Chapters 4-5: Mercury

Surface characteristics

heavily cratered ancient terrain like Moon and parts of Mars

Lava flow properties

• similar to the moon

• high eruption rates

• great travel distances

• drown their own vents

explosive volcanic activity

• evidence: >30 rimless depressions up to ~40km diameter

• distinctive deposits

• volatile content interference: up to ~1wt% volatiles in erupting magmas

• nature of volatiles: high abundance of sulfur

more volcanic evidence

• irregularly shaped, rimless, steep-sided pits up to ~30km diameter

• common location: floor of impact craters

• long sheet-like lava flows flooded much of northern hemisphere early in geologic history

• almost completely buried an old impact crater

mercury atmospheric pressure

• zero atmospheric pressure

• miniscule amount of magmatic volatiles

• <1Pa

• convecting eruption clouds cannot form

• shock waves form in region above vent

• io and moon too (no pressure, no atmosphere)

mercury volcanic composition is

basaltic lava flows

mercury eruption style is

high rate, long distance flows that drown their vents

Mercury impact velocities

highest mean impact velocity among terrestrial planets for asterodial bodies: over ~25km/s

Hokusai Crater

• 100km diameter

• transitional between central peak crater and peak ring basin

• continuous (lobate) and discontinuous ejecta

• secondary craters surrounding

• MOST EXTENSIVELY RAYED CRATER ON MERCURY

• rays extend over 1000km from the rim

simple crater dimensions on mercury

• D/d: 0.98

• coefficient: 0.18

• surface gravity 3.78m/s²

mercury impact craters compared to mars

• similar surface gravities (mercury = 3.78, mars = 3.72)

• Martian complex craters SHALLOWER tha equivalent sized Mercurian

• due to: target material properties - mars weaker target, mars also has wind/water processes that reduce crater-related topography through erosion and sedimentary infilling

Rachmaninoff Basin

• 290km diameter

• peak ring basin structure

• dark, smooth fractured deposits interior to the inner ring

• represent extensive deposits rich in impact melt that were emplaced during crater formation

Peak ring basin formation hypothesis on mercury

• has largest population of peak ring structures

• on large impact events: depth of impact melting may reach and even exceed depth of cavity floor

• when transient cavity uplifted - central, melted part has no strength

• only rings from unmelted portion can form out from crater

• explains why theres rings but no central peaks

key mercury values/stats

1. gravity 378cm/s²

2. mean impact velocity >25km

3. has most peak ring basins per area of all terrestrial planets

4. notable craters: Hokusai (~100km), Rachmaninoff (~290km)