Terrestrial Planetary Surfaces (cont.) Impact cratering

1. Terrestrial Planetary Surfaces (cont.)Impact cratering Crater chain on Jupiter’s moon Ganymede

2. Announcements • Reading Assignment • Chapter 9 (The Earth) • 3rd Homework now available for download from the course website (due March 6)

3. Dust devils on Mars Planetary Surfaces:Aeolian processes • Wind erosion has effected the surfaces of Earth, Mars, and Titan • Sand Dunes exist on all three bodies • Dust devils exist on Mars and Earth • Mars rover Spirit is still working because of a dust devil! • Dust devils are much larger on Mars Sand dunes on Titan

4. Planetary Surfaces:Fluvial processes • Fluvial processes are those associated with the flow of liquid (like water) • Examples can be found on Earth, Mars, and Titan • All 3 have an atmosphere! Gulley channels on Mars Methane lakes on Titan

5. Planetary Surfaces:Impact Cratering The dominant geological process in the solar system Impact craters are caused by the collision of meteoritic material with a planetary surface The number of craters on a body is a good indicator of the age of the surface Impacts have also affected the atmospheres of the gas giants

6. History of Craters • Galileo observed the moon and noted all the “spots” , but did not speculate about their origin • Robert Hooke (in 1665) made the first detailed drawings of a lunar crater with a better telescope in 1965. • Hooke considered an origin by impacts but felt it was not likely (what was the impactor?). • Hooke put forth the volcanic origin • For the next 300 years astronomers accepted this origin.

7. “Crater” Elegante (Pinacate basaltic fields in Mexico, just below Arizona border)

8. History of Craters (cont.) • In 1794 Ernst Chladni argued that meteorites were real phenomena, not just peasant’s fables of rocks falling from the sky. • A proposal that was greeted with scorn, but opinions were revised after a large “fall” at L’Aigle in 1803 • Roughly at the same time, the “void” of space was partly filled when on Jan. 1, 1801 Piazzi discovered asteroid Ceres from Palermo, Sicily • But it wasn’t until 1893 that the impact origin for lunar craters gained some acceptance

9. The first impact crater recognized on Earth was the Arizona Meteor crater

10. It was recognized in the early 1900’s that a high-speed impact resembled an explosion • Impact cratering studies took a major leap a bit later when the effects of nuclear weapons test were seen! • High-speed projectile experiments and later numerical modeling also demonstrated how craters are formed by impacts • It is now common knowledge that craters are the result of impacts from meteoric material

11. Bikini Atoll atomic bomb test July 1, 1946 PTYS/ASTR 206 Planetary Surfaces and Atmospheres 2/23/12

12. Direct evidence of an impact First, shatter cones High pressure polymorphs and shock features in quartz followed quickly

13. Crater Size-Morphology Progression Simple, bowl-shaped craters. Less than 15 km diameter on the moon Complex craters with central peaks and terraced rims. From 30 to 200 km diameter on the moon

14. Peak-ring craters. About 300 km diameter on the moon Multi-ring basins. Largest sizes, more than 500 km diameter on the moon

15. Another type of Multi-ring structure on Callisto and Europa Planetary Surfaces and Atmospheres 2/23/12

16. Earth’s Impact Craters Planetary Surfaces and Atmospheres 2/23/12

17. The population of impact craters can be used to establish relative dates of planetary surfaces

18. Impacts have brought us rocks from Mars and the Moon Planetary Surfaces and Atmospheres 2/23/12

19. An impact caused the greatest biological extinction in the last 100 Million years

20. At the beginning of Earth’s history, a gigantic collision with a Mars-size protoplanet may have created the Moon

21. The surfaces, and indeed the very existence, of the Earth, Moon and the rest of the solar system is affected by the impact of solid objects

22. Terrestrial Planetary Atmospheres “For the first time in my life, I saw the horizon as a curved line. It was accentuated by a thin seam of dark blue light – our atmosphere. Obviously this was not the ocean of air I had been told it was so many times in my life. I was terrified by its fragile appearance.” Ulf Merbold (1941 – ) German Astronaut

23. Planetary Atmospheres • A layer of gas which surrounds a world is called an atmosphere. • Need a gas in which the molecules collide with themselves more often than the planet to have an atmosphere • they are usually very thin compared to planet radius

24. Large cool objects more easily can retain an atmosphere • Requirements for an atmosphere • Appropriate chemical(s) in molecule form (H2, N2, CO2, etc.) • Collisions among particles more common than collisions with planet’s surface • Low enough temperature (cool) • Enough gravity (big) • More or less obvious for the gas giants, but also explains why Titan has an atmosphere, while Mercury and the Moon do not Planetary Surfaces and Atmospheres 2/23/12 Titan Earth Jupiter

25. Jupiter Saturn Neptune Uranus Earth Venus Mars Mercury Titan Triton Moon Galilean Satellites Pluto Recall that the temperature of an objects falls off with distance from the Sun Planetary Surfaces and Atmospheres 2/23/12

26. Planetary Surfaces and Atmospheres 2/23/12