Homework 8 will be posted shortly

Life on Titan?

Enceladus

Enceladus is small (500 km diameter)

 Young, crater-free surface regions with like those on Europa  Orbit resonance with Dione  South polar hot spot and ice plumes  Thin “atmosphere” of water vapor  Subsurface ocean!?

Enceladus

Ice Plumes from Enceladus Area of plumes is much warmer than surroundings - evidence of subsurface reservoir of liquid water

Enceladus feeds the outer E ring

Most likely, there is subsurface liquid water, simple organics, and water vapor welling up from below. Over billions of years, heating of this cocktail of simple organic molecules, water, and nitrogen could have led to some of the most basic building blocks of life.

Moons of Uranus No large moons, nothing of particular interest as far as the search for life

Ariel Umbriel Titania Oberon Caliban Sycorax Prospero Setebos Stephano Trinculo Cordelia Ophelia Bianca Cressida Desdemona Juliet Portia Rosalind Belinda Puck Miranda Named moons of Uranus What do these names have in common? They are all characters from the works of Shakespeare & Alexander Pope

Moons of Neptune One location of interest

Neptune’s TritonNeptune’s Triton < 40K icy volcanism. –Extremely cold (< 40K) objects made from volatile materials produce icy volcanism. –Huge geysers of nitrogen! –Pluto and the Kuiper Belt Objects may look and act similarly.

Very unlikely location for life

Solar system beyond Saturn Decline of probability of life –Main factor is temperature –Europa  Ganymede  Callisto  Titan  Enceladus ? Triton –Retrograde rotation  capture –Uneven surface: Cantaloupe terrain, Smooth parts, Frost deposits?, Wind streaks –Few impact craters  recent geological activity (10  100 Myr) Pluto and remaining moons –Too cold and too small –But, amino acids seen in meteorites

Time to reach for the stars!

Star A mass of gas held together by gravity in which the central temperatures and densities are sufficient for steady nuclear fusion reactions to occur.

A star’s color is indicative of its temperature

Spectral type Temperature Color Stars are often described by their “spectral type”, which is a function of its temperature

The required mass to have fusion reactions in the core is at least a few percent of the mass of the sun.

Nuclear fusion occurs in the core of a star. Fusion of hydrogen to helium is the nuclear process functioning over most of a star’s lifetime. We refer to this time as the Main Sequence lifetime

A convenient way to gain insight into the life and death of stars is through the “Hertzsprung-Russell Diagram”

Hertzsprung-Russell Diagram A plot of the temperature of stars against their brightness (luminosity)

Hertzsprung-Russell Diagram Stars do not fall everywhere in this diagram An HR diagram for about 15,000 stars within 100 parsecs (326 light years) of the Sun. Most stars lie along the “Main Sequence”

Hot stars (bluer) are found at the upper left hand end of the Main Sequence while cooler (redder) stars are found to the lower right. Stars are all classified according to temperature and spectral type, with the hotter stars called ‘O’ type stars and the coolest called ‘M’ type stars. The order of classification is: O-B-A-F-G-K-M

Stars live most of their lives on the “Main Sequence”. These stars generate energy by nuclear fusion of hydrogen into helium in their core.

Hotter “Main Sequence” stars are much less common than cooler Main Sequence stars Very rare Very common

Hotter stars have shorter Main Sequence lifetimes than cooler stars 10 7 yrs 10 8 yrs 10 9 yrs yrs yrs

A star “moves” on the HR diagram as it ages

Collapse of protostar to Main Sequence

Moving up Main Sequence

Hydrogen begins to run out in core. Expansion to giant

Depletion of fuel in core. Shedding of mass

Collapse of remnant - dead star

Increasing mass

Major Factors for life on the Surface of a Planet:  Location, location, location: –must lie within a star’s habitable zone

Major Factors for life on the Surface of a Planet:  Location, location, location: –must lie within a star’s habitable zone  Size is important: – Large enough to retain an atmosphere substantial enough for liquid water – Large enough to retain internal heat and have plate tectonics for climate stabilization

The Habitable Zone An imaginary spherical shell surrounding a star throughout which the surface temperatures of any planets present might be conducive to the origin and development of life as we know it. Essentially a zone in which conditions allow for liquid water on the surface of a planet.

The Sun’s Habitable Zone (today)

The Sun’s Habitable Zone (thru time) The Sun’s brightness (luminosity) has changed with time.

Habitable Zones for Different Stars

Lower mass (cooler) stars have smaller habitable zones

By contrast, the HZ of a highly luminous star would in principle be very wide, its inner margin beginning perhaps several hundred million km out and stretching to a distance of a billion km or more.

The size and location of the HZ depends on the nature of the star Hot, luminous stars – spectral types "earlier" than that of the Sun (G3-G9, F, A, B, and O) – have wide HZs, the inner margins of which are located relatively far out: To enjoy terrestrial temperatures: Around Sirius (Spectral type A1: 26 times more luminous than the Sun), an Earth-sized planet would have to orbit at about the distance of Jupiter from the star. Around Epsilon Indi (Spectral type K5: about one-tenth the Sun's luminosity), an Earth-sized planet would have to orbit at about the distance of Mercury from the star.

The size and location of the HZ depends on the nature of the star The situation becomes even more extreme in the case of a red dwarf, such as Barnard's Star (M4: about 2,000 times less luminous than the Sun), the HZ of which would extend only between about 750,000 and 2 million km (0.02 to 0.06 AU). However: if planets exist too close to its parent star, the development of life might be made problematic because the tidal friction would have led to synchronous rotation.  The same side of the planet will always face the star.