Lunar and Planetary Science XLVIII (2017) 1578.pdf

J and H filter photometry of Mercury and the other bright planets. R. W. Schmude, Jr.1 1(Gordon State Col- lege, 419 College Dr., Barnesville, GA 30204; [email protected]).

Introduction: Since April of 2014, I have meas- Venus. The J and H filters penetrate to altitudes in ured the brightness of Mercury, Venus, Mars, Jupiter Venus’ atmosphere where temperatures are ~700 K and and Saturn. I have used an SSP-4 solid-state infrared ~500 K, respectively. [6] These temperatures imply photometer along with filters transformed to the Mauna that a portion of the H filter light may come from ther- Kea J and H system. The purpose of this work is to mal emission. The J – H color index rises slowly with summarize some early results. Normalized magni- increasing solar phase angle. At a solar phase angle of tudes, color indexes and light curve results are present- 60° it equals 0.05 and is much lower than the corre- ed. sponding value for Mercury. This is somewhat surpris- The SSP-4 photometer contains a model G5851 de- ing since the temperature of Mercury is comparable to tector manufactured by Hamamatsu Corporation [1]. It that of Venus’s atmosphere at the levels probed by the is sensitive to light having a wavelength of between 0.9 J and H filters. The normalized magnitudes were fit to and 2.05 µm. The J and H filters are sensitive to the several equations and the selected equation was a cubic wavelength ranges 1.15 to 1.35 µm and 1.5 to 1.8 µm, polynomial with α > 0° [7]. Two factors may cause a respectively. [1] Transformation coefficients were brightness change at the altitudes penetrated by the J measured using the star-pair method. [2] All meas- and H filter. The longitudes facing Earth may have urements were corrected for atmospheric extinction. some effect on Venus’ atmosphere. More importantly, Mean extinction coefficients, measured between April diurnal changes may be significant. When Venus is 2014 and December 2016, are 0.092 and 0.073 magni- near superior conjunction, its central meridian is near tudes/air mass for the J and H filters, respectively. local noon,. As that planet moves towards quadrature, Measurements were made from Barnesville, GA (ele- the central meridian moves away from local noon. vation ~250 meters). Therefore, during the time that Venus is visible, the Results: In the following paragraphs, I will sum- central meridian moves though different times of Ve- marize my work for each of the bright planets. In all nus’ day. The simplest interpretation of this data is that cases, measurements were normalized to planet-Sun there are little or no brightness changes as a result of and planet-Earth distances of 1.0 astronomical units. changing longitude or diurnal changes during 2014- These normalized magnitudes and respective solar 2016. The evidence for this is that the standard error phase angles were fit to equations. of estimate for the selected cubic equations is near 0.04 Mercury. The writer made 76 brightness measure- magnitudes. [7] ments of that planet between May 2014 and October, 2016. The phase angle spread was ~40° to ~120° for Table 1: Preliminary photometric constants for Mercu- the H filter and was a little lower for the J filter. The ry and Venus in the J and H filters. selected normalized magnitudes, based on best-fit cu- Parameter Mercurya Venusb bic equations, are summarized in Table 1. The J(1,60) J(1,0) ‒ ‒5.24 ± 0.14 value is the normalized magnitude at a solar phase an- H(1,0) ‒ ‒5.11 ± 0.14 gle (α) of 60°. Mercury’s maximum surface tempera- J(1,60) 0.36 ± 0.12 ‒4.62 ± 0.05 ture at perihelion is 740 K. [3] Therefore, thermal H(1,60) ‒0.30 ± 0.12 ‒4.67 ± 0.05 emission may be a significant amount of near infrared J ‒ H, α = 0° ‒ ‒0.13 ± 0.20 radiation given off by the planet. The J – H color in- J ‒ H, α = 60° 0.66 ± 0.15 0.05 ± 0.06 dex for that planet is larger than the corresponding V ‒ J, α = 0° ‒ 0.85 ± 0.15 value for our Sun. The V – J color index for Mercury V ‒ J, α = 60° 2.26 ± 0.13 1.01 ± 0.06 is 2.26 which is much higher than the corresponding c p , J filter ‒ 0.54 ± 0.07 value for our Sun, 1.12. [4, 5] Therefore it is redder c p , H filter ‒ 0.36 ± 0.05 than our Sun in the 1.15 to 1.8 µm wavelength range. a Values are based on unpublished results; manuscript The J – H color index drops as the solar phase angle will be prepared in mid-2017. increases which means that at α = 0°, the J – H value is b Most values are from [7]. probably even higher than 0.66. This is consistent cp = geometric albedo with thermal emission being a significant part of the H filter light coming from Mercury. Mars. The nearly dust-free atmosphere of Mars is believed to have a negligible impact on that planet’s Lunar and Planetary Science XLVIII (2017) 1578.pdf

near-infrared brightness. Photometric constants for Furthermore, its geometric albedo drops from 0.535 for 2014-2015 [8] are summarized in Table 2. Mars has the V filter (wavelength = 0.54 µm) to 0.11 for the H nearly the same J – H color index as the Sun (J – H = filter. This is consistent with reflectance spectra. [11], 0.31) [4]. This is different from its reddish color at [12] Banfield and co-workers [13] have shown that the shorter wavelengths. [9] Since Mars’ surface tempera- south polar region on Jupiter is much dimmer in the H ture is so low, practically all of the near-infrared light band than the North Equatorial Zone. Therefore, Jupi- coming from it is believed to be reflected sunlight. Its ter undergoes small seasonal brightness changes. light curves for 2015-2016 are shown in Figure 1. Es- Saturn. This planet is different from the other four sentially, the normalized magnitude, scaled to a solar just discussed because of the influence of its bright ring phase angle of zero degrees, was used in computing system. There are undoubtedly extrasolar planets J(1,0) and H(1,0) values. The mean solar phase angle which have a similar ring system. The selected nor- coefficient for both the J and H filters was found to malized magnitudes in 2014 for Saturn are J(1,0) = equal 0.0151 magnitudes/degree in late 2015 and 2016. ‒10.76 ± 0.05 and H(1,0) = ‒10.40 ± 0.05 and the cor- As can be seen in Figure 1, the planet brightens by ~0.4 responding values in 2015 are: ‒10.76 ± 0.03 and magnitudes as the longitude changes from 0° W to 130° ‒10.51 ± 0.03. [14] These values are different because W. A similar situation was observed in 2014 to 2015 of the different ring tilt angles. In 2014 and 2015, the [8]. rings were tilted at respective angles of ~22.5° and ~24°. As for Jupiter, the J – H color index is much Table 2: Preliminary photometric constants for Mars lower than the corresponding value for our Sun. This and Jupiter. is consistent with reflectance spectra. [11] Parameter Marsa Jupiterb Acknowledgements. The writer would like to J(1,0) ‒3.39 ± 0.07 ‒9.54 ± 0.02 thank Gordon State College for a faculty Development H(1,0) ‒3.71 ± 0.07 ‒9.07 ± 0.02 Grant in the summer of 2014. This grant enabled me to J – H, α = 0° 0.32 ± 0.10 0.46 ± 0.03 purchase the SSP-4 photometer. He is also grateful to V – J, α = 0° 1.79 ± 0.07 0.19 ± 0.03 the library staff at Gordon State College. Pc, J filter 0.32 ± 0.02 0.22 ± 0.008 References: [1] Optec Inc. Model SSP-4 Solid-State Infrared Pc, H filter 0.32 ± 0.02 0.11 ± 0.004 a Photometer Technical Manual for Theory of Operation Most values are from [8]. and Operating Procedures, Lowell MI (2005). [2] Hall bMost values are from [10] c D. S. and Genet R. M. (1988) Photoelectric Photome- p = geometric albedo try of Variable Stars, Second, Revised edition, Will- mann-Bell, Richmond, VA. [3] Strom R. G. (2007) Figure 1: The normalized J and H filter magnitudes Mercury in Encyclopedia of the Solar System, second plotted against longitude for Mars based on measure- edition, McFadden, L. ‒A., Weissman P. R. and John- ments made between October 2015 and September son T. V. – editors, Elsevier, Amsterdam. [4] Roddier 2016. F. et al. (2000) Icarus, 143, 299‒307. [5] Cox A. N. (2000) Allen’s Astrophysical Quantities, fourth edition, The Athlone Press, London, p. 341. [6] Taylor F. W. (2014) The Scientific Exploration of Venus. Cam- bridge University Press, Cambridge, UK. [7] Schmude, R. W. Jr. (2017) Submitted to The Journal of the Royal Astronomical Society of Canada. [8] Schmude R. W. Jr. (2016) J. Assoc. Lunar & Planet Obs. 58, No. 2, pp. 42‒55. [9] Mallama A. (2007) Icarus, 192, pp. 404‒416. [10] Schmude R. W. Jr. (2017) J. Assoc. Lunar & Planet. Obs., in the press. [11] Binder A. B. and McCarthy D. W. Jr. (1973) The Astronomical Journal, 78, No. 9, pp. 939-950. [12] Clark R. N. and Jupiter. Photometric constants, based on measure- McCord T. (1979) Icarus, 40, pp. 180-188. [13] Ban- ments made in mid-2014, are summarized in Table 2. field D. et al. (1998) Icarus, 134, pp. 11-23. [14] [10] Measurements made in 2015 are consistent with Schmude R. W. Jr. (2016) Georgia Journal of Science, little or no change in Jupiter’s normalized brightness as 74, No. 2, Article 17. its phase angle changes from 10° to 0°. Jupiter’s J – H color index is ‒0.46 and is well below that of our Sun.