Brightening is due to a combination of observing geometry, as the comet approaches both the Sun and the Earth, and increasing back-scattering cross-section.
Total Magnitude(brightness) of comet:
M1 absolute magnitude of the comet (a baseline brightness value).
Δ: geocentric distance (distance between the comet and Earth, inAU).
r: The heliocentric distance (distance between the comet and the Sun, in AU).
K1: brightness slop
Dynamically new comets have a lower heliocentric brightening slope (kr), meaning they brighten at a slow rate.
Higher kr means the comet brightens more rapidly as it goes near the sun. Older comets tend to have more scattered kr values due to surface evolution and patchy mantles of refractory material.
New dynamically young comets often experience strong post-perihelion fading because they have more pristine, volatile-rich surfaces.
New comets appear intrinsically brighter than old comets because they -contain pristine, highly volatile ices that sublimate easily, have higher surface activity with a larger fraction of the nucleus actively.
The dynamically new comets produce more CO2 than CO, while dynamically old comets appear to be more CO-dominant.
source:
https://arxiv.org/html/2504.00565v1
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