Phys 214. Planets and Life

Transcription

1 Phys 214. Planets and Life Dr. Cristina Buzea Department of Physics Room (Please use PHYS214 in subject) Lecture 24. Searching for life in our Solar system. Mercury, Venus, Jupiter, Saturn, Uranus, & Neptune March 14th, 2008

2 Contents Textbook pages Biological tour of our Solar system Mercury, Venus, Jupiter, Saturn, Uranus, & Neptune

3 Mercury Mercury Earth Mass (M Earth ) Escape velocity (km/s) Orbital semimajor axis (AU) Orbital inclination ( o ) 7 0 Orbital period (days) Axial rotation (days) Axial inclination ( o ) Radius (km) Density (x10 3 kg m -3 ) Surface gravity (m s -2 ) Mean surface T (K) 443K (170C) 288K (15C) Max surface T(K) 700K (427C) 310K (37C) Min surface T(K) 100K (-173C) 260K (-13C) Atmospheric surface pressure (bar) Atmosphere O, Na, H 2 N 2, O 2 Satellites none 1 Largest known surface feature Caloris Basin Mt. Everest (1350 km diam) (8 km above sea-level) Larger than most moons, but smaller than Jupiter s moon Ganymede and Saturn s moon Titan.

4 Mercury The orbit of Mercury is the most eccentric of the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. The varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet s rotation around its axis, result in complex variations of the surface temperature. Each day and night last about three Earth months each. The dayside temperature is 427 o C, but with no atmosphere to retain the heat the temperature drops during the night to minus -173 o C. Mercury s orbit is inclined by 7 to the plane of Earth s orbit (the ecliptic), as shown in the diagram on the right.

5 Mercury Mercury high density suggests that like earth suffered an impact shortly after it went differentiation. The impact ejected much of its mantle into space, leaving a planet with a large iron core. This large impact would have ejected most of its water into space. Mercury s surface is flexed by tidal bulges raised by the Sun the Sun s tides on Mercury are about 17 times stronger than the Moon s on Earth. Mercury is also among the least-habitable bodies in the solar system because of lack of liquids and atmosphere.

6 Mercury (Left) part of the enormous Caloris Basin. Probably formed by a giant impact early in Mercury's history, this basin was subsequently filled by lava flows. Weird terrain at the antipodal point to the large Caloris basin. The shock wave produced by the Caloris impact was reflected and focused to the antipodal point, breaking the crust it into a series of complex blocks.

7 Mercury Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. Floors of deep craters near the poles are never exposed to direct sunlight, and have temperatures lower than the global average. Water ice strongly reflects radar, and observations reveal that there are patches of very high radar reflection near the poles The icy regions are believed to be covered to a depth of only a few meters, and contain about kg of ice. By comparison, the Antarctic ice sheet on Earth has a mass of about 4 x kg, and Mars south polar cap contains about kg of water. Two most likely sources of ice on Mercury are from outgassing of water from the planet s interior or deposition by impacts of comets. Even if Mercury has surface water, it will not be liquid because its surface temperature are almost always above the boiling point or far below the freezing point. Radar image of Mercury's north pole. Mercury may have ices brought by impacts in crater bottoms near its poles that have not been exposed to Sun light for billions of years.

8 Venus Venus Earth Mass (M Earth ) Escape velocity (km/s) Orbital semimajor Axis (AU) Orbital inclination ( o ) Orbital period (days) Axial rotation (days) Axial inclination ( o ) Radius (km) Density (x10 3 kg m -3 ) Surface gravity (m s -2 ) Mean surface T 733K (460C) 288K (15C) Atmospheric surface pressure (bar) 92 1 Atmosphere CO 2 N 2, O 2 Satellites none 1 Highest point on surface Maxwell Montes Mt. Everest (17 km above planetary radius) 8 km Venus is the only planet which spins backwards. Venus is completely cover by thick clouds of sulfuric acid and other toxic chemicals.

9 Venus The pressure on its surface is 92 times the one on Earth equivalent of a depth of 1 km in the ocean. The planet Venus is often referred to as the Earth s sister planet because it is almost the same size and density. With an Earth like atmosphere Venus global average temperature would be 35 o C. Venus has much higher temperatures and pressures than Earth. Based on its distance from the Sun, we would expect the surface of Venus to be a little hotter than the Earth. However the surface of Venus is much hotter than the Earth because it has a very thick atmosphere of carbon dioxide a greenhouse gas. Most of the carbon dioxide on Venus is still present in the atmosphere, unlike on Earth where most of the carbon dioxide on the Earth is dissolved in the oceans, forming carbonate rocks. CO 2 makes 96% of Venus atmosphere compared to less than 1% for Earth. Venus may have been more Earth-like in the past because the Sun was dimmer, putting out less solar radiation. Every planet in the solar system has strange and unexplained features. In the case of Venus it's the spin. Venus is the only planet which spins backwards, all the others move in the same direction as their orbit.

10 Map of Venus Topographic Map of Venus from Pioneer Venus

11 Venus False-color picture of Venus in ultraviolet light - acidic haze spread across Venus. The unusual clouds were discovered last July by ESA's robotic Venus Express spacecraft currently orbiting Venus. The bright haze is rich in sulfuric acid, created when an unknown process lifted water vapor and sulphur dioxide from lower levels into Venus' upper atmosphere. There, sunlight broke these molecules apart and some of them recombined into the volatile sulfuric acid. Over the course of just a few days the acidic clouds spread from the South Pole of Venus across half the planet.

12 Venus The high surface temperature rules out the possibility for life at the surface. The surface may have been habitable in the past. Now Venus lacks water and plate tectonics to cycle carbon dioxide. Crater counts suggest that its entire surface is less than one billion old, meaning that volcanism or plate tectonics reshaped it. Radar image of Venus taken by orbiting Magellan spacecraft

13 Venus Landers sent to Venus lasted less than 2 hours before being disabled by high temperatures and corrosive chemicals. Actual photographs of Venus s surface

14 Venus Radar images: dark = smooth surface, light = rugged surface Venus appears to be among the most geologically active planets in the solar system. Venus Express is able to detect gases in the lower layers of the atmosphere and variations in its temperature, possible signs of volcanic activity. The large radar-dark areas are probably tectonically formed basins that have been filled in by fluid lava flows, thus presenting a smooth surface to the Magellan radar system.

15 Image showing radio-thermal emission (emissivity) of a volcano about 2 km high. The hot surface of Venus shows clear signs of ancient lava flows. Three large impact craters with diameters ranging from 37 km to 65 km

16 Venus Computer generated three-dimensional perspective view of the surface of Venus from radar data

17 Venus Radar image of large circular domes 25km across Arachnoids are large structures of unknown origin that have been found only on the surface of Venus. They resemble spider-webs - concentric ovals surrounded by a complex network of fractures, and can span 200 kilometers. Over 30 arachnoids have been identified on Venus, so far. The Arachnoid might be a strange relative to the volcano, but possibly different arachnoids are formed by different processes.

18 Venus If in the past Venus had oceans and life arose, microbes might still survive in the clouds. At about 50 km altitude the greenhouse effect is weaker and droplets contain liquid water. The clouds are acidic, but their sulfur content might provide chemical energy for extremophiles to survive. If life exists on Venus today, it will most likely be found floating high in the atmosphere. Movie - composed by six different sequences of images obtained by the Venus Monitoring Camera onboard Venus Express in May 2006, December 2006, and January 2007 respectively. Distance from the planet s surface varies from to kilometres. All images were obtained through the camera s UV filter, at a wavelength of 367 nanometres. Movie: Cosmos Disk 4. Ch. 8. [41-46:00] Planet Venus (12 min)

19 Mars is a very important planet and we will speak about during the next two lectures. Monday March 17th Wednesday March 19 - Movie about manned spaceships to Mars - NOT ON THE WEB! Exam questions from this movie!

20 Jovian planets The four jovian planets are very different from terrestrial planets. - Massive, lower in density, composed mostly of H, He and H compounds (water, methane, ammonia) - Observations and theoretical models suggest they lack a solid surface similar to the one of Earth. Outer layers contain clouds of gaseous H, He and H compounds. Deeper the pressure increases and H and He are in liquid form. In Jupiter and Saturn the pressures are so high that H becomes metallic. They have cores of rocks, metals and H compounds in very different phases from those we know.

21 Jupiter Jupiter Earth Mass (M Earth ) Escape velocity (km/s) Orbital semimajor Axis (AU) Orbital inclination ( o ) Orbital period 11.8 years 365 days Axial rotation (days) Axial inclination ( o ) Radius (km) 70,000 6,371 Density (x10 3 kg m -3 ) Surface gravity (m s -2 ) Mean surface T 288K (15C) Cloud top T 120K (-153C) T at 1 bar pressure 165K (-108C) Rings few 0 Satellites > 39 1 Atmosphere H 2, He, (CH 4 ) N 2, O 2 Temperatures in upper atmosphere are very low, however they increase towards their deep interiors. At some altitude the atmosphere is hot enough for liquid water to exist! Movie - Jupiter1.mp4

22 Jupiter Storms and lightning on Jupiter = energy for life? Movie - Jupiter2.mp4

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24 Jupiter

25 Jupiter Temperature far below freezing at the cloud top, and increases with depth. Jupiter has several cloud layers, each formed as different type of gases condense. Clouds containing water can form at a depth of a little over 100 km (1% down from the highest cloud). Jupiter might host life in the clouds! The main impediment for life on Jupiter is strong vertical winds that would carry away organic molecules at depth at which the heat would destroy them. Life with some sort of buoyancy might stabilize it at a comfortable altitude. Movie. Cosmos Ch. 11. [53:00-56:00] Alien life on Jupiter (3 min)

26 Jupiter Could life survive on Jupiter if it were to arrive from somewhere else? Probably not. Jupiter has a strong magnetic field. Jupiter s aurorae are not due to solar winds, but probably to one of its satellites - Io. Io's volcanoes release particles, some of which become ionized and trapped by Jupiter's magnetic field.

27 Saturn Saturn Earth Mass (M Earth ) 95 1 Escape velocity (km/s) Orbital semimajor Axis (AU) Orbital inclination ( o ) Orbital period 29 years 365 days Axial rotation (days) Axial inclination ( o ) Radius (km) 58,000 6,371 Density (x10 3 kg m -3 ) Surface gravity (m s -2 ) Mean surface T 288K (15C) Cloud top T 89K (-184C) T at 1 bar pressure 135K (-138C) Rings many Satellites > 30 1 Atmosphere H 2, He, (CH 4 ) N 2, O 2

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29 Saturn In the shadow if Saturn The night side of Saturn is seen to be partly lit by light reflected from its own ring system.

30 Saturn Movie. SaturnRingsCrossing

31 Saturn

32 Saturn

33 Saturn Saturn`s North Pole. The bizarre hexagonal cloud pattern show stability even 20 years after Voyager. Movies of Saturn North`s pole show the cloud structure maintaining its structure while rotating. Movie. SaturnPole

34 Saturn At the South Pole lies a huge vortex that is a hurricane-like storm, showing no signs of dissipating. The storm is slightly larger than the entire Earth and carries winds of 550 km per hour, twice the velocity of a category 5 hurricane. Saturn is very similar to Jupiter that the same considerations regarding possible life should apply to it as well. The biggest obstacle to life being present in the atmospheres of Saturn is the strong vertical wind speeds that would rapidly carry organisms into the hot. In order for life to survive in the atmospheres of Saturn it must be large and buoyant, allowing it to maintain a stable altitude in the atmosphere.

35 Uranus and Neptune Uranus Neptune Earth Mass (M Earth ) Escape velocity (km/s) Orbital semimajor Axis (AU) Orbital inclination ( o ) Orbital period 83 yr 163 yr 365 days Axial rotation (days) Axial inclination ( o ) Radius (km) 25,360 24,620 6,371 Density (x10 3 kg m -3 ) Surface gravity (m s -2 ) Mean surface T 288K (15C) Cloud top T 53K (-220C) 54K (-219C) T at 1 bar pressure 75K (-198C) 70K (-203C) Rings several few Satellites > 21 > 8 1 Atmosphere H 2, He H 2, He N 2, O 2 If life exists in Uranus and Neptune, it will most likely be found in the oceans of liquid ices beneath the surface. Movie. Uranusmagnetic. Movie. Neptunespot

36 Uranus and Neptune Uranus and Neptude magnetic dipole are off center by 30% of planet s radius. One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giant planets, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water ammonia ocean.

37 Uranus and Neptune Uranus and Neptune are unlikely candidates for habitability, same as Jupiter and Saturn. They have colder atmospheres due to higher distance from the Sun. Vertical winds would make life unlikely. However Uranus and Neptune might have a habitable zone deep within their outer cores of water, methane and ammonia, forming some very odd oceans. At present we lack technology that would make the search for this life possible.

38 Life on Jovian moons Life on Jovian Moons will be discussed in a following lecture.

39 Spacecraft exploration of the Solar System Information on the world in our solar system comes from telescopic observations, and spacecraft exploration - manned or robotic. Robotic missions comprise: 1. Flyby - a spacecraft goes past a world just once and then continues on its way 2. Orbiter - orbits the world that it studies, allowing longer term observations 3. Lander or probe - are landing on the surface or probe the planet s atmosphere by flying through it. Some landers can carry rovers to explore wider regions. 4. Sample return mission

40 Spacecraft exploration of the Solar System Flyby - cheaper because of lower weight - cost of fuel. In addition to detailed images they carry instruments to measure magnetic fields, provide info on masses, densities, sample dust. Orbiters - can study a world for a much longer time than a flyby. Carry cameras, spectrographs, radar for precise altitude measurements. Is more expensive than a flyby. Trajectory of Voyager 2, which made flybys of each f the 4 jovian planets. Asteroid Eros photographed by NEAR spacecraft that landed on Eros after orbiting it for a year.

41 Spacecraft exploration of the Solar System Landers and probes Galileo spacecraft 1995 dropped a probe into Jupiter s atmosphere - collected temperature, pressure, composition data for about one hour of descent Cassini spacecraft carried a probe - Huygens - descended on Saturn s moon Titan Rovers on Mars - Spirit and Opportunity Spirit and opportunity landing on Mars. Artist s conception

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43 Next lecture Searching for life in our Solar System- Mars