life in the outer solar system - astronomy at the ... · surface gravity = 0.135 of earth's...

AST 309part 2:

Extraterrestrial Life

Life in the outer Solar System

Overview:

Prospects for life on:

1. Europa (Jupiter moon)2. Titan (Saturn’s moon)3. Enceladus (Saturn’s moon)

Life on Europa?

Juno arrives at Jupiterin 2016

NASA’s JUNO mission to Jupiter:

Science goals:

1. abundances (e.g. ratio of O/H)2. determine core mass3. map the gravitational and magnetic field 4. map the variation in atmospheric composition, temperature structure,

cloud opacity and dynamics

• Europa is the sixth of Jupiter's known satellites and thefourth largest; it is the second of the Galilean moons.Europa is slightly smaller than the Earth's Moon.

Life on Europa?

Galileo Galilei

1610!

Europa

• Spacecraft exploring the Galilean satellites:

Life on Europa?

Voyager 1 & 2

Galileo

Life on Europa?

Basic Properties of Europa:

Semimajor axis = 671079 kmOrbital period = 3.551810 daysHeliocentric Distance = 5.203 AURotational Period SynchronousOrbital inclination = 0.464 degreesEccentricity = 0.0101Radius = 1565 kmMass = 4.797E22 kgMean density = 2.99 g/cm3Surface Gravity = 0.135 of Earth'sEscape Velocity = 2.02 km/sGeometric Albedo = 0.6Surface Temperature = 128 K (-145 C)Surface Composition = Water IceTenuous O2 Atmosphere = Surface Pressure about 10-11 Earth's

Europa’s surface is smooth and young (no craters), and is covered withcracks:

Europa:

Icebergs on the surface movedby liquid water that later froze orby slushy warmer ice beneath?

Europa is tidally heated like Io (just less)and has maybe the youngest surface inthe outer solar system!

Europa is heated by tidal forces from nearby massive Jupiter and hasforced orbital eccentricity of 0.0094 from the gravitational interactionswith the other Galilean moons:

Europa:

That is exactly the same processthat drives Io’s intensive volcanism:

Europa is tidally heated like Io (just less)and has maybe the youngest surface inthe outer solar system!

2 models of Europa’sinterior:

Europa:

Artist’s conception of the icysurface of Europa

Europa:

Cycloidal features (“flexi”) near Europa’s south pole. These cycloidal cracks form inEuropa's solid-ice surface with the daily rise and fall of tides in the subsurface ocean(Gregory V. Hoppa, Randall Tufts, Richard Greenberg and Paul Geissler of the Lunaand Planetary Laboratory, University of Arizona). This image shows what appears tobe the most convincing evidence yet for a global ocean under Europa's icy crust.

Europa:

• the likely presence of a sub-surface ocean of liquid water(perhaps as much as 150 km deep) which could provide amedium and solvent for life.

• intense radiation from Jupiter's magnetosphere striking iceon Europa's surface and releasing oxygen, which if it findsits way into ocean could provide a fuel for life;

• the possible presence of undersea volcanic vents, whichcould furnish energy and nutrients for organisms.

Reasons why Europa is so interesting:

Europa:

Chaotic features seen in many images of Europa's icy surface are probably createdby Europa's tides, and are believed to be evidence of melt-through needed forexposing the oceans. The mixing of substances needed to support primative lifemay be driven by the tides on Europa, with maximum heights of 500 meters(much larger than Earth tides). Circulation of liquid water through cracksproduced by tidal forces could bring salts and organic compounds dissolved inthe water up to Europa's surface. This circulation also brings biologically usefulchemicals, such as formaldehyde (as well as organic compounds dumped onEuropa's surface by cometary impacts) down to the subsurface ocean. Otherchemicals, formed by radiation near the surface, such as sulfur, hydrogenperoxide, and free oxygen, would also provide primative life with sources ofenergy and nutrients. Hydrothermal vents would produce organic compounds(seen as dark material coloring cracks?) and provide a heat source. Underseavolcanism could also lead to large melt-throughs, and tidal heat, created byinternal friction could also melt the ice. The melted-through ice provides lightand surface chemicals to the oceans. Any creatures inhabiting these oceanscould use photosynthesis for energy.

Reasons why Europa is so interesting:

The questionreally is: isthe ice thin orthick?

Tides andunderseavolcanoescould play arole too!

Europa:

Let’s go there and find out!

Before we drill, we wouldsend a radar mappingprobe to measure thethickness of the ice crust!

Indirect evidence from large craters suggest athickness of 19 to 25 km. On Europa, largercraters become smoothed out with gentleconcentric rings. They are shallower than theircounterparts on other moons. This is becauseof the influence of a very thick cushion of ice(Schenk, 2002, Nature)

• The biggest moon of Saturn• Cold! 75 K (-180 C)

• Thick atmosphere! ~ 1.5 x pressure of Earthmostly nitrogen (95%) and methane (CH4)

Origin?– Probably outgassing after formation by accretion of icy

planetesimals/comets and subsequent evolution– Methane from cryovolcamism

Titan:

• Exploring Titan:

Titan:

Voyager 1 & 2

Cassini / Huygens

• Titan as seen from space:

Titan:

visible 938nm methane window composite

• Titan is big! 2nd largest moon in our solar system:

Titan:

Size comparison: our Moon,Earth and Titan

Simple model of internal structure

• Titan’s thick atmosphere:

Titan:

Layered structure in Titan’s atmosphere

Chemical composition determined by spectroscopy:

Titan:

Titan:

Titan:

Titan:

Source of Titan's atmosphere?Internal - original planetesimals contained dissolved/trapped volatiles -> interiorand are now outgassing to provide persistent atmosphere

External - delivery of comets (same as Earth, Venus, Mars) - evidence in highaltitude H2O and CO

Losses of Titan's atmosphere?Thermal escape - particularly H, also other atoms produced by dissociation ofmolecules at high altitudesIonization, stripping by surrounding magnetosphere and solar windCondensing -> ocean

Recycling?Methane is recycled in a similar way to water in the Earth's atmosphere throughthe hydrological cycle......

Titan:

Why is this important for astrobiology?

Titan’s atmosphere is remarkably similar to the Urey-Miller experiment and earlyEarth (except temperature)Is Pre-biotic chemistry possible on Titan?

But life?Remember that Titan is cold and all chemical reactions slow down, but still….

Ok, so let’s land on Titan!

Titan:

The Huygens lander:

Titan:

The Huygens lander’s descent:

Titan:

The Huygens lander, impact data:

Surface is like wet clay,packed snow or sand

Titan:

The Huygens lander:

Evidence forliquid methaneon the surface

Heating of thesurface by the probe causedmethane outgassing

Titan:

The Huygens lander:

Surface reflectivitymeasured on the landingsite with a lamp turned on(red line). The visibleportion of the spectrum isconsistent with laboratory-produced tholins, thoughtto be analogs of Titan'sphotochemical aerosols(black curves). Water ice islikely responsible for theabsorption seen at 1500-1600 nm. The decrease ofthe reflectivity withwavelength beyond 830nm is due to anunidentified material.

Enceladus:

Enceladus:

Enceladus has the highest albedo (>0.9) of any body in thesolar system. Its surface is dominated by fresh, clean ice.

At least five different types of terrain have been identified onEnceladus. In addition to craters there are smooth plains andextensive linear cracks and ridges. At least some of the surface is relatively young, probably less than 100 million years.This means that Enceladus must have been active until very recently (andperhaps is still active today).

Enceladus is much too small (500km) to be heated solely by the decay ofradioactive material in its interior at present. But briefly after its formation 4.5billion years ago short-lived radioisotopes may have provided enough heat tomelt and differentiate the interior. That combined with modest present dayheating from long-lived isotopes and tidal heating may account for thepresent day activity on Enceladus.

Enceladus:

Enceladus orbits in the densestregion of Saturn’s E-ring

Is it the source?

Enceladus is currently in a 2:1 mean motion orbital resonance withDione, completing two orbits of Saturn for every one orbit completed byDione. This resonance helps maintain Enceladus's orbital eccentricity(0.0047) and provides a heating source for Enceladus's geologic activity

Enceladus:

Cassini discovers jet-like plumes rising from the south polar regions.During “fly-throughs” the spacecraft detected mostly water vapor, as wellas minor components like molecular nitrogen, methane and CO2.

Enceladus:

Cassini measures mean density:1.6 g/cm3 => some % of silicates & iron

Now two heat sources:1. Radioactive decay2. Tidal forces

Possible molten core or magma pockets thatdrive volcanism!

Enceladus:

Enceladus:

The “Tiger Stripes” in the southpolar region.

Thermal map of cracks near southpole of Enceladus. High temperaturesaround 180 K (-135 F), normal = 72 K

Enceladus:

Pockets of water of salty ocean?

Postberg et al. (2011, Nature):"Alkaline salt water, together with the observed organic compounds and the thermal energy obviously present in the south polar region, could provide an environment well suited for the formation of life precursors.“

Cassini's Cosmic Dust Analyser instrument found that about 6% of the grains forming the E-ring were rich in sodium salts.

“Saturn's tiny moon Enceladus may be the best place to look for lifeelsewhere in the Solar System.” Bob Brown (Cassini mission)

Summary:

• Europa likely has a liquid sub-surface ocean andis heated by tidal forces

• Titan has complex organic chemistry and a thicknitrogen atmosphere (early Earth)

• Enceladus is geological active, has lots of water,plus some organic material and a heated interior