Research

Evolution of Distant Activity

Motivation

Long-period comets are some of the most pristine objects in our solar system. Spending most of their lifetime in the Oort cloud until perturbed inward by passing stars or galactic tides, they are believed to have undergone minimal processing, retaining much of their original volatile inventory. Therefore, these objects are valuable probes for connecting observed cometary properties to conditions in the pre-solar nebula and subsequent evolution, as they are considered to be mostly primitive with limited solar heating prior to discovery.

In the inner Solar System, comet brightness evolution can be characterized by an activity index (n) of roughly 2–4. Differences in brightening behavior between dynamically new and returning comets have been noted since the introduction of the Oort cloud hypothesis, with several studies finding that returning comets typically brighten more rapidly. However, these conclusions are based almost exclusively on observations within ~3 au of the Sun, where activity is dominated by water-ice sublimation.

Modern sky surveys are now discovering comets at distances beyond 5 to 10 au from the Sun, a range expected to increase with the upcoming Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST). However, the mechanisms driving comet activity at such distances remain poorly understood, as temperatures are too low for efficient water ice sublimation, the usual driver closer to the Sun (within 3–5 au). At these larger distances, activity is likely driven by the sublimation of more volatile ices such as CO and CO2, or by phase transitions in amorphous water ice. But the relative importance of these processes remains uncertain. Understanding the drivers of distant activity is essential not only for modeling the physical evolution of cometary nuclei, but also for interpreting future discoveries and planning future observations, including spacecraft missions like ESA’s Comet Interceptor. One way to gain insight is through the characterization of the observed brightening behavior of comets as they approach the Sun using well-calibrated photometry over a wide range of heliocentric distances.

LCO Outbursting Objects Key (LOOK) Project

My research is closely tied to the LCO Outbursting Objects Key (LOOK) Project, which is organized around two complementary efforts to understand activity and outbursts in small Solar System bodies using the global Las Cumbres Observatory telescope network. The first focuses on long-term, systematic monitoring of long-period comets; I lead this monitoring program, which provides uniform photometric measurements as comets evolve across a wide range of heliocentric distances. The second effort leverages alerts from wide-field surveys to identify and rapidly follow up outbursting objects across comets, asteroids, and centaurs. Together, these projects enable population-level studies of activity onset, outburst frequency, and evolutionary state, while also serving as a testbed for observing strategies and analysis tools needed for the Rubin/LSST era and future mission support.

Read the Year 1 Overview (Lister et al. 2022)

Long-term Monitoring of Distant Comets

My first LOOK paper uses the initial three years of uniform monitoring data to provide the first population-level view of how long-period comets brighten and change color as they approach the Sun. Using time-series photometry for 21 comets discovered beyond 5 au, I show that comets brighten more rapidly at large heliocentric distances, with the brightening rate systematically slowing closer to the Sun, and that this behavior is similar for dynamically new and returning objects. At smaller distances (< 3 au), comets exhibit consistent color changes indicative of increasing gas production driven by water-ice sublimation. Notably, returning comets undergo this transition closer to the Sun than dynamically new comets, suggesting differences in volatile layering or dust-to-gas ratios shaped by prior solar passages.

A New Empirical Brightening Model

The population-level trends revealed by the first years of LOOK monitoring motivated the development of new empirical models to better describe comet brightening at large heliocentric distances. Using the uniform, well-sampled LOOK light curves, I compared the traditional power-law brightening model to a more flexible framework in which the brightening rate varies with heliocentric distance. This variable-slope model captures the observed behavior that many comets brighten more rapidly far from the Sun, with the rate decreasing as they approach perihelion, and it consistently provides improved fits beyond ~3 au. By better reproducing early activity and enabling more reliable extrapolation from distant observations, this approach improves brightness predictions critical for follow-up coordination, survey interpretation, and mission planning.

Evolution of C/2024 E1 (Wierzchos) with JWST

Building on results from ground-based monitoring, we were awarded Director’s Discretionary Time with JWST (PI: Snodgrass) to observe the dynamically new comet C/2024 E1 at three epochs as it approaches the Sun. These observations provide a rare opportunity to directly probe the composition and evolution of a distant comet during the early stages of activity, linking changes in gas and dust production to its photometric and morphological evolution. By combining JWST spectroscopy with contemporaneous ground-based monitoring, this program will place strong constraints on the physical drivers of distant activity and provide critical context for interpreting future comet discoveries.

First JWST Spectrum at 7 au from the Sun

Results from the first JWST epoch have now been published. Observations of C/2024 E1 at 7 au with JWST/NIRSpec reveal water ice in the coma and clear evidence for CO2-driven activity, with no detectable emission from water or CO. The absence of CO (despite its higher volatility) suggests early depletion of near-surface CO prior to emplacement in the Oort Cloud. Analysis combining all three epochs to track the comet’s evolution toward the Sun is in progress.