2000 January 25, SPS 1020 (Introduction to Space Sciences) - Read TNSS Ch. 9 (Planet Earth) by Thursday, January 27. - Finish the Basics of Spaceflight (online) by Thursday, January 27. - We will have a reading quiz on Thursday, January 27, covering TNSS Ch. 2, 6, 9, and the Basics of Spaceflight handbook (Chapters 1-17). ------------------------------------------------------------------------- The Role of Collisions (TNSS Chapter 6): --------------------------------------- Any planetary surface is shaped by 4 primary geological processes: - Impact cratering - Tectonism - Volcanism - Gradation (incl. water and wind [or aeolian] erosion, but also micrometeorite erosion) The Late Heavy Bombardment ended 3.8 billion years ago. The kinetic energy (= 1/2 m v^2) of the impacts was enough to melt the surfaces of the terrestrial planets, until this time. Major impacts (objects with d > 1 km) occur now only about every 10^7 or 10^8 years. We therefore don't see many craters on Earth, since this is the timescale of terrestrial geology. (Recall the Appalachians are 300 million years old, and the Grand Tetons are 9 million years old.) Barringer Crater = Meteor Crater, 1 hour E of Flagstaff, AZ, on I-40: Best-preserved impact crater on Earth. 50,000 years old, 200 m deep, d = 1.2 km. Meteor was 50 m in diameter, mostly vaporized on impact. Impact had the energy of a 20 megaton nuclear bomb. (Largest ever exploded was 60 MT; Hiroshima explosion was 16 kT.) Gene Shoemaker's (1957) Ph.D. thesis: proved it was an impact crater, after many decades of doubt. (It was held to be a volcanic crater, not an impact crater.) He found *coesite* (shocked quartz) and *stishovite* (rock totally melted by impact). Other common types of impact rocks: Impact *breccias*: broken rock fragments, fused together. *Glass*: including *impact melt* and *tektites* (glassy Earth rocks, melted and thrown into space by impact, which later re-entered atmosphere and struck Earth. Barringer is a relatively small, *bowl-shaped* crater. Larger craters (10 < d(km) < 100, depending on composition and gravity of impacted body) throughout the Solar System have central peaks and slumped, terraced walls. Larger craters (d >> 100 km) often occur as *multi-ring basins* (e.g. Mare Orientale on the Moon, Valhalla on Callisto). Comet Shoemaker-Levy 9: Broke apart by Jupiter's tides in 1992. Pieces struck Jupiter in 1994 July. Made large clouds of dark, organic material ("tarry gunk") by heating Jupiter's atmosphere. (How many of you observed this?) Historical records of black spots appearing on Jupiter: perhaps occurs as often as every 50 years? Collisions with Earth: Large features on Earth that have been identified as impact features: Chesapeake Bay, Hudson Bay. Crater in Yucatan (d = 180 km) : attributed to have caused of the extinction of the dinosaurs, 65 million years ago. There is little doubt now that a large object (d = 10 km) struck Earth at this time. Aside from the crater, there is coesite, and a global blanket of iridium, a rare Earth element often found in meteorites. This coincides with a sudden decrease in microfossils in marine sediments, right at the K-T boundary (Cretaceous-Tertiary): the first evidence Walter & Luis Alvarez had of the extinction. Tunguska, Siberia, 1908 June 30: d = 50 m object, ~ 30 MT explosion (Hiroshima 18 kT, large H bomb 2 MT, largest ever 60 MT). Knocked down trees 50 km away, but left no crater. Impactor may have been a comet, made of light, icy material, and so vaporized and exploded high in Earth's atmosphere. Sound heard in London. Not many meteorites found, except for metallic micro-spheres. (Large impactors, like at K-T, move through Earth's atmosphere as if it isn't even there.) Crater counts: Relative ages of surfaces can be determined by counting craters' numbers and relative sizes. Oldest surfaces: most densely cratered. Saturation cratering: as denselt cratered as possible, any more will obliterate as many as are made. Particulars depend on strength and other properties of surface materials, but still, crater counts can yield remarkably precise information about impactor populations and Solar System history (which book covers in much detail). The future: You are about as likely to die from a meteorite impact as in a plane crash. Impacts are rare events: there are no reliable, undisputed records of a human ever being killed by one. However, a large impact would be catastrophic for the whole planet. > 500 known Earth-crossing asteroids; maybe > 20,000? Surveys woefully inadequate. Amor asteroids: 1.017 < r(perihelion)(AU) < 1.3 - Earth crossing Apollo asteroids: r(perihelion)(AU) < 1.107, a > 1 AU - Earth approaching Aten asteroids: a < 1 AU 1997 XF11 (March 1997): the infamous "never mind" episode. International Astronomical Union Circular forecast doom: better observations => announcement, "Never mind", on next day! => The Torino Scale (by Rick Binzel, MIT) 0 = No hazard, will not impact Earth (all known asteroids rate 0) 1 = May impact Earth, should be observed more ... 8-10 = Will impact Earth, and cause catastrophes that are: Local (8, once per 50-1000 yr), Regional (9, once per 1000-100,000 yr), or Global (10, rarer than once per 100,000 yr). Tunguska and Barriger crater were 8s (about like nuclear bombs). Dinosaurs wiped out by a 10. => Even if we learn to live in peace with each other, and in harmony with nature, humanity still needs to learn to live in space. If we never leave Earth, impacts will make us become extinct!