Spring 2011 - April 22
Dept of Physics & Astronomy
University of Maine, Orono, Maine
Presents
Jeff Morgenthaler, Ph.D.
Senior Scientist
Planetary Science Institute
Comets as Molecular/Atomic Physics Laboratories
Comets represent nearly ideal laboratories for the study of molecules and atoms that have long dissociative/ionization lifetimes. Interplanetary space can be likened to an infinitely large extreme-high vacuum vessel in microgravity. The Sun is a well-studied light and particle source. Comets, moving in well understood paths with respect to the Sun, release parent molecules, such as water and CO into this environment at radial velocities on the order of 1 km/s. Sunlight photodissociates and ionizes the parent molecules. Solar wind collisional processes also contribute. The parent molecules, their dissociation products and ions can interact with each other within a few thousand kilometers of most comets (the collision sphere), but then enter into a region of purely ballistic motion. Here, they are excellent test particles for interaction with sunlight and the solar wind. Molecules and atoms which exhibit strong resonance scattering of solar photons can be easily studied with remote sensing techniques. We will concentrate on OH, CO, and C. The outflow velocities of these species is well known, so the radial falloff of their emissions can be used to measure their lifetimes against all destructive processes. This provides a means of measuring these lifetimes, some of which are of order one week, and therefore
are difficult to study in earth-bound laboratories. We have used comet Machholz to measure the ionization lifetime of carbon and find it to be in agreement with calculated values. The surprising result is that previous comet research was conducted neglecting the effects of the solar wind and therefore used an ionization lifetime that was too large by a factor of up to 3. We will discuss the implications of this to cometary science and our plans to measure the CO and OH dissociative lifetimes.
Friday, April 22, 2011
3:15 pm
140 Bennett Hall
Refreshments are now served BEFORE the Colloquium talk
in Rm. 140 Bennett Hall at 3:00 pm