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Triton: The World that should not be.

Adric Riedel

Things that are wrong with Triton

• Wasn’t officially named for nearly 100 years

• Largest retrograde orbiter in the solar system

• Has an atmosphere• Has geysers• Looks like a cantaloupe• Might even have an underground ocean

or magnetic field

History of Triton: Discovery

• Discovered less than a month after Neptune itself, by William Lassell

• Unusual orbit was spotted very soon after discovery, possibly Neptune rotated retrograde

• By the 1920s it was known that Triton does indeed rotate retrograde relative to Neptune

Properties of Triton (Voyager)Discovered: October 10,

1846 (William Lassell)

Visited: August 25, 1989(Voyager 2)

Radius: 1,350 km Mass: 2.14e+22 kg

Distance from Neptune:

14.3 RN

354,800  km

Surface Gravity:

0.78 m/s2

Orbital Period: -5.87685 (retro) 165 year orbit (Neptune)*

Density: 2.07 gm/cm3

Rotation Period:

-5.87685 (tidal locking)*

Albedo: 0.78 (0.62-0.89)

Obliquity (relative to Neptune):

157.35° Surface Temp:

-235°C/ 38°K*

Orbital Eccentricity:

0° Magnitude:

13.47

A History of Triton: Properties• Triton was not officially named until 1948,

when Gerard Kuiper discovered Nereid (Wikipedia)

• Long believed to be the largest moon in the solar system

• In 1936, R. A. Lyttleton published a paper suggesting Pluto was an escaped moon of Neptune, and the event that separated it reversed Triton’s momentum. This is impossible; Triton is actually more massive.

A History of Triton: Atmosphere

• In 1978, astronomers detected a methane (CH4) band in Triton’s spectra; interpreted as a thin atmosphere.

• Further observations showed it was methane ice

• Triton is yellow-pink! (due to methane)• In 1980, Nitrogen is detected on Triton• Triton’s Nitrogen believed frozen or in

pools

Voyager 2• Encountered Neptune on August 25,

1989• Triton was imaged during late spring

in its southern hemisphere• Voyager 2 found an atmosphere• Pressure: 14×10-9 bar• Temperature: 38°K

• Composition: 99.9% N2, 0.01% CH4

The Interior• Believed to be differentiated rock and

ice (Ruiz, J.; Torices, A. 2000)• Some models (Ruiz, J.; Torices, A.

2000) suggest temperatures of 176 K between different layers of ice, sufficient to melt ammonia-methane ice.

• Possible tidal heating• Practically nothing is known for sure

The Surface• Surface of Triton appears to be new;

oldest is the cantaloupe terrain. Dimpled seems to be the youngest. (S.K. Croft, 1990)

• Surface is entirely ice, apparently a smooth, reflective glaze (no fine particle effects observed)

• Cryovolcanism via as-yet unknown mechanism

• Cryovolcanism possibly linked the odd cantaloupe terrain

http://www.arcadiastreet.com/cgvistas/images/sunrise_on_triton.jpg

The Surface• Wide variety of terrain• Cantaloupe terrain

possibly due to tectonics (S.K. Croft 1990)

• Possibly due to collapse• Some dimples (#5, #6,

#7) appear volcanic(S.K. Croft 1990)

The Surface• The current active area (as of 1989) is the

south pole, which seems to be retreating

• There is more N2 in the south polar cap than there is in the atmosphere (Triton’s atmosphere is the relocation of the polar ice caps)

• More cantaloupe terrain may be discovered underneath

• Cap disputed by Schenk & Moore (1993)

The Atmosphere: Structure• Various proposed thermal

structures (Elliot et al. 1999) based on occultation.

• Note the area from 50-25 km. Elliot had to assume that to allow for the known (by Voyager) surface temperature profile. The cooling process is unknown, or Triton may not be in atmospheric equilibriumhttp://www.cosmographica.com/gallery/portfolio/portfolio201/images/212-TritonSurface.jpg

(Elliot et al. 1999)

The Atmosphere: Structure

• Principal model is solar heating, conducted down to the surface where it’s radiated away.

• Geyser plumes level off at 8 km, clouds seen at 30 km. (Olkin 1997)

• Troposphere to 10 km (more recently 50 km, with energy radiating away above the surface) (Elliot et al. 1999). The lower regions are not homogenous (Elliot, 1998).

The Atmosphere: Structure• Apparent ionosphere from 125-130 km

(Stevens et al. 1992, Elliot et al. 1999)• Thermosphere is spherically

symmetric and isothermal (?) above 400 km (Stevens et al. 1992)

• Thermosphere reaches peak temperature of 102 K at 600 km (Olkin et al. 1997 using the early Voyager model), drops to 96K (75K with old model)

The Atmosphere: Future• Olkin et al. (1997) find that either the

atmosphere was twice as thick at 50 km during a 1995 occultation or the models are wrong

• Initially believed to be evidence of asymmetric atmosphere due to high winds and gravitational anomalies (unlikely given the 5.9 day rotation) (Elliot 1997)

• Eventually deemed to be an actual temperature (+5K) and pressure (x1.8) increase (Elliot 1998, Sicardy 1998)

The Future• Currently accepted hypothesis for Triton

is that it’s a captured Kuiper Belt Object like Pluto.

• This explains the similarities to Pluto/Charon and the retrograde motion

• The current model requires Triton to hit one of the original Neptunian sattelites, and then pick off most of the rest as its orbit stabilized

The Future• Triton is tidally locked to Neptune, and

may be subject to tidal heating (if not, it will be soon)

• The tidal forces may be responsible for the unusually high winds seen on Neptune

• Triton’s orbit seems stable enough that Triton won’t be destroyed in the next 5 Gyr

The Future• Proposals have been made since 2000 to fund a

return mission to Neptune, and determine the state of the rings, the composition and structure of Triton’s atmosphere, electromagnetic fields and physical composition. (Hammel et al. 2002 and subsequent)

• Current hopes are for a Cassini/Gallileo style orbiter mission, scaling from a simple orbiter to a mission with a Triton lander, Triton orbiter, Neptune Orbiter and Neptune atmospheric probe.

• Since Triton seems to be entering summer, we have time…

Bibliography• Croft, S.K. (1990) LPI 21 248C• Elliot, J.L (1997) hst.prop 3833E• Elliot, J.L (1998) STIN 9914316E• Elliot, J.L. et al. (1999) Icarus 143 425-428• Hammel, H.B. et al. (2002) ASPC 272 297H• Herbert, F. & Sandel, B.R. (1991) JGR 9619241H • Olkin et al. (1997) Icarus 129 178-201• Schenk, P. & Moore, J.M. (1993) LPI 24 1247S• Sicardy, B. et al (1998) DPS 30 4902S• Rothery, David. “Sattelites of the Outer Planets: Worlds in their own right”.

2nd ed. New York, Oxford: 1999• Ruiz, J. & Torices, A. (2000) LPI 31 1298R• Weissman, Paul; McFadden, Lucy-Ann and Johnson, Torrence.

“Encyclopedia of the Solar System”. San Diego, Academic Press: 1999• Wikipedia. “Triton (moon)”. http://en.wikipedia.org/wiki/Triton_%28moon

%29. January 29, 2005.Images from http://www.solarviews.com/eng/triton.htm and

http://www.tharsisartworks.com/Space%20Art%20Pages/neptune%20page.htm