P L U T O

Ken Derham

27 October 2016 Halesworth U3A Science Group

Illustrations reproduced from publicly available web pages of NASA and Wikipedia, with thanks

DISCOVERY OF PLUTO

• Pluto was discovered in 1930 by Clyde Tombaugh

• At the Lowell Observatory in Flagstaff, Arizona

• A “ninth” planet had previously been postulated by

Percival Lowell in 1905

• Named after the god of the underworld by 11 year

old English girl, Venetia Burney

BLINK COMPARATOR

SIZE AND DISTANCE OF PLUTO

• Pluto's diameter is 2374 km

• It is 40 AU / 7.5 billion kilometres from Earth

• Light (& radio waves) take 7 hrs to reach Earth from Pluto

• Pluto takes 248 earth years to orbit the sun

GRAPHIC REPRESENTATION OF SOLAR SYSTEM

BUT SOLAR SYSTEM IS MOSTLY SPACE

• If we tried to build a scale model of the solar system with Earth represented by a pea

• Jupiter would be 300 metres away

• Pluto would be 2.5 kilometres away and the size of a bacterium

• Our nearest star, Proxima Centuri, would be 16,000 km away

Bill Bryson – A Short History of Nearly Everything (2003)

STATUS OF PLUTO

• General Assembly of the International Astronomical Union (IAU), Prague August 2006

• Michael Rowan-Robinson represented UK as President of the Royal Astronomical Society

• Heated debate among astronomers over several days on the definition of a Planet

• Final debate chaired by Jocelyn Bell

• New definition of a Planet as a celestial body which:

• is in orbit around the Sun,

• has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and

• has "cleared the neighborhood" around its orbit.

• Pluto demoted to “dwarf planet” as it is one of several “trans-Neptunian” objects

Michael Rowan-Robinson – Stars ‘n tides

ORBIT OF CHARON AROUND PLUTO

NEW HORIZONS

• New Horizons space probe was launched on January 19, 2006

• New Horizons proceeded to Jupiter, making its closest approach on February

28, 2007, at a distance of 2.3 million kilometers

• The Jupiter flyby provided a gravity assist that increased New Horizons'

speed

NEW HORIZONS

• On January 15, 2015, the New Horizons spacecraft began its approach phase

to Pluto.

• On July 14, 2015, it flew 12,500 km above the surface of Pluto, making it the

first spacecraft to explore the dwarf planet.

• Having completed its flyby of Pluto, New Horizons has maneuvered for a

flyby of Kuiper belt object 2014 MU69 expected to take place on January 1,

2019, when it is 43.4 AU from the Sun

NEW HORIZON’S TOP 10 DISCOVERIES

• The complexity of Pluto and its satellites is far beyond what we expected.

• The degree of current activity on Pluto’s surface and the youth of some surfaces on Pluto are

simply astounding.

• Pluto’s atmospheric hazes and lower-than-predicted atmospheric escape rate upended all of the

pre-flyby models.

• Charon’s enormous equatorial extensional tectonic belt hints at the freezing of a former water

ice ocean inside Charon in the distant past. Other evidence found by New Horizons indicates

Pluto could well have an internal water-ice ocean today.

• All of Pluto’s moons that can be age-dated by surface craters have the same, ancient age—

adding weight to the theory that they were formed together in a single collision between Pluto

and another planet in the Kuiper Belt long ago

www.nasa.gov/feature/one-year-later-new-horizons-top-10-discoveries-at-pluto

NEW HORIZON’S TOP 10 DISCOVERIES

• Charon’s dark, red polar cap is unprecedented in the solar system and may be the

result of atmospheric gases that escaped Pluto and then accreted on Charon’s

surface.

• Pluto’s vast 1,000-kilometer-wide heart-shaped nitrogen glacier (informally called

Sputnik Planum) that New Horizons discovered is the largest known glacier in the

solar system.

• Pluto shows evidence of vast changes in atmospheric pressure and, possibly, past

presence of running or standing liquid volatiles on its surface – something only seen

elsewhere on Earth, Mars and Saturn’s moon Titan in our solar system.

• The lack of additional Pluto satellites beyond what was discovered before New

Horizons was unexpected.

• Pluto’s atmosphere is blue.

www.nasa.gov/feature/one-year-later-new-horizons-top-10-discoveries-at-pluto

COMPLEXITY OF PLUTO’S SURFACE

• A year-long download of data from New Horizons include images with resolutions of

400 meters per pixel

• They show new features as diverse as:

• Possible dunes

• Nitrogen ice flows that apparently oozed out to mountainous regions into plains

• Network of valleys thay may have been carved by materials flowing over Pluto’s surface

• “The surface of Pluto is every bit as complex as that of Mars”

• “Pluto is showing a diversity of landforms and complexity of processes that rival

anything we have seen in the solar system”

• “If an artist had painted this Pluto before our flyby, I would have called it over the top

– but that’s what is actually there”

Pluto’s 1,000 km wide nitrogen glacier is the largest known glacier in the solar system

SPUTNIK PLANUM

J

GEOLOGY OF PLUTO

• Pluto’s surface is composed of more than 98% nitrogen ice

• with traces of methane and carbon monoxide

• The face oriented towards Charon contains more methane ice

Whereas the opposite face contains more nitrogen and carbon monoxide ice

• Distribution of volatile ices is thought to be season-dependent and influenced by

“solar insolation”

• Pluto is one of the most contrastive bodies in the Solar System

• color varies between charcoal black, dark orange and white

• New Horizons found that Pluto's surface age is equally variable, with ancient, dark,

mountainous terrain being found alongside the bright, flat, effectively craterless

Sputnik Planum and various terrains of intermediate age and color

INTERNAL STRUCTURE

ATMOSPHERE

• New Horizon shows Pluto’s atmosphere extending as far as 1,000 miiles (1,600 km)

above the surface of the planet

• Previously only observed some 170 miles above the surface

• Shows seasonal changes, partly caused by the orbital and axial rotation of Pluto

• Surface pressure of the atmosphere is approx. 1 Pa (1/100,000th of earth’s)

• Temperature on surface is 40-60K

• Temperature increases with altitude to ~110K due to “greenhouse effect” of

methane in the predominantly nitrogen atmosphere

HAZE

• Despite the low density of atmosphere, there is

appreciable haze

• Probably consists of particles of non-volatile compounds

• Approx 20 layers of haze visible with about 10 km between

the layers

PLUTO’S BLUE ATMOSPHERE

• The blue ring around Pluto is caused by sunlight

scattering from haze particles common in Pluto's

atmosphere;

• scientists believe the haze is a photochemical

smog resulting from the action of sunlight on

methane and other molecules, producing a

complex mixture of hydrocarbons such as

acetylene and ethylene.

• These hydrocarbons accumulate into small

particles – a fraction of a micrometer in size –

which scatter sunlight to make the blue haze.

• The new infrared image, when combined with

earlier images made at shorter, visible

wavelengths, gives scientists new clues into the

size distribution of the particles

CHANGES IN ATMOSPHERIC PRESSURE

• Pluto shows evidence of vast changes in atmospheric

pressure

• and, possibly, past presence of running or standing liquid

volatiles on its surface

• – something only seen elsewhere on Earth, Mars and

Saturn’s moon Titan in our solar system

CHARON

• Charon’s equatorial extensional tectonic belt suggests the freezing of an

internal water ocean inside Charon in the distant past

• New Horizon’s view of Charon shows “pull apart” tectonic faults, expressed as

ridges, scarps and valleys – sometimes reaching 4 miles (5.5 km deep)

• Outer layer of Charon is primarily water ice

• It is thought that Charon could once have been warm enough to melt the ice

deep down

• But as Charon cooled the water froze and expanded lifting the outermost

layers

• A close-up of the canyons on Charon. Multiple views taken by New Horizons as it passed by Charon allow stereo measurements of topography, shown in the color-coded version of the image. The scale bar indicates relative elevation.

• Credits: NASA/JHUAPL/SwRI

CHARON’S DARK POLAR CAP

• Charon’s dark red polar cap is unprecedented in the solar system

• It may be the result of atmospheric gases that escaped from Pluto

• Thought to be caused by methane

AGE OP PLUTO’S MOONS

• All of Pluto’s moons that can be age-dated by surface craters

have the same, ancient age

—adding weight to the theory that they were formed

together in a single collision between Pluto and another

planet in the Kuiper Belt long ago