Colles (Planetary Geology)

Colles (singular collis, from the Latin word for "small hill"; almost always used in the plural colles to designate a group of hills or a knobby field) is an official descriptor term in International Astronomical Union (IAU) planetary nomenclature used to identify a cluster of small hills, conical knobs, or hummocky mounds on the surface of a planet, satellite, or dwarf planet. Ranked below a mountain (mons / montes), a flat-topped mesa (mensa), or a steep isolated volcanic dome (tholus), individual mounds within a colles province typically measure 0.5 to 15 km across and rise 50 to 500 meters above the surrounding planitia or plateau (planum). First mapped systematically across Mars in Mariner 9 and Viking imagery—where fields such as Deuteronilus Colles, Cydonia Colles, Acidalia Colles, and Tartarus Colles mark the eroded margin of the global Martian dichotomy—named colles have since been cataloged across the basaltic plains of Venus (Asherat Colles, Olosa Colles), the hydrocarbon and water-ice plains of Saturn's moon Titan (Bilbo Colles, Nimloth Colles), and the nitrogen-ice glaciers of Pluto (Coleta de Dados Colles, Challenger Colles). Because planetary colles form via four distinct geological pathways—periglacial scarp retreat, lava–ice phreatomagmatic explosions (rootless cones), sedimentary mud volcanism, and small-shield effusive volcanism—their morphology and spatial clustering serve as diagnostic indicators of buried ground ice and volatile reservoirs in astrobiology.

Because IAU nomenclature classifies landforms by topographic geometry rather than genetic mechanism, fields designated colles encompass four distinct geological processes across rocky and icy worlds:

1. Erosional Knobby Remnants of Scarp Retreat (Martian Dichotomy Fretted Terrain)

Along the 700-km-wide transition zone separating Mars's ancient Noachian southern highlands from its low-lying northern plains (Vastitas Borealis), groundwater sapping, mass wasting, and periglacial glaciation dissected the highland plateau into flat-topped mesas (mensae). As a flat-topped mensa shrinks through cliff retreat and sublimation of interstitial ground ice, its flat cap rock eventually erodes away, leaving behind a field of hundreds of isolated, conical or rounded bedrock hills (knobby terrain, mapped formally as colles, such as Deuteronilus Colles, Cydonia Colles, and Avernus Colles). Many of these hills remain skirted by concentric viscous flow features and radar-transparent lobate debris aprons composed of >80% water ice (H₂O) beneath a rocky sublimation lag (Sharp, 1973; Head et al., 2006).

2. Rootless Volcanic Cones (Lava–Ground-Ice Phreatomagmatism)

Across the young Amazonian flood-basalt plains of Tartarus Colles, Elysium Planitia, and Amazonis Planitia on Mars, high-resolution imagery from Mars Global Surveyor MOC and Mars Reconnaissance Orbiter HiRISE revealed dense clusters of tens of thousands of small pitted cones (50 to 600 meters in basal diameter with summit craters half the cone width), often superposed on or adjacent to larger 2–10 km bedrock knobs (Lanagan et al., 2001; Bruno et al., 2004; Hamilton et al., 2011):

• Morphometrically identical to rootless cones (pseudocraters) at Lake Mývatn and Eldhraun in Iceland, these cones lack deep magmatic feeder conduits ("roots").
• Instead, when a sheet of molten basaltic lava (~1,400 K) flows across water-saturated sediment or shallow permafrost ground ice, conductive heating vaporizes the subsurface H₂O into high-pressure steam. Repeated phreatomagmatic steam explosions burst upward through the liquid lava core of an inflating sheet or lava tube, ejecting spatter and scoria to build clusters of small cones aligned along tortuous lava pathways.
• On Mars, rootless cone fields in Tartarus Colles and Athabasca Valles map out the exact spatial footprint of shallow equatorial and mid-latitude ground ice at the time of Late Amazonian lava emplacement.

3. Sedimentary Mud Volcanoes (Dewatering of Outflow Basin Sediments)

Within the northern sedimentary basins of Acidalia Planitia (Acidalia Colles) and Utopia Planitia (Arena Colles) on Mars, orbital cameras and the Zhurong rover documented thousands of smooth, low-albedo domical mounds 300 to 1,500 meters wide with central summit pits and lobate flank aprons showing low thermal inertia (fine-grained silt/clay rather than solid basaltic rock) (Skinner & Tanaka, 2007; Oehler & Allen, 2010). These mounds are interpreted as sedimentary mud volcanoes formed when rapid burial of water-rich Hesperian outflow-channel muds beneath freezing cryospheric lids generated overpressured subsurface mud-and-methane/brine slurries that vented to the surface—bringing deep Noachian–Hesperian aquatic sediments directly to the modern surface where rovers can sample them for preserved organic biosignatures.

4. Distributed Basaltic Small-Shield Fields on Venus

On Venus, Magellan synthetic aperture radar mapped more than 550 shield fields—several of the most prominent formally named colles (such as Asherat Colles, Jurate Colles, Marake Colles, and Olosa Colles, named after sea deities). Each Venusian colles province spans 100 to 300 km and contains 50 to several hundred small basaltic shield volcanoes measuring 2 to 20 km in diameter and 100 to 400 meters high, each topped by a single summit pit (Crumpler et al., 1997). Unlike terrestrial cinder cones (which are built by explosive gas expansion at 1 bar), the 92-bar (9.2 MPa) Venusian atmosphere suppresses gas fragmentation, so small magma batches erupt effusively as low-angle basaltic shields from a single shallow melting anomaly.

In the outer Solar System, Cassini and New Horizons extended the descriptor colles to icy satellites and Kuiper Belt dwarf planets:

• Titan's Hummocky Water-Ice Hills (Bilbo Colles, Faramir Colles, Gandalf Colles, Nimloth Colles): Named by the IAU after characters in J. R. R. Tolkien's Middle-earth legendarium, Titan's colles are clusters of radar-bright, rounded hills (1 to 10 km wide, 100 to 300 meters high) rising above the dark equatorial longitudinal dune fields (undae) and organic plains. Spectral and SAR backscatter analyses indicate that these hills are ancient water-ice (H₂O) bedrock blocks fractured by regional compression or impact ejecta and heavily incised by liquid methane (CH₄) rainfall runoff (Radebaugh et al., 2007).
• Pluto's Convecting Iceberg Clusters (Coleta de Dados Colles, Astrid Colles, Columbia Colles, Challenger Colles): Named after historic spacecraft and space missions, the colles inside Pluto's Sputnik Planitia are clusters of angular hills 1 to 5 km across concentrated within the boundary troughs between 30-km nitrogen-ice convection cells. Because solid water ice (ρ ≈ 920 kg m⁻³ at 37 K) is significantly more buoyant than solid β-nitrogen ice (ρ ≈ 1,020 kg m⁻³), blocks of water-ice bedrock detached from the western highlands float like icebergs on the denser convecting N₂ glacier and are swept by surface currents into congested clusters ("colles") over the sinking downwelling boundaries (McKinnon et al., 2016).

What is the difference between colles, montes, mensae, and tholi?

In IAU planetary nomenclature, these four descriptors classify positive-relief landforms by size, slope, and shape. Montes (singular mons) are large mountains or mountain chains rising 1 to >20 km high. Mensae are flat-topped, steep-sided mesas bounded by cliffs. Tholi are isolated, steep-sided domical hills or small conical volcanoes (typically 10 to 100 km wide). Colles are clusters or fields of small hills, knobs, or mounds (typically 0.5 to 15 km wide and <500 m high) grouped together across a plain.

How do rootless volcanic cones form in Martian colles provinces like Tartarus Colles?

Unlike a normal volcano, which sits directly above a deep magma conduit coming from the mantle, a rootless cone (pseudocrater) forms when a horizontal surface lava flow spreads over wet mud or shallow ground ice. Heat from the lava flashes the underlying water ice into pressurized steam, which blasts upward through the molten lava sheet to build swarms of small scoria cones whose footprints trace the underlying ice table.

Why are Pluto's colles floating inside Sputnik Planitia?

At Pluto's cryogenic surface temperature (~37 K or −236 °C), ordinary water ice (H₂O) is rigid bedrock with a density of ~0.92 g cm⁻³, whereas solid nitrogen ice (N₂) is a denser, ductile glacier with a density of ~1.02 g cm⁻³. Because water ice is ~10% less dense than solid nitrogen ice, chunks of water-ice crust broken off the surrounding highlands float at the top of the nitrogen glacier and get herded by slow thermal convection currents into floating hill clusters (colles) along cell edges.

1. Sharp, R. P. (1973). "Mars: Fretted and chaotic terrains." Journal of Geophysical Research, 78(20), 4073–4083. https://doi.org/10.1029/JB078i020p04073
2. Lanagan, P. D., McEwen, A. S., Keszthelyi, L. P., & Thordarson, T. (2001). "Rootless cones on Mars indicating the presence of equatorial ground ice in recent times." Geophysical Research Letters, 28(12), 2365–2367. https://doi.org/10.1029/2001GL012932
3. Skinner, J. A., Jr., & Tanaka, K. L. (2007). "Evidence for and implications of sedimentary diapirism (mud volcanism) in the southern Acidalia Planitia, Mars." Icarus, 186(1), 41–59. https://doi.org/10.1016/j.icarus.2006.08.013
4. Hamilton, C. W., Fagents, S. A., & Thordarson, T. (2011). "Lava–ground ice interactions in Elysium Planitia, Mars: Geomorphological and geospatial analysis of the Tartarus Collescone group." Journal of Geophysical Research: Planets, 116(E3), E03004. https://doi.org/10.1029/2010JE003657
5. Radebaugh, J., Lorenz, R. D., Kirk, R. L., Lunine, J. I., Stofan, E. R., Lopes, R. M. C., & Wall, S. D. (2007). "Mountains on Titan observed by Cassini Radar." Icarus, 192(1), 77–91. https://doi.org/10.1016/j.icarus.2007.06.020