Astronomy190 - Topics in Astronomy Astronomy and Astrobiology Lecture 16 : Icy Worlds Ty Robinson

Questions of the Day How can icy moons support liquid oceans? How do we know that some icy moons have liquid oceans? What provides the energy for life on icy worlds?

Jovian Moons Decrease in density with distance from Jupiter –Io (3.5 g/cm 3 ), Europa (3.0 g/cm 3 ), Ganymede (1.9 g/cm 3 ), and Callisto (1.8 g/cm 3 )! Have a higher fraction of water ice than the Saturnian moons All are synchronous rotators

What powers the volcanism?

Tides

for a moon tidal friction stops its rotation until the tidal bulge is on the planet-moon line this is why our Moon, and the Jovian and Saturnian moons, synchronously rotate synchronous rotation probably set in a few million years after formation

Distance from Jupiter –Io 4.2 x 10 5 km = 5.9 R Jupiter –Europa 6.7 x 10 5 km = 9.4 R Jupiter –Ganymede 1.1 x 10 6 km = 15.4 R Jupiter –Callisto 1.9 x 10 6 km = 27.6 R Jupiter

To maximize tidal heating: -proximity -eccentricity Energy Generated: -Io = 2 W/m 2 (tidal) -Earth = 0.08 W/m 2 (radioactive) -Earth = 0.2 W/m 2 (hot spot)

Why are the orbits eccentric?

Europa Ice Lines Few craters Smooth

Europa’s Internal Structure km thick (-150C)

Evidence for an Ocean Lack of craters Chaotic terrain

More Evidence - Ridges

Europa’s Magnetic Field Europa’s orbit causes it to “cut through” Jupiter’s magnetic field lines magnetic field anomalies are observed around Europa this implies that part of Europa conducts salty ocean could conduct ocean may be 100km deep (twice the volume of Earth’s ocean!)

Can Europa Support Life? water nutrients energy –temp. differences at a rock/water boundary could come from ocean floor volcanism –chemical energy could support ongoing life UV production of H 2 O 2 and O 2 radioactive decay to form H 2 and O 2

Chemical Energy for Life for chemical energy we need a “disequilibrium” –chemicals are more likely to react in this state we can combine H 2 and O 2 to form water –2H 2 + O 2  2H 2 O –this may require initial energy to start –the equilibrium state depends on the environment –once equilibrium is reached, it can be perturbed by adding more reactants –as the disequilibrium system makes its way back to equilibrium, it releases energy that can be used to fuel metabolism –need a natural system that induces disequilibrium (e.g., hydrothermal vents)

Redox Reactions reactions that move electrons back and forth between reactants –there is an electron donor and an electron receptor –when an electron receptor accepts electrons, its net charge is reduced (electrons are negative) and it is said to be “reduced” –when an electron donor gives up electrons, it increases its positive charge and is said to be…”oxidized” –2H 2 + O 2  2H 2 O is a redox reaction two H donate their electrons to the O the O gets reduced, and the H gets oxidized

Questions? Respiration (in your cells) is represented as: C 6 H 12 O O 2  6CO 2 + 6H 2 O + energy Who is reduced? Who donates the electrons? C 6 H 12 O 6 ; C 6 H 12 O 6 O 2 ; C 6 H 12 O 6 C 6 H 12 O 6 ; O 2 O 2 ; O 2

Questions? Photosynthesis takes in sunlight and CO 2 to make sugar and O 2. What is the disequilibrium driving this reaction? sunlight is in disequilibrium with the atmosphere O 2 is in disequilibrium with the atmosphere CO 2 is in disequilibrium with the atmosphere O 2 and sugar are in diseqlibrium

Redox and Microbes Thiobacillus ferrooxidans uses: –2Fe /2 O 2 + 2H +  2Fe +3 + H 2 O + energy –No organic molecules or sunlight needed! similar reactions can also use H 2 and S compounds. –good reactions to power the origin of life –Fe, S found in hot springs and hydrothermal vents –water/rock interfaces have Fe and can produce H 2 Any geologically active world with liquid water will have chemical disequilibrium environments!

The Saturnian Moons higher methane and ammonia ice fractions Titan is bigger than Mercury!

Questions? Titan is larger than Mercury but is only half as massive. How can this be? Titan is colder than Mercury Titan has an atmosphere Mercury is made of iron Titan is composed of more ices, making it less dense

Titan

Why an Atmosphere? formed without atmosphere formed relatively rich in methane ices, nitrogen-bearing species outgassed secondary atmosphere of N 2 and CH 4 –these molecules are in the vapor phase at current conditions

Liquid on Titan

Enceladus

Tiger Stripes

Enceladus’ South Pole

Questions of the Day How can icy moons support liquid oceans? How do we know that some icy moons have liquid oceans? What provides the energy for life on icy worlds?