Comets
The following passage is adapted from an article entitled “What (Maybe) Didn’t Kill the Dinosaurs” (© 2009 by John Matson).
- The chunks of ice and dust
- that make their home in the
- Oort cloud, far beyond the
- orbit of Pluto, sometimes
- become dislodged and head
- into the solar system as
- streaky comets. Some
- disruptions, caused by
- passing stars and other
- interactions with the Milky
- Way galaxy, are severe enough
- to send Oort comets into orbits
- that buzz or even collide with
- Earth. New simulations
- have revealed a novel
- mechanism for their entry into
- our part of the solar system, a
- method that also suggests that
- comet showers may not have
- been strongly involved in
- major extinctions on Earth.
- Comet dynamics depend heavily
- on Jupiter and Saturn: their
- huge gravitational fields tend to
- keep objects away from Earth.
- Comets that manage to skirt
- Jupiter and Saturn, the
- conventional thinking goes,
- had to have originated in the
- outer reaches of the Oort cloud,
- where perturbations from
- outside the solar system can be
- felt most strongly and are writ
- large across vast cometary orbits
- that take hundreds of years to
- complete. Only during comet
- showers caused by close stellar
- passages, the theory holds, have
- extreme gravitational disruptions
- brought inner Oort cloud comets
- into the mix.
- A computer model by Nathan Kaib
- and Thomas Quinn has upended
- this thinking. They have found
- that the comets that manage to
- cross the Jupiter-Saturn barrier
- do in fact originate in large
- numbers in the inner Oort cloud,
- even in the absence of a large
- disruption causing a comet
- shower. Specifically, they found
- that the relatively nearby objects
- of the inner Oort cloud can be
- kicked into the reaches of the
- outer cloud via interactions
- with the massive planets. Those
- newly far-flung comets,
- suddenly enjoying a longer orbit
- and greater gravitational
- perturbations from interstellar
- space, can find their orbits so
- changed that, by the time they
- pass through the planetary
- region again, they slip past the
- gas giants. “They can basically
- hop over the Jupiter-Saturn
- barrier,” Kaib says.
- Kaib and Quinn estimate that
- more than half of the comets we
- observe streaking in from the
- Oort cloud reach our
- neighborhood via this route,
- and other researchers agree the
- simulation appears valid. “This
- mechanism, this dynamical
- path, as we call it, could work
- and could be a significant
- contributor,” says Paul
- Weissman, a senior research
- scientist at the NASA Jet
- Propulsion Laboratory.
- The new research presents a
- route for comet production “that
- goes some way” toward resolving
- discrepancies between the
- standard model and the
- observations, says Scott
- Tremaine, an astrophysicist at
- the Institute for Advanced Study
- in Princeton, N.J. “One of the
- issues is that the conventional
- view of the cometary formation
- process is so inefficient; in order
- to produce the number of comets
- that we see, you’d need a really
- massive protoplanetary disk, one
- that appears to be incompatible
- with our best estimates from
- other sources.”
- Kaib and Quinn used their
- newfound mechanism, as well as
- the number of observed comets,
- to estimate an upper limit on
- how much material could be in
- the inner Oort cloud. They then
- produced a statistical model of
- how many comets would have
- hit Earth in comet showers in
- the past several hundred million
- years. Their conclusion: large
- cometary showers were few and
- hence probably did not cause
- more than one extinction event.
- Using cometary dynamics to
- unwind the extinction history on
- Earth will likely meet with some
- controversy. Weissman notes that
- the extinction implications of Kaib
- and Quinn’s analysis would
- involve comet showers, not comets
- in general, and that even a
- diminished profile of showers
- does not rule out the role of comets
- in extinctions. One big strike is all
- that’s needed to, he points out.