Chapter 7 The Other Terrestrial Planets PDF

Preview

Summary

Lecture 8, Chapter 7, The Other Terrestrial Planets
Prof. Rayf Shiell...

Description

The Other Terrestrial Planets 7-14 November Quiz Time 1. Approximately how many times in one Earth year does Mercury go completely around the sun, as viewed from the distant stars? C) 4 times 2. Mercury is unique among the inner planets in having B) an iron core that occupies almost half its volume 3. Compared with that of Earth, Mercury’s magnetic field is D) weaker but still noticeable 4. When a planet, in its orbit, is closest to the Sun, it: E) spins faster than its average 5. Which planet(s) do not have one or more natural satellites (moons) orbiting them? B) Mercury and Venus 6. One chemical element that plays a major role in the colouring and chemistry of the Venusian atmosphere and clouds and is suspected of coming from volcanic eruptions, is? D) sulfur (as dust), sulfur dioxide, and sulfuric acid droplets 7. Which of the following physical properties of Venus are very similar in value to those of Earth? D) mass and radius and hence average density and surface gravity The Other (Non-Earth) Terrestrial Planets - These three planets are far more similar to the Earth than the other (outer) planets Mercury - Mercury is the closest planet to the sun - Its average distance from the Sun is 0.39AU o About 60 million km - It orbits the Sun once every 88 (earth days) Finding Mercury in the Sky - Mercury is bright enough to see with the naked eye, however, it is often difficult to see, since it is so close to the sun o It is never more than 28 degrees away from the sun - The best time to see it is soon after sunset or shortly before dawn The Rotation of Mercury - Mercury rotates once every 58.6 (Earth days) o This is the length of its sidereal day - In fact, its rotation period is exactly 2/3 as long as its (88-day) year

o This is because Mercury is locked in a tidal resonance with the Sun A Day on Mercury - A solar day in Mercury is the time between two successive sunsets or sun overheads Surface Temperature - A solar day on Mercury consists of 88 Earth days of daylight followed by 88 Earth days of darkness - Temperature ranges from +430 degrees down to -170 degrees The Surface of Mercury - Considerably darker in appearance than other terrestrial planets o The surface has a very low albedo (reflectivity) -

The surface of Mercury reveals lots of impact craters, scarps (cliffs or ridges) as the planet cooled and shrunk, and evidence of volcanoes and flowing lava in the pasts Mercury’s Atmosphere - Mercury’s surface gravity is too small to retain a substantial atmosphere o Its atmosphere is 1017 times less dense than Earths - Tiny amounts of some gases have been detected (hydrogen, sodium, oxygen) - There appears to be some ice in permanently shadowed craters near the polar regions - Some gases are pushed away from Mercury by the solar wind The Interior of Mercury - Earth’s iron core is 55% of the diameter of the entire planet, or 17% of its volume, whereas Mercury’s iron core is about 75% of the planet’s diameter, or 42% of its volume Venus - The most similar to the Earth overall o Only 5% smaller in diameter o 20% less mass than the Earth Finding Venus in the Sky - Venus is the brightest of the planets and stars seen in the night sky - It is often seen low in the western sky after sunset, or in the east before dawn The Phases of Venus - With even a small telescope, Venus is seen to have phases - It also appears to change in size o It looks smallest when it is fully illuminated - Its appearance depends on its position with respect to the Earth and Sun The Orbit and Rotation of Venus - Venus orbits the Sun once every 225 days, at an average distance of 0.72AU - Its period of rotation is even longer: 243 days The Retrograde Rotation of Venus - Venus rotates in the opposite direction from its orbit around the sun

o It rotates clockwise as seen from the north o This is called retrograde rotation Days and Seasons on Venus - The length of a solar day on Venus is 117 Earth days o Its retrograde rotation causes its solar day to be shorter than its sidereal day o The opposite is true for Mercury, Earth, Mars, etc. - The small tilt of Venus on its rotation axis (3 degrees) means it basically has no seasons (like Mercury) The Atmosphere of Venus - Venus is surrounded by a thick atmosphere o It is always completely covered in clouds o These clouds are made of sulfuric acid - Its atmosphere is o 95% carbon dioxide o 4% nitrogen o Like an early stage of Earth A Runaway Greenhouse Effect - The surface of Venus was originally thought to be warm and humid Temperature and Pressure - The temperature at the surface is about 750K - The thick atmosphere of Venus also creates very high pressures o Its surface pressure is 98x greater than on Earth The Interior of Venus - No seismic data is available for Venus - However, its interior is expected to be similar to Earth’s interior o Similar density - No magnetic field has been detected Making Sense of Venus’ Surface - Surface appears to be young o Relatively few craters o None of the craters are old - Venus does not have tectonic activity or erosion to renew its surface - It is believed it renews its surface as material melts and sinks into the crust Mars - Mars is the 4th planet from the Sun - Its average distance from the Sun is 1.52AU - Mars is midway between the Earth and Moon in terms of diameter and mass The Orbit & Rotation

-

Mars orbits the Sun once every 687 (Earth) days Mars rotates once every 24 hours and 37 minutes o Its solar day is 2 minutes longer Finding Mars in the Sky - The position, apparent brightness and apparent size of Mars depends on its location relative to the Earth and Sun - Brightest and largest at opposition, every 26 months, next in July 2018 Seasons on Mars - Mars’ rotation axis is tilted by 25 degrees with respect to its orbit around the Sun - As a result, there are seasons on Mars o Martian seasons last almost twice as long as Earth’s The Origin of Phobos and Deimos - Mars has two tiny moons called Phobos (fear) and Deimos (panic) - Phobos and Deimos were long through to have been captured asteroids - Recent research suggests instead that they formed in a debris dick after a giant impact The Interior of Mars - Mars appears to have a thin crust, a mantle and a core o Mars’ crust is thicker than Earth’s, since it was able to cool faster than Earth - Mars does not appear to have any ongoing plate tectonic activity o It may have had tectonic avitivty in its early history - Mars has only a weak magnetic field Early Exploration of Mars - The Soviet Mars 2 spacecraft was the first man-made object to impact Mara o Nov. 27, 1971 NASA’s Maven Mission - This Mars orbiter arrived in Sept. 2014 - It is analyzing the Martian atmosphere, and should help us to understand how the atmosphere has evolved o E.g., how gas is leaving the upper atmosphere of the planet Mars Curiosity Rover - Mars curiousity rover arrived on Mars in Aug. 2012 - This rover is much larger and more capable than its predecessors - It is looking for the chemical building blocks of life In 2021 – ExoMars Rover - In 2021, the ExoMars rover is due to land on Mars - This rover will extract and analyze samples from a depth of up to 2 metres o At this depth, the soil is protected from solar radiation and from reacting with the Martian atmosphere

- This rover will provide our best shot at detecting life in the soil of Mars Water on Mars - Mars is currently very cold and dry, and has little geological activity - However, in the past, it appears to have been warm and wet, with active volcanoes - Spacecraft which have visited Mars have found evidence that water flowed on Mars in the past, and probably still trickles in places today Where Has the Water Gone? - It has been lost to space - It exists under the surface as liquid water - It exists on or near the surface as ice The Atmosphere of Mars - Mars has a very thin atmosphere - It is made mostly of carbon dioxide (95%) o It also has some nitrogen, argon, oxygen and water - It is thick enough to support strong winds and huge dust storms Surface Conditions - The air pressure at the surface of Mars is very low compared with Earth o It is about 0.6% of Earth’s air pressure o Astronauts would need to wear pressurizes spacesuits -

However, the surface gravity would be quite comfortable o About 1/3 of the surface gravity on Earth

The average surface temperature on Mars is about -60 degrees Celsius o The coldest temperatures are found near the poles during the winter, with temperatures reaching -150 degrees Celsius o The warmest temperatures are found near the equator during the summer, with temperatures reaching +20 degrees Celsius - The surface of Mars is very dry and dusty - Features on the surface of Mars include towering volcanoes, deep canyons, craters, and polar ice caps - Mars appears to have been very active geologically, particularly early in its history Polar Ice Caps - Mars has permanent ice caps at both poles o They are composed mostly of carbon dioxide (dry ice) - In the northern summer, the carbon dioxide disappears, revealing residual layer of water ice at the north pole - The south pole is always covered by a layer of solid carbon dioxide o The ice caps shrink during the southern summer, but never disappear completely o There appears to be a layer of water ice beneath

The Challenges of Visiting Mars - Its surface is more hospitable for humans, compared to the other planets - A round trip of several years (or more) - Prolonged periods of weightlessness en route - Exposure to high levels of radiation - The psychological challenges of extreme isolation - Sending astronauts (rather than robotic spacecraft) would be extraordinarily expensive Terrestrial Planets: Atmospheres - Mercury: no significant atmosphere - Venus: 96% carbon dioxide - Earth: 78% nitrogen, 21% oxygen - Mars: 95% carbon dioxide Terrestrial Planets: Rotation - Mercury: 58.7 days (2/3 of a Mercury year) - Venus: 243 days (retrograde) - Earth: around 24 hours - Mars: around 24.5 hours Terrestrial Planets: Surface Temperatures - Mercury: -170 to + 430 degrees Celsius - Venus: averages + 480 degrees Celsius - Earth: averages + 15 degrees Celsius - Mars: -140 to +20 degrees Celsius Terrestrial Planets: Moons - Mercury: none - Venus: none - Earth: one large moon - Mars: two tiny moons: Phobos & Deimos...