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. Grundy^1,2, C.B. Olkin^2,3, L.A. Young^2,3, and B.J. Holler^2,4.

(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.

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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 20°N. The dotted curve is for an equatorial belt of ice between 20°N and 20°S latitude. Annual oscillations in these curves are due to the orbital motion of the Earth.

Data products associated with this paper

• Gzipped tar file of normalized, resampled Pluto spectra (see table below).
• Grand average Pluto spectrum.
• ADS catalog of refereed citations to this work.

Circumstances of Pluto Observations

UT date + hour	lon	lat	minutes
2001/07/04 08:59	197.7	27.7	120
2001/07/07 09:12	28.2	27.6	147
2001/07/08 08:18	333.9	27.6	192
2002/06/24 06:32	102.5	30.0	44
2002/06/25 06:18	297.4	29.9	46
2002/06/26 06:30	240.6	29.9	48
2002/07/15 07:15	248.0	29.5	60
2002/07/16 07:18	191.6	29.5	80
2002/07/17 06:56	136.1	29.4	96
2002/07/18 06:49	80.0	29.4	88
2002/07/19 07:04	23.0	29.4	78
2002/07/20 06:48	327.3	29.4	92
2002/07/21 06:49	270.9	29.4	96
2002/07/22 06:43	214.8	29.4	104
2003/08/09 06:49	226.2	31.1	96
2005/04/15 13:09	104.2	37.1	176
2005/05/09 13:04	191.8	36.8	188
2005/05/23 11:16	127.1	36.5	184
2005/05/24 12:02	69.0	36.5	268
2006/07/19 08:19	107.9	37.0	148
2006/07/24 07:50	187.2	37.0	100
2006/07/25 07:44	131.1	36.9	160
2006/07/26 07:43	74.8	36.9	168
2006/08/08 07:08	63.5	36.7	126
2006/08/25 06:48	186.2	36.5	136
2007/06/23 10:47	73.1	39.5	32
2007/06/25 10:40	320.7	39.4	36
2007/08/06 07:16	121.8	38.5	104
2007/08/07 07:30	64.9	38.5	80
2007/08/08 07:04	9.6	38.5	108
2007/08/09 06:54	313.6	38.5	112
2007/08/10 07:04	256.9	38.5	116
2007/08/11 07:39	199.2	38.5	124
2008/06/16 11:00	356.5	41.4	52
2008/06/17 10:56	300.3	41.4	64
2008/06/18 11:06	243.5	41.3	34
2008/16/19 10:38	188.3	41.3	84
2008/06/20 10:40	131.9	41.3	60
2008/06/21 10:41	75.5	41.3	60
2009/06/18 12:31	185.9	43.0	64
2009/06/19 12:08	130.5	43.0	80
2009/06/20 12:51	72.4	43.0	84
2009/06/21 12:24	17.1	43.0	58
2009/06/22 12:37	320.3	42.9	88
2010/05/08 14:06	278.2	45.5	80
2010/05/28 14:00	231.4	45.1	64
2010/06/24 10:31	158.1	44.5	92
2010/06/25 10:15	102.4	44.5	124
2010/06/30 10:13	180.7	44.4	96
2010/07/01 10:00	124.9	44.4	112
2011/05/22 14:46	153.2	46.8	52
2011/06/01 13:58	311.6	46.6	72
2011/06/02 13:53	255.4	46.6	76
2011/06/06 14:26	28.7	46.5	64
2011/06/07 13:55	333.6	46.5	40
2011/06/11 13:49	108.4	46.4	52
2011/06/12 11:54	56.6	46.4	64
2011/07/15 09:01	3.9	45.6	64
2012/05/12 13:30	248.8	48.4	104
2012/05/13 13:50	191.6	48.4	120
2012/05/14 13:55	135.1	48.4	92
2012/05/15 14:02	78.4	48.4	120
2012/05/16 14:02	22.1	48.4	96
2012/05/17 13:58	325.9	48.4	104
2012/09/09 06:12	344.0	46.3	40
2013/05/21 13:54	45.9	49.7	64
2013/06/01 12:47	148.7	49.5	134
2013/06/12 13:33	247.1	49.3	66
2013/06/26 10:38	185.1	49.0	96
2013/06/27 10:37	128.8	49.0	104
2013/06/30 10:31	320.0	48.9	100
2013/07/27 08:30	243.4	48.4	104
2014/08/20 07:25	278.9	49.3	64
2014/08/21 06:19	225.2	49.3	62
2014/08/23 07:02	110.8	49.3	100
2014/08/25 06:51	358.5	49.3	104
2014/08/26 06:42	302.5	49.2	108
2014/08/27 06:43	246.1	49.2	128
2014/08/28 06:42	189.8	49.2	112
2014/08/29 06:51	133.1	49.2	112
2014/08/30 06:53	76.7	49.2	120
2014/08/31 06:46	20.6	49.2	108
2015/07/03 10:52	40.5	51.6	128
2015/07/04 10:29	345.1	51.6	108
2015/07/05 10:58	287.6	51.6	72
2015/07/06 10:27	232.5	51.6	116
2015/07/07 10:27	176.1	51.5	116
2015/07/08 10:29	119.7	51.5	104
2015/07/10 10:29	7.0	51.5	104
2015/07/12 10:27	254.4	51.4	116
2016/07/04 10:32	233.6	52.8	104
2016/07/05 10:28	177.5	52.8	120
2016/07/06 10:29	121.1	52.8	116
2016/07/07 10:29	64.7	52.8	120
2016/07/08 10:17	8.9	52.7	135
2016/07/09 10:22	312.3	52.7	128
2017/05/21 14:41	127.9	54.7	68
2017/05/22 14:29	72.0	54.7	76
2017/05/23 14:30	15.7	54.7	80
2017/05/24 14:36	319.1	54.7	68
2017/05/25 13:57	264.2	54.6	139
2017/05/26 14:24	206.8	55.8	88
2018/05/15 14:08	52.1	55.8	116
2018/05/16 14:04	355.9	55.8	124
2018/05/17 14:06	299.5	55.8	124
2018/05/18 14:06	243.1	55.8	120
2018/05/19 14:04	186.8	55.8	124
2018/08/12 07:45	92.8	54.3	120
2019/06/12 12:14	223.5	56.4	112
2019/06/13 11:41	168.4	56.4	128
2019/06/14 11:29	112.5	56.4	134
2019/06/15 12:04	54.8	56.4	124
2019/06/16 11:42	359.4	56.3	120
2019/08/27 07:42	272.6	55.1	108
2019/08/28 07:35	216.5	55.1	116
2019/08/29 07:33	160.2	55.1	120
2019/09/01 07:46	350.7	55.1	100
2020/05/28 13:24	234.1	57.4	116
2020/05/29 14:05	176.2	57.4	120
2020/05/30 14:00	120.0	57.4	120
2020/05/31 13:21	65.2	57.4	128
2020/06/01 13:57	7.4	57.4	112
2020/08/31 06:44	297.8	56.0	143
2020/09/01 06:39	241.7	56.0	143
2020/09/02 06:42	185.2	56.0	128
2020/09/03 06:42	128.9	56.0	112
2020/09/04 06:36	72.8	55.9	128
2021/05/19 14:10	323.9	58.3	100
2021/05/20 14:07	267.7	58.3	100
2021/05/21 13:55	211.8	58.3	76
2021/05/24 14:09	42.2	58.2	100
2021/09/18 06:14	309.4	56.7	120
2021/09/19 06:22	252.8	56.7	112
2021/09/20 06:20	196.5	56.6	116
2021/09/21 06:20	140.2	56.6	120
2021/09/22 06:18	83.9	56.6	120
2022/05/31 13:07	314.6	58.8	116
2022/06/01 13:14	258.0	58.8	104
2022/06/02 13:12	201.7	58.8	124
2022/06/03 13:16	145.2	58.8	116
2022/06/04 13:14	88.9	58.8	116
2022/06/05 13:13	32.6	58.8	120
2022/08/11 08:02	230.5	57.8	160
2022/08/12 08:45	172.5	57.8	120
2022/08/13 07:28	119.2	57.8	120
2022/08/19 09:59	135.3	57.7	116
2022/08/22 13:43	331.6	57.7	144
2023/06/13 13:34	245.4	59.3	148
2023/06/14 13:34	189.0	59.2	140
2023/06/15 13:35	132.7	59.2	132
2023/06/16 13:41	76.1	59.2	132
2023/06/17 13:36	19.9	59.2	136
2023/06/18 13:40	323.5	59.2	132
2023/08/02 09:36	318.2	58.6	132
2023/08/03 09:01	263.2	58.6	112
2023/08/04 09:16	206.3	58.5	120
2023/08/05 09:16	150.0	58.5	116
2023/08/06 09:31	93.1	58.5	116
2024/06/26 13:05	122.1	59.6	168
2024/06/27 11:56	68.5	59.6	114
2024/06/28 12:08	11.7	59.5	140
2024/06/29 12:39	314.1	59.5	100
2024/07/01 10:45	205.9	59.5	140
2024/08/28 07:30	186.6	58.8	152
2024/09/02 09:21	260.6	58.7	136

(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)