September 1, 1892.] 



KNOWLEDGE 



167 



bee itself. The family is called Sti/hpiihn, from the chief 

 genus Sti/Inps. But we must defer the consideration of 

 these interesting parasites till our next paper. 

 [To be continued.) 



T 



THE CLIMATE OF MARS. 



Bj' E. W. Maunder, F.R.A.S., Asxhtant sniienntendinq the 



Sular and Spectroscopic Drpartnunts of the Rcnjal Obserriitori/, 



(jrcenuich. 



I HE analogy between Mars and the earth is 

 perhaps by far the greatest in the whole solar 

 system." Such, we have been pretty frequently 

 reminded during the past few weeks, was the 

 opinion of Sir W. Herschel, and it may seem 

 very presumptuous to attempt to traverse the dictum of so 

 great an astronomer. Still, it may be worth while to 

 look a little closely into what we know of the Red Planet, 

 in order to ascertain if the facts of the case really bear out 

 this view. There are two great points of difference between 

 Mars and the earth which strike ns at once as havmg a 

 very important bearing on the climates of the two planets ; 

 first, the greater distance of Mars from the sun, in volvmg 

 its receiving a much smaller supply of light and heat, and 

 next, the greater length of the Martian year, rendering the 

 effect of its seasons more pronounced. 



Under the first head we find that Mars only receives, on 

 the average, three-sevenths of the light and heat which 

 falls upon a similar area of the earth. This would be a 

 serious matter, even if it only meant that the mean 

 temperature of Jlars lay three-sevenths of the distance 

 from freezing-point to the mean temperature of the earth. 

 But as it is, we must take the absolute zero of temperature 

 as om- starting-point ; that is, as is well known, 273° 

 Centigrade, gi^"ing us fi-om 130° to l-tO° below zero for our 

 result. 



This will be the mean temperature of the whole planet. 

 But Mars must have its various zones, differing from each 

 other as much as similar zones differ on the earth, and 

 if we confine our attention to the equator of Mars as the 

 hottest region, we may get a .somewhat closer ap- 

 proximation to its condition, for it is easy to find a zone 

 on the earth, where, owing to the oblique presentation of 

 the surface towards the sun, the light and heat incident 

 on any square mile is but three-sevenths of that falling on 

 an equal area at -the equator. This we find in Lat. 02°. 

 We may therefore take the difference in mean tem- 

 perature which we find between Iceland and Archangel 

 on the one hand, and Cayenne and Singapore on the 

 other, as affording some indication of the difference 

 between the equatorial temperatures of the two planets. 



This is, of course, to put the best possible construction 

 on the matter, for the circulation which goes on con- 

 tinuallj' in air and sea tends greatly to diminish the 

 difference between the climates of the several terrestrial 

 zones ; so that Singapore is cooler and Iceland wanner 

 than it should be if we considered latitude alone. This 

 second mode of approaching the problem confirms the 

 'first to this extent at least, that it gives the mean 

 temperature of even the hottest region of Mars as below 

 0° C, and consequently the mean temperature for the 

 planet as a whole must be lower still. 



Nor is this all. We know that the atmosphere of Mars 

 is far less dense than that of our own world. This is not so 

 obvious a difference as the two already mentioned, but 

 it is not less certain, for it is a direct conclusion from the 

 smaller size and mass of Mars, two points which might not 

 at first seem to have anything to do with temperature. 



For the mass of Mars is only one-ninth of that of the 

 earth, and this implies, when its smaller diameter is taken 

 into consideration, that the force of gi'a\'ity at its surface is 

 less than two-fifths of that which prevails here. If the 

 strength of terrestrial attraction were diminished till it 

 only equalled the Martian attraction, our atmosphere 

 would at once expand upwards to more than two and a 

 half times its present extent, and the pressure on the 

 surface of the earth (as measured by a spring balance) 

 would be only 5j pounds instead of 14 pounds to the 

 square inch, and the aneroid barometer would read 11-| 

 inches. So, if we assume that there is the same quantity 

 of air above every square inch of the surface of Mars that 

 there is for our earth, it would extend to a greater height, 

 and exert a smaller pressure in this proportion. 



This makes the condition of " our nearest neighbour " 

 yet worse. To find a terrestrial parallel, we must not 

 compare Archangel or even Spitzbergen with Cayenne, but 

 must compare the summit of a mountain more than four 

 miles high in the Arctic regions with a place at sea-level 

 on the equator. The result would appear to be conclusive — 

 that the mean temperature even of the equator of Mars lies 

 far below the freezing-point. 



Of course, it might be that Mars had so extensive an 

 atmosphere that the pressure at the surface was as great 

 as here, or even greater. If we could imagine the pressure 

 double that which we experience, we might suppose that 

 such an atmosphere would go far to compensate for the 

 diminished supply of solar heat. But that would mean 

 a total atmosphere over the unit of surface more than five 

 times as great as we have here ; and we may feel 

 perfectly sure that such is not the case. 



For, under such circumstances, we could not perceive 

 the surface markings of the planet. Now there is a very 

 marked gradation in the distinctness of the surface 

 markings of the different planets. Those of the moon are 

 perfectly distinct ; no atmospheric veil hides them from 

 us. The marlcings on Mars are much less apparent, 

 and yet are tolerably distinct. It needs but a small 

 experience to prove that the trouble in distinguish- 

 ing them, down even to the very limit of telescopic 

 minuteness, lies far more with our own atmosphere than 

 with that of the planet. This present opposition seems to 

 be a hopeless one for English astronomers, but that is not 

 in the least because Mars is modestly concealing himself 

 behind any cloud veil ; it is simply that he rises so little over 

 our horizon that we have to observe him through a great 

 thickness of our atmosphere. Venus, again, is a far more 

 difficult object, and such of its markings as are permanent, 

 if any deserve that designation, are only to be made out 

 under specially good conditions and by trained observers. 

 Jupiter, of course, shows us a wealth of beautiful details, 

 but it does not require a long scrutiny to see that these are 

 purely atmospheric, and few observers will be prepared to 

 affirm that we have ever seen any part of its real surface. 

 It is clear, then, that so far as these four bodies are 

 concerned, the denser and less transparent atmosphere is 

 always found in connection with the larger planet. Mars, 

 on this principle, should have a less atmosphere than the 

 earth, not a greater one. 



Further, Prof. Langley and Prof. Pickering have shown 

 us that the loss of light of a ray entering our zenith is 

 probably just about one half, the rest being partly absorbed 

 by our atmosphere, but mostly reflected and diffused by 

 the minute dust particles with which it is filled. It is 

 clear, then, that an observer on Venus would have the 

 greatest difficulty in making out the chief features 

 of our geography, for a large proportion of the light 

 he received from us would come from our atmosphere, 



