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 28. Search for life on jovian moons. March 24th, 2008

2 Contents Textbook pages First several slides from life on Mars, Neptune and Uranus. Jovian moons Formation of jovian moons Composition of jovian moons Synchronous rotation of the moons Tidal heating Orbital resonance Jupiter s moons Io

3 Life on Jovian moons The largest Jovian moon Ganymede and Titan are larger than Mercury but smaller than the Earth. Three of Jupiter s moons Io, Europa, and Callisto, and Neptune s moon Triton are bigger than Pluto.

4 The formation of Jovian moons Most of the smallest Jovian moons are most likely captured asteroids and comets. The largest Jovian moon that appears to have been captured is Triton, the moon of Neptune it orbits backwards relative to its planet s rotation. The fact that many of the larger Jovian moons orbit nearly in the equatorial plane of their host world, moving in the same direction as their planet s spin, suggest that they formed from a rotating disk of gas and dust like a miniature solar system Triton

5 Composition of Jovian moons Planetary objects fall into several groups: - Giant planets with radii greater than 10,000 km and low densities - Terrestrial planets and terrestrial-like bodies - Icy satellites - Small asteroids Jovian moons are typically made of ice and rock. The outer Solar System was cold enough to allow ices to condense along with metal and rock. The average densities of most jovian moons are significantly lower than that of Earth they contain a lot of water ice. Within individual moons we see variation in composition reflecting the fact that the moons formed at different distances from a hotter, central planet. Jupiter s moons show a decrease in density with distance from Jupiter formed of a cloud of gas that was hotter in the center than in its outer regions.

6 Jupiter s satellites Io Europa Ganymede Callisto Io Europa Ganymede Callisto Earth s Moon Radius (km) Mass (10 20 kg) Density (g cm -3 ) Escape velocity (km/s) Average surface T (K) Orbit period (days) Io s density indicates it has no water, Europa is mostly rock with water ice, Ganymede and Callisto have more significant amounts of water ice compared to rock.

7 Composition of Jovian moons In addition to compositional variation within individual moons, there is also variation in composition of moons as we move from one planet to the next because of temperature differences in the solar nebula. Water ice condenses easily at temperatures near Jupiter. Methane and other ices condensed only at colder temperatures farther away from the Sun. Jupiter s moon contains significant quantities of water ice, but no other ices. Moons of more distant planets contain higher proportion of water ice compared to rocks, and contain other ices (methane in addition to water ice).

8 Synchronous rotation of the Moons Like our own moon, most of Jovian moons exhibit synchronous rotation they keep the same face turned toward their planet (they rotate at the same rate that they orbit their host planet) Synchronous rotation develops naturally for any moon that orbits close to its parent, and is the consequence of the same gravitational effect that lead to tides on Earth. Tidal force = the force of gravity exerted by one object on another is greater on the near side than the far side. Tidal forces stretches the Earth and creates tidal friction. Tidal friction allows Earth s rotation to pull the bulge slightly ahead of the Earth-Moon line => 1) The moon gravity is pulling back causing earth s rotation to slow down (1 second every 50,000 years) 2) The gravity of the bulge pulls the Moon ahead in its orbit causing the moon to move farther away from Earth.

9 Synchronous rotation of the Moons Calculation. Calculate the the tidal force that a planet exerts on its moon (per kg). F g = G M 1 d M 2 2 F = G tidal ( near side)! F ( on far )= = F side M g planet g planet rock ( d! r ) ( d + r ) 2 planet! moon M rock moon! G M M 2 planet! moon moon

10 Tidal heating - Jovian moons Planets gradually loose their internal heat (accretion, differentiation, radioactive decay), the smaller the planet the faster it cools. Several of jovian moons still have a source of internal heat, despite their smaller size. Io (one of Jupiter s moons) is the most volcanically active body in our solar system! Its internal source of heat is very different from that of planets = tidal heat due to tidal forces. Even though Jupiter s moon Io is similar in size to our geologically dead Moon, it is more geologically active than the Earth. This is because Io experiences strong internal tidal heating because of the strong tidal force from the massive Jupiter combined with its elliptical orbit which causes the strength and direction of the force to constantly change. We would expect most large satellites to have nearly circular orbits, and not elliptical. Why is Io s orbit elliptical?

11 Orbital resonance and Jupiter's moons Because of orbital resonance between the three of the moons of Jupiter (I = Io, E = Europa, G = Ganymede). Sequence of conjunctions as they orbit about Jupiter (J). The seventh configuration would complete one period of Ganymede, which is two periods of Europa and four periods of Io (Murray and Dermott, 2001)

12 The resulting tidal friction greatly heats Io's interior, causing molten rock to explode through the surface. Io's volcanoes are so active that they are effectively turning the whole moon inside out. Io's colors derive from sulfur and molten silicate rock kilometers plume. Io lacks water and extreme volcanic activity makes is improbable for hosting life. Jupiter s moon Io

13 Next lecture Searching for life on Jupiter s moons Europa, ganymede, Callisto, Saturn s moon Titan, Neptune s moon Triton The nature and evolution of habitability