Exploring Uranus at New Angles: Insights from New Horizons' Observations

In a recent study, Samantha N. Hasler and colleagues analyzed unique observations of Uranus captured by the New Horizons spacecraft at high phase angles, or the angle between the Sun, Uranus, and the spacecraft, as it viewed Uranus from various points in its orbit. These observations, taken in 2010, 2019, and most recently in 2023, provided insights into the energy balance, atmospheric properties, and overall appearance of the ice giant at specific angles that aren’t visible from Earth-based telescopes. Through these efforts, the team aimed to better understand Uranus as a representative of ice giant planets, which are prevalent among exoplanets in our galaxy.

Introduction

Historically, data about Uranus' heat balance and atmospheric behavior have been sparse, limited to flybys from Voyager 2 in 1986. Since then, scientists have had lingering questions about the differences between Uranus and its similar-sized neighbor, Neptune, which has a significantly higher internal heat source. By observing Uranus at high phase angles with New Horizons, researchers hoped to refine estimates of Uranus's energy budget and atmospheric scattering properties. This study also aimed to provide a foundation for interpreting data from future exoplanet imaging missions that will encounter similar challenges in capturing distant ice giants.

Methods

The research team used New Horizons' Ralph instrument, which includes the Multispectral Visible Imaging Camera (MVIC). MVIC observed Uranus in four color bands—Blue, Red, near-infrared (NIR), and a methane filter—across different seasons of Uranus’s year. The 2023 observations at a phase angle of 43.9° were supported by simultaneous images from the Hubble Space Telescope (HST) and amateur astronomers worldwide, allowing for a comparative study of Uranus at both high and low phase angles.

Observations and Data Reduction

To analyze Uranus's brightness, or flux, in each filter, the team processed images to correct for striping artifacts caused by the MVIC camera and accurately located Uranus within the images. Each observation’s exposure data was adjusted to calculate the radiance factor, or I/F, which measures how much light Uranus reflects. Hubble’s low-phase-angle data, processed for brightness, provided a reference point to compare against the high-phase-angle data collected by New Horizons.

Results

Hasler and her team found that Uranus’s brightness varied slightly across longitudes but showed consistency within the expected error margins. High-phase observations showed that Uranus was darker than predicted in the Blue and Red filters when compared to a Lambertian phase curve, which assumes even reflection. The lack of significant variation in the brightness over different regions suggests minimal atmospheric activity or distinct surface features, which aligns with Hubble’s observations that showed a relatively featureless disk, with the exception of a polar cap and a small bright spot near the pole.

Discussion

These results are valuable for future studies on exoplanets, as many ice giants, like Uranus, will be observed at high phase angles from Earth. Observing Uranus across different filters and angles provides a benchmark for interpreting similar data from distant exoplanets. The New Horizons data, combined with earlier Voyager data and recent Hubble support, gives a more comprehensive view of Uranus’s appearance and brightness variations over time and angle, crucial for future models of atmospheric behavior on ice giants.

Conclusion

This study underscores the importance of observing planetary bodies from multiple perspectives. While New Horizons observed Uranus at high-phase angles that are not accessible from Earth, support from Hubble and community observers enriched the data, confirming that Uranus lacks large-scale atmospheric variations. Such detailed observations of our Solar System’s ice giants will serve as a reference for interpreting images of similar exoplanets from upcoming missions, such as NASA’s Nancy Grace Roman Space Telescope.

Source: Hasler