234 CARNEGIE INSTITUTION OF WASHINGTON. 



enough to control a gaseous envelope in its earlier stages, a possible 

 atmosphereless stage is to be recognized. Just how massive a plan- 

 etary body must be to hold permanently an appreciable atmosphere 

 is not accurately computable at present, because of the uncertain 

 value of some of the factors involved.* A fairly safe conclusion 

 may perhaps be drawn from known celestial bodies. The moon 

 (g^j- of earth's mass) has no detectable atmosphere, nor has any 

 smaller body, whether satellite or asteroid, so far as known. Mars 

 (j\^ of earth's mass) has an appreciable, but apparently quite 

 limited, atmosphere. The limit between atmosphereless and atmos- 

 phere-bearing bodies probably lies between the two — /. e. , roundly 

 between one-eightieth and one-tenth of the earth's mass. The mass 

 of Mercury, unfortunately, is not known with satisfactor>' accuracy, 

 because it has no satellite and offers no other ready means of determi- 

 nation. Values all the waj- from one twenty-sixth to one- ninth of the 

 earth's mass have been assigned. Mercury gives no distinct signs 

 of atmospheric refraction, and its reflection of light (albedo) is very 

 low, even lower than that of the moon, and, like that of the moon, 

 is relatively 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 inaplies the aKsence of an atmosphere and 

 hydrosphere of sufficient value to give effective reflection of them- 

 selves 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 Mercur>" has no atmosphere that is effective as a weathering 

 or degradational agent, which is the point of geologic interest. 

 This brings the limit of appreciable atmosphere much nearer Mars 



* The following papers bear upon this subject : G. Johnstone Stoney : On the 

 Cause of the Absence of Hydrogen from the Earth's Atmosphere, and of Air 

 and Water from the Moon ; Poy. Dublin Soc, 1892. G. Johnstone Stoney : On 

 Atmospheres upon Plants and Satellites ; Trans. Roy. Dublin Soc, 2d series, 6, 

 1897 ; ibid., 1898, p. 305. T. C. Chamberlin : A Group of Hypotheses Bearing 

 on Climatic Changes ; Jour. Geol., vol. V, 1897, p. 653. G. Johnstone Stoney : 

 On the Presence of Helium in the Earth's Atmosphere and its Relation to the 

 Kinetic Theory of Gas ; Astrophys. Jour., vol. VIII. Dec, 1898, p. 316. S. R 

 Cook : On the Escape of Gases from Planetary Atmospheres According to the 

 Kinetic Theory; Astrophys. Jour., vol. XI, Jan., 1900, p. 36. G. Johnstone 

 Stoney : On the Escape of Gases from Planetary Atmospheres According to the 

 Kinetic Theory, No. I; Astrophys. Jour., vol. XI, May, 1900, p. 251 ; No. II, 

 idid., June, 1900, p. 325. G. Johnstone Stoney : Note on Inquiries as to the 

 Escape of Gases from Atmospheres ; ibid., vol. XII, Oct., 1900, p. 201. 



