Characteristics and Origins of the Solar System
Lecture 15
October 11, 2000
Mars, Part 1
>>>>>> 3D
representation of surface of Venus
Today
begin several 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. I
would bet 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
First
let’s start with some basic orientation information.
- View of the
solar system
- Right now,
Mars is 2.405 astronomical units from us.
- The
celestial coordinates of Mars are RA=11h09m, Dec=+6degrees50min. Plot it
up on your SC1 chart.
- Next June
25 it will be at 0.40 au at opposition, so keep your eyes on it in
the meanwhile. >>> Transparency with oppositions of Mars.
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 (Hey! What does this mean?)
- 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. A good
illustration is in Figure 9.2 of the book.
Some of the features you can see are
- 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 in Arizona and began careful observations of Mars. He thought he saw the famous Martian
Canals (excellent illustration is Figure 9.3). 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 . Check Table 9.2 of the text. Note that the chemical composition of
the atmosphere of Mars is almost identical to that of Venus. Notice also that 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.