This evening s announcements Homework 3 is graded and available for pickup at entry Quiz 4 will be held this Wednesday, March 12. Coverage: Feb. 25: origin of the solar system (chapter 6) Feb. 27: Earth, Moon, Mercury (chapter 7) Mar. 10: Mars, Venus as time permits (chapter 7) Homework 4 will be distributed March 12, due March 19
Frequently missed questions from Homework 3 5. Age of solar system: 4.5 billion years (no problem here). Evidence: Radiometric ages of meteorites
This evening s topics Terrestrial worlds, continued 1. Moon (continued) 2. Mercury 3. Mars 4. Venus (if time permits)
Rule of thumb: crater s diameter approx 10 times that of meteorite Interpretation of maria as impact basins Typical diameter about 1000 kilometers; implies dug by 100-kilometer meteorites Later filled with smooth, dark lava Lava wrinkles visible on surface Also some volcanic domes, which probably were the last vestiges of the activity that produced the lava 230
Age of lunar surface From radiometric dating of rock samples returned by Apollo astronauts Highlands: various samples range from 3.8 to 4.4 billion years Maria: 3.2 to 3.8 billion years, depending on location All lunar rocks are older than almost every known Earth rock Significance of ages of lunar rocks Highlands have about 10 times more large craters than do the maria 231
Many fewer craters have formed in last 3.8 billion years (since maria were resurfaced) than in the first 0.7 billion This means that most large craters formed within first billion years: a heavy early bombardment by meteorites The Moon has many old rocks and no known young rocks because most surface modification ceased about 3 billion years ago. Reason: being small, the Moon lost its internal heat rapidly, volcanoes could no longer be active. The only surface modification going on today is meteorite bombardment: continual impacts by small particles, formation of a kilometer-size crater every million years or so. 232
Mercury The least well-studied of the terrestrial planets Difficult to study with telescopes from the ground because it is always close to the Sun Until recently, little visited by spacecraft, only Mariner 10 flyby; Messenger in late 2007 Bulk density 5.5 grams per cubic centimeter: Earthlike But surface is Moonlike Impact craters & basins Lava flows, but without the strong dark coloration of lunar lava 233
New feature: scarps Typical example: crosses a crater No radioisotope dates for this planet because no surface samples returned to Earth; must guess ages of scarps Tentative explanation for scarps in terms of planetary processes Initial heating by radioactivity, true of all terrestrial planets Small ones (Moon, Mercury) cooled rapidly Formed thick, rigid crust: a lithosphere When the metallic core cooled, it shrank a lot: lithosphere shrank accordingly, causing scarps to form 234
Heavily cratered surface suggests that all the terrestrial planets underwent an early heavy bombardment, more support for the nebular theory. Sequence of events for Mercury, from stratigraphic sequence (layering of features on surface): 1. Formation of numerous large craters in early heavy bombardment 2. Lava flows, formation of lava plains 3. Cooling of planet, formation of scarps 4. Formation of numerous smaller impact craters No surface samples available yet, therefore no dates 235
Mars Bulk density about 4 grams per cubic centimeter Smaller in size than Earth but larger than Moon, Mercury Rotation period about 24 hours; length of day similar to Earth s Rotation axis tilted much as Earth s, so seasons are similar to Earth s 236
Major surface markings Polar caps (change with the seasons) Reddish color Light & dark areas Recent exploration Mars Exploration Rover Mission: Spirit and Opportunity robotic rovers have been on Mars since January 2004, taking pictures, studying surface & atmosphere by remote control from Earth Mars Global Surveyor orbiter made imaging and other studies 1999 2006 237
Mars Reconaissance Orbiter is currently operating. Camera can distinguish features as small as 1/2 meter on the surface. Atmosphere Mainly carbon dioxide Surface pressure Less than 1% of Earth s Equivalent to pressure at high altitude on Earth, about 130,000 feet Provides little shielding from solar ultraviolet 238
Liquid water cannot exist; it would either vaporize or freeze Wind: 100 mph not uncommon; blowing sand; dust storms; dunes; wind erosion Daily and seasonal temperature changes are strong but, generally, it s cold 239
Polar caps Composition Contain water (always frozen) and carbon dioxide snow and ice Seasonal vaporization & freezing of carbon dioxide component Act as cold storage for water and carbon dioxide 240
Other surface features Impact craters & basins Canyons Shield volcanoes Shield volcanoes on Earth Theory: caused by a hot spot, a convective plume originating at the outer boundary of the core Liquid rock breaks through & forms volcanoes Shield volcano appearance caused by more-liquid lava, compared to steeper-sided, cone-shaped volcanoes 241
Crustal plate moving over hot spot causes chain of volcanoes, example: Hawaiian Islands On Mars Typical in shape for shield volcanoes, but very large. No plate movement, so volcanoes build up in one place. Likely age (rough estimate!) 2 billion years from number of impact craters on slopes 242
Permafrost, a permanent layer of subsurface ice Evidence: peculiar craters with flow patterns around them probably result from impact in frozen soil Dry river beds or channels or arroyos Generally quite old; craters on top of them Outflow channels are the result of flash flooding wider at beginning, narrower at end. Detailed examination by Mars Reconaissance Orbiter shows some are typically filled with lava, although originally carved by flowing water. 243
Some gullies have been found by Mars Global Surveyor and confirmed by Mars Reconaissance Orbiter. Locations have very few impact craters, hence recently formed Appear to be fed by melting subsurface ice 244
Evidence for long-term climate change on Mars: the channels. Water could not flow extensively today Higher atmospheric pressure in past (2 billion years ago?) More greenhouse warming as a result, warmer also Or perhaps Mars is now in an ice age Recent MRO results indicate the warm period was less extensive than previously thought. They cast doubt on claims of possible early rainfall, for example. Flash flooding could have been caused by an impact melting subsurface ice. 245
What happened to Mars atmosphere? Escape: because of its relatively weak gravitational pull, Mars may have lost a significant part of its atmosphere over time Freezeout: a lot of water and carbon dioxide could be stored in permafrost and the polar caps. Cessation of volcanic activity around 2 billion years ago Planet s interior cooled due to small size Then atmosphere no longer replenished by gases emerging from volcanoes 246
Fits in with pattern: small size goes with less surface modification Mercury, Moon: smallest, most craters, least modified surface Mars: intermediate in all respects Venus, Earth: largest, fewest craters, most surface modification 247
Has there ever been life on Mars? Evidence for ancient water from Spirit and Opportunity Erosional, depositional landforms Blueberries - round stones that formed in water Other minerals that form in the presence of water Implies life could have arisen there. Life now? Maybe where there is underground liquid water. Why we want to know: life on Earth is all the same (DNA-based); can t draw reliable conclusions about origins from only one example. 248
Venus Similar to Earth in size, mass, density Major difference is atmosphere: surface invisible from Earth or space because of thick cloud layer Except: radio waves can travel through atmosphere Receive radio signals from a lander Radar: send radio signal, receive echo with information about surface roughness, elevation 249
Atmosphere of Venus Pressure at surface: 90 Earth atmospheres (equivalent to 1/2 mile deep under water on Earth) Venera 9 lander from former Soviet Union Images show red color from light filtered through clouds Venus s gravitational pull is similar to that of the Earth, so its atmosphere is much more massive than the Earth s. Average temperature at surface: 750 Kelvin Hottest planetary surface, hot enough to melt lead Rock generally melts above 1000 K 250
Composition: mostly carbon dioxide (CO 2 ) Explanation for the high surface temperature: the Greenhouse Effect of its very massive carbon dioxide atmosphere Venus may have started out with a transparent atmosphere like Earth s but the greenhouse effect ran away. Now that the surface is hot, it produces a lot of thermal radiation, mostly infrared. The atmosphere trapping the infrared is enough to keep the planet hot, even though almost all the sunlight is reflected from the cloud layer. 251
Surface features Source of best information is the Magellan orbiter, which carried out 4 years of radar mapping (early 1990 s) Impact craters more than on Earth but far fewer than on Moon Lava flows covering much of surface. May have occasional, planetwide bursts of volcanism. Volcanoes similar to shield volcanoes on Earth and Mars 252
Summary of terrestrial planets All formed by same process: solid-body accretion in the early solar nebula. All subject to similar processes during their history. Differences between them are largely explained by their different sizes and distances from the Sun. 253