The colours of Jupiter in 2021 (3) Photometric spectra of individual features and regions by Christophe.P

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The colours of Jupiter in 2021 (3) Photometric spectra of individual features and regions by Christophe.P

The colours of Jupiter in 2021 (3) Photometric spectra of individual features and regions by Christophe.P
Here is the last topic from my study.

North-South scans work wonderfully to measure the global state of the
planet, but are not suited to measure individual features such as the
GRS. In order to produce data about them, I have experimentated a second
method, which is producing photometric spectra of some precise regions
of the globe, equivalent to a spectroscopic analysis, but with only a
few points.

Values are simply found by measuring, when they are on the central
meridian, the ratio of the brightness of the feature / the brightness of
the whole globe, X the geometric albedo of the planet. Beofre that, the
brightness of the individual feature is recalculated to find the
brightness it would present, if it had the same geometric surface as the
globe. By "brightness", I understand the intensity of the feature/globe
measured on the photometric, unprocessed, image through a software (IRIS).

This method does not only work for individual spots, but also for belts
or zones providing that they present a rather homogeneous aspect
throughout all the longitudes. For example, in 2021, it is possible to
measure the difference in albedo/color of the two parts of the NEB.

Because the absolute values measured inside the CH4 band are, by nature,
very small in comparison with the other bands, I have made a specific

The reason why most of the values look to be much brighter than the
spectroscopic albedo from Karkoschka (grey profile), is simply because
the value for the whole globe includes the light gradient from center to
limb. Measuring features on the CM leads to much higher values, in general.

As for any analysis, this way to produce results also suffers from limits:

1) It is less relevant for belts or zones that are not homogeneous
throughout all the longitudes. For that reason, there is no graph for
the SEB, for example.

2) Values are not corrected for the gradient of light between the
equator and the poles. This means that it is only wise to compare date
inside a given domain in latitude, but we should be careful when trying
to compare values taken from different domains if they do not benefit
from the same angle of illumination by the Sun.

3) Some measurements have been made when features where quite away from
the central meridian. A coefficient has been measured to correct the
data in this respect, considering that the photometric variation of the
feature when turning from limb to CM was the same as its domain of
origin ; but of course, in the reality, this might not be exactly the
case. This could have led to errors, but I believe that they are reasonable.

Finally, I have made a few attempt at measuring values from past
apparitions, to compare, when features presented a widely different
aspect. I present two that were quite obvious, but some of the results I
have obtained in this respect were mixed. So take that with caution, of

I hope to see some of you at the EPSC to discuss this, and many other
things :)

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