5,2,9,10,14,18,- 1Southwest Research Institute, Boulder, CO, USA.
- 2Space Telescope Science Institute, Baltimore, MD, USA.
- 3University of Texas Institute for Geophysics, Austin, TX, USA.
- 4Lowell Observatory, Flagstaff, AZ, USA.
- 5Northern Arizona University, Flagstaff, AZ, USA.
- 6LESIA, Observatoire de Paris, Meudon, France.
- 7Southwest Research Institute, San Antonio, TX, USA.
- 8Konkoly Observatory, Budapest, Hungary.
- 9Max-Planck-Institut für extraterrestrische Physik, Garching, Germany.
- 10Université Paris-Saclay, CNRS, Orsay, France.
- 11Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
- 12LGL-TPE, Villeurbanne, France.
- 13Association of Universities for Research in Astronomy, Washington, DC, USA.
- 14NASA Goddard Space Flight Center, Greenbelt, MD, USA.
- 15SETI Institute, Mountain View, CA, USA.
- 16Institute of Space Science and Technology of Asturias (ICTEA), Universidad de Oviedo, Asturias, Spain.
- 17University of Texas at San Antonio, San Antonio, TX, USA
- 18Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
Makemake is one of the brightest and most methane-rich bodies in the trans-Neptunian region, with a spherical-equivalent diameter of ~1430 km and a high geometric albedo of pV ~0.8 [1,2]. Its near-infrared spectrum is dominated by strong methane (CH₄) ice absorption bands that appear broad and saturated—markedly different from those observed on other volatile-rich trans-Neptunian objects (TNOs) [3, and references therein]. Stellar occultation measurements revealed the absence of a global Pluto-like atmosphere, with a 1σ upper limit of 4–12 nanobar [1]. This result was interpreted as evidence for a strong depletion of nitrogen (N₂) ice, whose vapor pressure exceeds the microbar level even at Makemake’s coldest surface temperatures. Makemake’s volatile-rich surface and lack of a global atmosphere make it a compelling target for probing how surface volatiles—and CH4 in particular—evolve under irradiation and thermal cycling in the absence of atmospheric shielding.
Observations obtained with the JWST Near-Infrared Spectrograph (NIRSpec) provide unprecedented spectral coverage of Makemake from 1.0 to 4.8 μm. A previous analysis of the NIRSpec spectrum in the 3.9–4.8 μm range confirmed the presence of solid CH4 and placed tight upper limits on molecular N2 and carbon monoxide (CO) [4]. The same dataset also enabled the first measurement of the deuterium-to-hydrogen (D/H) ratio in CH4 ice on a TNO. The origin of methane on Makemake remains debated, with proposed scenarios ranging from primordial incorporation in the protosolar nebula [5] to production by internal geochemical processes and transport to the surface via cryovolcanism or other endogenic processes [6].
We present a comprehensive analysis of the full NIRSpec dataset, including the identification and modeling of CH4 absorption bands across the entire wavelength range, improved precision on the D/H ratio, and characterization of hydrocarbon irradiation products. In particular, we focus on tracing the chemical progression from CH4 to more complex hydrocarbons, such as ethane, ethylene, and acetylene, which are predicted by laboratory irradiation experiments and supported by previous near-infrared detections on Makemake [7,8].
In addition to compositional studies, we place our results in the broader thermal context of Makemake’s surface by referencing complementary JWST/MIRI measurements, which show a prominent mid-infrared excess in the 18–25 μm wavelength range, corresponding to brightness temperatures near 150 K [9]. This temperature significantly exceeds those expected from solar insolation alone. Possible interpretations include the presence of a localized thermally active surface region or an undetected dust ring composed of fine carbonaceous particles [9]. While no direct evidence for active outgassing has been observed, such phenomena remain plausible and underscore the need for continued monitoring.
The goal of this work is to better understand how volatile-rich TNOs evolve chemically and thermally under the combined effects of solar radiation, cosmic ray irradiation, and internal activity. By extending previous spectral coverage and conducting detailed modeling of hydrocarbon features, we provide new constraints on the irradiation chemistry, isotopic composition, and potential endogenic processes shaping Makemake’s surface. Our results contribute to a growing understanding of the diversity of TNO surfaces and the role of internal and external drivers in sculpting their volatile inventories.
[1] Ortiz, J. L., Sicardy, B., Braga-Ribas, F., et al. 2012, Nature, 491, 566
[2] Brown, M. E. 2013, ApJL, 767, L7
[3] Brown, M. E. 2012, Annual Review of Earth and Planetary Sciences, 40, 467–494
[4] Grundy, W. M., Wong, I., Glein, C. R., et al. 2024, Icarus, 411, 115923
[5] Mousis, O., Werlen, A., Benest Couzinou, T., & Schneeberger, A. 2025, ApJL, 983, L12
[6] Glein, C. R., Grundy, W. M., Lunine, J. I., et al. 2024, Icarus, 412, 115999
[7] Brown, M. E., Barkume, K. M., Blake, G. A., et al. 2007, AJ, 133, 284
[8] Brown, M. E., Schaller, E. L., & Blake, G. A. 2015, AJ, 149, 105
[9] Kiss, C., Müller, T. G., Farkas-Takács, A., et al. 2024, ApJL, 976, L9
How to cite: Protopapa, S., Wong, I., Johnson, P., Grundy, W. M., Emery, J. P., Lellouch, E., Holler, B., Glein, C. R., Kiss, C., Müller, T., Brunetto, R., Cartwright, R. J., Guilbert-Lepoutre, A., Hammel, H. B., Milam, S. N., Parker, A. H., Pinilla-Alonso, N., Raut, U., Santos-Sanz, P., and Stansberry, J.: JWST/NIRSpec Observations of Makemake: Hydrocarbon Chemistry and Surface Processes on a Methane-Rich Trans-Neptunian Object, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-968, https://doi.org/10.5194/epsc-dps2025-968, 2025.
