HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 93 



haps be drawn from known celestial bodies. The moon (1/81 of earth's 

 mass) has no detectable atmosphere, nor has any smaller body, whether 

 satellite or asteroid, so far as known. Mars (1/9.34 of earth's mass) 

 has an appreciable but apparently quite limited atmosphere. The 

 limit between atmosphereless and atmosphere-bearing bodies probably 

 lies between the two, i.e., roundly, between 1/80 and 1/10 of the earth's 

 mass. 



The mass of Mercury, unfortunately, is not known with satisfactory 

 accuracy, because it has no satellite and offers no other ready means 

 of determination. Values all the way from 1/26 to 1/9 of the earth's 

 mass have been assigned. Mercury gives no distinct signs of atmos- 

 pheric refraction, and its reflection of light (albedo) is very low, even 

 lower than that of the moon, and, like that of the moon, is rela- 

 tively much stronger for surfaces normal to the line of incidence and 

 of vision than for those oblique to it, which is characteristic of a rough 

 surface. All this implies the absence of an atmosphere and hydrosphere 

 of sufficient value to give effective reflection of themselves, or to develop 

 a good reflecting body by smoothing down the surface and filling up 

 the pores. On the other hand, certain lines of the planet's spectrum 

 have been thought to imply the presence of water- vapor, but this is 

 not conclusive. The probabilities seem to be that Mercury has no 

 atmosphere that is effective as a weathering or degradational agent, 

 which is the point of geologic interest. This brings the limit of appre- 

 ciable atmosphere much nearer Mars than the moon, and justifies the 

 provisional conclusion that if the young earth had no more than 1/20 

 of its present mass it probably possessed no atmosphere of appreciable 

 geological efficiency, but that, when it had gained 1/10 of its present 

 mass (radius probably about 2100 miles) an appreciable though rela- 

 tively slight atmosphere surrounded it. 



III. The Initial Atmospheric Stage. 



The origin of the atmosphere. — The origin of the atmosphere under 

 the gaseo-molten line of evolution has become familiar. Its origin 

 under the accretion hypothesis is so different as to present more points 

 of contrast than of similarity. When the growing earth reached a 



251; No. II, Ibid., June 1900, p. 325. G. Johnstone Stoney, Note on Inquiries as to 

 the Escape of Gases from Atmospheres, Ibid., Vol. XII, October 1900, p. 201. 



