Sky & Telescope - December 2024 - 69

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Photometric Spectra of Mars 11/17/2020
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tioned earlier, the GRS reddened noticeably from 2014 to
2020, but when I started this work in 2021, it had suddenly
become paler. However, in 2022, a more intense tint had
returned. My measurements of its albedo from 2021 to 2022
successfully quantify the variations; in 2021 the value was
almost equal to the geometric albedo of the whole globe in U
and B (36% and 41%), but in 2022, it had fallen to 27% and
34%, respectively. You can see that in the graph at the bottom
of the facing page. On the other hand, note that its albedo
did not change in the R band. The same brightness in red, less
bright in blue produces a change in color. I've also calculated
a slightly higher albedo in CH4: 12% versus 11% in 2021. If
correct, this agrees with the findings of professional astronomers
studying the planet.
Monitoring the Other Planets
Jupiter is by far the most dynamic planet to study using
photometry, but the other nearby worlds also yield interesting
results. Storms also appear in the atmospheres of Saturn,
Neptune, and Uranus, which stand out notably. On the ice
giants, these storms significantly increase the signal in the I
band and don't require high resolution to detect.
Mars is also an exciting subject to study using photometry.
While many albedo features remain fairly constant, the planet's
thin atmosphere can sometimes produce surprises. On
the evening of November 17, 2020, I was imaging Mars with
my friend Emmanuel Beaudoin as part of an international
team attempting to detect aurorae on it. That evening, we
caught an unusual feature. While we initially focused on the
possibility that we had succeeded in capturing aurorae, after
careful analysis we concluded that it was a kind of cloud.
In fact, it was a very long, fast-evolving cloud that appeared
beyond the planet's sunrise terminator.
Still, the exercise was a success, in that I was able to
measure the albedo of the feature across the spectrum. The
results show that the signal from the cloud was slightly
higher in the red and infrared bands. The professional
astronomer on the team thus hypothesized that it was likely
composed of carbon dioxide.
Using these techniques, you can contribute valuable
data to groups and organizations that monitor the
large bodies in the solar system, such as The Association
of Lunar and Planetary Observers (ALPO,
https://alpo-astronomy.org), the British Astronomical
Association (https://britastro.org), the French Astronomical
Society (http://www.astrosurf.com/planetessaf),
or the Planetary Virtual Observatory & Laboratory
(http://pvol2.ehu.eus/pvol2). My work here is inspired by
the extensive work that ALPO coordinator Richard Schmude
performed for decades. Currently, very few amateurs pursue
this approach to planetary imaging. But with more of us
studying the planets photometrically, we can help expand
our knowledge of our neighboring worlds.
¢ CHRISTOPHE PELLIER is a French planet imager and Vice
President of the Commission of Planetary Observations of
the French Astronomical Society. See more of his work at
www.planetary-astronomy-and-imaging.com.
sk yand tele scope .o r g * DECEMBER 2024 69
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1: Amazonis
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2: Valhalla
Entire Globe
3: Mare Cimmerium
4: South Polar Cap
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5: Terminator feature
LUCKY CATCH This graph plots
spectroscopic albedo of various
features on Mars recorded on the
night of November 17, 2020. At
the time, a long cloud (5) appeared
just beyond the sunrise terminator,
allowing the author to record a full
set of photometric measurements
before it disappeared.
5
Geometric albedo
https://www.alpo-astronomy.org https://www.britastro.org http://www.astrosurf.com/planetessaf http://pvol2.ehu.eus/pvol2 http://www.planetary-astronomy-and-imaging.com http://www.skyandtelescope.org
Sky & Telescope - December 2024

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