Lobate Scarp

A lobate scarp (formally designated by the descriptor term rupes, Latin for "cliff" or "scarp," in International Astronomical Union nomenclature) is an asymmetric, arcuate or sinuous tectonic landform on a solid-surface planetary body consisting of a steeply dipping fore-scarp cliff face backed by a gently sloping, convex-upward hanging-wall crest that represents the surface expression of a low-angle compressional thrust fault (dip angle θ ≈ 20° to 40°). Unlike Earth—whose lithosphere is broken into mobile plates where horizontal crustal shortening is accommodated at subduction zones and collisional mountain belts—Mercury, the Moon, and Mars are one-plate (stagnant-lid) terrestrial bodies. On these worlds, secular cooling of the silicate mantle and crystallization of a solid inner metallic core cause the interior volume of the planet to shrink, placing the entire rigid outer lithosphere into a state of global horizontal isotropic compression that fractures the crust along thousands of distributed lobate thrust scarps.

First discovered on Mercury in 1974–1975 by picking out Discovery Rupes in Mariner 10 television imagery (Strom, Trask & Guest, 1975) and mapped globally by NASA's MESSENGER spacecraft (2011–2015) and the Lunar Reconnaissance Orbiter (LRO, 2009–present), lobate scarps range across five orders of magnitude in scale: from gigantic Mercurian megascarps such as Enterprise Rupes and Discovery Rupes (up to 1,000 km long with 2.0 to 3.5 km of vertical relief) down to young, meter-scale lunar fault scarps such as the Lee-Lincoln scarp at the Apollo 17 landing site that have been active within the last 50 million years and continue to generate shallow moonquakes today. In planetary science and astrobiology, lobate scarps provide the primary empirical constraint on the thermal history, inner-core solidification rate, magnetic dynamo energetics, and volatile outgassing budgets of terrestrial planets.

Cross-Sectional Geometry and Depth of Faulting

In high-resolution laser altimetry (such as the Mercury Laser Altimeter [MLA] on MESSENGER, MOLA on Mars Global Surveyor, and LOLA on LRO), a lobate scarp displays a characteristic asymmetric topographic profile:

1. Steep Fore-Scarp Face: A cliff face inclined at 10° to 35° (locally approaching the angle of repose of fragmental regolith) that exhibits lobate or scalloped planform salients in map view where adjacent en-echelon thrust-fault segments have linked along strike.
2. Rounded Anticlinal Crest and Gently Sloping Back-Scarp: A broad, asymmetric hanging-wall ridge (10 to 60 km wide on Mercury and Mars; 0.1 to 1 km wide on the Moon) that slopes gradually back down to the regional datum, frequently accompanied by a narrow back-thrust scarp or crestal flexural-slip graben formed by bending stresses over the fault ramp.
3. Deformed Impact Craters as Strain Gauges: When a lobate scarp cuts across a pre-existing circular impact crater (such as the 65-km crater Rameau cut by Discovery Rupes, or the 82-km Rembrandt-hosted craters cut by Enterprise Rupes), the hanging-wall half of the crater rim is uplifted and thrust horizontally over the footwall floor, shortening the crater diameter parallel to the slip vector by 1.5 to 3.5 kilometers and enabling direct measurement of the fault dip angle θ via the geometric relation tan θ = Δh / ΔL_h.

Elastic dislocation modeling (using Coulomb and Okada boundary-element codes; Watters et al., 2002–2016) demonstrates that the large lobate scarps on Mercury and in the Noachian–Hesperian southern highlands of Mars are fault-propagation folds rooted above master thrust faults that dip at θ ≈ 25° to 35° and penetrate the entire brittle lithosphere down to the brittle–ductile transition (BDT) at depths of 25 to 40 kilometers.

Calculation of Global Radial Contraction (ΔR) from Fault Population Strain

Because a thrust fault of length L_i, mean vertical throw h_i (or fault displacement D_i = h_i / sin θ), and dip angle θ accommodates a horizontal crustal surface area reduction of ΔA_i = L_i D_i cos θ = L_i h_i cot θ, summing across a complete population of N lobate scarps mapped over a survey area A_s on a spherical planet of radius R_p yields the areal contractional strain ε_A and the corresponding global radial contraction (radius change ΔR) (Strom et al., 1975; Byrne et al., 2014):

ΔR = − ( R_p / 2 ) ε_A = − ( R_p / 2 A_s ) ∑_i=1^N L_i D_i cos θ

where the displacement-to-length scaling ratio γ = D_max / L for planetary thrust faults typically lies between γ ≈ 5 × 10⁻³ and 1.2 × 10⁻².

The Mariner 10 Thermal History Paradox

When Mariner 10 imaged 45% of Mercury during its three flybys in 1974–1975, Robert G. Strom, Newell J. Trask, and John E. Guest measured the lengths and heights of the visible lobate scarps (led by the 650-km-long, 2-km-high Discovery Rupes) and estimated that Mercury's radius had contracted by ΔR ≈ −1 to −2 kilometers since the end of the Late Heavy Bombardment (~3.9 Ga).

However, this small 1–2 km contraction created a major geophysical paradox that persisted for four decades:

• Mercury possesses an exceptionally large metallic iron–nickel core (R_core ≈ 2,020 km, or 83% of the planet's 2,439.7 km radius and ~60% of its total mass).
• Standard planetary thermal evolution models (Solomon, 1976; Schubert et al., 1988) showed that simple conductive cooling of the silicate mantle and solidification of even a modest inner metallic core—required to drive Mercury's active core magnetic dynamo via compositional buoyancy—must shrink Mercury's radius by 5 to 10 kilometers due to the thermal expansion coefficient of silicate/iron (α_th ≈ 3 × 10⁻⁵ K⁻¹) and the ~4%–5% volume reduction when liquid iron freezes into solid face-centered/body-centered cubic iron.

MESSENGER Global Mapping and Discovery of Small Young Scarps

Between 2011 and 2015, NASA's MESSENGER spacecraft imaged 100% of Mercury under low-sun illumination angles alongside Mercury Laser Altimeter profiles. In 2014, Paul K. Byrne, Christian Klimczak, Sean C. Solomon, Thomas R. Watters, and collaborators published a comprehensive global tectonic map comprising 5,934 individual contractional landforms (lobate scarps, high-relief ridges, and wrinkle ridges) spanning Up to Enterprise Rupes—the largest lobate scarp in the Solar System, stretching ~1,000 km across the Rembrandt impact basin with 3.2 km of vertical relief.

Summing the horizontal shortening across all 5,934 fault segments proved that *Mariner 10*'s high-sun lighting bias had missed the majority of Mercury's thrust faults: Mercury's true post-3.9 Ga radial contraction is ΔR = −5.0 to −7.1 kilometers, reconciling observation with inner-core solidification and thermal history models. Furthermore, in 2016, Thomas R. Watters et al. using low-altitude MESSENGER images (<20 m pixel⁻¹) discovered decameter-scale lobate scarps (h = 10–100 m, L = 2–20 km) cutting across small, crisp impact craters (<100 m diameter) that would be erased by impact gardening within <50 Myr—proving that Mercury remains tectonically active today.

On the Moon, where secular interior cooling since 3.8 Ga has been far milder because the lunar metallic core accounts for less than 20% of the lunar radius, Apollo panoramic cameras and the Lunar Reconnaissance Orbiter Narrow Angle Camera (0.5 m pixel⁻¹ resolution) have mapped more than 3,200 small, pristine lobate thrust-fault scarps distributed globally across both the lunar highlands and maria (Watters et al., 2010–2019).

Three lines of evidence establish that these lunar lobate scarps are among the youngest active tectonic faults in the Solar System (<50 million years old, and actively slipping today):

1. Crosscutting of Meter-Scale Craters and Boulder Trails: Lunar lobate scarps—which are typically 1 to 20 km long and 5 to 100 meters high—transect and offset small impact craters <10–20 m in diameter without being degraded by impact gardening, and exhibit bright, freshly exposed boulder fields and meter-wide crestal extension trenches created by seismic shaking during recent slip events.
2. Tidal Stress Modulation of Fault Orientations (Watters et al., 2015): Whereas purely isotropic thermal contraction of the Moon (ΔR ≈ −25 to −50 m, corresponding to ~5–10 MPa of isotropic compressional stress) would produce randomly oriented fault strikes, the strikes of the >3,200 mapped lunar lobate scarps show a statistically significant global alignment pattern controlled by the superposition of diurnal orbital tidal stresses (~0.05–0.15 MPa) raised by Earth's gravitational pull as the Moon travels between perigee and apogee along its eccentric orbit (e = 0.0549) and orbital recession despinning stresses.
3. Correlation with Apollo Shallow Moonquakes (Watters et al., 2019): Re-analysis of the 28 shallow moonquakes (body-wave magnitudes m_b = 1.5 to 5.5, hypocentral depths <50 km) recorded between 1969 and 1977 by the four-station Apollo Passive Seismic Network showed that eight of the epicenters fall within 30 km of mapped young lobate scarps, and that six of those eight events occurred when the Moon was near apogee, where combined tidal plus contractional shear stresses peak along the thrust fault planes. Evaluating these active seismic hazards is a central engineering priority for designing long-duration human habitats near the lunar South Pole under NASA's Artemis program.

1. Strom, R. G., Trask, N. J., & Guest, J. E. (1975). "Tectonism and volcanism on Mercury." Journal of Geophysical Research, 80(17), 2478–2507. https://doi.org/10.1029/JB080i017p02478
2. Watters, T. R., Schultz, R. A., Robinson, M. S., & Cook, A. C. (2002). "The mechanical and thermal structure of Mercury's early lithosphere." Geophysical Research Letters, 29(11), 1542. https://doi.org/10.1029/2001GL014308
3. Watters, T. R., Robinson, M. S., Beyer, R. A., et al. (2010). "Evidence of Recent Thrust Faulting on the Moon Revealed by the Lunar Reconnaissance Orbiter Camera." Science, 329(5994), 936–940. https://doi.org/10.1126/science.1189590
4. Byrne, P. K., Klimczak, C., Celâl Şengör, A. M., Solomon, S. C., Watters, T. R., & Hauck, S. A. (2014). "Mercury's global contraction much greater than earlier estimates." Nature Geoscience, 7(4), 301–307. https://doi.org/10.1038/ngeo2097
5. Watters, T. R., Weber, R. C., Collins, G. C., et al. (2019). "Shallow seismic activity and young thrust faults on the Moon." Nature Geoscience, 12(6), 411–417. https://doi.org/10.1038/s41561-019-0362-2