Webb and the Solar System

Jonathan I. Lunine, jlunine@astro.cornell.edu

The James Webb Space Telescope will peer into the depths of space and back in time to reveal the earliest epochs of our universe. But it also will look at our cosmic backyard, the Solar System, for clues to how planets form and evolve with time. Webb’s unprecedented combination of sensitivity, spatial resolution, and spectral range give it a unique set of capabilities for observing a wide variety of objects in the Solar System. With the implementation of moving-target tracking on the facility, detailed planning has begun. To that end, workshops on Webb Solar System observations were held at the 2008 and 2012 annual meetings of the Division for Planetary Sciences of the American Astronomical Society. At these workshops, planetary astronomers discussed the science potential and the challenges of observing the Solar System with Webb. This short report highlights a few of the types of observations that can be done with the facility and some challenges that must be addressed.

Kuiper Belt

In the Solar System beyond Neptune lies the Kuiper Belt, a dynamic, rag-tag collection of objects (called KBOs) left over from the formation of the Solar System. They range downward in size from the so-called dwarf planets Pluto and Eris—each about 2300 km across. KBOs have a wide range of colors, from gray to extremely red. Observations in the optical and thermal infrared, which can untangle the size and reflectivity of the brightest KBOs, indicate a broad range of visible-wavelength reflectivities, from coal black to almost perfectly reflecting. Spectroscopic studies from ground-based telescopes of the largest KBOs reveal variable compositions, with water ice, methane, nitrogen, and carbon monoxide on Pluto, but only methane detected on Eris. Most KBOs are simply too small, distant, or dark for ground-based spectroscopy to yield much information, and the wavelength range of such studies is limited to parts of the near-infrared.

With its wavelength range of 0.6–29 microns, and its ability to perform high-resolution spectroscopy (λ/Δλ = 3000), Webb will constrain isotopic ratios in the water ice and other components on Pluto and Eris, as well as provide surface-temperature monitoring through the nitrogen overtone band. Detection of non-water ices on surfaces of smaller or more distant KBOs, including the enigmatic Sedna, will be possible with Webb.  In total, Webb will perform on Kuiper Belt bodies, which have varied and complex surfaces (see Figure 1), the same range of compositional studies done now in the much closer, main asteroid belt.

Titan

The second-largest moon in the Solar System after Jupiter’s Ganymede, Saturn’s Titan has a dense and cold—94 K at the surface—nitrogen atmosphere with 1–5% methane, and a variegated surface of water ice, which is overlain by organics in solid and liquid form. Though only weakly heated by the Sun, Titan has a climate system that exhibits cloud formation, methane rain, and rivers, lakes, and seas of methane—as well as the stratospheric chemical product of methane, ethane. In some respects, the cycle of methane from surface-to-lower atmosphere mimics the hydrologic cycle on Earth, only the 29.5-year orbit of Saturn around the Sun means that the seasons on Titan—which has a tilt just a few degrees more than Earth’s—last seven years. The Cassini mission has observed seasonal changes on Titan over the past eight years, and will continue to do so until mission’s end in 2017, when about half a Titan year will have transpired. With its clouds and hazes at various altitudes, the complex nature of Titan’s atmosphere taxes the capabilities of optical, near-infrared, and mid-infrared instruments on Cassini.