Research

Lunar Water

The evidence for water on the Moon arises from multiple lines of evidence (neutron probes for hydrogen, ultraviolet reflectance [LRO], and near-infrared reflectance [Chandayan/M3, Spex]), though the evidence has been considered indirect because it traces only parts of the water molecule and possible H or OH from other carriers. Using SOFIA, water was detected uniquely, because the 6 micron emissivity feature of the surface arises from a bending mode of the H2O molecule requiring all three atoms. I am part of a legacy science team that is significantly extending the discovery paper, making the first images of the distribution of water across the lunar surface. We hope to provide constraints on whether the water is inside lunar regolith materials, as opposed to being outgassed and migrating over the surface, eventually ending up at the permanently shadowed regions near the poles that are the focus of multiple upcoming NASA and international space agencies' missions. The study of natural lunar water becomes more urgent as human exploration will inevitably pollute the moon to the extent that its natural state is overwhelmed.

Interstellar Medium

One of my research projects research focuses on the nature of diffuse interstellar clouds. I use infrared and radio observations to measure the amount of dust and gas, which showed that there are clouds of molecular gas that are not bright in CO. This material is now known as "dark gas" (named by a gamma-ray astronomer). In my work, I identified isolated, nearby, 100 solar mass clouds that are not forming stars. The first paper identified the clouds and made an assessment of their contents using Planck to measure the dust content and Arecibo to measure atomic gas. We found the locations of excess infrared emission are spatially coherent and identified three hypotheses for the nature of the apparent "dark gas". The second paper showed the "dark gas" is not cold, atomic gas, which would have shown 21-cm absorption in new Arecibo observations. The third paper examined the effect of dust property evolution in diffuse clouds, which contributes to the apparent far-infrared excess but falls short of explaining all of it, which still requires molecular gas. The fourth and final paper of the series examined pairs of diffuse clouds with significant far-infrared excess and compared them to shock models, making the point that motion of the clouds through their surrounding medium leads to shocks that enhance molecule formation.

Cometary debris trails

Comets are composed of ice and rock. My work on the larger pieces of rocky material that are produced by comets suggests the comets in the inner Solar System are mostly rock. I learned this by studying the trail of debris that comets leave behind in their orbits. This is the same type of material that makes meteor showers when the Earth passes through them. Using wide-field infrared images, we can observe them in the sky even when we can't see the individual meteoroids; they are heated by sunlight and emit infrared radiation as they cool. The infrared dust trails from comets were a surprise discovery in the first infrared telescope to survey the sky, in 1983.

Here is a link to my 2007 paper describing the results of the observations, and an openly accessible preprint version of the article.

Split comet 73P/Schwassmann-Wachmann 3

Here is a link to my 2009 paper describing the results of the observations, and an openly accessible preprint version of the article. The article shows how the fragments of the comet are in three classes: the major fragments (which look like individual comets themselves, mostly with tails, and of which there are more than 50), the debris trail (which is the swarm of debris that fills the comet's orbit, like a meteoroid stream, and makes the diagonal line of brightness across the entire image), and dust (which makes the tails from each of the major fragments). The theoretical part of the paper explains the dynamics of each class of debris. The smaller fragments are strongly affected by a "rocket effect" that is caused by the gases being released from thir surfaces as they sublimate after being exposed to sunlight. The dust dynamics are dominated by radiation pressure, which sweeps them back into tails that point away from the Sun. The debris trail particles feel slight radiation pressure and are dispersed along the orbit by the combination of that slight radiation pressure and the small kick in speed that they got during release from the comet. I show that the debris trail particles must be "dry" with no ice, or else they would feel the rocket effect that so strongly dominates the small fragments seen in the Hubble images.

Independent work on this image was posted by P. Birtwhistle.

A short article in Planetary Times about this image.

A webpage with some other images from the split-up including the Hubble ones.

The exploding comet Holmes

Here is a link to my 2010 paper describing the results of the observations; an openly accessible preprint version of the article is available too.

The main thing in this paper is determining the properties of the explosion that spread the ejecta around the comet in the observed pattern. The Spitzer Space Telescope images are probably the best images of the ejecta pattern. I make a simple theoretical model for the explosion being the result of a subsurface cavern of amorphous ice, which explosively released gases until it broke through the surfcae

A short news article on the Spitzer website about this.

A nice news article in Astronomy Now.