Palus (Planetary Geology)

Palus (plural paludes, from the Latin word for a "marsh", "swamp", "bog", or "fen") is an official descriptor term in International Astronomical Union (IAU) planetary nomenclature used to designate a comparatively small, flat plain—smaller than a lunar mare ("sea") or a planetary planitia ("lowland plain")—on the surface of Earth's Moon and Mars. First introduced into lunar cartography in 1647 and 1651 by the telescopic selenographers Johannes Hevelius and Giovanni Battista Riccioli, who imagined that dark, smooth lunar basins were liquid seas (maria) and that smaller, semi-bright marginal plains bordering mountain ranges were coastal marshes (paludes), the descriptor is retained today for three major lunar paludes (Palus Putredinis, Palus Epidemiarum, and Palus Somni) and since 2006 has been extended to Martian sedimentary and volcanic plains—most famously Aeolis Palus on the northern floor of Gale Crater (the landing site and traverse basin of NASA's Curiosity Mars Science Laboratory rover) and Cerberus Palus in Elysium Planitia. Within lunar petrology and astrobiology, planetary paludes encompass two of the most thoroughly sampled in-situ geological environments in the Solar System: the Apollo 15 Hadley–Apennine mare basalt and KREEP landing site in Palus Putredinis, and the Noachian–Hesperian habitable lakebed mudstones of Aeolis Palus on Mars.

When Giovanni Battista Riccioli and Francesco Maria Grimaldi published their landmark lunar map in Almagestum Novum (1651)—establishing the nomenclature framework still used by the IAU today—they assigned water-themed Latin descriptors according to feature size and optical albedo: broad dark basins were named Oceanus and Mare ("sea"), embayments were named Sinus ("bay"), small isolated dark patches were named Lacus ("lake"), and flat, intermediate-albedo plains flanking the edges of major maria were named Palus ("marsh").

While several 17th- and 19th-century palus names (such as Palus Nebularum, "Marsh of Mists", inside eastern Mare Imbrium) were discontinued by the IAU in the 20th century, three canonical lunar paludes remain in official nomenclature:

1. Palus Putredinis ("Marsh of Decay") and the Apollo 15 Mission (1971)

Located at 27.4° N, 0.0° E along the southeastern margin of the 1,145-km Mare Imbrium impact basin (~3.85 to 3.92 Ga), Palus Putredinis is a 180-km-wide level volcanic plain bounded to the southeast by the 4.5-km-high fault-block front of Montes Apenninus, to the north by the flooded crater Archimedes, and to the west by the Archimedes Montes (Spudis & Ryder, 1985):

• Hadley Rille (Rima Hadley) and Imbrian Flood Basalts: Between July 30 and August 2, 1971, NASA's Apollo 15 astronauts David Scott and James Irwin explored the eastern margin of Palus Putredinis aboard the first Lunar Roving Vehicle, collecting 77.3 kg of lunar rock and core samples along the rim of Hadley Rille—a 135-km-long, 1.2-km-wide, 370-m-deep sinuous volcanic channel (collapsed lava tube / thermal erosion channel). Radiometric Ar–Ar and Rb–Sr dating of the Apollo 15 olivine- and quartz-normative mare basalts proved that Palus Putredinis was flooded by low-viscosity iron-rich basaltic lavas at 3.25 to 3.35 Ga, roughly 500 to 600 million years after the Imbrium impact basin formed.
• The "Genesis Rock" (Sample 15415) and Apennine Bench KREEP Volcanism: On the edge of Palus Putredinis at Spur Crater, Scott and Irwin recovered Lunar Sample 15415 ("the Genesis Rock"), a pristine ferroan anorthosite (>98% calcic plagioclase, anorthite An_96–97) dated to ~4.4 to 4.5 Ga, confirming that the Moon's primary terra crust crystallized by plagioclase flotation atop a global lunar magma ocean. Beneath the thin mare basalts of Palus Putredinis lies the Apennine Bench Formation (~3.85 Ga), the Moon's premier exposure of pre-mare KREEP basalt (enriched in incompatible K [potassium], REE [Rare Earth Elements], and P [phosphorus], plus radiogenic U and Th) alongside emerald-green pyroclastic glass beads (Sample 15426, erupted from 400 km depth in the lunar mantle at 3.3–3.4 Ga).
• Luna 2 First Extraterrestrial Impact (1959): On September 14, 1959, the Soviet Union's Luna 2 spacecraft impacted eastern Palus Putredinis at 29.1° N, 0.0° E (west of Autolycus Crater), becoming the first human-made object to reach the surface of another celestial body.

2. Palus Epidemiarum ("Marsh of Epidemics")

Situated in the southwestern lunar nearside at 32.0° S, 28.2° W south of Mare Humorum and Mare Nubium, Palus Epidemiarum is a 300-km-wide rectangular trough of dark mare basalt (~3.2–3.5 Ga) intensively fractured by a crisscrossing network of linear extensional grabens (Rimae Ramsden, up to 130 km long and 2 km wide—lunar equivalents of fossae) and bordered by the concentric multi-wall volcanic caldera crater Capuanus (which contains dome-like volcanic constructs).

3. Palus Somni ("Marsh of Sleep")

Located on the northeastern nearside at 14.1° N, 45.0° E between Mare Tranquillitatis and Mare Crisium, Palus Somni (~163 km wide) exhibits a distinctive intermediate optical albedo (0.12 to 0.16) and brownish-tan color contrasting sharply with the dark blue-gray titanium-rich basalts of Mare Tranquillitatis (albedo 0.06–0.08). Clementine and Lunar Reconnaissance Orbiter multispectral mapping shows that Palus Somni is a shallow structural bench where thin mare basalt flows are intimately mixed by impact gardening with bright highland feldspathic ejecta from nearby Proclus Crater (cryptomare mixing).

In the 21st century, the IAU adapted the descriptor palus for small, topographically enclosed plains on Mars where liquid water or low-viscosity flood lavas ponded inside larger basins:

1. Aeolis Palus in Gale Crater: A Long-Lived Habitable Martian Lakebed

Aeolis Palus is the smooth, crescent-shaped plain (~20 km wide and 70 km long, at −4,400 to −4,500 meters below the Martian datum) lying between the northern wall of the 154-km Gale Crater (~3.8 to 3.6 Ga, Noachian–Hesperian boundary) and its 5.5-km-high central sedimentary mountain, Aeolis Mons (informally Mount Sharp). On August 6, 2012, NASA's Mars Science Laboratory (Curiosity) rover touched down on Aeolis Palus at Bradbury Landing (4.59° S, 137.44° E) and traversed across it toward the base of Aeolis Mons (Grotzinger et al., 2014, 2015):

• Fluvial Conglomerates and the Peace Vallis Alluvial Fan: Soon after landing on Aeolis Palus, Curiosity imaged lithified pebble conglomerates (Link and Hottah outcrops) containing rounded, imbricated gravel clasts 2 to 40 mm in diameter transported out of the northern crater rim via Peace Vallis by sustained ankle-to-hip-deep streams flowing at 0.2 to 0.75 m s⁻¹ (Williams et al., 2013).
• Yellowknife Bay (Sheepbed Mudstone) and Neutral Freshwater Habitability: At the distal toe of the Peace Vallis alluvial fan in Yellowknife Bay on Aeolis Palus, Curiosity drilled into the Sheepbed mudstone (John Klein and Cumberland drill holes), a fine-grained lacustrine (lakebed) sediment deposited in a calm, freshwater lake that persisted for thousands to millions of years (Grotzinger et al., 2014). CheMin X-ray diffraction and SAM gas chromatography–mass spectrometry revealed:
  1. ~20 wt% authigenic trioctahedral smectite clay (ferrian saponite) formed in situ by aqueous alteration of basaltic olivine along with magnetite (Fe₃O₄), calcium sulfates (anhydrite, bassanite, gypsum), and amorphous material;
  2. Neutral pH, low salinity, and mixed redox state (coexisting Fe²⁺/Fe³⁺ and S²⁻/SO₄²⁻ couples capable of supporting chemolithoautotrophic extremophiles);
  3. Indigenous Martian organic molecules—including chlorobenzene (150–300 ppb), dichloropropanes, thiophenes, and long-chain decane, undecane, and dodecane alkanes derived from carboxylic/fatty acid precursors in the Cumberland mudstone (Freissinet et al., 2015)—alongside fixed nitrogen (nitrate, NO₃⁻ at 70–1,100 ppm) and background seasonal cycles of atmospheric methane (CH₄, 0.24 to 0.65 ppbv) with transient plumes up to 7–21 ppbv (Webster et al., 2018).

Centered at 5.7° N, 148.2° E inside western Elysium Planitia at the terminal mouth of the Athabasca Valles outflow channel (fed by Cerberus Fossae), Cerberus Palus is a 467-km-wide level basin covered by interlocking kilometer-scale polygonal plates that drifted apart, rotated, and refit like broken polar sea ice. Planetary geologists have debated two end-member models for Cerberus Palus: (1) a frozen dust-covered pack-ice sea ponded after a catastrophic ~5–20 Ma groundwater outburst flood from Cerberus Fossae (Murray et al., 2005), versus (2) an ultra-fluid, insulated platy-ridged basaltic flood-lava pond whose solidified surface crust broke into drifting plates over an underlying molten lava sheet (Jaeger et al., 2007), or mud-volcano / magma–ice interaction across both regimes. In the classical Oxia Palus quadrangle bordering eastern Chryse Planitia, the smooth clay-bearing basin Oxia Planum preserves a 3.9-Ga Fe/Mg-smectite and opaline-silica delta complex targeted by the European Space Agency's Rosalind Franklin (ExoMars) rover and its 2-meter subsurface drill.

Why are flat plains on the Moon and Mars called "paludes" (marshes)?

In the 1600s, early astronomers looking through low-power telescopes thought the dark, smooth plains on the Moon were oceans (maria) and seas, and they assumed that smaller, slightly brighter flat plains along the edges of lunar mountain ranges were shallow coastal swamps or marshes (paludes, singular palus). Today, the IAU preserves the classical Latin term palus as a purely morphological label for small, flat plains inside or along the margins of larger basins—even though lunar paludes are solid volcanic basalt and Martian paludes are ancient dry lakebeds or lava plains.

Why did NASA land the Curiosity rover in Aeolis Palus on Mars?

Orbital mapping by the Mars Reconnaissance Orbiter showed that Aeolis Palus—the flat northern floor of Gale Crater—sits at the bottom of a 4.5-km-deep topographic sink where an ancient river (Peace Vallis) poured water and sediment off the crater rim into a flat alluvial fan and terminal lakebed, directly adjacent to the 5.5-km-high layered mountain Aeolis Mons (Mount Sharp). Landing on flat, boulder-free Aeolis Palus allowed Curiosity to touch down safely and sample ancient freshwater mudstones (Yellowknife Bay) before climbing Mount Sharp to read how Mars transitioned from a wet, clay-forming climate to an arid sulfate desert.

What was the significance of the Apollo 15 samples returned from Palus Putredinis?

Landing on Palus Putredinis between Hadley Rille and the Apennine Mountains in 1971 allowed Apollo 15 astronauts to sample three foundational eras of lunar history in a single mission: (1) the 4.46-billion-year-old "Genesis Rock" (ferroan anorthosite) from the Moon's primordial magma-ocean crust; (2) the 3.85-billion-year-old Imbrium impact basin and KREEP volcanic rocks; and (3) 3.3-billion-year-old mare basalts and deep-mantle pyroclastic green glass beads containing trace interior water.

1. Spudis, P. D., & Ryder, G. (1985). "Geology and petrology of the Apollo 15 landing site: Past, present, and future understanding." Eos, Transactions American Geophysical Union, 66(43), 721–726. https://doi.org/10.1029/EO066i043p00721
2. Grotzinger, J. P., Sumner, D. Y., Kah, L. C., Stack, K., Gupta, S., Edgar, L., et al. (2014). "A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale Crater, Mars." Science, 343(6169), 1242777. https://doi.org/10.1126/science.1242777
3. Freissinet, C., Glavin, D. P., Mahaffy, P. R., Miller, K. E., Eigenbrode, J. L., Summons, R. E., et al. (2015). "Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars." Journal of Geophysical Research: Planets, 120(3), 495–514. https://doi.org/10.1002/2014JE004737
4. Jaeger, W. L., Keszthelyi, L. P., McEwen, A. S., Dundas, C. M., & Russell, P. S. (2007). "Athabasca Valles, Mars: A lava-draped channel system." Science, 317(5845), 1709–1711. https://doi.org/10.1126/science.1143315