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1445 Introductory Astronomy I. Chapter 7 Mercury, Venus and Mars R. S. Rubins Fall, 2010. The Inner Planets. Key Features of the Terrestrial Planets.
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1445 Introductory Astronomy I Chapter 7 Mercury, Venus and Mars R. S. Rubins Fall, 2010
Key Features of the Terrestrial Planets • Earthis unique in its oceans, which cover about 70% of its surface, and an atmosphere loaded with “free” oxygen. • Mercuryhas the largest temperature swing of 620oC (over 1100oF), varying from a maximum of 450oC (840oF) during the day to a minimum of –170oC (–275oF) at night. • Venushas a runaway greenhouse effect, caused by its dense toxic atmosphere, which produces a surface temperature 465oC (870oF), which is hot enough to melt lead. • Marscontains both the highest mountain – Olympus Mons, (roughly 17 miles tall, or 3 times the height of Mt. Everest), and the deepest and longest valley – VallesMarineris(over 5 miles deep and roughly 2500 miles long.
Mercury: the Once Forgotten Planet In 1974, Mariner 10photographed 45% of Mercury’s surface. This Mariner 10 photo shows fog-filled canyons at sunrise.
Mercury Data • Average distance from Sun: 0.39 AU. • Mass: 5.5% of Earth (0.055 ME). • Radius: 38% of Earth (0.38 RE). • Average density: 98% Earth density – surprisingly high for a small planet. • Orbital eccentricity 0.21; cf. Earth (0.02) and Venus (0.01). • Siderial revolution period about the Sun (1 Mercury year) is 88 Earth days. • Siderial rotation period (1 Mercury day) is 58.7 Earth days. • The ratio of the two periods is 58.7/88 = 2/3. • The solar day (or synodic rotation period), which is the time from dawn to dawn, lasts two Mercury years, which is 176 Earth days.
Mercury’s Locked Periods: Year 1 In one siderial year (88 Earth days), Mercury rotates 1½ times; i.e. the siderial day is (2/3) x 88 = 58.7 Earth days.
Mercury’s Locked Periods: Year 2 • After 2 years Mercury returns to its initial position. • Thus the dawn-to-dawn period (the synodic day) on Mercury is 3 x 58.7 = 176 Earth days.
Summary of Periods for Mercury • A solar day (from noon to noon) on Mercury takes 176 Earth days or 2 siderial years (88 Earth years each).
Sunrise on Mercury: Artist’s Impression • Chosen as a Scientific American “Wonder of the Solar System”, the Sun, viewed from Mercury, appears about two and a half times larger than on Earth.
About Mercury 1 • Kepler’s 3rd Law[P2 = a3] indicates P is 88 days, so that Mercury orbits the Sun at almost twice the speed of the Earth.. • There are no seasons on Mercury, since its rotational axis is not tilted like that of the Earth. • As a result, craters at the poles receive no direct sunlight, and so may contain water ice. • Mercury lacks has only traces of an atmosphere because of its small mass, which makes the escape velocity very low. • Because of its lack of an atmosphere and proximity to the Sun, Mercury has the greatest temperature extremes in the solar system, varying from about 850 K (1100oF) to 90 K (–300oF).
About Mercury 2 • Of the “traditional” planets, only Pluto has an orbit more elliptical than Mercury. • Because Mercury’s orbit is so elliptical, the diameter of the Sun as seen from its surface would be roughly 50% bigger at its perihelion than at its greatest separation, varying from about three times to twice its diameter as seen from the Earth. • Newton’s Laws do not work perfectly for Mercury, since its perihelion point shifts with each orbit about the Sun. • In 1845, this lead Leverrier to predict another planet, Vulcan, even closer to the Sun, which would affect Mercury’s orbit. • Vulcan was never found, but Einstein showed in 1915 that Mercury’s orbit could be explained by his General Theory of Relativity.
Mercury’s Unusual Orbit The change of apheleon (the point in the orbit furthest from the Sun) with each rotation of Mercury, provided the first quantitative test of Einstein’sGeneral Theory of Relativity.
About Mercury 3 • Never more than 27o of arc from the Sun, Mercury can only be seen from Earth just before sunrise or just after sunset. • Even the Hubble telescope has not been used on Mercury, in case its delicate optics are pointed too close to the Sun. • The 2008and 2009 flybys of the MESSENGER probe has revealed many volcanoes and smooth plains probably formed by volcanic deposits. • Because of its remarkably high density for a small planet, Mercury must be the most iron-rich planet in the solar system. • A hypotheses for the plentiful iron on Mercury is that during its formation, it was struck by a large object, which ejected much of the mantle, vaporizing the lighter, more volatile elements.
Mercury and Earth’s Moon • The diameters of Mercury and the Moon are roughly 4900 km and 3500 km respectively, and there is some resemblance in their heavily cratered surfaces.
Mariner 10 View: Craters and Plains • Although heavily cratered like the Moon, there is less overlap between craters than on the Moon, and the crater walls are less steep because of the larger gravity.
Scarp Formation on Cooling A scarpis a cliff formed by the vertical moment of a section of the planet’s crust.
Mariner 10 View: Edge of Caloris Basin The Caloris Basin, the edge of which was photographed by Mariner 10, is a crater of about 800 mi in diameter (or twice the area of Texas), which is surrounded by a ring of mountains about 2 km (6,500 ft) high.
Formation of the Caloris Basin The Caloris Basinwas probably formed by a major impact that occurred early in the planet’s life, the shock waves from the impact producing a hilly landscape on the opposite side of the planet.
Mercury Messenger 2008 1 • The following observations were made in the first Messenger “flyby’ of January 2008: • i.Mercury is filled with very subtle blue and red areas; • ii. wrinkle-like ridges are an indication that Mercury is shrinking; • iii. Mercury shows evidence of extensive volcanic activity.
Mercury Messenger 2008 2 The Spider, consisting ofmore than 100 narrow ridges radiates from a 50 km wide depression, at the center of the Caloris Basin. The Caloris Basin, shown with colors intensified.
Mercury Messenger 2008 3 • The unknown blue material indicates that Mercury might still be volcanically active – a surprising observation.
Mercury Messenger 2008 4 • The 2nd flyby of Mercury took place in October 2008, and produced 1200 photos, revealing 30% of the surface never before seen, extending the observations to about 95% of Mercury’s surface. • Messenger crossed the planet’s equator at a height of just 125 miles and a speed of 14,800 mph. • Messenger’s data has shown that about 40% of Mercury’s surface consists of hardened lava flows, compared to about 20% on the Moon. • A 3rd flyby took place in September, 2009. • When Messenger returns in March 2011, its speed should be slow enough for it to begin orbiting the planet, so that much more detailed observations will be made.
Mercury Messenger 2008 5 • Messenger’s 2nd flyby revealed the Rembrandt Crater, which is 430 miles across. • Rembrandt was probably formed about 3.9 billion years ago by an impacting space-rock. • The original floor was never filled in by later lava flows, and parts are still intact.
A mysterious, unexplained bright spot observed on Mercury. Mercury Messenger 2009 2
Mercury Messenger 2009 3 Lava-filled Crater
Mercury’s Partially Molten Core • In a recent experiment, radar signals bounced off Mercury in 2007 have shown that Mercury wobbles as it rotates, indicating that it has molten fluid in its core. [To see the difference fluid makes, compare how differently raw eggs and hard-boiled eggs spin on a table.] • This result is surprising because Mercury’s small size means that it loses heat easily, so that its iron core would have been expected to have solidified long ago. A partially fluid core would explain the weak magnetic field first detected by NASA’s Mariner 10 mission in 1974, and now being studied in detail by Messenger.
The High Density of the Planet Mercury • Mercury, with a density just 98% that of the Earth, is the second densest planet in the solar system. [If Mercury were made of the same material as the Earth, the number would be appreciably smaller than 98%, since the gravitational compression would be much less on the muich tinier planet Mercury.] • It is likely that the relatively high density of Mercury is due to a higher percentage of iron-rich compounds, compared to the constituents of the Earth. • The several theories proposed to explain the high density of Mercury all involve mechanisms by which the lighter, more volatile compounds are boiled off by the solar radiation.
About Venus 1 • Because of its proximity, only the Moon outshines Venus in our sky. • Venus, when observed, is never far from the Sun. • As the Morning Star, Venus heralds the rising Sun; as the Evening Star, it is a postscript to the setting Sun. • Venus spends nine month as either the Morning Star or Evening Star, with a gap of about 50 days between them, as it passes in front of or behind the Sun. • In daytime, the white shape of Venus may sometimes be seen against the blue of the sky. Napoleon saw Venus in this way when making a speech, interpreting the sight as a sign of victory for his campaign in Italy.
About Venus 2 • The closer Venus is to the Earth, the more it approaches the crescent phase, with only 1/6 of her surface visible at its closest position. • Unlike our other neighbor Mars, Venus has not easily revealed its secrets, because it is completely covered by a thick cloud cover of sulfuric acid, which prevents the study of its surface from the Earth. • Ten Soviet Venera and Vega spacecraft landed on Venus between 1970 and 1984, each being destroyed within an hour by the extreme pressures and temperatures. • The US spacecraft Magellen, which circled Venus for four years, was able to resolve features of the planet’s surface
Lava Plains on Venus Over a dozen Soviet “Venera” spacecraft have landed on Venus, none surviving for much more than an hour.
Venus Data • Average distance from Sun: 0.72 AU. • Mass: 81.5% of Earth (0.815 ME). • Radius: 95% of Earth (0.95 RE). • Average density: 95% Earth density. • Siderial revolution period: 224.7 Earth days. • Siderial rotation period: 243 Earth days (retrograde). • Solar day (or synodic rotation period): 116.8 Earth days. • Tilt of rotation axis: 177o. • Surface temperature: roughly 750 K at all times and latitudes.
The Day on Venus • The solar day on Venus, which is the time between consecutive noons, takes just under half a Venusian year.
About Venus 3 • Venus is of similar size and composition to the Earth. • Like Earth, Venus has a complex evolving climate, fueled by geological activity. • At one time, the atmospheres of both planets consisted mainly of carbon dioxide (CO2). • However, Venus became hotter than the Earth because it is 30% closer to the Sun, and also lacks a magnetosphere, so that the two planets developed quite differently. • On the Earth, the oceans absorbed the CO2, leaving a small amount of nitrogen, which became the major component of our atmosphere. • On Venus, the water boiled off, so that the CO2 remained in the atmosphere.
Effect of the Slow Spin Rate • Venus is the only planet with a retrograde rotation. • If one were to look down from above its north pole, it would rotate clockwise (towards the west), so that observed from Venus, the Sun would rise in the west. • Because of its very slow spin rate, the magnetic field on Venus very much smaller than on Earth. • Since it has no magnetosphere, there is no protection on Venus from the solar wind, which is a stream of high-energy charged particles. • As a result, hydrogen, helium and oxygen are blown away by the solar wind much faster on Venus than on Earth, which explains how Venus lost its original water to space.
About Venus 3 • The surface temperature of 750 K (900oF), resulted from a “runaway greenhouse” effect, caused by the absorption of the IR radiation from the ground by both CO2 and H2O molecules. • The very high surface temperature is enough to make rocks glow. • Because of the thick cloud cover and the lack of wind, the temperature difference between poles and equator is small. • The atmosphere is unbreathable for us, consisting of about 96% carbon dioxide (CO2) and 4% nitrogen. • The atmospheric pressure is about 100 times greater than on the Earth’s surface, equivalent to being ½ mile under water.
Venusian Landscape Most of Venus consists of lava plains and gently rolling hills, although the highest mountain is about 2 km higher than Mt. Everest.
Bright Spot on Venus 2009 • First observed by an amateur astronomer in New York State in July 2009, it was confirmed by the European Space Agency’s Venus Express spacecraft. • It may have been caused by a volcanic eruption.
Mars Data • Average distance from Sun: 1.52 AU. • Mass: 10.7% of Earth (0.107 ME). • Radius: 53% of Earth (0.53 RE). • Average density: 72% Earth density. • Siderial revolution period (Martian year): 687 Earth days. • Siderial and solar rotational periods (Martian day): both a little more than 24½ hr (cf. 24 hr on Earth ). • Tilt of rotation axis 25o (cf. 23.5o on Earth). • A daily temperature range of –105 oF to 14 oF (cf. –280 oF to 800 oF on Mercury), was measured by the Mars Pathfinder (1997).
