678 



SCIENCE 



[N. S. Vol. XXX. No. 776 



OXYGEN AS WELL AS WATER PEOVED TO EXIST IN 

 THE ATMOSPHEEE OF MAES 



In Science for January 29, 1909, I an- 

 nounced that I had determined the amount of 

 water vapor in the atmosphere of Mars by 

 quantitative measurements of the relative in- 

 tensification of little a in its spectrum, as 

 exhibited in the Mars-moon spectrograms 

 taken by Dr. Slipher at the Lowell Observa- 

 tory. Through the kindness of Dr. Lowell, I 

 have been enabled to continue the examina- 

 tion of these plates, and with improved facili- 

 ties, I am now able to add the definite estab- 

 lishment of a relative intensification of the 

 oxygen bands in the spectrum of Mars. 



Li my previous communication, an earlier 

 paper in the Philosophical Magazine was re- 

 ferred to, but not quoted, in which I ascribed 

 Dr. Slipher's success to his having made use 

 " not of the comparatively feeble ' rain-band ' 

 near D which has been the subject of much 

 contention in the past, but of the much more 

 powerful water-vapor band ' a ' in the extreme 

 red." This might be interpreted as meaning 

 that though little a is intensified in the spec- 

 trum of Mars, the rain-band near D is unaf- 

 fected, although I had no intention of making 

 such an assertion. Dr. Slipher did, indeed, 

 say that " the spectrum of Mars shows no 

 selective absorption not found in that of the 

 moon photographed under the same condi- 

 tions " ;^ but his obvious meaning is that 

 other Martian absorption bands, though 

 doubtless present on these plates, are too 

 feeble to be certainly distinguished in the 

 presence of telluric bands of the same wave- 

 length. I should say the same myself of any 

 immediately apparent intensification of the 

 rain-band or of the oxygen bands in the 

 spectrum of Mars; but it is well known that 

 by sufficiently delicate methods, and by taking 

 the average of a large number of observations, 

 almost vanishingly small quantities can be 

 evaluated. The reliability of the measurement 

 must be tested by its probable error, and by a 

 thorough investigation of the possible sources 

 of error. I have now made such an investiga- 



' Astrophysical Journal, 28, p. 403, December, 

 1908. 



tion of the relative intensity of great B in 

 the spectra of Mars and the moon at equal 

 altitudes, and find that B in Mars is more in- 

 tense by an amount eight times as great as 

 the probable error, thus confirming the exist- 

 ence of oxygen in the atmosphere of the 

 planet. Lowell Observatory Bulletin No. 41 

 may be consulted for the details of the obser- 

 vation. 



In Science for March 26, 1909 (p. 500), Pro- 

 fessor Campbell reproves me for not knowing 

 that " the effects of oxygen and water vapor 

 on Mars were no more visihle in the region 

 \ BkOO-X 6900 of the spectrum than were the 

 effects of oxygen and water vapor existing on 

 the moon!" (Italics and exclamation point are 

 Professor Campbell's.) The reason why I did 

 not mention facts which Professor Campbell 

 considers so obvious as to require only his 

 statement to prove them, is that I already had 

 evidence at that time that great B (A 686Y) is 

 more intense in Mars; but because the prob- 

 able error of the measurement with the appa- 

 ratus then used was large, I waited until im- 

 proved apparatus and more reliable results 

 could be obtained before making the an- 

 nouncement. 



Eight entirely independent series of meas- 

 ures were made on four plates, each containing 

 three spectrograms. No computations were 

 made until after the last measurement had 

 been completed, and I had no knowledge what- 

 ever of the significance of the result until the 

 computations were finished. Since every one 

 of the eight series gave a positive result, and 

 since the method was so guarded as to elim- 

 inate every source of possible error which is 

 laiown to me, I have no hesitation in an- 

 nouncing the intensification of great B in the 

 spectrum of Mars as a fact. Nevertheless, I 

 must warn any one who seeks to repeat the ob- 

 servation that its verification will demand 

 exceptional facilities, a long apprenticeship in 

 the art of delicate photometric comparisons, 

 and a good deal of patience and persistence. 

 The measurement is much more difficult than 

 the by no means easy one of the intensifica- 

 tion of the little a band of water vapor in 

 Mars. In illustration of the difficulty of the 



