Mensa (Planetary Geology)

Mensa (plural mensae, from the Latin word for "table") is an official descriptor term in International Astronomical Union (IAU) planetary nomenclature for a flat-topped prominence or tableland bounded on all or most sides by steep cliff-like scarps—the planetary equivalent of a terrestrial mesa or butte. Adopted by the IAU in 1973 after NASA's Mariner 9 orbiter imaged vast regions of flat-topped outlier plateaus dissected by steep-walled valleys along the global Martian dichotomy boundary, mensae occur on Mars, Jupiter's volcanic moon Io (e.g., Tvashtar Mensae, Capaneus Mensa), and Saturn's moon Titan (initial albedo/topographic features). On Mars, extensive belts of mensae—most famously Deuteronilus Mensae, Protonilus Mensae, Nilosyrtis Mensae, Aeolis Mensae, Cydonia Mensae, and Nepenthes Mensae—record the multi-billion-year scarp retreat and glacial dissection of the ancient Noachian southern highlands along the northern lowland boundary. Within astrobiology and Martian paleoclimatology, radar sounding by the SHARAD (Shallow Radar) instrument aboard the Mars Reconnaissance Orbiter proved in 2008 that the smooth lobate debris aprons (LDAs) and lineated valley fill (LVF) surrounding mid-latitude Martian mensae are not rock-dominated talus piles, but debris-covered glaciers composed of >80% to 90% pure water ice (H₂O) hundreds of meters thick, representing the largest non-polar reservoir of accessible water ice on Mars.

In his classic 1973 analysis of Mariner 9 imagery, Caltech geologist Robert P. Sharp coined the term fretted terrain to describe the complex transition zone along the Martian dichotomy—especially between longitudes 0°E and 90°E north of Arabia Terra and Syrtis Major (30°N–50°N)—where the heavily cratered Noachian upland plateau breaks up northward into a labyrinth of flat-floored, steep-walled valleys separating hundreds of isolated, flat-topped mensae (10 to 100 km across and 0.5 to 2.5 km high) and smaller conical knobs.

Stratigraphic Control of Flat Tops and Steep Scarps

Like the sandstone-capped mesas of the Colorado Plateau (such as Monument Valley) on Earth, a planetary mensa owes its flat summit and steep bounding cliffs to differential erosion of a horizontally stratified crust:

1. Resistant Caprock Unit: The flat upper table of a Martian mensa is armored by a coherent, mechanically strong upper layer—typically Hesperian or Late Noachian flood basalts, welded impact breccias, or indurated volcanic ash/sulfate beds—that resists vertical downwasting.
2. Weak or Volatile-Rich Basal Substrate: Beneath the caprock lies a weaker, poorly consolidated, or ice-cemented phyllosilicate/megaregolith unit. When tectonic fractures (such as fossae), groundwater sapping, basal ice sublimation, or glacial scouring undermine this softer basal unit along faulted valleys, the overhanging caprock collapses along vertical joints, causing the bounding cliffs to retreat laterally (parallel scarp retreat) while preserving a flat summit plateau.
3. Evolutionary Continuum (Upland Plateau → Fret Valley → Mensa → Knob): As parallel scarp retreat widens the intervening fret channels over hundreds of millions of years, peninsulas of the southern highland plateau are pinched off into isolated flat-topped mensae (e.g., Deuteronilus and Cydonia Mensae); continued lateral cliff erosion eventually consumes the flat caprock altogether, reducing the mensa to a pointed conical spire or knobby terrain outlier rising from the northern plains of Acidalia or Utopia Planitia.

One of the most important discoveries in modern Martian science emerged from investigating the gently sloping aprons that encircle nearly every mensa in the mid-latitudes of Mars (30°–55°N and S), particularly across Deuteronilus Mensae, Protonilus Mensae, and Nilosyrtis Mensae:

1. Lobate Debris Aprons (LDAs), Lineated Valley Fill (LVF), and Concentric Crater Fill (CCF): Extending 10 to 30 km outward from the base of steep mensa scarps are convex-upward, tongue-shaped aprons (300 to 900 m thick) with steep lobate snouts whose cross-sectional profiles match the Nye (1952) parabolic plastic-flow profile of terrestrial ice sheets (h(x) = [2 τ₀ (L − x) / (ρg)]^1/2 for basal yield stress τ₀ ≈ 50–100 kPa). Where aprons from opposing mensa walls converge inside narrow fret valleys, viscous shear compresses their surfaces into longitudinal ridges and grooves (lineated valley fill), identical to coalescing valley glaciers and medial moraines in the Dry Valleys of Antarctica (Squyres, 1978, 1979; Head et al., 2006).
2. SHARAD Radar Proof of Bulk Water Ice (Holt et al., 2008; Plaut et al., 2009): For three decades, planetary geologists debated whether LDAs around Martian mensae were "rock glaciers" consisting mostly of rocky talus lubricated by 10%–30% interstitial pore ice, or true debris-covered glaciers composed of massive water ice protected from sublimation by a thin (1–10 m) surface lag of rock debris. In 2008–2009, John W. Holt, Jeffrey J. Plaut, and colleagues used the 20-MHz Shallow Radar (SHARAD) sounder on NASA's Mars Reconnaissance Orbiter to beam radio waves completely through lobate debris aprons in Deuteronilus Mensae and Eastern Hellas, detecting bright basal radar reflections from the underlying bedrock floor at depths of 400 to 800 meters with minimal signal attenuation.
3. Permittivity Confirmation of Pure Water Ice (ε' ≈ 3.15): By dividing the geometric thickness Δz of the apron (measured from MOLA laser altimetry of the surrounding plain) by the two-way radar travel time Δt, the bulk relative dielectric permittivity ε' = (c Δt / 2Δz)² was measured directly at ε' = 3.1 to 3.3 (with dielectric loss tangent tan δ < 0.005). Because pure hexagonal water ice (Ice I_h) has a laboratory dielectric constant of ε' = 3.15 whereas silicate basaltic rock has ε' = 7 to 9, SHARAD proved that the aprons surrounding Deuteronilus, Protonilus, and Nilosyrtis Mensae consist of 80% to >90% pure water ice capped by a sublimation lag only a few meters thick!
4. Milankovitch Obliquity Ice Ages: General circulation models (Madeleine et al., 2009; Head et al., 2006) demonstrate that during periods of high Martian axial obliquity (ε_obl ≈ 35° to 45°, occurring repeatedly over the past 10 to 500 million years due to chaotic secular spin-orbit resonances), solar insolation at the Martian poles increases strongly, sublimating water vapor from the polar caps and precipitating it as meters of snow and glacier ice onto mid-latitude mensa escarpments. When Mars's obliquity drops back to its current value (25.19°), a rocky sublimation lag accumulates over the glaciers, preserving >1.5 × 10⁵ km³ of mid-latitude glacial water ice (enough to cover the entire planet in ~1–1.5 m of water).

A second, genetically distinct class of Martian mensae occurs inside the giant canyon troughs of Valles Marineris and within Hesperian chaos terrains:

• Interior Layered Deposit (ILD) Mensae (e.g., Candor Mensa, Hebes Mensa, Juventae Mensa, Ganges Mensa): Rising 3 to 6.5 km from the floors of enclosed Valles Marineris chasmata are enormous freestanding, flat-topped mounds of finely bedded light-toned sedimentary rock. Orbital spectroscopy by OMEGA and CRISM demonstrates that these interior mensae are rich in hydrated magnesium and calcium sulfate evaporites (kieserite MgSO₄·H₂O, polyhydrated sulfates, and bassanite/gypsum CaSO₄·2H₂O) interbedded with crystalline ferric oxides (hematite). They record Hesperian lacustrine evaporite deposition, groundwater upwelling springs, or sub-ice volcanism inside deep canyon basins subsequently exhumed by aeolian deflation.
• Chaos-Terrain Mensae (e.g., Iani Chaos and Aureum Chaos blocks): Where pressurized subsurface aquifers ruptured the Hesperian cryosphere to feed Martian outflow channels, the undermined highland plateau fractured into hundreds of tilted, flat-topped mensa blocks 2 to 20 km wide standing above the collapsed chaos floor.

How was the "Face on Mars" in Cydonia Mensae explained by high-resolution spacecraft imagery?

In July 1976, NASA's Viking 1 orbiter photographed a 2-kilometer-long eroded mensa in Cydonia Mensae (40.75°N, 350.54°E) at a low solar illumination angle (~20°) and coarse spatial resolution (43 m/pixel), where shadows cast by crags along the flat-topped summit created a pareidolia illusion resembling a human face. When re-imaged at 1.5 m/pixel by Mars Global Surveyor (MOC, 1998–2001), 13.7 m/pixel in 3D stereo by Mars Express (HRSC, 2006), and 25 cm/pixel by the Mars Reconnaissance Orbiter (HiRISE, 2007), the landform was shown to be a thoroughly natural, wind- and mass-wasting-eroded remnant mesa typical of hundreds of surrounding Cydonia Mensae outliers along the Martian dichotomy boundary.

How did SHARAD radar prove that the aprons around Deuteronilus Mensae are made of pure water ice rather than rock?

Radio waves travel at different speeds and suffer different attenuation depending on the dielectric permittivity (ε') of the material they pass through: silicate rock has ε' ≈ 7–9 and strongly absorbs radar waves within tens of meters, whereas pure water ice has ε' = 3.15 and is nearly transparent to 20-MHz radio waves. When SHARAD transmitted radar pulses through 500-to-800-meter-thick lobate debris aprons around Deuteronilus Mensae, the pulses penetrated all the way to the flat bedrock floor and reflected back with virtually zero energy loss and a wave speed corresponding to ε' = 3.15, proving that the interior of the aprons is >85% pure glacial water ice protected by a thin surface blanket of rocky dust.

Why are Martian mensae important for future human exploration and astrobiology?

Because liquid water and water ice cannot survive exposed at the surface in Mars's warm equatorial and mid-latitude zones, locating abundant, shallow water ice at low-to-mid latitudes where solar power and thermal conditions are favorable for human landers is a top exploration priority. The buried debris-covered glaciers surrounding Deuteronilus, Protonilus, and Phlegra Mensae (at 35°–45°N) contain over 150,000 cubic kilometers of pure water ice within a few meters of the surface, while also preserving a continuous stratigraphic ice-core archive of Martian climate cycles and potential cryo-preserved organic molecules over the past several hundred million years.

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. Squyres, S. W. (1978). "Martian fretted terrain: Flow of erosional debris." Icarus, 34(3), 600–613. https://doi.org/10.1016/0019-1035(78)90048-990048-9)
3. Holt, J. W., Safaeinili, A., Plaut, J. J., Head, J. W., Phillips, R. J., Seu, R., et al. (2008). "Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars." Science, 322(5905), 1235–1238. https://doi.org/10.1126/science.1164246
4. Plaut, J. J., Safaeinili, A., Holt, J. W., Phillips, R. J., Head, J. W., Seu, R., et al. (2009). "Radar evidence for ice in lobate debris aprons in the mid-northern latitudes of Mars." Geophysical Research Letters, 36(2), L02203. https://doi.org/10.1029/2008GL036379