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The Mystery of Mercury’s Orbit Mercury’s (and Uranus’) orbit inconsistent with Newton’s Law of Universal Gravitation French astronomer Urbain Leverrier had accurately predicted presence of a planet outside Uranus - Neptune - to explain Uranus’s orbit Suggested the presence of a planet called Vulcan inside Mercury’s orbit - doesn’t exist Required Einstein general theory of relativity 40 years later Deals with gravitational effect on space - warping Accurately predicted Mercury’s orbit

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Mercury

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As viewed from Earth, Mercury can be seen only near times of greatest eastern or western elongation. At greatest western elongation (when the planet is farthest west of the sun in the sky), Mercury rises about 1 1/2 hours before sunrise. At greatest eastern elongation (when the planet is farthest east of the sun in the sky), Mercury sets about 1 1/2 hours after sunset. The Orbit of Mercury At an average distance of only 58 million kilometers (36 million miles) from the sun, Mercury takes a mere 88 days to go around its orbit.

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Mercury Transit of the Sun Movie

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Comparison of Venus and Earth Parameter Venus Earth Distance from Sun (AU) .72 1.00 (million KM) 108 150 Sidereal Period (year) (earth days) 225 365 Rotation Period (day) (earth days) 243 1.00 Direction of rotation Retrograde Direct Equatorial Diameter 0.96 1.00 Escape velocity (km/Sec.) 10.3 11.2 Inclination of axis 3 23.5 Seasons No Yes VENUS

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Comparison of Venus and Earth Parameter Venus Earth Surface Temperature 480C (900F) 15C (60F) Surface atmospheric pressure 90 1 (atmospheres) Atmospheric gases CO2 N2, O2 Cloud cover Total Partial Surface Solid 3/4 water Number of satellites 0 1 VENUS

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Differences Between Venus and Earth Venus rotation rate is very slow and in retrograde direction. Venus surface consists of 1 plate; earth has 9 plates. Venus has little or no magnetic field. Venus' atmosphere pressure is 90 times that of earth. Dominant gas in the Venus atmosphere is carbon dioxide. 6. Venus' surface temperature is 900° F. 7. Venus has very little water vapor in its atmosphere. 8 Venus has a very strong greenhouse effect. 9. There is no water on Venus' surface. 10. Venus has a very dense cloud cover.

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Venus Cloud covered, thick atmosphere - carbon dioxide - greenhouse affect - surface temperature near 900ºF - rains sulfuric acid

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Surface features unknown because of heavy cloud cover until the Magellan spacecraft mapped surface with radar. The surface of Venus is covered with abundant lava flows and tectonic features, along with a few large impact craters. In the following images dark and light areas correspond to how well radio waves are reflected, not visible light. Nonetheless, geological features stand out well. Images of Venus’ Surface Russians probes actually landed on Venus four times in the 70’s - probes lasted up to two hours before failure. NASA’s Pioneer Venus sent probe into atmosphere - had on-board instrument (mass spectrometer) built at UTD. Venera 13

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Two of Venus' relatively rare impact craters Shield volcanoes like this one are common on Venus. Radar Images of Venus’ Surface Tectonic forces have fractured and twisted the crust in this region. Circular cracks and volcanic bumps make up a corona, probably caused by pressure of a mantle plume below.

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Venus orbit tilted 3.4º to the ecliptic Transits of Venus only occur in pairs about 8 years apart every ~120 years. Transit occurred in 2004 - next in 2012 Edmund Halley one of few to observe first transit in 1677 after invention of telescope - called upon future astronomers to observe subsequent transits. Captain Cook traveled to Tahiti to observe transit of 1769 in an attempt to estimate the distance from the Earth to the Sun using triangulation and the parallax effect - discovered Hawaii on the way. Measurements not very accurate: - intense sunlight filtering through Venus' atmosphere fuzzed the edge of the disk and decreased the precision with which Cook could time the transit - his measurements disagreed with those of ship's astronomer Charles Green, who observed the transit beside Cook, by as much as 42 seconds. Venus Transits

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- Cook and Green also observed the "black drop effect." When Venus is near the limb of the sun - the critical moment for transit timing - the black of space beyond the Sun's limb seems to reach in and touch the planet - made it hard to say just when the transit began or ended. - a problem for observers elsewhere - observations of Venus' 1769 transit from 76 points around the globe not precise enough to set the scale of the solar system. Astronomers didn't manage that until the 19th century when they used photography to record the next pair of transits.

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Venus Transit of the Sun

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Earth Only known world on which humans can live without protective covering - only known oasis of life.

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The Moon Earth is the first planet with a moon - surprisingly large compared to Earth - major mystery of the solar system

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Mars The last of the terrestrial planets. A world of wonders with extinct volcanoes that dwarf the largest mountains on Earth, a great canyon that runs nearly 1/5 of the way around the planet, polar caps of frozen carbon dioxide and water ice, and dried up river beds

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Mars Comparison of Venus, Earth and Mars Parameter Venus Earth Mars Distance from Sun (AU) .72 1.00 1.52 (million KM) 108 150 228 Sidereal Period (year) (earth days) 225 365 687 Rotation Period (day) (earth days) -243 1.00 1.03 Direction of rotation Retrograde Direct Direct Equatorial Diameter 0.96 1.00 0.53 Escape velocity (km/Sec.) 10.3 11.2 5.0 Inclination of axis 3 23.5 25.2 Seasons No Yes Yes

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Mars Comparison of Venus, Earth and Mars Parameter Venus Earth Mars Surface Temperature 480C (900F) 15C(60F) -60C(-76F) Surface atmospheric pressure 90 1 1/200 (atmospheres) Atmospheric gases CO2 N2, O2 CO2 Cloud cover Total Partial Rare Surface Solid 3/4 water Solid Number of satellites 0 1 2

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Moons of Mars - Captured Asteroids? Phobos - 13 km across Diemos - 8 km across

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60 km Olympus Mons - largest shield volcano in the solar system - three times as high as Mt. Everest - 600 km across Dried up river bed?

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Life on Mars? Mars is the only planet on which we have searched for life Early Mars may have been similar to early Earth Conclusive evidence that free-flowing water existed on Mars at one time Has all the chemical ingredients necessary for life as well as energy from sunlight and now dormant volcanoes CO2 atmosphere May have significant amounts of subsurface ice - residual volcanic heat may create pockets of liquid water where life could exist Microbacteria developed in labs can survive simulated Martian conditions

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Viking Landers Viking 1 and Viking 2 landed on Mars in 1976 - specific mission to look for life. Equipped with robotic arms to scoop up soil samples and deposit them in a closed container for treatment. Arms pushed aside rocks to get at shaded soil less likely to have been sterilized by UV light.

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Viking Life Detection Experiments on Mars Living organisms alter their environment - they breathe, eat, grow, and produce waste Three experiments were designed to detect signs of living organisms by treating soil samples in a closed environment (a container):

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Viking Life Detection Experiments on Mars Gas exchange - Looked for changes in the atmosphere caused by metabolism of organisms in the soil. Soil sample fed nutrient in a carbon dioxide (CO2) atmosphere. Organisms eat nutrients and release gases like CO2, methane, oxygen and hydrogen into the container. Some gases were found but were thought to be due to chemical reactions between the nutrient water and the soil.

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Viking Life Detection Experiments on Mars Labeled release - Looked for CO2 breathed into the atmosphere Soil sample fed radioactive nutrient Organisms would release radioactive CO2 into the container Some gases were found but were thought to be due to the chemical reactions between the nutrient and the soil

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Viking Life Detection Experiments on Mars Pyrolytic release - Looked for radioactive carbon in soil sample Soil sample in radioactive CO2 atmosphere illuminated by ultraviolet light to simulate sunlight Soil sample then heated to 650°C to decompose any growth material in the soil Soil contained some radioactive carbon - did not represent life Conclusion: Some positive results found in each experiment. However, not sufficient to confirm life as we know it Explanation - unusual chemical activity

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Fossil Life in Meteorites Possible fossil life found in meteorite discovered in Antarctica - hotly debated - Meteorite came from Mars - air trapped in rock matches known Martian atmosphere Age of meteorite 4.5 billion years - solidified shortly after Mars formed - resided on Mars throughout period when Mars may have been warmer and wetter Ejected from Mars 16 million years ago by large meteor impact Landed on Earth 13,000 years ago in Antartica Well preserved: not exposed to water erosion or industrial contaminants

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While on Mars, meteorite was infiltrated by water - carbonate mineral globules formed in water About 3.6 billion years old - when Mars was warmer and liquid water may have existed - older than impact (asteroid?) that launched meteorite into space Oxygen and carbon in globules contain isotopes characteristic of Mars Showed presence of polycyclic aromatic hydrocarbons (PAM) unlike any on earth - associated with life Iron rich materials characteristic of fossil remains

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Pictures of meteorite taken with electron microscope reveal shapes that look much like nanobacteria in existence on Earth - fossils of microscopic life on Mars? Hotly contested - other scientists claim they are mineral formations.

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Carbonate globules are the key to biogenic activity on Mars Globules formed in fractures of rock in liquid water Globules are younger than the rock Globular features resemble earth microorganisms, earth biogenic carbonate structures and microfossils Globules contain PAH's CONCLUSION: Alternative explanations exist for each globular phenomenon taken individually Collectively, they are evidence for primitive life on Mars