Modern Astronomy

Lecture 3

August 29, 2003

 

Mars

 

Today begin two  lectures on Mars.  Mars is a terrestrial planet we know a lot about, and it has always been associated with the question of life elsewhere in the universe. Today that connection is closer and more compelling than ever.  Compared with other cosmic environments we know about, it is not that  hostile.  My expectation is that  in the next twenty years we will know fairly definitively whether life has ever existed on Mars, or if it even exists at the present time.

 

Orientation

 

Mars is currently the brightest object in the evening sky, visible in the southeast at about 10PM.  Its distance from the Earth is 0.33 astronomical units. 

 

Just the facts, Maam

 

  • Diameter: 6378 kilometers versus 12756 for Earth (53%). >>>> famous transparency. Mars is a significantly smaller “ball in space” than Earth.
  • Mass 6.44 ‰ 1023 kilograms (11 % that of Earth).
  • Density 3.9 grams/cc
  • Major axis of orbit 1.52 au.
  • Period of orbit 1.88 years.
  • Eccentricity of orbit 0.093 (rather high for a major planet).

 

Basic Observations and History of Study of Mars

 

            With telescopes on Earth you can make out surface features on Mars. Some of the features you can see are  All of these could be seen with the telescope on the roof, provided observing conditions were good. 

  • Recurrent features such as Syrtis Major.  With these you can tell the rotational period of Mars is almost exactly that of Earth,  24.6 hours.
  • Polar caps, seemingly like Earth.
  • Seasonal variations in the prominence of the surface features.
  • All of these observations were available between 100 and 150 years ago, and led to the idea that Mars was an Earthlike planet.

 

Percival Lowell and the Martians

 

            At the end of the last century and the beginning of this one, Percival Lowell set up an observatory at Flagstaff, Arizona and began careful observations of Mars.  He thought he saw the famous Martian Canals. These were believed to be the construction of  intelligent Martians (see the 1960 movie Mars, the Angry Red Planet).

 

Mid Twentieth Century Physics and the Inhabitability of Mars

 

            A number of discoveries were made with the improved knowledge of physics that we could bring to astronomical observations in the twentieth century.  These made it clear that Mars could not really host intelligent life forms

  • The atmospheric pressure is about 8-9 millibars, far too low to support liquid water.  The atmospheric pressure here on Earth is about 1013 millibars at sea level. At the cruise altitude of commercial aircraft it is about 300 to 350.  To reach

8-9 you have to go 20 miles up.

  • The chemical composition of the atmosphere of Mars is quite different .  It is primarily carbon dioxide, like Venus.   The abundance of Argon is 1.6 % rather than 0.006%.  It turns out that this (apparently stultifyingly) dull detail is actually an extremely important clue to the ancient history of Mars.
  • Because of the lack of an atmosphere, there are extreme variations in the surface temperature.

 

 

 

            Today’s class will deal with the discoveries we have made with unmanned spacecraft over the past 30 years.  These findings are fascinating feats of discovery in their own right, but they also provide the empirical groundwork and basis we need for obtaining insight into the geological history of Mars, and deal with the intriguing question of life (ancient or modern) on Mars.

Let’s begin with a look at Mars and how it stacks up relative to the Earth

http://mars.jpl.nasa.gov/science/index.html

            A good collection of Mars images is contained at

http://mars.jpl.nasa.gov/gallery/images.html

 

            After looking at these images, go outside tonight and look at Mars, 0.33 au distant, in the southeast in the early evening.

 

 

The Flotilla of Mars Spacecraft

 

            The following spacecraft have successfully completed missions to Mars.

  • Mariner 4 (1965)… flyby of southern hemisphere
  • Mariner 9 (1971) ….orbiter
  • Viking 1 and 2 (1976)….orbiter and lander
  • Pathfinder (1997)…lander and rover
  • Mars Global Surveyor (1997)…orbiter

 

In addition to these, there were a number of other US and Russian missions to Mars, which were unfortunately lost when Martians hiding in “The Face” emerged to destroy the spacecraft.  There are also ambitious plans for the future, but I will wait to describe those at the end, when we can understand why they would be so scientifically desirable. Four spacecraft are on the way to Mars right now, and will arrive in December and January.  By the middle of next semester, you will see pictures of the surface of Mars returned from these landers. 

 

 

 

   As a result of these missions, we found an amazing variety of landforms and surface

Geology.  This is what you were not able to see with the telescope on the roof.  

 

Main Surface Features of Mars

                  

1.  North-South Asymmetry:  The southern hemisphere is higher in altitude than the northern, and heavily cratered.  The northern hemisphere is lower (like the lunar Maria) and has regions with few or no craters, and other evidence of geology or hydrology.

http://ltpwww.gsfc.nasa.gov/tharsis/global_paper.html

 

2. Impact craters:  Craters are seen all over the planet, so the surface retains information from the Age of Bombardment.  Interestingly, and importantly, some of these craters show evidence of liquid ejecta, or seepage from crater walls.

http://www.msss.com/mars_images/moc/MENUS/crater_list.html

 

3. Volcanos: The northern hemisphere of Mars has prominent shield volcanos.  Some of them have slopes with numerous impact craters, so they have evidently been dormant since the early days of  the solar system.  However, the largest, Olympus Mons, has crater-free slopes, so it has been active well after the age of bombardment, and may still be active.

http://www.msss.com/mars_images/moc/MENUS/volcanic_list.html

 

4. Valles Marineris: There is a large crack in the crust of Mars, evidently caused by

Tectonic forces, which stretches across most of one hemisphere. See Figures 9.12 and 9.15 in your book.  There have been observations of haze in the bottom of this canyon in the Martian morning, which is believe to be due to water ice below the surface.  A good picture of this is seen in the URL under section 1. above. 

5. Runoff  Channels and Outflow Channels:  Perhaps the most exciting development was the discovery of flow channels which are accepted to be ancient waterways.  These channels are therefore dry river beds.  These may be divided  into outflow channels, which were apparently caused by huge eruptions of subsoil water, and runoff channels (also called valley networks) which have more of a network of tributaries, and are found in more heavily cratered  (and therefore older) parts of Mars.  Some scientists (and the books they write) state fairly categorically that the runoff channels were a result of rainfall in an ancient Martian climate.  There are potent objections to this interpretation, so I believe this question is more controversial than is sometimes presented.  An exciting new recent development  is evidence that water has very recently flowed on the surface of the planet.

http://barsoom.msss.com/http/ps/channels/channels.html

 

6. Polar Caps. ..  The polar caps are a combination of water ice and carbon dioxide ice.  The seasonally-variable component is carbon dioxide, the permanent component is water ice.  The fact that water ice is known to be present was a reason the Mars Polar Lander was sent to this part of  the planet, but they got it too.

7. Hebes Chasma and Ancient Lakes:  There are a number of depressions and basins that have rock formations which appear to be sedimentary deposits.  This would then suggest that they were lakes in which standing water was present for long periods of time.

http://www.msss.com/mars_images/moc/science_paper/f2/index.html

 

8. The Surface of Mars: The Viking landers and Mars Pathfinder returned abundant pictures from the surface of Mars.  These pictures show an eerily Earth-like scene.  Pathfinder was deliberately chosen to land in an ancient water channel, and the rocks appeared to have been deposited in an ancient flash-flood.

http://antwrp.gsfc.nasa.gov/apod/ap000514.html

 

 

         Next time I will discuss how all of this fits into our view of the geological history of Mars.