Astronomy 1001/1005 Paul Woodward Fall, 2007

Transcription

1 Mars This is the material on Mars that we covered up to about the end of the week before the first midterm exam, and which therefore should be reviewed in studying for the midterm. Astronomy 1001/1005 Paul Woodward Fall,

2 The orbit of Mars is just barely visibly elliptical, and this is why it was the planet that drove Kepler to try ellipses rather than circles. The spin axis of Mars is tilted, like the earth s, and therefore Mars has seasons. 2

3 Fig. 9.5: Interior structure of a generic terrestrial planet. 3

4 Mars is an intermediate case between the earth and Venus, on one hand, and Mercury and the Moon on the other. Due to its smaller size, Mars has had time to cool, so that its rigid lithosphere goes to great depths, and any earlier tectonic motions have ceased long ago. Mars is intermediate between the earth and Venus on the one hand and the moon and Mercury on the other. It is large enough to have had significant heat generation in its core through radioactive decay, leading to volcanic activity in its early history, and it is small enough to have cooled well before the present era, developing a thick, rigid lithosphere and a solid metallic core. Mars is therefore an intermediate case from the point of view of its internal structure. It is also an intermediate case from the point of view of its atmosphere. Mars is massive enough to hold an atmosphere, unlike Mercury and the moon, but it is not massive enough to have retained a dense atmosphere over the entire age of the solar system, like Venus or the earth. 4

5 Viewed from the earth, Mars is a reddish object. Its blood-like color inspired the Greeks and later the Romans to name it after their gods of war. The name of the Roman god, Mars, has persisted. Mars, seen with the 100-inch telescope on Mt. Wilson 5

6 Mars, seen with a small telescope with attached video camera by a student, Rolf Karlstad. Karlstad observing equipment setup. A video camera records the images at a rate of 60 per sec. in a digital format saved on video tape. The camera can take pictures in infrared light. The images are later aligned and composited on a PC. This gives a manual kind of adaptive optics, which, together with the patience and persistence of the observer, explains the image clarity. 6

7 When viewed through a powerful telescope, surface markings are visible, as well as polar caps, from which the rotation of the planet and the alternation of its seasons can be observed. Two views of Mars, showing the rotation of the planet. 7

8 An image of Mars taken with the Hubble Space Telescope 8/24/03. This is the sharpest color picture ever taken of Mars from Earth. Mars, due to its proximity and similarity to the earth, has continually inspired theories that it harbors intelligent life. Seasonal changes were attributed to the growth and retreat of vegetation, and linear markings on the planet, real and imagined, were attributed to irrigation canals. 8

9 A map of Mars drawn by Italian astronomer Giovanni Schiaparelli, who in 1877 reported the discovery of strange markings on Mars. Some thought the long straight markings evidence for intelligent life on Mars, while still others could not find these markings at all in their telescopes. (from Mars, p. 35) Maps of Mars drawn by Italian astronomer Giovanni Schiaparelli, who in 1877 reported the discovery of strange markings on Mars. Some thought the long straight markings evidence for intelligent life on Mars, while still others could not find these markings at all in their telescopes. (from Mars, p. 35) 9

10 Lowell s observations, apparently much enhanced by his active imagination, helped to fuel interest in the possibility of life on Mars. (from Mars, p. 191) (from Mars, p. 190) 10

11 Evidence of a dense Martian atmosphere, certainly a prerequisite, many believed, for life as we know it, was visible from the earth. Storms are visible on Mars from the earth. The different appearance of the planet in light of different wavelengths gives evidence for an atmosphere as well. A storm on Mars. 3 views from the Lowell Observatory, Before storm (left), first day of storm (center), and eighth day of storm (right). 11

12 The composition and density of the Martian atmosphere has been determined by space probes. Previous estimates by Lowell and others set the density of the atmosphere more than 10 times too high. The CO 2 in the atmosphere was detected by its absorption of reflected sunlight from the surface, and was known to be 30 times the amount in the earth s atmosphere. This helped to fuel the idea of a dense Martian atmosphere, dense enough for the ice caps to be water ice. In its fly-by visit to Mars, Mariner-4 transmitted its radio signals through the atmosphere, revealing its surface pressure to be only about 1% that of the earth s atmosphere. This, together with the measured amount of CO 2 and the -125 C temperature of the polar caps, meant dry ice, not ice. 12

13 Water on the surface of Mars would immediately either freeze or evaporate. The present Martian atmosphere does not permit liquid water to exist on the surface. However, the atmosphere of Mars may have been quite different very long ago. The density and temperature profiles (with altitude) of the atmospheres of the earth and Mars are compared 13

14 This Viking orbiter view of the edge of Mars uplifted and rough Argyre impact basin shows the Martian atmosphere, composed predominantly of CO 2 and with a surface pressure only 1% of that of the earth s atmosphere. (from Mars, p. 87) 14

15 We have learned an enormous amount about Mars from the Mars Global Surveyor, which has taken high resolution images of Mars and also used a laser altimeter to map its topography. The following maps were made from the laser altimeter data, using a color scale where blue denotes low regions, green regions of intermediate (average) altitude, and as the altitude increases, the colors go from yellow to red to white. These color choices make the lowest regions look like oceans and lakes, but of course there is now no liquid water on the surface. The following black and white images are less suggestive but equally detailed. Like maps of the earth, they give a flat representation that distorts regions near the poles. 15

16 Global view of Mars, with the Hellas impact basin just below and to the left of center. Color shows topography based upon laser altimeter data. Global view of Mars, with Utopia Planitia near the center. Color shows topography based upon laser altimeter data. 16

17 17

18 Here laser altimeter data are combined with color images. Topography of the Tharsis region of Mars. 18

19 Evidence for the action of liquid water on Mars: Liquid water is fundamental to life as we know it. Therefore, the presence of liquid water on Mars, either now or in the past, is an essential prerequisite for life on Mars, either now or in the past. The first pictures of Mars, from the Mariner-4 spacecraft in 1965 dashed the hopes of many that Mars could support life. The Mariner-4 spacecraft flew by Mars in July, 1965, providing the first close-up views of Mars in 21 images it sent back. (from Mars, p. 55) 19

20 (from Mars, p. 55) One of the 21 images of the Martian surface sent back to earth by Mariner-4 in July, Hopes for life on Mars sank as a result of these moon-like scenes. Evidence for the action of liquid water on Mars: Despite the present, obviously dry surface of Mars, there is much evidence for water on the planet. Water exists at present in the form of ice in the northern polar cap, and, presumably, in the layers, several km thick, under both polar caps. There is also evidence for a thick layer, perhaps as much as a km deep, of permafrost beneath the present Martian surface at mid to high latitudes. Water, even in the form of ice, is not stable at mid latitudes, and will sublime into the atmosphere, only to freeze out at the poles. 20

21 A close-up view of the north polar region of Mars White areas are believed to be water ice. The alternating whitish and reddish layers may be clean and dirty ice. They may record shortterm climatic changes. These layers have almost no craters and are therefore believed to be very young (perhaps a hundred million years). (from Mars, p. 93) Mapping subsurface Martian Water: The Mars Odyssey Mission used measurements of both neutron and gamma ray fluxes from Mars to deduce the amount of water that is likely to be trapped in frozen form within the first meter or so of the Martian surface. Because Mars lacks a magnetic field and has a thin atmosphere, cosmic rays bombard the surface and produce neutrons. These neutrons can be observed directly by Mars Odyssey. Also, gamma rays are emitted as a result of the interaction of these neutrons with the nuclei of the atoms of the Martian surface material. These gamma rays, which have a spectrum that indicates the composition of the Martian surface, were also observed by Mars Odyssey. Putting all this data together results in the map on the next slide, showing the concentration of hydrogen (water) in the subsurface. 21

22 Mapping subsurface Martian Water: The Mars Odyssey observations indicating subsurface water last year were disappointing for the northern hemisphere. Here many scientists had speculated that an ocean may once have existed. More recent observations showed that as the overlying layer of CO 2 ice evaporated in the Martian spring, the water ice beneath became visible to the Mars Odyssey measurements. Now it is clear that there is more subsurface water in the northern than in the southern hemisphere, as had originally been expected. 22

23 Here we see the northern hemisphere of Mars. Here we see the epithermal neutron flux in the northern hemisphere of Mars in winter and in summer, when the overlying layer of CO 2 ice has evaporated. 23