Astronomy 1 - Lecture November 9, 1998

Venus, Its Atmosphere and Planetology Mars, an overview

Assignment for November 16. READ Chapter 10 on Mars and go through the True-False, Fill-in-the-Blank and Review and Discussion Questions at the end of the Chapter. There will be a quiz on Nov 16 using two of those questions.

MARS

Major Properties of Mars

Visual
	Size of Mars = 25" maximum (opposition 2003) 
		at present 5" arc in diameter, Magnitude = 1.5
		in the Constellation of Leo (near rear leg)
		At present rise at 2 am (2.0 AU away)

Orbit
	Mars's Orbit (Eccentricity 0.0934 and Time of Perihelion) 
			related to observation [figure 10.1]
		perihelion = 1.38 AU (Aquarius) (Would be August on Earth)
		aphelion = 1.67 Au (Leo) (Would be February on Earth)
		(Ratio of Solar brightness:  perihelion/aphelion = 1.46)

		Orbit Period = 1.881 years = 687 days

	Synodic Period = 780 days = 2 years and 50 days
	Opposition delayed 50 days each year (1 month 20 days)
		It takes 365/50 = 7+ oppositions to get back to 
			the favorable one (15 years)
		One in 1973, 1988, the next in 2003
	Next Opposition April 27, 1999 (16" arc, 0.58 AU away)

Satellites	size		orbit			period
		(km)
	Phobos	19x27	9300 km   2.6 R		7h 39m	(faster than rotation)
	Diemos	11x15	23,500 km 6.9 R		30h 18m

	Orbits are circular, equatorial  = sychronous rotation
	Phobos (name means Fear) 
		close with large craters - no soil
	Diemos (name means Panic) 
		far away and dust covered (compositions different from Mars)
	Probably captured asteroids with density 2000 kg/m3

Rotation
	Rotation Period very similar to Earth 24h 37.4m 
	(41 minutes longer than that of the Earth)
	Each day near opposition Mars 
		central longitude changes by 10 deg/day so it require 36 	
		days (a month ) to view the whole surface of Mars
	Tilt of Axis = 24 degrees (This produces seasons on Mars)

	Where is the Northern Summer?
		heliocentric longitude of N pole = 353 deg with latitude = 63.3 deg
		means N pole points away from Sun at longitude 353
	Longitude of perihelion = 336 deg 
		(So North pole points away from the Sun at perihelion)
		and the Southern Hemisphere is toward the Sun at perihelion 
		and away from the Sun at aphelion 
		(since Mars moves slowly at perihelion, the southern hemisphere 
		has a longer winter and a shorter hotter summer) 
		The northern hemisphere has a longer summer and shorter winter
		and hence there are less dramatic changes in the northern polar cap.
		[The Southern cap is CO2 over H2O - the hotter southern summer creates
		  dust storms that deposit dust at the cooler northen pole, and makes
		  it darker and warms up faster when sun hits it. Hence it is not as 
		  big.]

	Southern Summer in Mid-September (Using Earth positon in orbit)
		(Northern Winter)
	      Southern Winter in Mid-March   
		(Northern Summer)

Physical Properties



	Radius = 3397 km (equatorial) =  0.53 R(Earth) ~ 2 R(Moon)
	oblate 1/154 
		(Mercury, Venus and the Moon have 0 oblateness 
		 while the Earth has 1/298.)
	Mass = 0.11 M(Earth)
	Average Density = 3,900 kg/m3 (compare Earth at 5500 kg/m3)
	Lower denity implies smaller or non-iron core

	Surface Gravity = 0.38 g
	Escape Velocity = 5.0 km/s
	Albedo = 16%
	Surface Temperature 130 K to 250 K (-20 C)

Atmosphere
Composition  	CO2	95.3%
		N2	 2.7% 
		(enriched in 15N to 14N suggests 
		  denser atmosphere in past)
		Ar	 1.6%  (evidence of a former denser atmosphere - remnant)
		O2	 0.13%
		H20	  0.03% 
			(Water can not liquify at the pressure of the atmosphere)
			This is very dry atmosphere
			UV dissociation of H2O and 
				loss of H2 from the planet
			Maybe water in rocks

Pressure (surface) = 0.007-0.008 bar
	Varies 5% due to CO2 condensation/evaporation (Polar Cap sources)
Troposphere to 30 km at times.  Water and CO2 clouds
Winds - enough to create dust storms (particles only micrometres in size)

Magnetic Field
	No magnetic field or magnetosphere to deflect the solar winds
	Solar Winds can hit the atmosphere and strip off the gases
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ASIDE: Escape Velocity of Satellites of Mars
	Mass of Diemos = 4.8x10-9 M(mars) = 5.3x10-10 M(Earth)
	Radius (Diemos) = 6 km = 9.4x10-4 R(Earth)
	Since v(escape) ~ (M/R)1/2	and v(escape Earth) = 11.2 km/s
	v(excape Diemos) = (5.3x10-10/9.4X10-4)1/2 x 11.2 km/s = 0.0084 km/s = 8.4 m/s		
		If can run off the satellite if you can run the 100 m in 12 seconds.
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NEXT:
	Surface Features (from Viking orbiters)
		Mariner 4 (1965), Mariner 9 (1971), Viking Orbiter (1976),
	Ice Caps 
	Search for Life (Viking 2) and Meteors from Mars (1997)
	Theory of History of Mars (1997)