I. How does Mars appear from Earth?

Red -- Roman god of war

Orbits outside Earth: •

• P =1.88 Earth-years
• average distance from sun = 1.5 AU

Telescope view: •

• diameter = 0.53x Earth's (2x moon)
• surface features visible: •
• 1877-1910: scientists speculated life might exist:
• sometimes surface markings disappear:

Watch features move as Mars rotates: (movie)

• one rotation takes 24 hours 40 minutes
• rotation axis is tipped by 25°: seasons

Mars has two tiny moons:

• "Phobos" & "Deimos"
• small & orbit close to Mars
• measure P and a of orbits, find mass:
  • Mars is 1/10 Earth's
• Mars's density = mass/volume = 3.9 g/cm³ •

II. What have spacecraft visits taught us about Mars?

• first flyby in 1965 (Mariner 4)
• first landers = Viking 1 & 2 (1976)
• Pathfinder lander & rover (1997)
• modern orbiters (MGS, Odyssey, Express)
• Spirit & Opportunity rovers (2004)

1) Atmosphere:

• 95% CO₂
• few% N₂ and Argon
• almost no water vapor or O₂
• similar to proportions on Venus
• low density: air pressure is 1% of Earth's
• no ozone layer:

Why so different from Venus and Earth? •

(a) Mars smaller, less internal heat, less outgassing:

• "primordial" atmosphere thinner

(b) more of Mars's atmosphere leaked away...

III. What determines whether a planet will retain an atmosphere?

(a) Gravity tends to hold gas particles to planet

(b) Gas particles escape if their (thermal) speed is greater than the planet's escape velocity:

• small mass planets tend to leak gas faster
• large radius planets tend to leak gas faster
• use equation to find V_esc for a given planet:
  • 11.2 km/sec for Earth
  • 5.0 km/sec for Mars
  • 2.4 km/sec for moon

(c) hotter gases = faster particle motions

• hotter planets lose gas faster

(d) in atmosphere with mix of gases: •

• lighter particles move faster & escape more easily

Fig. 23-16: •

• dots mark planets (Temp & Vesc)
• curves mark velocity of gas particles
  • H₂ (W=2) lightest
  • CO₂ (W=44) heaviest
• if gas moves faster than planet's V_esc (curve is above point) that gas will leak from planet

On Mars:

• H₂ & He escaped
• heavier gases stayed
• but no ozone:
• rich in Argon (W=40):
  • Mars's atmosphere was once thicker
• where did the CO₂ go?

Summary -- Mars's atmosphere was once thicker:

• contained water
• probably warmer (trapped in heat)
• not sure how much thicker & warmer

• Read Sec 22-2 starting at "Water on Mars" (pp. 491-496).