Comets1

Asteroids & Some Comets Visited by Spacecraft 2

Comparison of Comet Nuclei Imaged by Spacecraft 3

Kuiper Belt & Oort Cloud4

Since the early 1990’s, scientists have discovered a number of objects with orbits beyond Pluto.

Many of these objects are found to reside in a ‘disk’ that is more or less parallel to the ecliptic of our Solar System.

This region is the Kuiper Belt.

Other objects are in highly inclined and/or elliptical orbits and are in the ‘scattered disk’.

A region known as the Oort cloud is hypothesized to exist and contain numerous comets.

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image: NASA/JPL-Caltech

The Oort Cloud- interstellar space- Voyager 1 will reach it in ~300 years (but will not have power)

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Most comets have highly elliptical and long period orbits. They are believed to come from the Oort cloud that surrounds the solar system.

In contrast, short period comets (ones that return relatively quickly) may come from the Kuiper belt.

Comets in the Kuiper belt have orbital paths that are in the same plane as the planets, whereas the orbital planes of comets from the Oort cloud are inclined at a variety of angles relative to the orbital plane of the planets (hence ‘cloud’).

Comets: Overview

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As comets approach the Sun, the ices sublimate and produce a thin shell of vapor (a coma)and often a tail.

In addition to a dust tail, gases that come off of the comet are ionized and produce a plasma tail.

After many passes by the Sun, a comet can become depleted in volatiles and no longer exhibit a bright tail.

When this happens it may not be visible to the naked eye from Earth.

Comets: Overview

8Comets: Overview

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image: NASA/JPL

Comets: Overview 11

Comets are objects that are rich in gases, ices, and small rocky fragments (dust).

They tend to have highly elliptical orbits and can go hundreds of AUs from the Sun.Their return time to Earth can be several years or thousands of years.

The volatiles include water, ammonia, CO2, CO, and others. Comets are often thought of as ‘dirty snow’ or ‘icy dirt’. They also contain a variety of organic compounds.

Impact flash on Tempel 1 during Deep Impact mission

Comet Wild 2 & Stardust 12

The Stardust mission collected material from the coma of comet Wild 2.

In addition, it acquired images of the comet’s nucleus.

The material collected has since been returned to Earth and is being examined by scientists to understand the composition of the early Solar System.

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image: NASA/JPL-Caltech

Aerogel (pictured at left, held by Dr. Peter Tsou of JPL) was used to capture the comet particles.

Very strong, lightweight material, ~99% air.

Very low thermal conductivity.

stardust.jpl.nasa.gov

Comet Wild 2 & Stardust

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Capsule (and comet material) returned to Earth on Jan. 15, 2006.

Tracks in aerogel are used to find the comet (and other) particles.

Collectors captured cometary material as well as interplanetary dust particles (IDPs).

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Comet material includes a range of minerals formed under a wide range of temperatures!

Comet Wild 2 & Stardust

Organic compounds present, including N-bearing forms, aliphatic (hydrocarbon chains) that are larger than found in ISM, and even glycine (an amino acid; also present in C-chondrites).

16Comet Wild 2 & Stardust

What does the presence of high-temperature minerals (CIA-like phases) mean for origin of cometary material? Aren’t comets formed in cold regions?

ANRV309-EA35-19 ARI 20 March 2007 17:9

Figure 2Equilibrium diagram showing which minerals are stable between 900 and 1800 K in a nebulaof solar composition at 10!3 bar (after Davis & Richter 2003). At 900 K, half the atoms (0.55)in a CI chondrite are in minerals; S and other volatile elements are in the gas. Minerals stableabove 1400 K are found in refractory inclusions; minerals stable below 1400 K predominate inchondrules and matrix material. Only three minerals condense entirely from the gas oncooling—corundum (Al2O3), forsterite (Mg2SiO4), and Fe,Ni metal—the remainder form byreaction between solids and gas. Liquids are unstable unless the total pressure or the dust/gasratio is increased 10–100 " .

probably formed in under a few hundred thousand years (see below) in a uniquely 16O-rich environment during the most energetic phase of disk evolution (Wood 2004).They commonly have outermost rims of forsterite with minor refractory minerals thatclosely match the mineralogy of AOAs, suggesting that AOAs and forsterite rims onCAIs are coeval and postdate CAIs. Preliminary Al-Mg dates suggest that AOAs maybe #0.5 Myr younger (Itoh et al. 2002). Both types of refractory inclusions formedin a highly reducing (solar), 16O-rich environment before chondrules. Roughly equalproportions of AOAs and CAIs are present in nearly all chondrite groups, but theirtotal volume varies enormously from 0.01 to 10% (Table 1). CAI sizes in each chon-drite group are very roughly correlated with chondrule size, although the range in agiven chondrite may be very large. CAIs show mass-independent isotopic anomaliesthat are larger than those in chondrules; however, they are much smaller than those in

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17Comet Wild 2 & Stardust

What does the presence of high-temperature minerals mean for origin of cometary material? Aren’t comets formed in cold regions?

Material from inner solar system (high-T regions near Sun) were mixed with materials farther out.

No evidence that 26Al was present; solids formed ~1.7 Ma after CAIs! This means that mixing was happening for a while during solar system evolution.

Comet Tempel 1 and Deep Impact18

The Deep Impact mission traveled to comet Tempel 1 and sent a projectile into the surface, creating a flash and ejecting material.

Instruments looked at the ejected material and found evidence for ices (blue regions in upper right image), silicate minerals, clays, carbon, and organic material.

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The comet before impact, the flash during and immediately after impact, and the comet after impact.

Comet Tempel 1 and Deep Impact

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Arrows a and b point to large, smooth regions.

The impact site is indicated by the third large arrow.

Small grouped arrows highlight a scarp (a cliff or steep slope along the edge of a plateau) that is bright due to illumination angle. They show a smooth area to be elevated above the extremely rough terrain.

1 km

Comet Tempel 1 and Deep Impact

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The impactor created a ~100 meter crater in Tempel 1, but the images of the crater were not great and NASA/scientists wanted something better.

It was decided that the Stardust mission could be sent to Tempel 1 to re-image the crater.

It arrived 5 years ago, on February 15, 2011.

Comet Tempel 1 and Deep Impact

Stardust @ Tempel 1

22Comet Tempel 1 & Deep Impact & Stardust

pre-impact (Deep Impact) post-impact (Stardust)

Image credit: NASA/JPL-Caltech/University of Maryland/Cornell

Comet Hartley 2 (Deep Impact Mission)23

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Imaged by Deep Impact in Nov. 2010, Hartley 2 has large chunks of ice that are swarming around the nucleus, kind of like tiny comets around the main comet. Some are basketball size.

This comet is apparently very rich in CO2 ice (dry ice), and sublimation of this ice produces the large jets coming out of the nucleus.

Comet Hartley 2 (Deep Impact Mission)

The Rosetta Mission25

Target: Comet 67P/Churyumov-GerasimenkoLaunch: March 2004

Arrival: August 2014 Landing: November 2014

26The Rosetta Mission

27The Rosetta Mission

28The Rosetta Mission

29The Rosetta Mission

H isotopes (D/H ratios) from 67P suggest cometary water is not the source for water on Earth, but the debate will continue!

How variable is cometary water? What processes affect isotopes?