NICER / ISS Science Nugget
for December 2, 2021
Swapping Charge with the Sun
How can comets - cold, icy objects left over from the formation of our Solar System - produce up to a gigawatt of luminosity in X-ray light that is typically produced by gas with a temperature of millions of degrees? When heated by the Sun, comets form large, unbound atmospheres that consist mostly of H2O, CO2, and CO. Comets emit X-rays because highly charged ions in the solar wind capture one or more electrons from the atmosphere's neutral gas molecules when they collide. This process is known as charge exchange. These ions, such as O7+ and C6+, are stripped of all or most of their electrons, and they typically capture electrons into a highly excited quantum state. When the ions relax, they emit X-ray photons. Charge-exchange emission occurs anywhere an ionized plasma encounters a cold, neutral gas; it has been observed in planetary atmospheres, the geocorona, supernova remnants, and star-forming regions.
Charge-exchange emission is highly diagnostic of the plasma conditions at the comet, such as the velocity and ion composition of the solar wind and the chemical composition of the comet's neutral gas. Comets provide a unique environment that cannot be replicated in the lab; therefore, comets provide natural laboratories to study atomic and molecular physics. A research team led by Dr. D. Bodewits (Auburn University) is using NICER to study the interactions between comets and the solar wind. NICER is currently observing an exciting object: comet 67P/Churyumov-Gerasimenko (67P for short), which was the target of ESA's Rosetta mission. Rosetta carried an extensive suite of instruments (including plasma diagnostics capability) to 67P, orbited the comet for two years, and dropped a lander on its surface. At the time, the comet was poorly visible from Earth, but is now relatively nearby (~60 million km). NICER observations are providing global context to the local measurements conducted by Rosetta, revealing how comets respond to time-variable solar-wind conditions (flares, shocks, coronal mass ejections), and investigating the source of some surprising emission features at X-ray energies higher than those expected from the solar-wind charge-exchange process.
Figure: The X-ray spectrum of Comet 67P obtained by NICER clearly shows emission features consistent with charge exchange. The measurement (red) significantly exceeds the modeled background (green). Emission lines visible in the spectrum correspond to ionized carbon and nitrogen around 375 eV and oxygen between 570-590 eV. NICER observations enable us to compare, for the first time, in-situ measurements of a cometary plasma environment with remote X-ray observations of the global comet–solar-wind interaction.