Insider’s Cassini: Dr. Larry Esposito and Cassini’s Ultraviolet Imaging Spectrograph

Todd J. Barber, Cassini lead propulsion engineer

I recently enjoyed the opportunity to interview yet another Cassini principal investigator (PI) for my "Insider's Cassini" column. I've known Dr. Larry Esposito of the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP) for nearly 20 years, but this is the first chance I've had to delve into the nifty results of his Cassini instrument, the ultraviolet imaging spectrograph (UVIS).

Dr. Esposito is from Schenectady, N.Y., and unlike me can probably spell his hometown without Googling. He attended my alma mater MIT to obtain his undergraduate degree before pursuing a PhD from U-Mass Amherst. He then moved to LASP in 1977 and he's been there ever since, opining he's "never had a real job" as he climbed the ranks from research associate, lecturer, associate professor, and finally to full professor of astrophysical and planetary science. From his early work at LASP, he became "addicted to space missions" and has never looked back, working on such diverse projects as Pioneer Venus, Mars Observer, Phobos (a Russian mission), Galileo, Voyager, Mars '96, and of course Cassini. He also has made observations with the Hubble Space Telescope and has been the principal investigator for a number of Venus mission proposals. Even though he was on the imaging team for Pioneer 11 and Venus Express, most of his career has been devoted to ultraviolet spectroscopy of the planets, a perfect fit for UVIS.

UVIS is one of four Optical Remote Sensing (ORS) instruments on Cassini, similar to the ultraviolet instruments on Pioneer Venus, Voyager, and Galileo -- but with higher spatial, spectral, and time resolution. It has a significant data volume requirement (though not as much as a visible-light camera) and can provide useful observations to aid all five Cassini science disciplines -- Titan, icy moons, rings, magnetosphere, and Saturn itself.

Dr. Esposito told me UVIS is on nearly all the time, with his goal of UVIS "never being turned off" given its ability to obtain rich science data from almost every target. UVIS can take images and spectra simultaneously, basically enabling each pixel to have its own spectrum (something new on Cassini in ultraviolet wavelengths). UVIS is about the size of a small piece of carry-on luggage, with a mass of 15 kilograms and a peak power usage of only 14 watts. Speaking of carry-on luggage, I must share the anecdote of UVIS' trip to JPL from Colorado. The instrument initially went to JPL in checked baggage, but the case arrived dented. After that, it enjoyed a one-way journey to LAX in its own economy seat (next to a LASP engineer), where that ticket was charged to a "Mr. U. V. Experiment"! Fortunately, U.V. had an uneventful flight halfway across the country.

UVIS is essentially a telescope with four apertures -- two imaging spectrometers and two point detectors. The two UVIS spectrometers are called the EUV (extreme) and FUV (far) imaging spectrometers; UVIS covers a wavelength range of 55 to 192 nanometers (recall visible light roughly spans 390-750 nanometers). As Dr. Esposito put it, UVIS has a "butterflies'-eyes view," given these insects see in ultraviolet wavelengths.

The UVIS point detectors include a hydrogen/deuterium absorption cell, which measures this critical ratio important for solar system formation models (and Lyman-alpha in the hydrogen spectrum), and a high-speed photometer which observes distant stars and our sun during occultations, up to 1,000 times per second!

I asked Dr. Esposito about UVIS science results during cruise, and he had no shortage of material to discuss. UVIS observed Venus, connecting the data sets of Pioneer Venus (from the past) and ESA's Venus Express (from the future). It also calibrated its performance on Earth's moon, a well-studied target, during Cassini's 1999 Earth swing-by. UVIS briefly studied an asteroid in the main belt as well before a multi-month campaign to investigate the largest planet. It made time-variable observations of the Jovian system, thus providing "big-picture" context for Galileo's measurements. Even six months out from Saturn, UVIS began nearly continuous observations of the Saturnian system in December 2003, and made a surprising discovery of a plethora of atomic oxygen of unknown origin.

During tour, UVIS helped confirm water emanating from Saturn's icy moon Enceladus. In fact, the amount of water found was consistent with the amount of atomic oxygen measured upon Saturn approach, assuming water was breaking down into hydrogen and oxygen.

It's impossible to recount seven years of science results since Cassini began its tour of the Saturn system in a relatively brief column, so when Dr. Esposito offered a one-sentence science summary by discipline, this was too good an offer to ignore.

For the magnetosphere, UVIS found Saturn's environment is dominated by neutral species, which is unlike both Earth and Jupiter, a great surprise. For Saturn itself, UVIS has been able to measure the structure of the upper atmosphere using stellar and solar occultations.

At Titan, UVIS found connections between aerosols, haze particles, and organic molecules (methane, ethane, ethylene, acetylene, etc.) in the upper atmosphere via stellar limb occultations and reflected sunlight. For the icy satellites, UVIS was a key player in determining the morphology and composition of icy geyser jets.

"Lots of great papers" were obtained from this work, which basically showed the jets were mostly water in the form of vapor and ice crystals ejected at supersonic speeds that escape Enceladus. Finally, UVIS has contributed greatly to the knowledge base for Saturn's rings as well. Ultrafine structure was determined, sometimes at scales less than 10 meters (33 feet)! UVIS detected individual particles, clumps of particles, and waves in the rings. Many were surprised to see how dynamic the rings are, showing discernible changes since Voyager. Saturn's equinox in 2009 and the earlier UVIS discovery of wakes in Saturn's A-ring provided a new paradigm for ring studies, blowing away the idea of the rings being a flat, homogeneous collection of particles. UVIS helped measure the size and shape of ring particles, detecting F-ring clumping that "comes and goes," and (in the big picture) helped address the question of the age of Saturn's rings. Not bad for a mere seven years of work!

I closed our interview by asking Dr. Esposito about his plans for UVIS in the solstice mission. He said he was particularly interested in ring occultations over the next seven years. More than 100 of them were executed previously, and 99 more are planned, including some that will occur slowly enough to measure ring particles or clumps as small as one meter (3.3 feet) in size. He also looks forward to seeing the effects of Saturn's changing seasons on his measurements.

Finally, he's especially interested in the proximal orbits in 2017, which should allow measurements of the mass of Saturn's ring system, helping sort out questions of the age and origins of the rings themselves. Dr. Esposito concluded by lauding the mission, its technical success, all the science teams, his UVIS team in particular, and wonderful collaborations with our European partners. He said Cassini was "a helluva mission -- well designed, well managed, and well operated."

I couldn't agree more, Dr. Esposito -- thanks for your time, and may UVIS remain healthy and productive until the glorious conclusion of this storied mission.