Sky & Telescope - December 2024 - 63

UNLESS OTHERWISE NOTED, ALL IMAGES ARE COURTESY OF THE AUTHOR; HUBBLE IMAGE OF
JUPITER: NASA, ESA, A. SIMON (GODDARD SPACE FLIGHT CENTER), AND M. H. WONG (UNIVERSITY OF
CALIFORNIA, BERKELEY) AND THE OPAL TEAM / SCIENCE PHOTO LIBRARY
How to get more science out of your solar
system images.
A
mateur astronomers have long been the primary
resource for monitoring the planets. Typically, we
enjoy taking color images of the planets with the
primary goal of resolving fine details. These pretty pictures
help us follow dust storms on Mars as they develop, turbulence
in the upper clouds of Venus, or upheavals in the
atmospheres of the outer giant planets. Amateurs' contributions
are especially valuable because professional astronomers
have neither the equipment nor the resources to focus
on the major planets for extended periods (unless a spacecraft
is visiting one of them).
But there's more to color images than just pretty pictures -
they contain valuable information about ongoing phenomena
in the atmospheres of the bodies within the solar system. It's
difficult to monitor subtle changes visually at the eyepiece and
in photographs, but through the use of photometric filters,
imagers can expand the amount of useful data they record,
which can enrich our understanding of our neighboring
worlds. Here's how to do it.
Photometry and Color
Photometry allows us to improve our understanding of celestial
objects by analyzing the intensity of light we receive from
them in different wavelengths.
In planetary science, the key to measuring color is its
albedo - the amount of sunlight a surface reflects. This measurement
is typically expressed as a percentage, or on a scale
from 0 (having no reflection) to 1 (perfect reflection). Albedo
explains why objects have color. For example, a blue object
displays a higher albedo in blue than in green, and even more
so with respect to red wavelengths. This is readily apparent
if you compare the color channels of an image in a graphics
program such as Adobe Photoshop. Accurately measuring the
differences in albedo requires a change in one's approach to
planetary imaging.
An excellent visual manifestation of albedo changes is the
evolution of the color of Jupiter's Great Red Spot (GRS). From
2014 to 2020, the GRS reddened noticeably, with the deepest
color occurring in the 2018 apparition. Experienced amateurs
active before 2000 will certainly remember when the
GRS was a pale, pink feature hardly visible at the eyepiece.
Observers often only detected it due to the presence of its
bright, whitish Red Spot Hollow, which the storm draws from
the South Tropical Zone.
Changes in the brightness and color of the GRS are
fairly obvious, but other planetary phenomena may only be
detected through photometry. For example, using photometric
measurements scientists find that the albedo of the
planet Venus at ultraviolet (UV) wavelengths can double
in a matter of a few years. The highest value of 40% albedo
occurred in 2006 then slowly decreased for six years, reaching
a low of 20% in 2012 before the trend reversed. This is
a significant change yet would hardly be discernible when
comparing images from the two periods - the only way to
detect this is to measure the intensity of the light reflected
from the planet through photometry.
Photometric Technique
So, how do you perform photometric measurements of the
planets? The method I prefer is differential photometry, as
described by the American Association of Variable Star Observers
(AAVSO) at https://is.gd/photometryguide. This technique
compares the measured intensity of the targeted planet to
that of known reference stars across multiple wavelengths.
VARIABLE PLANET These images show Jupiter's central region
between 2011 and 2018. Each year, the planet displays striking variation
in the colors of its belts and zones, and the Great Red Spot's brightness
and hue change noticeably. But how can we measure such variations?
Color as Information
2015
2016
2017
2018
sk yand tele scope .o r g * DECEMBER 2024 63
https://www.is.gd/photometryguide http://www.skyandtelescope.org
Sky & Telescope - December 2024

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