L02 review fixes: Grand Tack resonance, Nice model AU range, Halley a, Oberg caption

Four review-driven fixes for Lecture 02 (Formation & Orbits):

- Grand Tack: 2:3 -> 3:2 mean-motion resonance (body line 595, caption
  line 602). The lecture's own resonance table at line 458 establishes
  the convention "first number = inner body's orbit count" (e.g.
  Pluto-Neptune 3:2 = Neptune does 3, Pluto does 2). Under that
  convention, Jupiter-Saturn must be 3:2 (Jupiter inner does 3, Saturn
  outer does 2). The earlier 2:3 was inverted.

- Nice model body text: "all between ~5 and ~17 AU" -> "all within
  ~20 AU" to match the Nesvorny 2018 figure shown (Neptune starts at
  ~20 AU). Caption already explains the difference vs the original
  Tsiganis 2005 numbers.

- Halley's comet: a = 17.8 AU (body) -> 17.83 AU to match the figure
  caption value already in use.

- Oberg 2011 snowline caption: removed "N_2 ice lines" and "C/N/O
  budget" — Oberg 2011 Fig. 2 only shows H2O, CO2, and CO snowlines and
  the y-axis is "C/O ratio" (not C/N/O). Caption now matches what the
  figure actually shows, with explicit Fig. 2 attribution.

Rejected adversarial flags:
- HL Tau license CC BY 3.0 vs 4.0 (ESO has migrated to CC BY 4.0;
  current caption is correct).
- Roche limit explanatory prose (algebra and physics are right; the
  "additional factor" wording is awkward but defensible).
- Disk lifetime ~3-5 Myr citing Haisch 2001 (canonical reference;
  modern surveys are already noted in the same paragraph).

Author: timlichtenberg

book/02_formation_orbits/formation_orbits.md

@@ -79,7 +79,7 @@ Ice lines for other species ($\mathrm{CO_2}$, $\mathrm{CO}$, $\mathrm{N_2}$, $\m
 :width: 600px
 :align: center
 
-Sequential condensation of major volatile species across a protoplanetary disk and the resulting C/O ratio in solids and gas as a function of radius. The H$_2$O snow line roughly triples the available solid surface density; further out, the CO$_2$, CO, and N$_2$ ice lines set the C/N/O budget of solids and gas at each radius, controlling the volatile inheritance of forming planets. Adapted from {cite:t}`Oberg2011`.
+Sequential condensation of major volatile species across a protoplanetary disk and the resulting C/O ratio of grains (dashed) and gas (solid) as a function of disk radius. The H$_2$O ice line at $\sim 2$ AU lowers grain C/O and raises gas C/O; the CO$_2$ ice line at $\sim 10$ AU shifts gas C/O further upward; and beyond the CO ice line at $\sim 40$ AU the gas C/O reaches unity. The dotted line marks the solar reference. The radial sequencing of these ice lines controls the volatile composition that forming planets inherit from their formation zone. Reproduced from {cite:t}`Oberg2011`, Fig. 2.
 ```
 
 ### Observational evidence: ALMA
@@ -422,7 +422,7 @@ The variation is only about $\pm 0.5$ km s$^{-1}$ around the mean, because Earth
 
 **Application: Halley's comet.**
 
-For a dramatic contrast, consider Halley's comet ($a = 17.8$ AU, $e = 0.967$):
+For a dramatic contrast, consider Halley's comet ($a = 17.83$ AU, $e = 0.967$):
 
 | Point | Distance $r$ | Velocity |
 |-------|-------------|----------|
@@ -580,7 +580,7 @@ Disk surface-density response to an embedded planet, showing the outer-Lindblad
 
 ### The Nice model
 
-After the gas disk has dispersed, the giant planets can still undergo migration through interactions with a remnant disk of planetesimals. The **Nice model** {cite:p}`Tsiganis2005` proposes that the giant planets formed in a more compact configuration (all between $\sim 5$ and $\sim 17$ AU) and subsequently underwent a dynamical instability, likely triggered when Jupiter and Saturn crossed their mutual 2:1 mean-motion resonance ({numref}`fig:nice-model`). This instability scattered Uranus and Neptune outward to their current orbits, disrupted the primordial Kuiper Belt, and may have triggered a spike of impacts throughout the inner solar system (the "Late Heavy Bombardment," although its timing and intensity remain debated).
+After the gas disk has dispersed, the giant planets can still undergo migration through interactions with a remnant disk of planetesimals. The **Nice model** {cite:p}`Tsiganis2005` proposes that the giant planets formed in a more compact configuration than today (all within $\sim 20$ AU) and subsequently underwent a dynamical instability, likely triggered when Jupiter and Saturn crossed their mutual 2:1 mean-motion resonance ({numref}`fig:nice-model`). This instability scattered Uranus and Neptune outward to their current orbits, disrupted the primordial Kuiper Belt, and may have triggered a spike of impacts throughout the inner solar system (the "Late Heavy Bombardment," although its timing and intensity remain debated).
 
 ```{figure} figures/nesvorny2018_nice.avif
 :name: fig:nice-model
@@ -592,14 +592,14 @@ Evolution of the giant planets through the Nice-model instability. Left panel: s
 
 ### The Grand Tack
 
-The **Grand Tack hypothesis** {cite:p}`Walsh2011` proposes that Jupiter first migrated inward to $\sim 1.5$ AU via Type II migration, then reversed course when Saturn caught up and became trapped in the **2:3 mean-motion resonance** with Jupiter: the combined torques from their mutual interaction with the disk drove both planets outward ("tacking," as in sailing). This inward-then-outward migration ({numref}`fig:grand-tack`) would have scattered and depleted material in the inner solar system, potentially explaining the small mass of Mars and the compositional structure of the asteroid belt.
+The **Grand Tack hypothesis** {cite:p}`Walsh2011` proposes that Jupiter first migrated inward to $\sim 1.5$ AU via Type II migration, then reversed course when Saturn caught up and became trapped in the **3:2 mean-motion resonance** with Jupiter: the combined torques from their mutual interaction with the disk drove both planets outward ("tacking," as in sailing). This inward-then-outward migration ({numref}`fig:grand-tack`) would have scattered and depleted material in the inner solar system, potentially explaining the small mass of Mars and the compositional structure of the asteroid belt.
 
 ```{figure} figures/walsh2011_grand_tack.avif
 :name: fig:grand-tack
 :width: 600px
 :align: center
 
-The Grand Tack scenario from a Walsh et al. (2011) simulation. Top panel: mass evolution of the four giant planets during the disk phase. Bottom panel: semi-major axis evolution; each curve is labelled with the planet's name on the track. Jupiter migrates inward to $\sim 1.5$ AU via Type II migration; once Saturn catches up and the two planets become trapped in their mutual 2:3 mean-motion resonance, the combined torques reverse the migration and both planets move outward. The episode depletes solid material in the inner disk, with consequences for Mars's small mass and the asteroid belt's compositional structure. Reproduced from {cite:t}`Walsh2011`.
+The Grand Tack scenario from a Walsh et al. (2011) simulation. Top panel: mass evolution of the four giant planets during the disk phase. Bottom panel: semi-major axis evolution; each curve is labelled with the planet's name on the track. Jupiter migrates inward to $\sim 1.5$ AU via Type II migration; once Saturn catches up and the two planets become trapped in their mutual 3:2 mean-motion resonance, the combined torques reverse the migration and both planets move outward. The episode depletes solid material in the inner disk, with consequences for Mars's small mass and the asteroid belt's compositional structure. Reproduced from {cite:t}`Walsh2011`.
 ```
 
 ### Observational evidence for migration