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I would test whether the ancient farside crust grew locally with an early thermal/compositional gradient or acquired a substantial later accreted layer. The research programme should estimate a globally consistent crustal mass and chronology before tuning either mechanism to a single attractive map.

First, map crustal geometry with independent density constraints. Jointly invert GRAIL gravity and laser-altimeter surface topography, allowing spatially varying density and porosity rather than converting every anomaly to thickness. A new broad-band seismic network should include farside stations, long-lived polar power systems and a communications relay. Natural impacts and moonquakes, supplemented by calibrated active sources near landing sites, would constrain layer velocities, attenuation, crustal interfaces and the crust-mantle boundary. Crossing ray paths and receiver-side conversions are essential to distinguish a thick crust from an incorrect density or velocity assumption. A coherent buried layer or age-correlated discontinuity across the farside would favor addition or stacking; a gradual hemispheric compositional gradient would better support asymmetric local growth. Neither pattern is guaranteed to survive later impacts.

Second, obtain samples that address the initial contrast. Collect geographically distributed ancient highland rocks from both hemispheres, using basin ejecta, central peaks and drilling to sample beneath the impact-gardened regolith. Return samples with mapped geological context. Mineral chemistry and bulk Ca–Al–Mg–Fe abundances would test a primary refractory gradient; crystallization ages would date crustal construction, while shock-reset ages would date overprinting. Isotopic and trace-element measurements could identify an accreted population, but similar isotopes are possible for material derived from the same early debris system. A young deposit draped over older crust would favor accretion; coeval flotation-related crust with a systematic chemical gradient would favor local asymmetric differentiation. Multiple sites are necessary because a single basin's ejecta can mix distant sources.

Third, measure the thermal and volcanic consequences. Place shielded, sufficiently deep heat flow probes in representative highlands and mare terrains. Combine their geothermal gradients and measured thermal conductivity with orbital gamma-ray constraints on heat-producing elements. Map mare volumes and eruption ages, rather than only their exposed areas, to separate melt production from storage and eruption efficiency. A strong correlation between late volcanism, subsurface heat producers and high inferred mantle temperatures would support radiogenic amplification, while old crustal asymmetry without that correlation would show that later heating is not the entire cause.

Finally, compare forward models against withheld observations. Couple irradiation, vapor transport, condensation, lunar magma ocean mixing and crystallization for the thermal scenario; couple orbital evolution, impact deposition, mixing and later viscous relaxation for accretion. Account for later true polar wander before treating present-day hemispheric orientation as the original formation geometry. Require each model to conserve mass and energy and to reproduce the independent thickness, chemistry, age and volcanic data within their uncertainties. Vary unknown initial conditions and compare predicted observables at sites not used for calibration. The discriminating outcome is a conjunction such as a coherent accreted unit with a consistent emplacement age, or an ancient chemical gradient quantitatively consistent with transport under Earth irradiation. A lower gravity misfit alone cannot establish the historical mechanism, because gravity is a nonunique inverse problem.

I would prioritize an accessible exposure of ancient, fine-grained lacustrine deposits associated with a river delta, preferably with nearby hydrothermal deposits or groundwater alteration that offer a second environmental window. A good target maximizes three separate probabilities: past habitability, preservation of a biosignature, and practical access to the preserved material. A spectacular former water channel alone need not score well on the second or third.

First choice: a stratigraphically resolved ancient lake basin. Look for a closed or formerly overflowing basin fed by an inlet valley, a river delta with coherent layering, and adjacent fine mudstone deposited below strong wave or channel disturbance. Delta foresets and bottomsets, channel relationships and laterally continuous lacustrine deposits provide a testable paleoenvironment, rather than relying on an isolated fan shape. An inlet plus an outlet and consistent shoreline elevations can constrain lake level and water throughput. Select Noachian or early Hesperian units where available, while avoiding the assumption that all younger environments were uninhabitable.

Quiet lacustrine deposits can concentrate material from a wider drainage area; clay minerals provide large surfaces for adsorption of organic matter. Rapid burial and limited exposure to oxidizing fluids favor preservation. Carbonate or silica cements can trap chemical or microscopic evidence, although later diagenesis may erase or redistribute it. A near-neutral, not excessively saline paleoenvironment is attractive for known life, but should be inferred from mineral assemblages and sedimentary context rather than assumed from the presence of water.

This strategy has direct primary-source precedents. An ancient lake mudstone investigated by a rover preserved evidence for near-neutral pH, low salinity and variable oxidation states, making a plausible habitat for chemolithotrophy. This established past habitability, not past life.^[ref] Independently, rover images of another basin established a delta-lake succession followed by high-energy floods. That distinction helps choose fine background deposits rather than assuming the coarsest flood layers are the best biosignature archive.^[ref]

Second choice: ancient water-rock reaction systems. Target exhumed hydrothermal deposits associated with old volcanic terrain, basin-floor alteration, or impact-generated hydrothermal circulation. Especially useful settings contain silica, clay minerals, carbonates and reduced iron or sulfur phases in a coherent geological association. Chemical gradients between reacted fluids and surrounding water can supply free energy for chemolithotrophy; rapid mineral precipitation may entomb biosignatures. Habitable cooler mixing zones are more promising than assuming the hottest vent was habitable. Mineralogical mapping of thick, ancient basin deposits on Mars has motivated a deep-water hydrothermal alteration interpretation, providing a physically grounded alternative to shallow lakes. The interpretation remains a hypothesis to test in situ, not a detection of life.^[ref]

Additional targets: protected groundwater archives. Old fractures, mineral veins and groundwater-fed sedimentary deposits exposed by recent erosion can sample former subsurface habitats protected from surface ultraviolet radiation. Evaporites can preserve inclusions and record water chemistry, but strongly acidic or hypersaline terminal brines may be poor habitats, and a salt deposit alone does not distinguish a long-lived ecosystem from brief evaporation. Choose a traverse that can test the relation between these deposits and the lake or hydrothermal alteration succession.

Preservation and sampling can overturn the apparent ranking. Favor ancient rocks that were buried for most of their history and only recently exhumed, with little thermal alteration, pervasive oxidation or recrystallization. Drill beneath weathered rinds and sample fresh bedrock; loose modern regolith mixes unrelated sources. Cosmic ionizing radiation penetrates deeper than ultraviolet radiation, so shallow abrasion is not complete protection. Laboratory work reported by National Aeronautics and Space Administration shows that survival depths depend on molecule, mineral matrix and accumulated dose; meter-scale burial or a short exposure history can be important, rather than one universal safe drilling depth.^[ref]

The landing ellipse needs safe terrain and a feasible traverse to several related exposures. Orbital spectroscopy, imaging and surface topography should establish the initial target; field observations must then test stratigraphy, sedimentary textures and alteration history. Combine organic chemistry, isotope ratios, mineral associations and microscopic structures in their geological context, using contamination controls and samples from neighboring noncandidate material. Organic molecules, carbon isotope fractionation or a cell-like shape alone can have abiotic explanations. A convergent suite of contextual observations is a stronger case for ancient life than any single signal.

Hydrostatic equilibrium means that the local pressure gradient balances gravity, with no large acceleration or strength-supported departure from that balance. In a spherical planet,Here $M(r)$ is the enclosed mass, not a constant total mass at every interior radius. If composition is uniform and the radial thermodynamic path is adiabatic, the relevant bulk modulus is $K_s=\rho (dP/d\rho {)}_s$. The chain rule then gives the Adams-Williamson relationA phase transition or composition jump must be treated separately; its density change is not obtained by integrating one smooth homogeneous-adiabatic equation through it.

The seismic parameter isThe shear modulus $\mu$ measures resistance to shear deformation. Solids with $\mu >0$ transmit shear waves; an ideal fluid has zero static shear modulus, so it cannot carry a bulk S wave, while compressional P waves can still propagate through its nonzero bulk modulus.

Limitations include uncertain seismic coverage, temperature and composition tradeoffs, nonadiabatic boundary layers, phase changes, lateral heterogeneity, rotation and finite material strength. Small asteroids and comet nuclei commonly retain irregular shapes because their self-gravity is too weak to overcome strength and impact history; a few larger bodies, such as Ceres, are much closer to global hydrostatic figures. A spherical shape alone is not a complete proof of present hydrostatic equilibrium.

Two complementary remote constraints are precise spacecraft tracking, which measures gravitational harmonics and constrains mass distribution when combined with shape and rotation; and tidal or rotational measurements, such as Love numbers, librations and spin precession, which constrain deformability, moment of inertia and possible fluid layers. Magnetic induction measurements in a time-varying external field can provide an additional conductivity constraint, but are not a unique density inversion.

For the June 2018 lunar picture, start with a regolith and fractured crust, a largely anorthositic crust tens of kilometres thick, a silicate mantle, a deep partially molten region and a small iron-rich core. GRAIL and Apollo-based models gave a global mean crustal thickness around $34$-$43$ km.^[ref] A representative seismic model has a partial-melt boundary near radius $480$ km, a liquid outer-core boundary near $330$ km and a solid inner-core radius near $240$ km; these are model-dependent radii from the centre, not depths below the surface.^[ref]

The long-running Apollo network had only four principal stations on the near side, with poor global coverage and strong scattering in the fractured near-surface material. Far-side structure and weak deep reflected phases were therefore difficult to constrain. The historical date matters: the answer does not import later Mars seismology or later lunar-core refinements into the 2018 premise.

The geomorphology of active normal faulting records a competition between accumulated tectonic displacement and the ability of erosion, transport and deposition to modify it. A hanging wall moves down relative to a footwall, producing a fault scarp, mountain-front relief, and accommodation space for a sedimentary basin. Repeated earthquakes can build a large front even though each individual surface break is much smaller.^[ref]

The principal controls and their observable effects are:

Useful observations therefore include scarp profiles, repeated offsets of dated surfaces, drainage organization, and continuity along strike. A high rugged front is not by itself evidence of a high present fault slip rate; conversely, a muted or buried scarp need not mark an inactive fault.

A surface-reaching normal fault earthquake commonly produces a down-facing step towards the hanging wall, accompanied by extension across the break. A fresh free face can expose bedrock, polish or striations, roots or offset soil. The actual rupture is usually irregular: fault slip changes along strike, separate strands overlap or step, and tension fissures, small grabens and secondary cracks distribute part of the extension.

Exposed competent rock or very thin cover: a relatively narrow, sharp break can closely follow the principal fault plane. A steep free face can be recognizable immediately, then weather and shed debris. The face need not be vertical or maintain the same height along strike.

Loose granular or poorly consolidated cover: the main shear can splay upwards through alluvium or regolith. Open fissures, step-like scarps, rotated blocks, synthetic and antithetic normal faults, and a small intervening graben can form across a wider zone. Collapse and raveling quickly build colluvium at the toe. A single rupture event can therefore create several parallel breaks; counting them does not count earthquakes.

Thick weak or cohesive cover: deeper fault slip can be transferred into distributed shear strain, tilted beds, broad warping or monoclines, with tension cracks above a propagating tip. A discrete break may be attenuated or absent if the fault remains blind. Cover thickness alone is not enough to predict the response: cohesion, grain size, layering, saturation, rupture displacement and fault depth also matter. Saturated loose sediment can additionally develop soil liquefaction, sand blows or lateral spreading; these are secondary shaking effects and do not independently locate the tectonic rupture.

The observed vertical step, crack aperture and total width of disturbed ground should be measured separately. Reconstruct the total displacement across all strands and account for the original ground slope. Landslides or gravitational spreading can imitate normal-fault scarps, so map connections to the main fault and displaced markers rather than assuming every open crack is primary tectonic slip.