The Methane Ice Plains and Dark Tholin Terranes of Makemake
Makemake moves along a moderately eccentric orbit ($e \approx 0.16$) inclined at nearly 29 degrees to the ecliptic plane. Completing a single revolution around the Sun every 305 Earth years, the planet experiences extreme seasonal shifts driven by its fluctuating heliocentric distance, which ranges between 38.4 AU at perihelion and 52.8 AU at aphelion. At these immense distances, solar flux is reduced to less than 0.1 percent of its intensity at Earth, driving surface temperatures down to a staggering 30 to 35 Kelvin (-243 to -238 degrees Celsius).
Surface Composition and Spectral Signatures
Spectroscopic observations conducted across visible and near-infrared wavelengths reveal that Makemake’s surface is dominated by frozen volatile compounds, primarily methane ($CH_4$), ethane ($C_2H_6$), and nitrogen ($N_2$). Methane ice exists in at least two distinct structural phases: large, millimeter-to-centimeter-sized crystalline grains that form bright, highly reflective sheets, and fine-grained icy matrices mixed with photolytic byproducts.
Exposure to solar ultraviolet radiation and galactic cosmic rays over billions of years breaks down methane and nitrogen molecules, initiating complex radiolytic reaction chains. These processes synthesize heavy organic polymers known as tholins. Tholins impart a distinct reddish-brown coloration across large swathes of Makemake's surface, similar to the dark Equatorial regions seen on Pluto. However, unlike Pluto, which retains massive reservoirs of volatile nitrogen ice that drive active glacial cycles, Makemake exhibits a higher ratio of methane to nitrogen, suggesting a distinct evolutionary history of volatile depletion.
Atmospheric Dynamics and Stellar Occultation
For several years following its discovery, theoretical models predicted that Makemake might possess a transient global atmosphere similar to Pluto's—expanding via sublimation when approaching perihelion and collapsing into surface frost as it recedes toward aphelion. However, a critical milestone in understanding Makemake occurred on April 23, 2011, when the dwarf planet passed directly in front of an 18th-magnitude star, an event monitored by multiple ground-based observatories across South America.
The stellar occultation data revealed an abrupt light curve drop without the gradual dimming characteristic of a thick atmospheric envelope. Astronomers concluded that Makemake lacks a global atmosphere down to a surface pressure limit of roughly 4 to 12 nanobars. Instead, volatile gas is likely confined to localized, transient micro-atmospheres hovering over warmer, tholin-rich surface patches, while colder methane ice sheets absorb volatile vapors out of the ambient environment.
Density, Interior Structure, and Satellite System
The discovery of Makemake's sole known moon, S/2015 (136472) 1—informally designated MK2—by the Hubble Space Telescope in April 2015 provided the missing key to calculating the dwarf planet's mass. Orbiting at a distance of approximately 21,000 kilometers with a period of roughly 12 days, MK2 is an extraordinarily dark object, reflecting less than 1 percent of incident light compared to Makemake's overall high average visual albedo of approximately 0.81.
By tracking MK2's orbital motion, planetary scientists determined Makemake’s mass to be approximately $3.1 \times 10^{21}$ kilograms. Combined with precise volume measurements derived from thermal imaging and occultation profiles, Makemake’s mean density is estimated at $1.7$ to $1.9 \text{ g/cm}^3$. This density points to a differentiated internal structure consisting of a substantial rocky silicate core surrounded by a dense water-ice mantle and an outer volatile crust composed of methane, nitrogen, and carbon monoxide frosts.
Geological Regimes and Thermal Heterogeneity
Thermal infrared measurements from space-based observatories indicate that Makemake’s surface is far from uniform. The planet displays significant thermal heterogeneity, divided into two primary geological domains: vast, ultra-bright plains covered in pure crystalline methane ice, and dark, low-albedo terrain patches enriched in reddish tholins.
The bright methane plains exhibit high thermal inertia, remaining exceptionally cold because they reflect the vast majority of absorbed sunlight. In contrast, the dark hydrocarbon patches absorb solar radiation more efficiently, heating up by several Kelvin relative to their surroundings. This temperature differential drives localized volatile transport, where sublimating methane gas migrates away from the warmer tholin deposits and cold-traps onto the cooler, high-albedo ice sheets, perpetually renovating the surface appearance of this remote world.
