fehim yamak calgavsubsection*{Abstract}
We examine a hypothetical collision in which a Moon-mass body ((M_{leftmoon}approx 7.35\times 10^{22},mathrm{kg})) impacts the early Earth, in contrast to the (sim 10\text{–}15,mathrm{km})-scale Chicxulub impactor. Such an event belongs to the giant-impact regime that is commonly invoked in lunar origin studies. The resulting energy deposition, angular-momentum redistribution, and disk formation would not merely create a crater or trigger a mass extinction; it would reset the Earth–Moon system into a new dynamical and thermal state. We summarize the expected evolution over timescales from hours to (10^8) years, with emphasis on the post-impact magma ocean, protolunar disk evolution, tidal re-equilibration, and the long-term implications for planetary habitability.
subsection*{1. Introduction}
The giant-impact hypothesis is the dominant framework for lunar origin, and modern work shows that the Earth–Moon system is strongly constrained by angular momentum, isotopic similarity, and post-impact disk evolution [web:70][web:88][web:90][web:96]. In this hypothetical case, the impactor is not Mars-sized but Moon-sized, which places the event at the threshold where the entire Earth system can be globally melted or vaporized. This is therefore best treated as a planetary-reconstruction problem rather than a surface-impact problem.
subsection*{2. Basic Parameters}
Let the target be Earth with mass (M_{oplus}=5.97\times 10^{24},mathrm{kg}) and radius (R_{oplus}=6.371\times 10^{6},mathrm{m}), and let the impactor mass be (M_{leftmoon}=7.35\times 10^{22},mathrm{kg}). The mass ratio is
[
gamma=\frac{M_{leftmoon}}{M_{oplus}}approx 1.23\times 10^{-2}.
]
This ratio is small compared with Earth, but it is enormous compared with the Chicxulub projectile and is fully within the giant-impact regime [web:52][web:70][web:88].
For a characteristic impact speed (v_{mathrm{imp}}=10\text{–}20,mathrm{km,s^{-1}}), the impact kinetic energy is
[
E_k=\frac{1}{2}M_{leftmoon}v_{mathrm{imp}}^2
approx 3.7\times 10^{30}\text{–}1.5\times 10^{31},mathrm{J}.
]
This is many orders of magnitude above Chicxulub-scale energy and approaches the range relevant to Moon-forming giant impacts [web:70][web:96][web:99].
subsection*{3. Immediate Physical Effects}
The first effect is global shock heating. A substantial fraction of Earth’s outer layers would melt, and in the most energetic realizations a global magma ocean and silicate-vapor atmosphere would form [web:79][web:80][web:101]. Oceans would flash to steam, the atmosphere would become opaque and superheated, and the surface would be temporarily uninhabitable.
Because the collision carries enormous angular momentum, the Earth–Moon system would be rearranged into a new rotation state. Modern studies show that lunar formation is highly sensitive to the available angular momentum budget, with canonical impacts requiring (J_0gtrsim 2J_{EM}) in some formulations, and post-impact equilibration or angular-momentum loss mechanisms becoming important in many models [web:57][web:88][web:90]. A Moon-mass impact is therefore not simply a thermal event; it is also a rotational-dynamical event.
subsection*{4. Protolunar Disk and Satellite Formation}
A key consequence is the formation of a circumterrestrial debris disk. Depending on impact angle, velocity, and the pre-impact spin states, the disk may be sufficiently massive to accrete into a moonlet or even a Moon-like satellite [web:70][web:73][web:95][web:96]. Disk evolution is expected to occur on timescales of (10^2) to (10^5) years, with turbulent mixing, vapor condensation, and accretion redistributing mass and angular momentum [web:99][web:94].
If the impact occurs in a high-angular-momentum regime, the post-impact structure can approach a synestia-like state, where no sharply bounded planet exists initially and the Earth–disk system forms a continuous, rapidly rotating, partially vaporized configuration [web:95][web:57]. In such cases, the final Moon and Earth may differ significantly from the present pair.
subsection*{5. Thermal Recovery}
The thermal evolution proceeds in stages. First, the Earth cools from a vaporized or magma-ocean state over (10^2) to (10^5) years, developing an initial crust. Then, over (10^6) years and beyond, atmospheric condensation, ocean reformation, and long-term volcanism reshape the surface [web:80][web:83][web:101]. The exact pace depends on the impact energy, atmospheric loss, and whether volatile retention remains sufficient for secondary ocean formation.
A useful energetic comparison is the Earth’s gravitational binding energy,
[
U_{oplus}approx \frac{3}{5}\frac{GM_{oplus}^2}{R_{oplus}}sim 2.2\times 10^{32},mathrm{J}.
]
The Moon-mass impact energy is below this value but is still large enough to melt a major fraction of the silicate envelope and to drive a global geological reset.
subsection*{6. Long-Term Geological State}
On (10^6) to (10^8)-year timescales, the planet would transition into a new tectonic and atmospheric regime. A new crust would form, mantle convection would reorganize, and the hydrosphere and atmosphere would re-equilibrate [web:83][web:84][web:101]. If a moon survives or reforms, tidal evolution would dominate the future Earth–Moon angular momentum exchange [web:71][web:72][web:90].
Importantly, the present Earth–Moon system is not a trivial outcome of any collision. It is constrained by angular momentum, isotopic chemistry, and the dynamics of the protolunar disk. Recent surveys indicate that even rotating-body impacts struggle to satisfy all constraints simultaneously, and some canonical scenarios remain under tension [web:96][web:97]. Thus, a Moon-mass impact would not simply reproduce today’s Earth with a larger crater; it would place the planet on a different evolutionary branch.
subsection*{7. Biospheric Consequences}
Surface life would be sterilized. Deep biosphere survival is highly unlikely under the more energetic realizations because the thermal anomaly, mantle overturn, and volatile loss would exceed the tolerance of known extremophiles. Even if life re-emerged later, the biosphere would restart from an entirely new geological and chemical baseline.
subsection*{8. Implications for the Solar System}
The Solar System as a whole would remain largely intact. The Sun and outer planets would not be mechanically destroyed by a localized Earth impact. However, Earth’s spin state, the Moon’s orbit, and the Earth–Moon barycentric evolution would be altered, with subtle consequences for long-term secular interactions [web:86][web:74]. In short, the event is catastrophic locally but not a system-wide planetary unbinding.
subsection*{9. Conclusion}
A Moon-mass impact on Earth is best understood as a planetary reset event. It would produce immediate global melting or vaporization, create a protolunar disk, alter Earth’s rotation, and likely generate a new Moon or new satellite architecture. The recovery timescale to a stable, habitable, Earth-like planet is measured in millions to tens of millions of years, and the resulting world would almost certainly not resemble the present Earth.
subsection*{Selected References}
[1] Canup, R. M. and Asphaug, E. (2001), Origin of the Moon in a giant impact near the end of the Earth’s formation.
[2] Pahlevan, K. and Stevenson, D. J. (2007), Equilibration in the aftermath of the lunar-forming giant impact.
[3] Ćuk, M. and Stewart, S. T. (2012), Making the Moon from a fast-spinning Earth.
[4] Lock, S. J. and Stewart, S. T. (2017), The origin of the Moon within a terrestrial synestia.
[5] Meier et al. (2024), A Systematic Survey of Moon-Forming Giant Impacts. II. Rotating bodies.
