Quiz 3 is available for pickup in front Extra credit corrections: for up to 4 of the questions you missed: Look up or figure out the correct answer. Write a sentence or two explaining what you did wrong or why the right answer is correct. Just giving the answer will not earn credit! Turn it in on paper at the beginning of class Monday, March 15. Or by email until midnight that night. If possible, please avoid attachments.
There was an error in the answer key of the Resolution and the Mirror version. The question reading, In general, the farther from the Sun a planet is, the it is found to be is (e), more than one of these. The answer given in the answer key was (d), which is the correct answer to a di erent question. If you gave (e) as the answer, 1 point has been added to your score. Test corrections are needed only if you did not answer (e). If you submit test corrections, when I reply I will confirm that I have amended your score.
Extra Credit Assignment 4 is now on line. Mastering Astronomy Due Friday, March 5 at 5:30 PM. After the due date, you can still work on it, but you won t get credit for it. Covers this week s material on the terrestrial planets. Planetarium reports from last week are now due.
Mercury The least well-studied of the terrestrial planets Di cult 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, to enter Mercury orbit 2011 Bulk density 5.5 grams per cubic centimeter: Earthlike But Mercury s lower mass means that its interior is less compressed by gravity than Earth s 231
But surface is Moonlike Impact craters & basins Extensive lava flows, but without the strong dark coloration of lava on the Moon 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 232
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 Heavily cratered surface suggests that all the terrestrial planets underwent an early heavy bombardment, more support for the nebular theory. 233
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 234
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 tilt similar to Earth s, so seasons are similar to Earth s 235
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 236
Mars Reconnaissance 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 237
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 238
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 239
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-fluid lava, compared to steeper-sided, cone-shaped volcanoes 240
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 241
Permafrost, a permanent layer of subsurface ice. Evidence: Peculiar craters with flow patterns around them probably result from impact in frozen soil Recent discoveries from radar studies with Mars Reconnaissance Orbiter (MRO) 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 MRO shows some are filled with lava, although originally carved by flowing water. 242
Some gullies have been found by Mars Global Surveyor and confirmed by MRO. Locations have very few impact craters, hence recently formed Appear to be fed by melting subsurface ice 243
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. 244
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 245
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 246
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 ice or 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. 247