University Lowbrow Astronomers

Why is a Comet when it Spins?

by Kurt Hillig
Printed in Reflections:  April, 1997.

The printing process used in producing Reflections isn’t well suited for reproducing high dynamic range images.  But everyone who’s looked at the head of Hale-Bopp recently at high power has commented on the concentric half-rings surrounding the nucleus; Peter Alway’s recent photos through the 24” scope at Peach Mountain clearly show six or seven rings, and hints of as many as nine!

Rings around cometary nuclei aren’t really all that rare - I’ve got a photo of Hyakutake that shows a ring, and many other comets have exhibited these as well.  These features all seem to share some common characteristics:  they generally cover an arc of less than 180 degrees, and they are most distinct on the sunward side of the comet.

Some have attributed these to shock waves as the gases escaping from the comets surface slam into the solar wind at relative speeds of a few hundred miles per second.  But the bow shock is actually quite a bit further from the nucleus than the rings, and it’s hard to come up with a mechanism that propagates such extreme density waves “upstream” against the ejected gas, with such remarkable uniformity.

What’s really going on here is that we’re seeing the fortuitously obvious results of two phenomena:  the nucleus is rotating, and gas and dust emission doesn’t occur uniformly across its surface.

The “evaporation” of the surface of a comet is driven by sunlight; solar intensity at Earth’s orbital distance is about a kilowatt per square meter.  Unlike the Earth, where it takes weeks for the surface to warm up in the sun, it takes only a few seconds for the comet’s surface to start evaporating when it rotates into the light (and only a few seconds to cool off again at “night”).  But the comet’s nucleus isn’t a simple sphere, it’s an irregular mass of ices, dust and rock - and not just water ice, the frozen gases include ammonia, carbon monoxide and carbon dioxide, carbonyl sulfide and carbon disulfide, hydrogen cyanide and cyanogen, sulfur dioxide and hydrogen sulfide, and many others.

Observations over the past year have clearly shown that these materials are not uniformly distributed, and research has shown that the dust, water, and carbon disulfide (among others) come off the surface at different rates, suggesting that the comet may be segregated into clumps of different materials.  Could this be the result of several generations of accretion during its formation?  We’ll have to wait until we can visit it to know for sure.  But the result of this segregation is that some parts of the surface are more active than others; and the “hot spots” are the sites where the multiple jets that have been seen in recent months arise.

What appears to be happening is that over the past several weeks, one of these hot spots has come to dominate the dust emission from the comet.  And, like water zooming out from a spinning lawn sprinkler, this one jet has given rise to a series of concentric, expanding shells around the nucleus.

There are two sites on the World-Wide Web where you can see this.  Terry Platt of Starlight Xpress (a color CCD camera manufacturer) took a series of 12 color images between 1900 UT on March 28 and 0415 UT on the 29th, using a 12.5” tri-schiefspiegler at f/20.  Point your browser at http://www.demon.co.uk/astronomer/ (this page no longer seems to be available) to see an MPEG movie made from these images.  Also, you can go to http://www2.jpl.nasa.gov/comet/anim22.html to see a similar movie made by Brad Wallis.  Terry Platt describes his images like this:

“The process seen is the formation of an intense ‘C’ shaped arc of dust which rotates and expands over about 5 hours on the sunlit side of the nucleus.  As the jet moves into darkness, its activity rapidly subsides and so very little is seen on the ‘dark’ side.  The ‘C’ arc continues to expand and becomes a new ‘hood’ after about 10 hours, after which the process repeats as the jet comes back into sunlight.  Most of the activity is on the ‘afternoon side’ of the nucleus and this leads to the asymmetric appearance of the coma.”

Peter Alway measured the separation between the shells to be ca. 1.1 mm on the negatives taken with the 24” scope; this corresponds to a separation of about 7300 miles, and given HB’s observed rotation period of 11.47 hours, they are found to be expanding at about 650 MPH.  Interestingly, this is fairly close to the average speed of a gas molecule at a temperature of 200K, and a very reasonable number if the dust is being driven by evaporating gas from the comets surface.  I haven’t tried to calculate the effective pressure at the surface; if you care to try, you can assume the comet is 25 miles across, and it’s observed to be evaporating at a rate of ca. 100 tons of gas per second...

Incidentally, researchers have now seen spectral signatures of silicate minerals and of sodium atoms (presumably boiled off from the dust grains by sunlight) in Hale-Bopp, along with dozens of small molecules and ions - many never seen before in comets.

[A photograph of the comet rings within Hale-Bopp by Peter Alway.  This is one of the photographs mentioned above].

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This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
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