Mars: some basics

1. Mars: some basics • Radius = 3375 km (cf Earth’s 6378 km) • Mass = 6.42 x 1023 kg ( ~0.1 Earth’s mass) • Surface gravity = 3.8 m s-2 (~ 1/3rd Earth’s gravity) • Orbital semi-major axis = 1.52 Astronomical units • Eccentricity = 0.0934 (cf. 0.0167 for Earth)  perihelion at 1.378 AU, aphelion at 1.662 AU  insolation ranges from 52.5% to 36.2% average terrestrial values • Current surface temperatures – –87 to 25o C (air temperatures) - -70 to 10o C (soil temperatures • Present-day atmospheric pressure – 6 millibars on average • Atmospheric composition – predominantly CO2 • Axial tilt = 24.5o – but variable (cf. 23.5o for Earth) • Rotation period = 24.6 hours • Orbital period = 687 days

2. Mars: global views Late Northern summer – from Mars Global Surveyor, Wide-angle camera Tharsis & 4 large volcanoes 

3. The ages of Mars • The Noachian era: 4.5 to 3.5 billion years ago. Early intense impact cratering; extensive volcanic activity, some plate tectonics; much denser atmosphere, at least during earlier stages; considerable erosion  extensive (sporadic?) surface water (lakes, maybe even oceans). • The Hesperian era: 3.5 tp 2.0-2.5 billion years ago. Low impact rate; continued volcanic activity, but at a lower level; minimal (no?) tectonics; continued atmospheric depletion; water primarily underground in massive, frozen deposits, but some continued river formation, possibly as a result of local melting and `breakouts’. • The Amazonian era: 2.0-2.5 billion years ago to present day. Low impact rate; sporadic volcanic activity – primarily in Tharsis regions (Olympus Mons, Ascraeus Mons, Pavonis Mons & Arsia Mons); water almost exclusively frozen underground, but some percolation and occasional surface breakouts.

4. Magnetic fields and tectonics • Planetary magnetic fields are due to an internal dynamo, generated by a rotating, molten core  correlated with vulcanism & tectonics • Magnetic fields shield planetary atmospheres from high-energy ions in the solar wind  moderate atmospheric erosion • Present-day Mars has no detectable dipole field. • Satellite measurements show that Noachian features (southern uplands) are magnetised, but Hesperian features are not. • Noachian Mars had a strong field which had decayed significantly by age ~3.9 billion years. • Molten core in early Mars maintained partly by short-lived radioactive elements • Some evidence for mild tectonic activity, primarily Thaumaric mountain range near Tharsis and the five giant Mars volcanos

5. Martian volcanoes Clouds over Tharsis both from Mars Global Surveyor Olympus Mons

6. Water on Mars - now • Crater morphology suggests massive frozen deposits underground  meteoric impacts generate muddy slurry which forms `rampart’ craters. • Many rampart craters have `softened’ features, as if partially melted  best explained by relaxation (flow) of underground ice deposits. • Minimum size of rampart craters decreases with increasing latitude  underground ice lies closer to the surface (~200 metres depth) near the Poles. • Mars Odyssey searched for water in surface layers – detected no traces near Martian equator, but 20-50% ice composition within the top few metres at latitutudes > 60o. • Polar ice caps have (relatively) permanent core of water ice, supplemented by larger seasonal deposits of CO2 frost. • Evidence for surface water flows (narrow gullies in canyon side) and possible Martian glaciers (in Promethii Terra) within late10-20 Myrs. Possible flood (5 million cubic metre/sec ~ 125 x Mississippi river) in Valle Martis within last 20 Myrs. Note that ice sublimes if exposed directly on present-day Mars.

7. Recent water flow Canyon gullies in Newton Canyon gullies in Hellas

8. Polar Caps Viking & MGS images of the permanent South polar ice cap.

9. Water on Mars: then • Extensive morphological evidence for water erosion – dried river beds & ox-bow lakes, canyons (eg Valles Marineris), hillside gullies • Possible evidence (shorelines) of a substantial North Polar ocean (Oceanus Borealis) during Noachian times  perhaps sufficient water to cover the entire planet’s surface to a depth of >100 metres. May have survived (episodically) into Hesperian times. • Evidence for sedimetary features – e.g. crater Crommelin, imaged b Mars Global Surveyor. • Mineral deposits in Martian meteorites – most contain salts, showing evidence of exposure to brine, probably similar to terrestrial seawater. Most recent exposure was probably only 670 Myrs ago. • Mineral deposits on the present-day Martian surface e.g. haematite concentration in Terra Meridiani (Mars rover site) – cf. mineral deposits near hot springs on Earth. • Late, episodic floods could be triggered by internal heating (residual magma), climate changes induced by changes in axial tilt; episodes of violent volcanic activity & consequent atmosphere enhancement.

10. Sustained water flow Nanedi Vallis – MGS images

11. The Martian atmosphere • Current atmosphere has 0.7% pressure of Earth’s atmosphere, and is almost pure carbon dioxide. • Scaling from measurements of Ar abundance in Martian meteorites, the primordial Noachian atmosphere may have been 15 to 70 times more dense  10-50% pressure of Earth’s atmosphere, including extensive water vapour component. • Primordial Martian atmosphere likely included significant greenhouse gases plus extensive cloud formation  surface temperatures could have been significantly warmer than today, although note that the Sun was 75% present-day luminosity. Current studies centre on two options: cold, wet early Mars, average temperature lower than present-day; warm, wet early Mars, with near-terrestrial temperatures. • Primordial atmosphere survived until early Hesperian times (based on studies of gases trapped in meteorite ALH84001); depleted relatively rapidly as the Martian magnetic field decayed. • Possible local(?), short-lived atmospheric enhancements due to significant volcanic episodes as late as Amazonian era (Tharsis volcanic eruptions).