
Once formed, an ice-sheet cools everything about it and chills the
climate of its hemisphere. It is a perpetual storehouse of cold. Mars
has no such general glaciation in either hemisphere, and the absence of
it, which is due to lesser precipitation, together with the clearness
of its skies, accounts for the warmth which the surface exhibits and
which has been found so hard hitherto to interpret. Could our earth but
get rid of its oceans, we too might have temperate regions stretching
to the poles.