I study planetary atmospheres and their interactions with the space environment.
Planetary upper atmospheres, which contain both a charged (the ionosphere) and neutral (the thermosphere)
component, are important transition regions between the dense meteorological atmosphere below and the
vacuum of space above. They mediate the exchange of particles, energy, and momentum between these two environments
and also host Low Earth Orbit (LEO) satellites. The study of planetary upper atmospheres helps
us to understand the evolution of atmospheres, aiding in determination of planetary habitability,
as well as to adapt and protect the technologies that society is becoming more dependent on, such
as satellite communications and positioning.
Research
Most of my research combines ground-based and space-based observations with computer models in order to improve understanding of
planetary upper atmospheres and their interactions with the space environment. A few examples of active and past areas of research are shown below.
Giant planet ionospheres and H3+
Giant planet ionospheres are dominated by a unique ion, H3+, which radiates strongly in the near-IR. This makes it an efficient probe of the properties of giant planet upper atmospheres, as it can easily be observed at Jupiter, Saturn, Uranus, and Neptune (finally!) using ground-based telescopes and now also JWST.
JWST
We have a lot of recent and upcoming JWST giant planet results. E.g., Jupiter (GO-03665, GO-08147), Saturn (GO-05308), Uranus (GO-05073), and the largest (so far) JWST Solar System program, GO-07570, which will track Uranus and Neptune for a complete solar cycle to uncover how their magnetic environments and auroral emissions respond to the changing solar wind. These programs have been led by Tom Stallard and Henrik Melin for the most part.
It took JWST less than 1 hour to do what decades of ground-based searches could not: detect H3+ at Neptune. It also revealed Neptune's IR aurora for the first time! See the paper, led by Henrik Melin. Surprisingly, JWST also revealed that H3+ densities were consistent with predictions, and instead the lack of prior detection stemmed from an atmosphere that had cooled substantially from the Voyager 2 flyby (and the power of JWST's spatial resolution). Unsurprisingly, this was a pretty big news event as well.
Some of the first JWST/NIRSpec Jupiter data (ERS-1373) revealed surprising small-scale structures in Jupiter's ionosphere above the Great Red Spot. See the paper, led by Henrik Melin.
Colleagues and I mapped the temperatures in Jupiter's upper atmosphere using NASA's InfraRed Telescope Facility (IRTF), a 3m telescope on Mauna Kea in Hawaii. We found that the hottest part of Jupiter's upper atmosphere was directly above its iconic Great Red Spot! This unexpected result may hold clues for solving the so-called "energy crisis" in giant planet upper atmospheres, and generated a suprising amount of news coverage, including radio and TV interviews.
Following the previous result, we used the 10m Keck II telescope to create global high-resolution H3+ temperature maps. We found that upper atmospheric temperatures descrease steadily from the auroral polar regions. Such a temperature gradient was also found at Saturn, thus the implication for both planets is that the strong Coriolis forces due to the rapidly rotating giant planets are overcome, and the aurorae are the dominant heat sources for the upper atmosphere! This result also generated significant press, largely due to James O'Donoghue's impressive animations and outreach.
Jupiter's H3+ Dark Ribbon
Combining >13,000 narrow-band H3+ images taken more than 20 years ago, we recently found that there is a "dark ribbon" of H3+ that identifies Jupiter's magnetic equator. This study was led by Tom Stallard, and was also a popular news item.
Jupiter's IR auroral variability
Much of our current Jupiter work is focused on providing ground-based support for the Juno spacecraft. While Juno was just arriving at Jupiter in 2016 we used the 10m Keck II telescope to monitor the response of its IR aurorae to variations in the solar wind.
Saturn's rings are draining away to the planet as a dusty rain of ice particles under the influence of Saturn’s magnetic field and gravity. This mass transfer process, first hinted at by Voyager observations in the 1980s, transforms the chemistry of Saturn's upper atmosphere, producing bright and dark bands of ionospheric H3+ at latitudes that are connected magnetically to specific regions in the rings. Using observations from the 10m Keck II telescope, combined with model simulations, we monitor the mass loss from Saturn's rings and the effect in Saturn's upper atmosphere.
Our most recent study finds that Saturn's rings are losing mass so rapidly they could potentially disappear within ~100 million years, meaning that we may be lucky to observe the beauty of this cosmically short-lived phenomenon. This study, led by James O'Donoghue, has also been covered by >100 news outlets so far.
In 2017, following an incredibly successful 13 years in orbit at Saturn, the Cassini spacecraft executed 23 orbits between Saturn's rings and its atmosphere before finally plunging dramatically to its death. These unique orbits allowed for the first in situ measurements of Saturn's upper atmosphere. One of the most dramatic suprises was that Saturn's rings also transfer a tremendous amount of mass to Saturn's equatorial atmosphere, primiarly in the form of sub-micron grains and gas. As a Cassini Participating Scientist, I analyzed the impact of this material influx on Saturn's ionosphere, finding that chemistry becomes significanly more complicated and leads to a number of unexpected heavy molecular ion species. Future ground-based observations of these ion species may be an effective means of monitoring the mass loss from Saturn's rings in the post-Cassini era.
Mercury
The Rapid Imaging Planetary Spectrometer (RIPS)
Along with Boston University colleagues Jeff Baumgardner (instrument lead) and Carl Schmidt (science lead) I have begun commissioning a unique new planetary instrument. RIPS is designed to conduct "lucky imaging" observations of extended objects, such as Mercury's extended exosphere or the torus of neutral and charged material in Jupiter's magnetosphere, originating ultimately from Io's volcanism. By taking video rate data and then only using the few crisp images that are unaffected by atmospheric seeing, RIPS can obtain high-quality simultaneous spectra and images.
In March 2018, we installed RIPS at the Perkins telescope in Flagstaff, Arizona and obtained first-light observations. Weather was poor, but when we finally had an opening in the clouds RIPS performed exactly as hoped, allowing us to map the spectral behavior of Mercury's exosphere across its disk.
RIPS was installed at AEOS in July 2018. As AEOS is an adaptive-optics telescope it can help to compensate for distortion of Mercury's light, given that nighttime observations are only possible through the long atmospheric path to Mercury after sunset (or before sunrise). We obtained a diverse set of RIPS observations at AEOS, including the exospheres of Mercury, the Moon, and Jupiter's moons, Io and Europa. This movie demonstrates the capability of RIPS to map the morphology of Mercury's Na atmosphere above its disk in only a few hundred seconds using spectral slit scans.
At Gemini North
