Add a giant impact atmosphere loss figure to Lecture 4

The impact erosion section described the loss mechanisms in words only. The notes and the slides now show Figure 1 of Roche et al. (2025, The Planetary Science Journal), an SPH time series of an oblique giant impact onto an Earth mass planet with a primordial envelope, with the lost particles marked in green. The figure is reproduced under its CC BY 4.0 license.

The text now names the two loss channels, near field loss from ejecta plumes at the impact site and far field loss from the shock that breaks out on the rest of the globe, and states how the loss fraction scales with impact velocity and impact angle.

Author: timlichtenberg

book/04_differentiation_magnetospheres/differentiation_magnetospheres.md

@@ -354,6 +354,16 @@ $$ (eq:ground-motion-blowoff)
 
 the overlying atmospheric column is expelled. One-dimensional shock-hydrodynamic calculations of this mechanism show that the loss is far from total: even in the Moon-forming impact, the ground over most of the globe moves at only a few km s$^{-1}$, well below Earth's $v_\mathrm{esc} \approx 11$ km s$^{-1}$, so the event removes only $\sim 20\%$ of the pre-impact atmosphere and most of the atmosphere survives {cite:p}`GendaAbe2003`. A surface ocean changes this conclusion substantially: vaporisation of the ocean and its lower **shock impedance** (the product of density and wave speed, which sets how efficiently ground motion transmits into the layer above) couple the ground motion to the atmosphere far more efficiently, enhancing the loss {cite:p}`GendaAbe2005`. This ocean effect has been invoked to explain why Venus (whose protoplanets likely lacked surface oceans) retains roughly 50 times more $^{36}\mathrm{Ar}$ than Earth: remnants of the noble-gas-rich primordial atmosphere survived on Venus but were lost from ocean-covered proto-Earth {cite:p}`GendaAbe2005`.
 
+Three-dimensional SPH simulations now resolve both loss channels within a single event ({numref}`fig:impact-atmosphere-loss`): close to the impact site, vapour-rich ejecta plumes expel the overlying atmosphere directly (**near-field loss**), while the shock wave that travels through the interior breaks out at the surface on the far side and ejects atmosphere wherever the ground motion approaches the escape velocity (**far-field loss**) {cite:p}`Roche2025`. Across a large suite of such simulations the loss fraction rises steeply with impact velocity and falls towards grazing geometries: head-on collisions at several times the mutual escape velocity can strip most of a primordial envelope, whereas oblique impacts near the escape velocity remove only of order a few percent per event {cite:p}`Roche2025`.
+
+```{figure} figures/roche2025_impact_loss.avif
+:name: fig:impact-atmosphere-loss
+:width: 100%
+:align: center
+
+Atmospheric loss during a giant impact, in snapshots from a smoothed-particle hydrodynamics simulation of a moderately oblique collision (impact parameter $b = 0.3$) between a $1.05\,M_\oplus$ planet carrying a primordial H$_2$-He envelope and an impactor holding one tenth of the total colliding mass, striking at twice the mutual escape velocity. Particles are coloured by material: target core (grey), target mantle (orange), atmosphere (blue), impactor core (brown), and impactor mantle (yellow); particles that become unbound and are lost from the planet are coloured green. Near the impact site, ejecta plumes drive near-field loss; the impact shock (white dashed line) travels through the planet, breaks out at the surface, and drives far-field loss over the rest of the globe. In this event 34% of the atmosphere is lost. Figure 1 of {cite:t}`Roche2025`, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).
+```
+
 The implication for Earth is that any **primordial H$_2$-He envelope** captured from the protoplanetary disk was not removed in a single blow. Its removal required the combination of repeated impact erosion during accretion and sustained thermal escape driven by the young Sun's high XUV (X-ray and extreme-ultraviolet) output ({ref}`Lecture 5 <lecture05>`), leaving behind a secondary outgassed atmosphere of the kind described in the previous subsection. For Mars, whose escape velocity ($\approx 5$ km s$^{-1}$) is less than half of Earth's, both erosion channels are more effective at equal impact energy. A Borealis-scale impact ({cite:t}`Marinova2008`) plausibly removed a substantial fraction of the earliest atmosphere, and the long-term solar-wind-driven escape after the dynamo shutdown did the rest ({ref}`Lecture 10 <lecture10>`).
 
 The balance between delivery and erosion is a key uncertainty in reconstructing the volatile histories of the terrestrial planets: the same planetesimal population that delivers volatile-rich material also erodes the growing atmosphere, and the net outcome depends on the impactor size distribution, velocity distribution, and volatile content {cite:p}`Schlichting2015`. Earth's present atmospheric and ocean inventory likely reflects the integrated outcome of both processes superimposed on the secondary outgassing from the final magma ocean.

book/04_differentiation_magnetospheres/figures/roche2025_impact_loss.avif

@@ -909,6 +909,17 @@ @article{Nicholls2024
     url     = {https://doi.org/10.1029/2024JE008576}
 }
 
+@article{Roche2025,
+    author  = {Roche, Matthew J. and Lock, Simon J. and Dou, Jingyao and Carter, Philip J. and Kegerreis, Jacob A. and Leinhardt, Zo{\"e} M.},
+    title   = {Atmospheric loss during giant impacts: mechanisms and scaling of near- and far-field loss},
+    journal = {The Planetary Science Journal},
+    volume  = {6},
+    pages   = {149},
+    year    = {2025},
+    doi     = {10.3847/PSJ/add929},
+    url     = {https://doi.org/10.3847/PSJ/add929}
+}
+
 @article{Hirschmann2012,
     author  = {Hirschmann, Marc M.},
     title   = {Magma ocean influence on early atmosphere mass and composition},

book/references.bib

@@ -582,6 +582,12 @@ \section{Accretion, melting, \& core formation}
   {\tiny\color{ipsText!60} Source: Melosh \& Vickery (1989); Schlichting et al.\ (2015); Genda \& Abe (2003, 2005)}
 \end{frame}
 
+\begin{frame}
+  \centering
+  \makebox[\linewidth][c]{\includegraphics[width=0.95\paperwidth, height=0.74\paperheight, keepaspectratio]{roche2025_impact_loss}}\\
+  {\footnotesize Atmospheric loss in a giant impact: ejecta plumes drive near-field loss at the impact site, while the shock breaking out on the far side drives far-field loss; green particles are lost. Roche et al.\ (2025), CC BY 4.0.}
+\end{frame}
+
 \begin{frame}{Delivery versus erosion}
   \begin{itemize}
     \item The same planetesimals that erode also deliver volatiles; the net budget depends on impactor sizes, velocities, and volatile content