194 Publications of the Measurements of The

194 PUBLICATIONS OF THE

MEASUREMENTS OF THE RADIATION FROM THE PLANET MERCURY

By Edison Pettit and Seth Β. Nicholson

The total radiation from Mercury and its transmission through a water cell and through a microscope cover glass were measured with the thermocouple at the 60-inch reflector on June 17, 1923, and again at the 100-inch reflector on June 21st. Since the thermocouple is compensated for general diffuse radiation it is possible to measure the radiation from stars and planets in full daylight quite as well as at night (aside from the effect of seeing), and in the present instance Mercury and the comparison stars were observed near the meridian between th hours of 9 a. m. and 1 p. m. The thermocouple cell is provided with a rock salt window 2 mm thick obtained from the Smithsonian Institution through the kindness of Dr. Abbot. The transmission curves for the water cell and microscope cover glass have been carefully de- termined : those for the former may be found in Astrophysical Journal, 56, 344, 1922; those for the latter, together with the curves for rock salt, fluorite and the atmosphere for average observing conditions are given in Figure 1. We may consider the water cell to transmit in the region of 0.3/a to 1.3/x and the covér glass to transmit between 0.3μ and 5.5/1,, although some radiation is transmitted by the cover glass up to 7.5/a. The atmosphere acts as a screen transmitting prin- cipally in two regions 0.3μ, to 2,5μ and 8μ to 14μ, respectively. It is chiefly through this latter band that planetary radiation (which may be defined as radiation emitted by the planet due to the warming of its surface) reaches us, although a small amount is also transmitted by the weak band 4.5μ to 6μ. The diameter of the image of the planet when observed with the 100-inch telescope on June 21, was 0.55 mm, while the diameter of the receivers is 0.40 mm. The phase angle being 70°, the image covered 50 per cent of the area of the receiver with about 18 per cent of the image overlapping at the horns. As the deflections were too great, the registering apparatus1 was

used at 1/5 scale distance. Under these conditions Mercury gave a deflection of 132.2 mm on direct exposure, but only 33.1 mm when a microscope cover glass was interposed in the beam, and 9.0 mm when the water cell was used. Cap ella gave a deflection of 23.0 mm on direct exposure. The transmission of radiation from Mercury through water cell and cover glass as a percentage of the total radiation observed, together with similar data for the Moon, Jupiter, Saturn, the Sun and two stars is given in Table I. The data for the Moon were obtained by taking drift curves across the disk on March 24 when the phase angle was about 85°, some- what greater than that of Mercury on June 21. The curves

Transmission—per cent Wave-length—μ Fig. 1. Transmission curves to. the atmosphere above Mt. Wilson (the unlabelled curve having eight minima between 0 and 8^, and two maxima · between 8 and 14m), microscope cover glass, fluorite, and rock salt.

which were taken along the line connecting the middle of the limb with a point near the middle of the terminator, have nearly the form of a right angle triangle, showing that the radiation for a point along the line observed is proportional to its distance from the terminator, being zero at the terminator and rising to a maximum at the limb. Drift curves taken through the water cell show that the percentage of transmission through this cell is practically uniform from the limb to a point at about 70 per cent of the distance to the terminator.

Here the water cell deflection becomes so small that further observations must be made before definitive transmissions can be obtained. No cover glass ratio was obtained for the Moon, but the value given here is inferred from observations on the ' Moon made with a thermocouple provided with a cover glass window, which gave 33.3 per cent transmission for the water cell. TABLE I Transmission of Radiation Object Water Cell Cover Glass Rock Salt Mercury 6.9 24.5 100 Moon 8.0 24.0 ' Jupiter 72.4 86.6 Jupiter dark band 70.3 " Saturn 70.5 " Saturn Ring 69.7 " Sun 67.5 92.2 Capella 60.0 93.2 Vega 77.6 93.1 It will be noted that the transmissions of the cover glass for the Sun, Capella, and Vega are nearly the same, deviating' little from the mean 92.8 per cent. We may infer from this that the lack of perfect transmission is due to radiation reflected from the cover glass when it is interposed in the beam, since practically no radiation is emitted by these bodies outside the region of high cover-glass transmission. Indeed this proves to be the case since a measurement of the reflected radiation from a cover glass when a beam of sunlight falls upon it at normal incidence gives 7.3 per cent. Applying this as a cor- rection to the second and third columns and taking the differ- ences we obtain Table II, which represents the radiation found in the regions 0.3μ to 1.3/a, 1.3/i. to 5.5^ and 8/x to 14/x, respectively. TABLE II Transmission Corrected for Reflection from Screens 0.3^ to 1.3/a 1.3/a to 5.5/¿ 8αι to 14/ι Mercury 7.4 19.0 73.6 Moon 8.6 17.3 74.1 Jupiter 78.1 15.3 6.6 Jupiter dark band 75.8 Saturn 76.0 Saturn Ring 75.1 Sun 72.8 27.2 0 Capella 64.8 35.2 0 Vega 83.7 16.3 0 1Astrophysical Journal, 56, 343, 1922.

The transmission in the fourth column then, may be taken to represent the planetary radiation entering our atmosphere through the transmission band 8μ to 14μ. Several facts are at once apparent: (1) Mercury, like the Moon, emits a large amount of planetary radiation, which may indicate that, like the Moon, it has little or no atmosphere; (2) The spectral distribu- tion of the radiation from Mercury is almost precisely that of the Moon, hence we may conclude that the two bodies have nearly the same surface temperature; (3) The radiation from Saturn's rings is practically all reflected sunlight. This might be expected from a consideration of the small value of the solar constant at Saturn's distance and the nature of the rings; (4) There is some planetary radiation from Jupiter. Conclusion (1) is also to be expected in view of the evidence from visual observations of the planet's surface, and its appear- ance during transit over the Sun's disk. One hesitates to ac- cept conclusion (2), that the temperatures of Mercury and the Moon are the same, however, in view of the relative nearness of the planet to the Sun. We may approach the problem from another angle by com- paring directly the intensity of the planetary radiation from the Moon and Mercury. If the Moon were at such a distance that its image were of the same diameter as the receiver, it would give a deflection equal to the average deflection obtained over its surface, when due allowance is made for the fact that the total area of the receiver does not receive radiation except at full phase. This quantity would be independent of the Moon's distance as long as this condition prevails. We may obtain the average deflection over the Moon's surface in still another way. The insolation, and hence the planetary radiation is isometric along small circles on the Moon's surface whose planes are parallel to the terminator, if we may neglect the possible effects of rotation. This is also true of the reflected light. Now, since the rate of radiation from a point on the Moon's surface along a line joining the middle of the limb and the middle of the terminator is proportional to its distance from the terminator for a phase angle of 85°, as was found from the drift curves of March 24, this must also be true for any line parallel to it. By

integration or mechanical quadratures it is easy to show that the average deflection which would be obtained by exposing a small receiver to an image of the Moon near first quarter i$ , equal to the value at the middle of the limb multiplied by the factor 0.43. This also is independent of the Moon's distance. Let us apply the first method of determining the average radiation independently of the distance to Mercury and the second method to the Moon and compare the results. The observed deflection from Mercury was 132 mm and since only 50 per cent of the receiver was covered by the image this deflection must be multiplied by the factor 2 in order to make it comparable with that for the Moon. This deflection then becomes 264 mm. In the case of the Moon, the observed deflection was about 300 mm. As one junction was used, the other being thrown out of the circuit, this deflection must be multiplied by the factor 4/5 to allow for the reduced resistance, and it must further be multiplied by the factor 2, since single deflections were observed. After making these corrections and multiplying by 0.43 we obtain the average deflection from the , Moon, which is 206 mm. Hence the planetary radiation per unit area is nearly the same from Mercury as that.-from the Moon, the ratio being 264/206. Again we must conclude that the surface temperature of Mercury cannot be greatly different from that of the Moon. It is hoped that a more complete comparison between the radiation frotruthese two bodies can be made in the near future. , An attempt was made to detect heat radiated from the dark side of Mercury. When the receiver was brought close to the limb a deflection of about 1.5 mm was obtained, but when the receiver was brought to the same distance from the terminator, approaching from the darkened side, a deflection of 4 mm was obtained. This may indicate that some radiation is epiitted by the dark side of Mercury, but further observations will be necessary to establish this point. Any radiation from the dark side of Mercury is an indication of a short rotation period, and this perhaps explains why the average temperature of the illuminated side of Mercury is nearly the same as that of the Moon. Mount Wilson Observatory, July, 1923.