Near-Infrared Spectral Monitoring of Pluto's Ices II: Recent Decline of CO and N2 Ice Absorptions

Published in 2014: Icarus 235, 220-224.

W.M. Grundy1,2, C.B. Olkin2,3, L.A. Young2,3, and B.J. Holler2,4.

(1) Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff AZ 86001.

(2) Visiting/remote observer at the Infrared Telescope Facility, operated by the University of Hawaii under Cooperative Agreement NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program.

(3) Southwest Research Institute, 1050 Walnut St. #300, Boulder CO 80302.

(4) Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, 1234 Innovation Dr., Boulder CO 80303.

Download the PDF preprint.


Abstract

IRTF/SpeX observations of Pluto's near-infrared reflectance spectrum during 2013 show vibrational absorption features of CO and N2 ices at 1.58 and 2.15 μm, respectively, that are weaker than had been observed during the preceding decade. To reconcile declining volatile ice absorptions with a lack of decline in Pluto's atmospheric pressure, we suggest these ices could be getting harder to see because of increasing scattering by small CH4 crystals, rather than because they are disappearing from the observed hemisphere.


Fig. 2. Secular evolution of Pluto's CO and N2 absorptions as measured by taking the sinusoidal fit to the ensemble data set for each parameter and then refitting just the constant term to each year's data separately. Diamonds (far-left axis) are for the equivalent width of the CO ice absorption at 1.58 μm while circles (near-left axis) are for the equivalent width of the N2 ice absorption at 2.15 μm. Squares are for the blue shift of the 1.19 μm CH4 band as measured by cross-correlation with a model based on pure CH4 ice absorption coefficients from Grundy et al. (2002). Diamonds and squares are offset in time by a quarter year to minimize overplotting. The wavy, gray, broken-line curves (arbitrarily normalized, and only applicable to the equivalent width data) show secular declines produced by geometric effects with ice-covered regions rotating out of view as the sub-Earth latitude moves north. The dot-dash curve is for ice confined to Pluto's southern hemisphere, while the plain dashed curve is for ice on the southern hemisphere plus latitudes up to 20N. The dotted curve is for an equatorial belt of ice between 20N and 20S latitude. Annual oscillations in these curves are due to the orbital motion of the Earth.


Data products associated with this paper

Circumstances of Pluto Observations
UT date + hourlonlatminutes
2001/07/04 08:59197.727.7120
2001/07/07 09:12 28.227.6147
2001/07/08 08:18333.927.6192
2002/06/24 06:32102.530.0 44
2002/06/25 06:18297.429.9 46
2002/06/26 06:30240.629.9 48
2002/07/15 07:15248.029.5 60
2002/07/16 07:18191.629.5 80
2002/07/17 06:56136.129.4 96
2002/07/18 06:49 80.029.4 88
2002/07/19 07:04 23.029.4 78
2002/07/20 06:48327.329.4 92
2002/07/21 06:49270.929.4 96
2002/07/22 06:43214.829.4104
2003/08/09 06:49226.231.1 96
2005/04/15 13:09104.237.1176
2005/05/09 13:04191.836.8188
2005/05/23 11:16127.136.5184
2005/05/24 12:02 69.036.5268
2006/07/19 08:19107.937.0148
2006/07/24 07:50187.237.0100
2006/07/25 07:44131.136.9160
2006/07/26 07:43 74.836.9168
2006/08/08 07:08 63.536.7126
2006/08/25 06:48186.236.5136
2007/06/23 10:47 73.139.5 32
2007/06/25 10:40320.739.4 36
2007/08/06 07:16121.838.5104
2007/08/07 07:30 64.938.5 80
2007/08/08 07:04 9.638.5108
2007/08/09 06:54313.638.5112
2007/08/10 07:04256.938.5116
2007/08/11 07:39199.238.5124
2008/06/16 11:00356.541.4 52
2008/06/17 10:56300.341.4 64
2008/06/18 11:06243.541.3 34
2008/16/19 10:38188.341.3 84
2008/06/20 10:40131.941.3 60
2008/06/21 10:41 75.541.3 60
2009/06/18 12:31185.943.0 64
2009/06/19 12:08130.543.0 80
2009/06/20 12:51 72.443.0 84
2009/06/21 12:24 17.143.0 58
2009/06/22 12:37320.342.9 88
2010/05/08 14:06278.245.5 80
2010/05/28 14:00231.445.1 64
2010/06/24 10:31158.144.5 92
2010/06/25 10:15102.444.5124
2010/06/30 10:13180.744.4 96
2010/07/01 10:00124.944.4112
2011/05/22 14:46153.246.8 52
2011/06/01 13:58311.646.6 72
2011/06/02 13:53255.446.6 76
2011/06/06 14:26 28.746.5 64
2011/06/07 13:55333.646.5 40
2011/06/11 13:49108.446.4 52
2011/06/12 11:54 56.646.4 64
2011/07/15 09:01 3.945.6 64
2012/05/12 13:30248.848.4104
2012/05/13 13:50191.648.4120
2012/05/14 13:55135.148.4 92
2012/05/15 14:02 78.448.4120
2012/05/16 14:02 22.148.4 96
2012/05/17 13:58325.948.4104
2012/09/09 06:12344.046.3 40
2013/05/21 13:54 45.949.7 64
2013/06/01 12:47148.749.5134
2013/06/12 13:33247.149.3 66
2013/06/26 10:38185.149.0 96
2013/06/27 10:37128.849.0104
2013/06/30 10:31320.048.9100
2013/07/27 08:30243.448.4104
2014/08/20 07:25278.949.3 64
2014/08/21 06:19225.249.3 62
2014/08/23 07:02110.849.3100
2014/08/25 06:51358.549.3104
2014/08/26 06:42302.549.2108
2014/08/27 06:43246.149.2128
2014/08/28 06:42189.849.2112
2014/08/29 06:51133.149.2112
2014/08/30 06:53 76.749.2120
2014/08/31 06:46 20.649.2108
2015/07/03 10:52 40.551.6128
2015/07/04 10:29345.151.6108
2015/07/05 10:58287.651.6 72
2015/07/06 10:27232.551.6116
2015/07/07 10:27176.151.5116
2015/07/08 10:29119.751.5104
2015/07/10 10:29 7.051.5104
2015/07/12 10:27254.451.4116
2016/07/04 10:32233.652.8104
2016/07/05 10:28177.552.8120
2016/07/06 10:29121.152.8116
2016/07/07 10:29 64.752.8120
2016/07/08 10:17 8.952.7135
2016/07/09 10:22312.352.7128
2017/05/21 14:41127.954.7 68
2017/05/22 14:29 72.054.7 76
2017/05/23 14:30 15.754.7 80
2017/05/24 14:36319.154.7 68
2017/05/25 13:57264.254.6139
2017/05/26 14:24206.855.8 88
2018/05/15 14:08 52.155.8116
2018/05/16 14:04355.955.8124
2018/05/17 14:06299.555.8124
2018/05/18 14:06243.155.8120
2018/05/19 14:04186.855.8124
2018/08/12 07:45 92.854.3120
2019/06/12 12:14223.556.4112
2019/06/13 11:41168.456.4128
2019/06/14 11:29112.556.4134
2019/06/15 12:04 54.856.4124
2019/06/16 11:42359.456.3120
2019/08/27 07:42272.655.1108
2019/08/28 07:35216.555.1116
2019/08/29 07:33160.255.1120
2019/09/01 07:46350.755.1100
2020/05/28 13:24234.157.4116
2020/05/29 14:05176.257.4120
2020/05/30 14:00120.057.4120
2020/05/31 13:21 65.257.4128
2020/06/01 13:57 7.457.4112
2020/08/31 06:44297.856.0143
2020/09/01 06:39241.756.0143
2020/09/02 06:42185.256.0128
2020/09/03 06:42128.956.0112
2020/09/04 06:36 72.855.9128
2021/05/19 14:10323.958.3100
2021/05/20 14:07267.758.3100
2021/05/21 13:55211.858.3 76
2021/05/24 14:09 42.258.2100
2021/09/18 06:14309.456.7120
2021/09/19 06:22252.856.7112
2021/09/20 06:20196.556.6116
2021/09/21 06:20140.256.6120
2021/09/22 06:18 83.956.6120

(see this 2013 paper for details regarding data up through 2012/09/09, and this paper for details of the 2013 data)

(contact Bryan Holler, Will Grundy, Leslie Young, and Cathy Olkin for details on 2014 and newer data)