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Climate zones on Pluto

The unusual geometric configuration of Pluto such as higher mean obliquity of 115° and higher amplitude ${\textstyle \pm }$ 10° of obliquity oscillation of over a time period of 2.8 million years produce very unusual climate zone on the draft planet. These climate zones are delineated based on astronomically defined boundaries based on sub-solar latitudes which are not associated with the atmospheric circulations. Five climate zones are assigned on the dwarf planet: tropics, arctic, tropical arctic, diurnal, and polar. The synchronous alignment (tidally locked mutually or binary pair) of Charon, the largest moon, with Pluto dictates that the moon has the same climate zone structure as the dwarf planet does have.

Haze with multiple layers in the atmosphere of Pluto. Part of the plain Sputnik Planitia with nearby mountains is seen below. Photo by New Horizons, taken 15 min after the closest approach to Pluto.

Pluto is an icy body in the kuiper belt of the solar system. The kuiper belt objects (KBOs) are the third zone of the solar system, beyond the terrestrial bodies (planets, moons, and asteroid belt) and gas giants and their moons. Pluto is the most prominent body in the KBOs. The surface of the planets is primarily composed of methane (CH₄), nitrogen (N₂), and carbon monoxide (CO) ices in various spatial abundances and distribution. Though Pluto is small compared to typical planets, the body possesses an atmosphere. The atmosphere of the dwarf planet is very thin compared to the Earth and it also consists of multiple layers of hazes. The atmosphere of Pluto composed mainly of nitrogen (N₂) with trace methane (CH₄) and carbon monoxide (CO). Along with this thin atmosphere, the dwarf planet has very unusual climate zones which are the product of the unusual geometric configuration of Pluto. The climate zones of Pluto are defined and delineated as the which regions (latitudes) are tropics, arctics, or both. Long term climate cycles of planetary bodies (e.g., Earth) are associated with the obliquity variation of the parent body, orbital eccentricity cycle and axial precession cycles. However, in the case of Pluto, the orbital eccentricity cycle and axial precession cycles have less influence in the climate cycles of the dwarf planet than the obliquity variation. Since the obliquity variation of the dwarf planet influences the climate cycles, the climate zones of Pluto were determined based on the obliquity variation only.

Characteristics and boundaries

There are five climate zones on Pluto which are defined by the sub-solar latitude. The climate zones have some specific boundaries. However, the latitude ranges of the climate zones expand and shrink in response to the obliquity range of Pluto from a minimum of 103° to a maximum of 127° over the 2.8 million year oscillation period. That means some of the zones have permanent latitude ranges whereas some zones also change its boundaries based on the variation of the axial tilt over the millions of year time periods. The permanent boundary means the latitude zone that experiences certain characteristics always regardless of the obliquity variation of the dwarf planet over its 2.8 million year oscillation period.

Tropics

Pluto climate zones as defined by astronomical cycles. For clarity these zones are depicted in three panels for each of the predominant seasonal effects. Pluto's New Horizons basemap is shown for reference (image credit: NASA/JHUAPL/SwRI).

The tropics of Pluto is defined as the latitude ranges where the Sun can reach overhead point or zenith point on at least one time during its one orbital period around the Sun. Currently, the tropics stretches from the latitude of 60°N to 60°S with the current obliquity of 120°. However, when the obliquity goes down to minimum of 103°, the tropics expand to the maximum latitude ranges of from 77°N to 77°S. At this period, the tropics cover most of the surface of the dwarf planet encompassing around 97 percent of the planet surface. In contrast, when the obliquity rises to maximum of 127°, the tropics contrast down to the minimum latitude ranges of from 53° N to 53° S. At that time, the tropics only covers 80 percent of the surface area of the planet. This 53° N to 53° S latitude ranges are called the permanent tropics of Pluto. Since the permanent tropic covers at least 80% of the surface therefore most of the Pluto surface always lies in the tropical area. The 53° N to 53° S region always receive direct overhead sunlight during every orbital period of Pluto and this zone is called permanent tropics. The tropical zone of Pluto overall shows the greatest range of albedo variation.

Arctic

Pluto's arctic is defined as the latitude ranges that experiences a period of continuous sunlight during summer seasons and experiences continuous dark during winter season. The arctic zones of the dwarf planet ranges from 90°N to 37°N in the northern hemisphere and in the southern hemisphere this southern arctic ranges from 90°S to 37°S. Combining these two arctic cover around 40 percent of the total surface of the planet and these zones are called the permanent arctic zones. The surface constitutional data of Pluto shows that the permanent arctic zones are co-located with N₂ rich surface. A long period of continuous winter typically lasting a more than a century is experienced by the permanent arctic zones in every Pluto orbital period of 248 years over the 2.8 million year obliquity cycle. Due to the variation in the axial tilting of Pluto, the arctic zones also expand and contrast over the period of 2.8 million year oscillation period. With the current obliquity of 120°, the arctic zones extend from 30°N to 90°N in the norther hemisphere whereas it is 30°S to 90°S in the southern hemisphere, combining two zones cover 50 percent of the total surface area. The maximum latitude ranges of the arctic extent to near-equatorial latitude of 13°N and 13°S when the obliquity goes down to minimum of 103°. At that time, the arctic circles cover around 78% of the total surface area. The high obliquity induced in latitude variations in arctics zones extending down to near-equatorial regions indicates that the maximum amount of solar insolation is received (and therefore the maximum energy budget) at the poles.

Diurnal

The diurnal climate zone of Pluto is defined as the latitudinal ranges where day and night cycle occur for each and every orbital rotation throughout the entire orbital year. This zone experiences continuous daily sunrise and sunset. The high mean obliquity and oscillation cycle makes the latitude ranges of 13°N and 13°S to experiences a diurnal circle. This narrow latitudinal band of 13°N and 13°S is, symmetric to equator, called permanent diurnal zone. Pluto experiences the narrowest band of diurnal zone (13°N and 13°S) when the obliquity comes down to minimum of 103° in around ~0.8 million year ago. The permanent diurnal zone covers an area of 22 percent of the total areas of the dwarf planet. The region experiences a continuous sunrise and sunset for each and every Pluto rotation period of 6.4 days over a time period of 10 million years. The present day diurnal climate zone of Pluto spans from 30°N to 30°S, encompasses an area of 50% of the total surface area, with the axial tilt of 120°. As obliquity changes to rise from current obliquity of 120°, the diurnal zone will expand to the maximum from 37°N to 37°S (covering a total area of 60 percent of the planet) which will be reached in ~0.6 million year. The morphology of this zone is characterized by a nearly uniform width of dark equatorial bands and comprises a zone of the mostly sharply contrasting albedo.

Tropical arctic

The tropical arctic zone covers the overlapping regions of the tropical and arctic zones. This unique climatic zone is only possible to objects that has an obliquity range between 45° - 135°. Since the obliquity of Pluto envelops between a minimum of 103° to maximum of 127°, therefore, the dwarf planet also has a tropical arctic climate zone. Due to high obliquity, most of the surface of Pluto experiences both tropical and arctic climate experiences over the 2.8 million years obliquity cycle. The tropical and arctic zones extend as close to 77° and 13° in both hemispheres, respectively. The tropical arctic zone overlaps between tropical and arctic zones extending between 13°N to 77°N and 13°S to 77°S. At this latitudinal range the zones cover an area of 75 percent of the total surface area and the majority of the Pluto's surface areas experiences the tropical arctic cycle. However, like the oscillation of tropical (between 53 - 77° latitude) and arctic (between 13 - 37° latitude) zones, the tropical arctic zone also expands and contrast in response to the Pluto's obliquity cycles of 2.8 million year. Only the latitude range between 37° to 57° in each hemisphere remains stable over the period, therefore, this bands of latitudinal ranges are called the permanent tropical arctic zone of Pluto (covers 20% of the total area). The maximum extend of the tropical arctic zones spans between 13 - 77° latitude in both southern and northern hemispheres (covers about 75% of the total surface area). With the current obliquity of 120°, the present day tropical arctic extends from 30°N to 30°S encompassing an area of 50% of the Pluto's surface. The tropical arctic zone is the area that experiences maximum cyclical extremes and optimum for long-term seasonal layering (e.g., Al-Isidis latitudes).

Polar

The polar zone of the Pluto is the areas where the Sun never reaches the zenith or overhead point at any time of the orbital period of the dwarf planet over its 2.8 million year obliquity cycle. The polar zone has a radius of 13° from each pole. The polar zone always experiences arctic climate and does never experiences tropical climate. The zone spans from 77°N to 90°N and from 77°S to 90°S, encompassing an area total of 3% of the planet. The regions of 77° to 90° latitudes of each hemisphere is called as permanent range of the polar climate zone. The polar zone is the region that consistently experiences the longest duration of arctic winter and summer during each and every orbital rotation of Pluto. However, the maximum extend of the polar zone can spans up to 53°N to 90°N and from 53°S to 90°S with an area total of 20% of the planet. At present day the polar zones extend between 60 - 90° of both the both northern and southern hemisphere (covers an area of 13% of the planet). Pluto's biggest moon Charon also has a polar zone consisting of polar cap.

Correlation to surface morphology

The diurnal zone of Pluto between 13°N to 13°S never experiences any period of continuous either of the arctic summer or winter. The equatorial regions of Pluto predominated consist of a dark band of distinct boundaries. This equatorial dark band region is coinciding with the permanent diurnal climate zone of Pluto. The high albedo terrain of diurnal zone remains warm enough that no cold trapping is attracted for new volatile. The absence of any interval of arctic winter darkness also preclude the possibilities of volatile cold trapping in the high albedo diurnal zone areas. On the other hand, the lower albedo region in the diurnal zone has reflectivity to solar insolation. These higher albedo areas radiate away absorbed energy at night for every rotation of the planet and become a cold trap for volatile. The temperature cycle near the equator (in the diurnal zone) maintain a sharp albedo variations between the bright and dark regions.

The correlation between the surface compositional distribution of methane (CH₄), nitrogen (N₂) and the climate zones are somehow speculative. Volatile CH₄ is widely distributed outside the Tombaugh Regio and Cthulhu Regio at latitudinal bands of the northern boundary of Cthulhu. On the other hand, volatile N₂ is concentrated in the latitudes above 30°N coinciding the permanent arctic climate zone which spans from 37°N to the pole. Some areas on Pluto's surface experience both tropics and arctic climate since both of the climate zones oscillate in response to the obliquity cycle. A substantially large area falls under tropical arctic climate state due to the oscillation of the climate pattern. The layering of the al-Isidis Monts at 37°N latitude is speculated to be form due to the long-tern climate cycle in this latitude. The latitude of 37°N is considered as a boundary latitude involving the mixing of tropical (overhead Sun) and arctic (constant dark winter) season.

See also

• Pluto
• Geology of Pluto
• Geography of Pluto
• Atmosphere of Pluto