Halfway Around the World for 5 Minutes of data Learning about the atmosphere of Pluto Jeff Regester Greensboro Day School

Outline History of Pluto and Outer Solar System science Occultations New Horizons

Pluto’s Discovery Discovered by accident, on purpose by Clyde Tombaugh, Lowell Observatory, 1930 PL started first of 3 searches in 1905 PL died 1916 PL became interested in astronomy, esp Mars, in 1894 Pluto disc 1930, by CT using blink comparator

The Edgeworth-Kuiper Belt proposed by Kenneth Essex Edgeworth 1943 Journal of the BAA proposed by Kuiper 1951, but thought it was long-gone, scattered by the (then-thought) Earth-mass Pluto by 1970s, rate of short-period comets too great to be from Oort cloud, plus short-period comets clustered near plane of solar system.

Leftovers

Moons Charon, discovered in 1978 by Jim Christie, USNO  MPluto = MMercury/20

Moons Nix and Hydra, discovered in 2005 by Alan Stern et al, using HST

Discovery of Atmosphere, 1988 Study of the Plutonian atmosphere by the occultation of a star on June 9, 1988. If Pluto had no atmosphere, the light curve would have vertical sides, but instead, it has sloped sides, whose exact shape can be used to determine the density profile of the atmosphere (how the scale height changes at different altitudes). Based on the density profile and the low gravity of the planet, the average molecular weight can be determined, and is about 28, which suggests that (diatomic) nitrogen molecules are the primary component of the atmosphere. Absorption at specific wavelengths also suggests the presence of small (1%?) amounts of methane. The atmosphere appears to consist of an isothermal (constant temperature) outer atmosphere, and a complex (variable temperature) lower atmosphere, possibly including an inversion layer (a region where temperature decreases with altitude, as in our stratosphere). If so, that would make Pluto the only planet other than the Earth known to have an inversion layer. (Elliot, Dunham, Bosh, Slivan, Young, Wasserman, Millis, Kuiper Airborne Observatory, Icarus)

Occultation an eclipse of a star

1992 Discovery of KBOs Jewitt & Luu 1992 QB1

In 2004...                                                                                                A comparison of the size of Sedna, the largest known KBO in 2004, to various planets and moons. (NACO Team, 8.2-meter VLT (Yepun), ESO, apod040827)

...and 2005

HST Keck

Kepler’s Third Law Dysnomia’s orbit P=16d  MEris= 1.27 MPluto Houston, we have a problem!

We’ve been here before. Ceres, Pallas, Juno, Vesta, Astraea, Hebe, Iris, Flora, Metis, Hygiea, Parthenope, Victoria, Eunomia Ceres disc 1801; vest 1807; astrea 1845; Eunomia in 1851

2006, The IAU acts goodbye “minor planets” RESOLUTION 5A The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way: (1) A "planet"1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies". 1 The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories. 3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. goodbye “minor planets”

RESOLUTION 6A The IAU further resolves: Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

Occultation Overview Temperature Pressure Extinction Flux Position in shadow Bending angle => => Refractivity =>

Chris L. Peterson, Cloudbait Observatory, Colorado

Lower Atmosphere Inversion (Ref: Elliot, Person and Qu 2003) Small Planet Case No Ray Crossing Geometric Optics Clear Atmosphere

12 June 2006 Occultation: Reconstructed Shadow Path Predicting Occultations Ain’t Easy!

At the mercy of the clouds.

Occultations 12Jun2006 18Mar2007 31Jul2007

Mt. Canopus Obs, Hobart, TAS

Luxury!

Conclusions from 2006 data Pluto's bulk atmosphere (geometry): 1988 to 2006, pressure has increased by 0.98 ± 0.09 µbar, a factor of 2.17±0.21 For N2 surface vapor pressure equilibrium, this implies an increase in surface temperature of 1.2-1.7 K. Pressures consistant between 2002 and 2006 Pluto’s upper atmosphere (model fit): Non-isothermal. dT/dr = -0.127±0.028 K/km Average (103.9±3.2 K) same as 2002 (104±2 K, isothermal fit), and 1988 (104.0±7.3 K). 99.4±3.1 K (ingress, 30.0 S, summer), 105.5±3.5 K (egress, 53.2 N, winter) despite ~1500 less insolation averaged over the winter latitude, so not tied to insolation (in a straightforward way) Pluto’s lower atm, clear assumption (inversion): As in 1988 & 2002, not isothermal. Temperature inversion around 1210 - 1220 km. Ingress & Egress are qualitatively similar, but the density perturbations differ in detail. Pluto’s lower atm, haze assumption (inversion, removing haze from model fit) Top of haze poorly constrained. Temperature perturbations qualitatively similar to those seen on Earth, Jupiter, Titan

Observations - Mt. John (Blue channel; l < ~627 nm)

Observations - Mt. John (Red channel ; l > ~627 nm)

Mt. Canopus

Observations - Portable Telescope (Tasmania)

New Horizons on its way launched 19Jan2006 on an Atlas V passed Pluto one year ago, 28Feb2007; will pass Saturn’s orbit 8Jun2008

July 13th, 2015

Questions?