[meteorite-list] Unique Observations of Comet McNaught Reveal Sprinkling Nucleus

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 23 Feb 2007 23:15:29 -0800 (PST)
Message-ID: <200702240715.XAA17070_at_zagami.jpl.nasa.gov>

ESO Education and Public Relations Dept.

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Text with all links and the photos are available on the ESO Website at URL:
http://www.eso.org/outreach/press-rel/pr-2007/pr-07-07.html
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Contacts:

Colin Snodgrass, Emmanu Jehin
ESO, Chile
Phone: +56 2 463 3069, +56 2 463 3054

For Immediate Release: 23 February 2007

ESO PR Photo 07/07

The Celestial Whirligig

Unique Observations of Comet McNaught Reveal Sprinkling Nucleus

Comet McNaught, the Great Comet of 2007, has been delighting those who have
seen it with the unaided eye as a spectacular display in the evening sky.
Pushing ESO's New Technology Telescope to its limits, a team of European
astronomers have obtained the first, and possibly unique, detailed
observations of this object. Their images show spectacular jets of gas from
the comet spiralling several thousands of kilometres into space, while the
spectra reveal the presence of sodium in its atmosphere, something seen very
rarely.

Comet C/2006 P1 (McNaught) has rightly earned the title of a 'Great Comet'
-- one so bright in the sky that such an occurrence could be expected just
once in a generation (see ESO 05/07).

The problem for astronomers was that the comet stayed very close to the Sun
and so was only visible very low on the horizon, making it impossible for
most professional telescopes to study it in detail. One telescope, however,
was up to the challenge: ESO's New Technology Telescope (NTT), at La Silla.

"We had previously pointed the NTT very low to observe the planet Mercury,
which is very close to the Sun and is therefore only visible low in the sky
just after sunset. I realised that we could take advantage of the same low
pointing limit to observe the comet while it was near the Sun," said ESO
astronomer Colin Snodgrass [1].

>From the 29th January onwards, the comet was thus observed with the NTT,
revealing in detail the heart of the comet. This was no easy feat as even
with the NTT it was only observable for half an hour after sunset. Luckily,
the NTT has another major advantage: it is equipped with the versatile ESO
Multi Mode Instrument (EMMI), which can provide both imaging and
spectroscopic observations across the visible wavelength range. This meant
that the maximum amount of scientific data could be taken during the limited
time available for observations.

The unique images reveal three clear jets of gas, which are seen to spiral
away from the nucleus as it rotates, like a Catherine Wheel firework.

"These jets are produced when sunlight heats ices on the surface of the
comet, causing them to evaporate into space and create 'geyser' like jets of
gas and small dust particles, which stretch over 13,000 km into space --
greater than the diameter of the Earth -- despite the fact that the nucleus
of the comet is probably less than 25 km in diameter," explained Snodgrass.

By comparing images like this taken at different times, astronomers should
be able to calculate how fast the nucleus rotates from the changing pattern
of jets.

Other images also reveal that while the gas forms spiral jets, the large
dust particles released from the comet follow a different pattern, as they
are thrown off the comet's surface on the brightly lit side towards the Sun,
producing a bright fan, which is then blown back by the pressure of sunlight
itself.

As well as taking images, the astronomers were able to investigate which
gases were present in the comet's atmosphere [2] using spectroscopy. The
usual gaseous species have been detected, such as cyanide, carbon, and
ammonia, whose analysis will help the astronomers to determine the activity
level of the comet and its chemical type.

But already in the first high resolution spectrum obtained on 29 January,
the astronomers noted something quite unusual.

"We detected two very bright lines -- the brightest of the whole spectrum
taken on this day as a matter of fact -- close to 589 nm and quickly
identified them as belonging to neutral sodium atoms," said Emmanu Jehin
(ESO). "Further measurements showed this sodium emission to be extending
over more than 100,000 km in the tail direction and fading rapidly with
time."

Such lines have only been detected in the greatest comets of the past
century like C/Ikeya-Seki in 1965, C/West in 1976 and C/Hale-Bopp in 1997,
for which a very narrow sodium tail was even photographed. This straight
neutral tail appears in addition to the dust and ionised gas tails when the
comet is close to the Sun.

"Its origin lies most probably in the dissociation of the cometary dust
grains," said Jehin. "In very active comets, which are also usually the ones
which pass closer to the Sun, the dust grains are vaporised under the
intense heat and start releasing sodium atoms which then react to the solar
radiation and emit light -- at the very same yellow-orange wavelength of the
lamps on our streets."

Sodium has also been observed around Mercury and the Moon forming a very
tenuous atmosphere. But closer to us, at 90 km altitude in our atmosphere,
there is the so-called 'sodium layer'. The origin of that layer is not well
known but might be coming from the ablation of meteoroids that are burning
(due to their high entry speed in the atmosphere) at the same altitude. As
most shooting stars (or meteors) originate from comets (annual showers like
the Eta Aquarids and Orionids originate from comet P/Halley, the Leonids
come from comet P/Tempel-Tuttle, and the Perseids from comet
P/Swift-Tuttle), the sodium in those dust particles might just be the same.
As a kind of gift to the astronomers that layer is used by observatories
like Paranal to produce with a laser an artificial star that allows for the
correction of atmospheric turbulence!

Notes

[1] The team is composed of Colin Snodgrass, Emmanu Jehin, and Olivier
Hainaut (ESO), Alan Fitzsimmons (Queen's University, Belfast, UK), and Jean
Manfroid and Damien Hutsemers (Universitde Lie, Belgium). These results were
presented in a Circular Telegram to the International Astronomical Union
(IAU CBET 832).

[2] When a comet is approaching the Sun, the ices trapped in the small
nucleus sublimate, sometimes in the form of very strong gaseous jets,
dragging in the process a lot of dust particles into space and forming a
dusty atmosphere -- called the coma -- of several thousands of kilometers
around the nucleus. All those molecules and dust particles are then pushed
in the direction opposite to the Sun (by the solar radiation pressure),
creating the gaseous and dust tails of the comet.
Received on Sat 24 Feb 2007 02:15:29 AM PST