1. ESCI 214: Mars: How it all got there. (Overview of Martian Evolution)
Patrick J. McGovern
Lunar and Planetary Institute

2. Martian Timescale
Noachian: sometime after accretion (oldest known geology) to Ga.
Hesperian: 250M-1.7Gyr between Noachian and Amazonian.
Amazonian: Ga to present.
Timescales may change due to new MGS discoveries concerning craters.
Sample return vital for absolute calibration.

3. Formation of Crustal Dichotomy
Cratered Southern Highlands.
Thick crust, decreasing thickness to north.
Cratered Northern Lowland Basement.
Nearly constant-thickness crust.
Hypotheses fall into two categories:
Endogenic (internal processes).
Exogenic (external forces).

4. Pole-to-pole Slope
This is a pole-to-pole view of Martian topography from the first MOLA global topographic model [Smith et al., Science, 1999]. The slice runs from the north pole (left) to the south pole (right) along the 0° longitude line. The figure highlights the pole-to-pole slope of 0.036°, such that the south pole has a higher elevation than the north pole by ~6 km. This global-scale slope was likely present for most of Mars' history and controlled the surface and subsurface transport of water indicated by images of outflow channels and valley networks. The regional high (in orange) in mid-southern hemisphere latitudes corresponds to the western edge of the topographic annulus that encircles the massive Hellas impact basin. In the figure warm colors correspond to high elevations and cold colors correspond to low elevations.

5. Pole-to-pole Crustal Slice
Global slice of the crustal structure of Mars along 0° E longitude as derived from gravity and topography data from the Mars Global Surveyor spacecraft that is currently mapping the red planet. In the figure the south pole is at the far right and the north pole is at the far left. For illustrative purposes the crustal structure is vertically exaggerated and is about 40 km thick under the northern plains and 70 km thick at high southern latitudes. The sloping region under part of the southern highlands (yellow/orange) and the uniform thickness region under the northern lowlands (blue) and Arabia Terra region (green) represent two distinct crustal provinces. The global dichotomy boundary occurs at the lowlands/Arabia Terra (blue/green) transition. This boundary does not correlate with the crustal structure, which indicates that the geological manifestation of the boundary is primarily due to surfical rather than internal processes.

6. Crustal Dichotomy: Endogenic
Crustal thinning of North by mantle convective tractions.
Can thin crust while leaving non-uniform surface expression.
Avoids difficulties with impact scenarios.
Numerical simulations of convection have not demonstrated physical plausibility (working on it!)

7. Crustal Dichotomy: Endogenic
Plate Tectonics.
Good way to generate constant-thickness crust.
Predicts northern hemisphere basement should be significantly younger than southern hemisphere, inconsistent with observed buried craters in north (unless plate tectonic era is in extreme early history of Mars).
No strong evidence for plate tectonic structures (e.g., spreading ridges, subduction trenches).

8. Crustal Dichotomy: Exogenic
A single giant impact.
Large enough impact could remove sufficient volume of crust.
Impact of size required plausible given known abundance of large impactors in early solar system.
Dichotomy boundary is complex and does not appear to be well-fit by a single circle.
Inferred Crustal thickness shows no sign of such a basin/rim.
No apparent ejecta blanket.

9. Crustal Dichotomy: Exogenic
Multiple large impacts.
Large enough impacts could remove sufficient volume of crust, and create rougher boundary.
Strong evidence for multiple ancient Northern Lowland basins (topo and grav).
Dichotomy boundary is complex and even multiple impacts cannot account for observed complexity.
Why did the impactors strongly favor the northern hemisphere?

13. Magnetic Anomalies
Banded anomalies found mostly in Noachian southern hemisphere terrains.
Intensities require rock magnetization in mag. Field much stronger than current one.
Locations suggest Noachian origin for magnetism.
Core Dynamo in Noachian generated magnetic field which magnetized the Martian crust.
Anomalies apparently obliterated by the (old) large impact basins.
Dynamo ceased working early.

16. Formation of Tharsis
Huge volcanic surface load from Tharsis depressed the lithosphere, creating a trough ringing the region.
Trough contains most of the negative gravity anomalies on the planet.
The Tharsis trough and the pole-to-pole slope control the slope of the Noachian valley networks.
Much of Tharsis was formed by end of Noachian.

21. Hesperian Activity Widespread deposition of Volcanic Plains.
Many such plains subsequently (but still in Hesperian) undergo compressional faulting, as demonstrated by parallel sets of wrinkle ridges. Ridge orientations controlled by lithospheric stresses from large rises. Such plains occur near Tharsis (Syria, Solis, and Lunae Plana) and in the northern lowlands (buried but recently uncovered by MOLA).
Activity at Alba and Elysium volcanic rises.
Ongoing volcanic/tectonic activity at Tharsis.
Valles Marineris troughs are formed (mostly).
The outflow channels are formed (mostly).

26. Amazonian Activity Tharsis Volcanism
Ongoing volcanism and deposition of smooth deposits covering northern plains.

27. “Recent” Activity (Late Amaz.)
Tharsis Montes and Olympus Mons Volcanism (latter has youngest volcanic flows seen on Mars: no superposed craters).
Gully formation (no superposed craters).

28. Verifiably Ongoing Activity
Slope Streaks: long, thin, usually dark.
Attributed to dust avalanches, influenced by melting of water.
Volatile cycling between poles and atmosphere (2 examples):
Polar pits formed by sublimation of CO2.
Polar cap mass changes observed in gravity.

29. Verifiably Ongoing Activity
Slope Streaks in Olympus Mons Aureole (MOC image).

31. Thermal Evolution
Thermal history inferred from studies of Gravity/topography ratios (admittance).
High G/T = strong lithosphere = low heat flux “q”.
Low G/T = weak lithosphere = high heat flux “q”.
Result: old features = high q, young = low q.
Inference: Planet cooled with time
Consistent with observations:
One Plate planet.
Early shutdown of dynamo.
Decline in volcanic activity with time (young volcanism restricted to Tharsis and Elysium).