Mercury and Mars
A stripped metal core beside a smaller world that kept its rock.
From the pair notes by Giulia Santopietro and Madison Carroll.
Mercury
- Distance
- Inside ~0.5 AU
- Density
- 5.43 g/cm³
- Year
- 88 Earth days
- Solar day
- 176 Earth days
- Core
- ~70% of the mass
- Moons
- None
Mars
- Build
- Rockier, mantle kept
- Density
- 3.93 g/cm³
- Year
- ~687 Earth days
- Sol
- 24 h 39 min
- Named scars
- Hellas, Borealis
- Moons
- Phobos and Deimos
Question 1
Formation pathway
Both planets formed by accretion from the terrestrial disk. Mercury grew inside about 0.5 AU and ended extremely rich in metal. The preferred pathway is a hit-and-run collision that stripped most of the mantle. Mars is the rockier of the two. Its density is 3.93 g/cm³, against Mercury’s 5.43. The notes do not call for a mantle-stripping impact on Mars.
Mercury
MESSENGER puts the core at about 70% of the mass and about 85% of the radius. That is the extreme metal enrichment.
Mars
Lower density, and no mantle-stripping impact required. The rock stayed.
Mercury’s core, two ways of saying the same enrichment. The drawing uses 85% of the radius. Mars is shown as rock only. These notes do not give Mars a core fraction.
Mercury
The core is about 85% of the radius. The metal reaches almost to the surface.
Mars
Density 3.93 g/cm³. No mantle-stripping impact needed.
Mercury
The same core is about 70% of the mass. MESSENGER is the measurement behind both figures.
Mars
Density 5.43 against 3.93 is the contrast these notes use. They do not assign Mars a core percentage.
Evidence. MESSENGER core fractions. Densities 5.43 and 3.93 g/cm³. Hit-and-run is the preferred pathway, not a closed proof.
Still open. The notes call hit-and-run preferred. They do not treat it as the only pathway that has been ruled in.
Question 2
Collisions and captures
Mercury’s headline collision is the hit-and-run itself, one or more impacts that stripped most of the mantle. It has no moon. Later, the Late Heavy Bombardment left Caloris. Mars kept its mantle. Its named scars are Hellas and the Borealis dichotomy. Phobos and Deimos are either captured asteroids or impact debris.
Mercury
Mantle stripped. No moon. Caloris is the later basin the notes name.
Mars
Hellas is about 2,300 km wide and more than 7 km deep. Borealis is the thin northern crust, the dichotomy. Two moons, origin unsettled.
Evidence. Named basins: Caloris, Hellas, Borealis. Mercury has no moons.
Still open. Phobos and Deimos may be captured asteroids, or they may be impact debris. The notes keep both.
Question 3
Volatiles and air
Mercury accreted dry and reduced. Any early atmosphere left quickly, less than 0.3% of the mass. What remains is a thin exosphere, plus polar ice sitting in shadowed craters. That ice is a cold trap. Mars once had a thicker atmosphere and a magnetic field. MAVEN measures ongoing loss of carbon dioxide and water. The ice that remains is a remnant of a former climate.
Mercury
Ice in permanent shadow. A cold trap, not a leftover ocean.
Mars
Ice as a climate remnant. The air and the water are still leaving.
Evidence. Early Mercury atmosphere under 0.3% of the mass. MAVEN for ongoing CO2 and H2O loss at Mars.
Question 4
A constrained future
Mercury’s orbit is chaotic over the next 5 billion years, with about a 1% chance of a collision with Venus or the Sun. It carries the highest risk of being engulfed when the Sun becomes a red giant, on a 5 to 7 billion year window. Mars will lose ice as the Sun brightens. Its weak gravity cannot hold a thick atmosphere, and its obliquity stays chaotic. Red-giant engulfment is far less likely. The notes add that Earth’s orbit stays stable in most simulations.
Mercury
Highest engulfment risk. A small chance of hitting Venus or the Sun before that.
Mars
Ice sublimes. The atmosphere does not come back. Engulfment is the unlikely case.
Evidence. About 1% collision odds over ~5 Gyr. Engulfment window ~5–7 Gyr for Mercury. Mars is the lower risk of the pair.
Still open. “About 1%” and “far less likely” are the bounds in the notes, not a calendar date for either planet.