1B11 Foundations of Astronomy Mercury, Mars and Venus Liz Puchnarewicz

1B11 Mercury This image is a composite of photos taken by the Mariner 10 spacecraft, which flew past in 1973 and It showed a world that resembles the Moon. It is pocked with craters, contains huge multi-ring basins, and many lava flows. The craters range in size from 100 metres to 1,300 kilometres. The Caloris basin is 1,300 kilometers in diameter, and was probably caused by a projectile larger than 100 kilometers in size.

1B11 Venus This beautiful image of Venus is a mosaic of three images acquired by the Mariner 10 spacecraft on February 5, It shows the thick cloud coverage that prevents optical observation of the surface of Venus. Only through radar mapping is the surface revealed.

1B11 Mars This image is a mosaic of the Schiaparelli hemisphere of Mars. It was taken by the Viking Orbiter in The center of this image is near the impact crater Schiaparelli, 450 kilometers (280 miles) in diameter.

1B11 Mercury, Mars and Venus PlanetDiameter (Earth=1) Mass (Earth=1) Density (water=1) Surface pressure (Earth=1) Mercury Venus Earth Mars Moon

1B11 Mars – essential information Radius : 3398 km Mass : 0.11 x mass of the Earth Mean density : 3.9 g cm -3 Average temperature : -58 O C Atmosphere 95% CO 2 with a pressure of 6 mbar No magnetic field today No evidence for plate tectonics Abundant volcanism in recent past Two small moons (Phobos and Deimos)

1B11 Interior of Mars Mars is believed to have a thin crust, mantle and core. Core radius is between km. It has polar caps in the north and south. In the winter, the caps cover 30% of the hemisphere and are made up of CO 2 frozen out of the atmosphere. They retreat to a few % in the summer – the north cap is H 2 O ice, the south a mixture of CO 2 and H 2 O. Layered deposits indicate a cyclical deposition of dust and ice.

1B11 Mars – surface features Mars is smoother in the north – it is younger, has fewer craters and is relatively low (-1 to 2 km wrt 6mbar pressure level) with volcanic plains. In the south it is much older (~4 billion years), cratered and high (~2-3km). It has 3 major impact basins, Hellas (2200 km across), Argyre and Isidis. These date from the age of heavy bombardment. Olympus Mons is a giant shield volcano, 600 km across, 26 km high and less than 500 million years old. The Tharsis Ridge contains Olympus Mons and three similar shield volcanoes - there are also many smaller volcanoes.

1B11 More surface features on Mars The Vallis Marineris is 5000 km long, 100s of km wide and up to 9 km deep. It’s believed to be caused by tectonic activity – possibly faulting associated with Tharsis. This topographical image of Mars shows the Vallis Marineris, Tharsis Ridge and the Hellas impact basin. The crust is very thick (~120 km) so inhibited tectonics.

1B11 Water channels Mars was once believed to have “canals”. We now interpret many surface features to be the result of erosion by water. 1.Outflow channels : Caused by catastrophic floods from the southern highlands and canyons to the north plains, due to the sudden release of ground water. 2.Sinuous Valleys : Mostly in the southern hemisphere. These are short, stubby tributaries thought to be due to ground- water sapping. 3.Dendritic channels : Maybe due to rain – but ground water is considered the most likely cause.

1B11 Evidence for water High resolution images show features possibly due to very recent (possibly ongoing) discharges of water from canyon and crater walls. How much water is there? Erosional features account for more than 6 million km 3 (equivalent to 40m spread over Mars.) In total, this is probably more than m. => Suggests a permafrost and groundwater layer which is several km thick!

1B11 Water phase diagram Pressure (bar) LIQUID GAS SOLID MARS Temperature (K) Liquid water is not stable - to remain liquid it requires a higher temperature and pressure than on Mars. There is evidence for climate change on Mars however, possibly a denser CO 2 atmosphere in the past.

1B11 Is there Life on Mars? Before space exploration, Mars was considered the best candidate for harboring extraterrestrial life. In 1976, three biology experiments aboard the Viking landers discovered unexpected and enigmatic chemical activity in the Martian soil, but provided no clear evidence for the presence of living micro organisms in the soil near the landing sites. But billion years ago, Mars had a thicker CO 2 atmosphere so the average temperature was probably higher than 0 O C, so water would have existed in liquid form. Earth was in the same state at the time. So did life evolve? The search is on for fossil remains – and the question of life on Mars at some time in the distant past remains open.

1B11 Venus Venus is the Earth’s “sister” planet – it has a similar size, mass, density and volume. However it is very different from the Earth. It has a heavy CO 2 atmosphere with almost no water vapour. Its clouds are composed of sulphuric acid and at the surface, the pressure is 92 times that of the Earth at sea level. Heat is trapped in the atmosphere by the greenhouse effect and raises the temperature to about 482 O C. One day on Venus = 243 Earth days One year = 223 days, so its day is longer than its year. And it rotates from east to west.

1B11 Venus – the essential facts Radius : 6050 km (0.95x the radius of the Earth) Mass : 0.82x the mass of the Earth Density : 5.25 g cm -3 (Earth, 5.52 g cm -3 ) Rotation period : 243 days (retrograde) Atmosphere : pressure = 90bars, 90% CO 2, 3.5% N 2 Severe greenhouse effect => surface temp ~ 460 O C Total cloud cover (composed of H 2 SO 4 ) No magnetic field No moons

1B11 Venus revealed This image compares the surface image of Venus with the “clouds” image. The clouds are mostly sulphuric acid and reach 65km above the surface (cf 16km on the Earth). The surface shows mountains, plains, high plateaux, canyons, volcanoes, ridges and impact craters.

1B11 Venus height (km) tops of clouds 1bar upper cloud deck (H 2 SO 4 ) haze layer clouds thin out clear atmosphere surface Only 3% of sunlight reaches the surface. An extreme greenhouse effect traps the surface heat to prevent heat from escaping. Heat is carried by strong winds so that day- night temps vary by less than 10K. Wind speeds in the cloud deck reach 240km/h -50 O C 460 O C 95 O C 10 O C winds: 1-3km/h

1B11 Venus’ surface features 1.Highlands Two continental-sized regions: (a)Aphrodite – equatorial, approx 10,000 x 2000 km with a maximum altitude of 4-5km (b)Ishtar – lies 60 O -75 O North, is the size and shape of Australia and reaches an altitude of 11km (Maxwell Mountains) 2. Impact Craters Uniform distribution => surface age of million years, => global re-surfacing. Small craters (<3km) don’t form because small meteors are vaporized.

1B11 Venus North-South divide North Mountainous with uncratered upland plateaux, resembling continents on Earth. Ishtar Terra is the great northern plateau and measures 1000km x 1500km. South Flat, rolling terrain. Seems to consist of vast lava plains.

1B11 Volcanoes and craters Venus is almost completely covered in flat, volcanic plains with 10,000’s volcanic plains and shields. Plains are punctuated with approx 1000 craters from ~10km to over 100km in diameter. Impact craters have a great deal of structure – 1.Central peaks 2.Terraced walls 3.Shocked surfaces 4.Flooded floors. Dense atmosphere => no extensive ray systems

1B11 Volcanism and tectonics Volcanoes are generally large shield volcanoes (eg Maat Mons). Thousands of smaller volcanoes cover the lowlands. It is not clear whether these volcanoes are active (Magellan orbiter) – but there is evidence for lava channels. Sulphur compounds in the atmosphere suggest on-going release of gases from volcanoes (outgassing, cf the Earth). Tectonics The surface of Venus is relatively smooth, (cf ocean basins and continents on Earth) suggesting no planetwide crustal plates. Any tectonic activity seems to concentrated in the Highlands. Maybe the crust is more plastic and heat losses are local and volcanic.

1B11 The greenhouse effect Calculating the equilibrium temperature. Planetary surfaces are heated by sunlight: R D Planet, radius R Sun, Luminosity L SUN Total energy intercepted by the planet:

1B11 Venus Albedo, A is the fraction of energy incident on a planet that is reflected back to space So total energy absorbed by the planet is: This energy will warm the planet. Assuming the planet then radiates as a blackbody of effective temperature T eff :

1B11 Equilibrium vs measured temp Putting in the numbers and measuring D in AU gives: For Venus, A = 0.76 and D = 0.72 AU.  T eff = 230K But the measured surface temperature is approx 730K! The difference of 500K is due to the CO 2 greenhouse effect. CO 2 transmits visible light but absorbs the near IR, trapping heat which is radiated by the surface.

1B11 Planetary greenhouse effects Remember Wien’s law: For T = 230 K, …which is absorbed by CO 2. planetTemp increase by greenhouse effect Venus+500K Earth+35K Mars+5K NB: the Earth has ~60 bars of CO 2 locked up in carbonate rocks.

1B11 Mercury This image is a composite of photos taken by the Mariner 10 spacecraft, which flew past in 1973 and It showed a world that resembles the Moon. It is pocked with craters, contains huge multi-ring basins, and many lava flows. The craters range in size from 100 metres to 1,300 kilometres. The Caloris basin is 1,300 kilometers in diameter, and was probably caused by a projectile larger than 100 kilometers in size.

1B11 Mercury – the essential facts Mass – 0.06x the mass of the Earth Radius – 0.38x the radius of the Earth Density – 5.4g cm -3 NO atmosphere Surface temperature - daytime :190 O C, night : -180 O C Weak magnetic field (1/60 th of the Earth’s) – probably from surface rocks High mean density => large (solid?) iron core 35% of surface imaged by Mariner 10 (1973-4) => heavily cratered surface, approx 4 billion years old

1B11 Is there ice on Mercury? Ground-based radar images of the poles give echoes characteristic of water ice. Impact craters near the poles are in perpetual shadow, so have a temperature of approx –170 O C which is cold enough for ice. The thickness of the ice must be at least the wavellength of the radar (12.5cm).