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 29. Search for life on jovian moons. Habitability. March 26th, 2008

2 Contents Textbook pages Search for life on: - Jupiter s moons Europa, Ganymede, & Callisto - Saturn s moons Enceladus & Titan - Neptune s moon Triton - Life in our Solar System - Conclusions

3 Europa evidence of subsurface oceans The level of tidal heating on Europa and Ganymede might be just right for life. Surface: bright white - entirely covered with ice; smooth, giant cracks in the ice crisscross the surface. few impact craters -> very young surface. Evidence for a subsurface ocean of water: 1) the MOST convincing evidence - the magnetic field of Jupiter is able to induce a magnetic field in Europa consistent with a salty ocean beneath its crust 2) The lack of large impact craters on the surface - large impacts will break the thin crust causing water and slushy ice below to flood out and resurface the crust. 3) Much of Europa s surface appears chaotic and clogged with huge iceberglike blocks. This is consistent with a thin icy crust that has been broken into pieces by tidal forces below which is a subsurface ocean of water. Landscape suggest that liquid water or slush ice has welled up from below, breaking apart the surface and then freezing in place.

4 Europa evidence of subsurface oceans Fractured ice planes The closest photograph ever taken of Europa.

5 Europa models The internal structure of Europa from gravitational field measurements: Model 1. thin icy crust (up to 3 km), warm convecting ice layer, thick rocky mantle, central iron core. Model 2. thin icy crust, 100 km deep subsurface ocean of water, thick rocky mantle, central iron core. Model 3. thin icy crust, warm convecting ice layer, subsurface ocean of water, thick rocky mantle, central iron core.

6 Jupiter s moon Europa The floating ice shell is heated by flexing and squeezing of Europa s tides. This tidal heat keeps the ocean liquid at a temperature near 0 C, even though the surface is very cold at a temperature of about -170 C. The cold surface ice can crack, while the ice below is hotter due to the tidal heat, so it can slowly flow like a glacier.

7 Life on Europa? Life in the subsurface ocean -would most likely obtain energy from tidal heating. In the subsurface ocean beneath Europa s icy crust, if life exists, it most likely originated close to volcanic vents on its ocean floor. The complexity of any life present in Europa s subsurface ocean is mainly limited by the amount of available energy to sustain it. Recent research indicates that enough carbon exists to support an underwater biosphere. Europa movie (4 minutes) A possible scenario for life on Europa. Image credit: Richard Greenberg Artist's concept of the cryobot and hydrobot. Estimated concentrations of major elements in Europan oceanic water compared with seawater on Earth.

8 Jupiter s moons with subsurface oceans of water Europa, Ganymede, and Callisto - moons of Jupiter that show evidence for subsurface oceans of water beneath their icy crusts.

9 Jupiter s moon Ganymede Ganymede -the largest moon in the Solar System 1) the largest magnetic field of any moon. 2) the only moon to have its own internal magnetic field indicative of a molten convecting core (radioactive decay?). 3) a small part of its magnetic field varies with Jupiter s rotation indicates an electrically conducting material under the surface salty ocean? Less tidal heating on Ganymede suggests a thicker ice cover than on Europa- at least 150 km. The higher pressure in Ganymede s interior is high enough to allow high-density forms of ice beneath any liquid water ocean - > no rock water boundary and less energy for life than on Europa. Magnetic field of the Jovian satellite Ganymede embedded into the magnetosphere of Jupiter. The image is based on the reported Galileo measurements.the green color denotes closed field lines.

10 Jupiter s moon Ganymede Ganymede has both young and old surface regions, separated by sharp boundaries. The young surfaces were created by water eruptions with subsequent freezing. Left. Fresh craters Right.Parallel ridges and troughs that are the principal features in the brighter regions of Ganymede. Resolution of the Galileo images is 74 meters.

11 Jupiter s moon Callisto 200 kilometer thick band of ice just beneath the moon's surface light blue stripe - potentially a salty layer of liquid water up to 10 km thick interior of rock and ice Entire surface packed with craters dating probably from the heavy bombardment. Dark dust covering the low-lying areas, with ridges and crests bright white. Its interior does not seems to be fully differentiated probably very few radioactive decay (it has never been heated enough to melt its ice component). Magnetic measurements made during Galileo flybys indicate Callisto's magnetic field is variable because Callisto too has a subsurface liquid layer (water contains a small amount of ammonia or other antifreeze, up to 5% by weight.). Tidal heating might is probably responsible for the subsuraface liquid.

12 Jupiter s moon Callisto The conditions for life appear to be less favourable on Callisto than on Europa because of: -the lack of contact with differentiated rocky material and -the lower heat flux from the interior of Callisto. Magnetic field around Callisto. The bending of the field lines indicates the existence of an electrically conducting layer in the interior. The red line shows a trajectory of the Galileo spacecraft during a typical flyby

13 Saturn s moons Enceladus size Earth Titan Moon Mimas Enceladus Thetys Dione Rhea Titan Iapetus Earth s Moon Radius (km) Mass (10 20 kg) Density (g cm -3 ) Orbit period (days)

14 Saturn s moons Mimas Enceladus Thetys Dione Rhea Titan Iapetus (Movie) Iapetus

15 Saturn s moon Enceladus Cryovolcanism - ice geysers erupt on Enceladus along surface fractures in the moon's south polar region. Geysers arise from near-surface pockets of liquid water with temperatures near 0 o C compared to moon's surface temperature of -200 o C. The ice geysers also likely produce Saturn's faint but extended E ring. Enceladus seems to have a substantial subsurface liquid probably ammonia/water mixture. Surprisingly, small Enceladus may have subsurface habitable zones. Enceladus movie 1, 2

16 Saturn s moon Titan UV and infrared image of Titan Titan - the second-largest moon in the solar system and the only moon with its own atmosphere (pressure ~ 1.6 times the one of Earth s; thickness of atmosphere km). Outgassing - the main source of its atmosphere Like the Earth, Titan has an atmosphere made mostly of molecular nitrogen. Other components are hydrocarbons like methane and ethane. The origin of N 2 in Titan s atmosphere is the breakdown of ammonia (NH 3 ) by ultraviolet light from the Sun. Methane should be rapidly destroyed in Titan s atmosphere, yet it is still present in appreciable amounts. Probably because is continually evaporating from the surface and the interior. Hydrocarbons rain down on the surface, forming enclosed seas, lakes, and ponds. Titan is roughly the same size as Mercury, yet Titan has an atmosphere while Mercury does not. This is because Titan is much colder, allowing molecules to be trapped in its atmosphere.

17 Saturn s moon Titan In 2005 Huygens probe landed on Titan, showing a world similar to the Earth in many respects. Landscape shows strong evidence that a liquid, possibly methane, has flowed on the surface, causing erosion.

18 Liquid seas and lakes on Titan. Titan.

19 Internal water-ammonia ocean on Titan The Huygens probe showed most of it to be solid. Since then, geological features such as dunes, channels, lakes, impact craters have been documented. Three years after their discovery, Cassini observed these features for a second time and reported a systematic drift (by 31 km) of these features compared to their expected position. Conclusion - Titan has an internal water-ammonia ocean buried below several tens of kilometers of ice that mechanically decouples the crust from the interior. Titan internal structure: Atmosphere Water Ice & methane compounds Liquid water High-pressure ice Rocky core

20 Dunes on Earth Life on Titan? The atmosphere of early Earth was probably similar in composition to the current atmosphere on Titan. Dunes on Titan The Miller-Urey experiment has shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances can be generated. Enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. Temperature profile derived from Voyager data. The temperature of Titan's surface is -179C. Even though Titan has liquid methane on its surface, some internal heat, and plenty of carbon-containing compounds, it is not a suitable place for life as we know it because it is far too cold, and methane is not a very good biological solvent. However, life could exist in the subsurface ocean of ammonia/water mixture.. Movie 1, Movie 2, Movie 3, Movie 4

21 Neptune s moon Triton Triton orbits in the opposite direction to Neptune s rotation - probably captured by Neptune s gravity. Triton s source of internal heat is mostly associated with tidal heat. Voyager 2 images showed active geyser-like eruptions of nitrogen gas and dark dust particles several kilometers into the atmosphere. Triton is one of only three objects in the Solar System known to have a nitrogendominated atmosphere (Earth & Titan). Triton has the coldest surface known in the Solar System - that most of its nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice. Triton's surface indicates a long history of melting - differentiated core - radioactive decay - heat sufficient to maintain an underground ocean. If Triton has a subsurface ocean, it will most likely consist of water mixed with ammonia, methane, or other melted ices. Triton Earth s Moon Radius (km) Mass (10 20 kg) Density (g cm -3 ) Orbit period (days) 6 27 Surface T (38K) -235C 250K (-23C)

22 Jovian Moons habitability- conclusions Europa Ganymede Callisto Titan Enceladus Triton Sequence of Jovian moons in the most likely order of decreasing habitability: Europa, Ganymede, Callisto, Titan, Enceladus, Triton. Life might exist on 6 jovian moonss in our Solar System that might have liquids on their surface or beneath the surface!

23 Life in our Solar system - Conclusions In clouds? In subsurface In clouds? In the water-ammonia oceans beneath the surface In subsurface oceans In the surface seas and lakes of methane? Planetary bodies that might harbour life (in the most likely order of decreasing habitability): Mars, Europa, Ganymede, Callisto, Titan, Enceladus, Triton, Venus, Uranus, Neptune, Jupiter, Saturn.

24 Next lecture Habitability & Extrasolar planets