Pluto the Kuiper Belt and Trans Neptunian Objects

Pluto, the Kuiper Belt, and Trans. Neptunian Objects 1

What about Pluto? 2

Discovery of Pluto Calculations indicated that Neptune’s orbit was being purturbed => another planet beyond Neptune. Found in 1930 by Clyde Tombaugh. Too small to account for the perturbations. They were later shown not to exist – no need to invoke planet beyond Neptune! 3

Pluto: basic data Semi-major axis Orbital period Inclination Axis tilt Eccentricity Diameter Mass Density Temp Uranus 19. 2 AU 84 yrs. 0. 8° 98° 0. 04 4. 0 DEarth 15 MEarth 1. 3 g/cm 3 55 K Neptune 30. 1 165 1. 8° 30° 0. 01 3. 9 17 1. 6 55 K Pluto 39. 5 248 17° 120° 0. 25 0. 19 0. 0025 2. 0 44 K 4

Orbit inclined 17º from Ecliptic, with a high eccentricity (sometimes inside Neptune's orbit) 5

HST hemispheric images Bright regions were speculated to be impact craters where ice has been exposed. 6 Mean density: rock/ice Spectroscopy: thin atmosphere of predominantly nitrogen, showing large pressure variations over the years. Thought to be seasonal.

Pluto and Charon • Smaller mass ratio than any planet/moon, same density. • Both objects tidally locked • Binary orbit gave precise mass estimates of both • Rare eclipses in 1985 -1991 gave diameters from timing measurements. Then could work out precise densities. Angular separation here is only 0. 9” 7

Now five moons found Charon (New Horizons) Hubble image showing Pluto, Charon, Nix and Hydra found in 2005, Kerberos 2011, Styx 2012, all with Hubble 8

New Horizons • Launched January 2006, flew past in July 2015. • Goals are to understand surface composition, temperature, thin atmosphere of Pluto, study Charon, look for more satellites, examine other Kuiper Belt object. 9

Plain of frozen nitrogen (“Sputnik Planum”), no impact craters found, age <10 million years. 10

Sputnik Planum may be impact basin, possibly 1 -3 km lower than surroundings. Ice may be from cryo-volcanoes or atmospheric freeze-out At northern edge there appear to be flowing glaciers. Flyover 11

Other areas show a range of ages, up to >4 billion years old. A long history of geologic activity. But crater-dating uncertain because of uncertainty in population of impactors in outer Solar System. 12

Elevation maps of the two candidate ice volcanoes 50 km One of two possible ice volcanoes. If so, internal heat source is unknown! Although nitrogen-dominated, frozen hydrocarbons give thin atmosphere blue color. Atmosphere loss rate is low, so it’s permanent, but should still vary seasonally. May be fed by ice volcanoes. 13

Charon • Long fractures. Also may have ice volcanoes. IR spectra show ice crystals of water and ammonia. Sunlight would soon break these down if not replenished. Surface has age variations. Charon flyover. 14

Moon system • Moon orbits all prograde. • Only Charon is tidally locked. • Others spin rapidly, Nix retrograde. Spins are slower closer to Pluto. • Hydra spin period only 10. 3 hours. Nix and Hydra spin chaotically, due to combined gravity of Pluto and Charon. • Charon’s gravity may be preventing Pluto from tidally locking the other moons. • All presumably formed by fragmenting collision with Pluto? Although why not more moons? Speculation that some merged. • Moons’ orbit animation. Nix chaotic spin animation 15

What is Pluto…? • Pluto and Charon are similar to Triton in density (mixtures of rock and ice) • Characteristics of objects formed in the outer solar system. • The discovery of more, similar objects beginning in early 1990’s means that Pluto is just one of the “Trans. Neptunian Objects – icy bodies orbiting beyond Neptune. The “Kuiper Belt” is a zone from 30 to 50 AU containing great majority of TNOs. • Orbit inclinations can be typically 20°. 16

• If orbit determined and distance from Sun and Earth known, then measurement of amount of sunlight reflected (optical) and reradiated (infrared) allows size and albedo (fraction of light reflected) to be found. • Other sizes from stellar occultations, again if orbit is known. • Some are binaries and masses can be found, thus densities. • May be 35, 000 TNOs larger than D=100 km. >1200 known. Probably many more smaller ones. Smallest found has R about 1 km. • Pluto probably wouldn’t have been called a planet if discovered now! 17

Kuiper Belt and TNOs are leftover planetesimals thought to be scattered outward by the late outward migration of Neptune. Total mass of Kuiper Belt 0. 06 - 0. 25 Earth masses. 18

Eris and other large TNOs Orbit of Eris – the most massive known TNO 19 Eris and Dysnomia from HST

Other large Trans-Neptunian Objects Makemake Haumea 20

Reminder of definitions • A planet is a spherical object orbiting a star, is not a star itself, and has swept out its path • A dwarf planet is a spherical object orbiting a star that has not swept out its path (Pluto, Eris, Ceres, a few other TNOs), and is not a satellite. Note Pluto and Eris are also TNOs, Pluto is also a KBO, and Ceres is an asteroid. 21

Mass and Size Functions Whenever you have a large number of objects with various masses, useful to describe the number as a function of mass, N(M), or size, N(R). Constrains theories of their origin. Useful for Kuiper Belt, Asteroids, impact craters, Saturn’s ring particles, stars, gas clouds, galaxies. Often have many small objects and a few large ones, which we can try to describe with a “power law” mass function: N(M) α M-β Gives relative importance of large and small objects (not total numbers – this depends on constant of proportionality) 22

A Stellar Mass Function 23

For KBOs with measured radii, find N(R) α R-4 If they all have the same density then M/R 3 = constant, so M α R 3, so R-4 α M-4/3, and N(M) α M-4/3 Now can ask, for example, how many are there of mass M 1 vs. 10 x. M 1? N(M 1)/N(10 x. M 1) = M 1 -4/3/(10 x. M 1)-4/3 = 104/3 = 21. 5 24

Can also ask: is most of the mass in larger KBO objects or smaller ones? For example, how much mass in objects of mass M 1 vs. objects of mass 10 x. M 1? If you have N objects of mass M, the total mass is Mx. N. So for N=N(M), total mass is Mx. N(M) α Mx. M-4/3 or M-1/3. So relative mass is M 1 -1/3/(10 x. M 1)-1/3 = 101/3 = 2. 2 So somewhat more mass in lower mass objects. Recall β was -4/3. Note if β > -1, more mass in higher mass objects. 25

The mass function is a constraint on our understanding of the origin of the population. KBOs represent leftovers from planet building process. Sizes affected by • growth through collisions (and gravitational focusing when they get massive enough), • shattering by collisions, which depends on speed and composition (held together by material strength, or by gravity (rubble piles)? ), • ejection from Solar System, etc. Mass function helps us understand the importance of each process. 26

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True color, HST Animation 28