07a

Search for Life in the Universe: 

Search for Life in the Universe Chapter 7 Mars (Part 1)

Outline: 

Outline Fantasies Herschel Schiaparelli Lowell General Public Habitability of Mars Basic Mars Data Mars Today Geography Geological History Liquid Water on Mars Climate History

William Herschel (17381822): 

William Herschel (17381822) Most famous astronomer of his time Discovered Uranus Catalogued double stars and nebulae Reinvented the hypothesis that nebulae were made of stars, i.e., galaxies (without knowing about Kant) Measured rotation of the planets Discovered Martian day of 24 hours 37 minutes Mars fantasies Mars has a [substantial] atmosphere It has inhabitants They enjoy life similar to ours

Schiaparelli (18351910): 

Schiaparelli (18351910) Respected astronomer Director of the Milan observatory Followed European astronomical tradition of looking for linear features on the sky Long eye observations: lead to imaginary images Mars Fantasies 79 linear features, which he called Canali (channels) Mistranslated into English as canals Conjured an advanced civilization that built artificial waterways Maybe similar to Suez Canal (opened in 1869)

Lowell (18551916): 

Lowell (18551916) Important astronomer On the faculty at MIT Founded Lowell Observatory (Flagstaff, AZ) Predicted Pluto, discovered after his death at Lowell Observatory Mars fantasies Believed Schiaparelli and proceeded to map ~200 canals Correctly identified seeing as a major problem Deduced that most of Mars was dry Thought polar caps were made of ice Canals did not show up in photographs Canals did not show up under good seeing Illustrates the pitfalls of bias and its ultimate demise

General Public: 

General Public H. G. Wells (18661946): “War of the Worlds” Motivated by Lowell Radio broadcast by Orson Wells (1938) created panic “Face” of Mars Looks like a face with poor resolution Looks completely natural with better resolution Tabloids Still coming up with invented stories Misrepresenting real photographs Sell well because people are uninformed and gullible

Basic Mars Data: 

Basic Mars Data Semi-major axis: 1.52 AU Orbital Period: 1.881 (Earth) yrs Equatorial radius: 3,397 km Mass: 0.107 Earth masses Rotation period: 24 hr 37 min Surface gravity: 0.38 Earth gravity Atmosphere: 95% CO2, 2.7% N2, 1.6% Ar Average surface pressure: 0.005 bar

Mars Today: 

Mars Today Martian seasons Axis tilt 25 (cf., 23.5 for Earth) Elliptical orbit  closer to the Sun during the summer in the southern hemisphere  harsher seasons there Seasonal sublimation of polar caps: change pressure and CO2 Seasonal cycle: 1/3 of the atmospheric CO2 Water on Mars Substantial amount of water Water ice seen when northern polar cap retreats Southern polar cap does not evaporate, reason not known Liquid water is not stable on Mars today  sublimation Water vapor often seen in atmosphere, including clouds Hydrogen detected by Mars Odyssey

Geography: 

Geography Southern hemisphere: high elevation, dense cratering Northern hemisphere: flat lowland, lower cratering density, features like lava flow Hellas basin: large impact crater, heavily cratered inside  old (4.4 byr?) Tharsis bulge: volcanic, 210 km high, contains Olympus Mons (600 km across, 25 km high) Valles Marineris: tectonic canyon, as long as USA, 4 times deeper than the Grand Canyon

Geological History: 

Geological History Dating Crater density Radiometric dating of Martian meteorites (next lecture) Noachian era 4.53.7 byr ago Heavy bombardments Hesperian era 3.71.0 byr ago Volcanic activity decreased strongly in the last 3.5 byr Amazonian era 1.00 byr ago Martian meteorites show crystallization as recent as 180 myr ago Some volcanism still taking place

Liquid Water on Mars: 

Liquid Water on Mars Noachian era (heavily cratered) Channels like terrestrial river systems Small craters missing and large craters have breaches Origin of water debated: rainfall + runoff v. sapping from below Erosion gradual  water could survive longer Hesperian and Amazonian eras Result of floods No source reservoir  water emerged from below Occurred sporadically throughout these eras, some recently Probably linked to volcanism: volcanoes show flood channels on some of their sides Ocean? Most of flood channels drain to northern lowlands Some evidence of “shoreline”

Climate History: 

Climate History Atmosphere warmer in the past Greenhouse effect due to CO2 Present Sun: liquid water with 400 x present atmospheric CO2  3 bar, 1/30 Earth/Venus CO2 Dimmer Sun: CO2 not enough; need some CH4 (?) Loss of CO2 to space Polar caps insufficient No good evidence for carbonates Loss via impacts Loss via the solar wind, which is not well deflected because of weaker magnetic field (core cools faster) Loss of H2O Loss via impacts UV breaks up H2O and H2 escapes easily