3^8 



NA TURE 



[February 13, 1908 



and he stated that "water alone could do this.' 

 The author entirely disagrees with this interpretation, 

 "the only proof," as he says, "he gives that the 

 caps are frozen water." He points out that for 

 w-ater to be blue it must be deep, and this cannot ' 

 be so on Mars because its surface is so level. 



He finally indicates that there are two very im- 

 portant pieces of evidence which point to a lack of 

 water vapour on the Martian planet — the spectro- 

 scopic evidence, which must be taken into account, 

 entirely negatives the view of the presence of water 

 vapour; and Dr. Johnstone Stoney's proof that 

 aqueous vapour cannot exist permanently there, or 

 on any planet, unless its mass is at least a quarter 

 that of the earth. As the mass of Mars is only one- 

 ninth that of our earth, the planet must have parted 

 with its water vapour many, many centuries ago. 



In the next chapter the important question of the 

 probable temperature of the planet is taken in hand, 

 and the author shows to his own satisfaction, and 

 probably to that of the majority of his readers, that 

 the temperature must be far too low for the possibility 

 of any formation of a high form of organic life. 

 He introduces also a note stating the view on this 

 point given previously by Prof. J. H. Poynting, who 

 showed that unless an assumption be made that there 

 exists some quality in 'the atmosphere of Mars entirely 

 different from any found in our own, the temperature 

 of Mars cannot be as high as the value given to it by 

 Prof. Lowell. 



The author ingeniously considers the condition of 

 the Martian atmosphere as being intermediate be- 

 tween that of the earth (a dense atmosphere) and 

 that of the moon (practically no atmosphere). He then 

 refers to many researches on lunar radiation as re- 

 gards measurements made on portions of the surface 

 exposed and unexposed to the sun's rays. He recalls 

 the important function of a planetary atmosphere, like 

 that, for instance, of our own earth, in retaining and 

 cumulating solar heat and reducing radiation into 

 space. He finally deduces that the Martian condi- 

 tions of temperature must approximate more to 

 those of the moon than to those of the earth. Further, 

 he lays great stress on the impossibility of the 

 seasonal change at the Martian poles being an 

 apparent freezing and thawing of water, and he ex- 

 presses his view in the follovv-ing words : — 



" H the moon, even at its equator, has not its tem- 

 perature raised above the freezing point of water, 

 how can the more distant Mars, with its oblique noon- 

 day sun falling upon the snow-caps, receive heat 

 enough, first to raise their temperatures to 32° F., 

 and then to melt with marked rapidity the vast frozen 

 plains of its polar regions? " 



In the chapters referred to above the author has 

 presented his views as to the extreme probability of 

 a very low temperature and of the absence of water 

 vapour on Mars, and consequently replies to the ques- 

 tion " Is Mars Habitable? " in the negative. 



In the remaining portion of the book he makes an 



alternative suggestion as to the cause or origin of 



the surface markings and changes recorded on the 



planet. Just as he stated he had to part company 



NO. lOOS VOL. 77] 



with Prof. Lowell when he considered the latter's 

 deductions drawn from the discussion of his facts, so 

 we must part cojnpany with Dr. Wallace, and dis- 

 agree with his views on the peculiar, and what seems 

 unique, origin of tlie planet Mars. 



It may be recalled that Prof. W. H. Pickering, 

 next to Prof. Lowell, has made the most minute 

 study of the Martian surface details during the last 

 decade or more. Further, he had the advantage of 

 making his observations under practically similar 

 climatic conditions, and, in addition, he has also 

 closely studied the lunar features under specially fine 

 instrumental and atmospheric conditions. 



Prof. Pickering's suggested origin of the Martian 

 canals is that they, like the rifts and streaks on the 

 moon, are caused by volcanic action due to internal 

 stresses set up by the cooling of the planet's heated 

 interior. Dr. Wallace refers here to Prof. Pickering's 

 (vork, and, like him, looks upon the canals and oases 

 as the results of cooling. 



In order, however, to create conditions on a planet 

 which, when cooling, should be capable of producing 

 an enormous network of fissures of large dimensions, 

 and thus give a representation of the chief surface 

 markings as seen on Mars, he suggests the follow- 

 ing very ingenious but very questionable mode of 

 planetary formation, rather straining even the very 

 flexible nieteoritic hypothesis. 



He supposes that the planet began to be formed on 

 the principle of the meteoritic hypothesis, but that 

 the aggregation of the meteorites involved in the pro- 

 cess took place so slowly ihat the heat generated by 

 the bombardments was lost equally quickly by radi- 

 ation. So gradual, he suggests, did this state of 

 things occur that the planet attained its present size, 

 minus about 50 to 100 miles of the radius, having 

 grown to this dimension " as a solid and cold mass." 



He then tells us that this cold mass, in its revolu- 

 tion round the sun, at a later stage of its life, passed 

 through at each revolution a large and dense mass ot 

 meteorites. So violent were the impacts that the 

 " inpour of the fresh matter first heated and later on 

 liquefied the greater part of it, as well perhaps as 

 a thin layer of the planet's original surface." 



In this way the author produces a thin shell of 

 liquid or plastic material covering a solid and cold 

 interior, which he requires for the explanation of the 

 surface features of Mars. At the termination of this 

 series of annual bombardments this thin shell of 

 heated material would rapidly cool, and, as it is super- 

 imposed on a globe of cool matter, craterlets would 

 first be formed, and subsequently large fissures due 

 to contraction. The fissures would have no regard 

 for the equator, but would cross from one hemisphere 

 to the other, as the canals are recorded to do. 



The superficial tensions would render the crack- 

 eventually very broad and deep, and where they 

 crossed each other, holes, giving the appearance of 

 oases, would be formed. In time, both fissures and 

 oases would gradually crumble away at their sides, in 

 consequence of the alternate expansion and contrac- 

 tion of the material, due to the presence or absence 

 respectively of. the sun's heat. 



