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Caption:

The following chart is the predicted light curve (visual magnitude as a function of time) of Mars, according to the most recent ephemeris data. Magnitude data is sampled with a 2 days interval and there might be inaccuracies for objects changing brightness very rapidly during the course of a few days. For comets there could be large discrepancies between the observed and predicted brightness because of their highly dynamic behaviour.

enter image description here

In addition to the large peak (from about +2 to -1 magnitude) around January 2025, I see a small wiggling behavior, about 10 oscillations per year, with an amplitude of about 0.1 magnitude.

Why is the light curve of Mars so jagged?

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    $\begingroup$ None of the other planets have such jagged light curves $\endgroup$
    – Harrychink
    Commented Jun 9 at 9:41
  • $\begingroup$ FWIW, I have a Horizons magnitude plotting script in this answer to a related question astronomy.stackexchange.com/a/53620/16685 $\endgroup$
    – PM 2Ring
    Commented Jun 10 at 6:22

2 Answers 2

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My first guess was that this was "something to do with the moon" since there seems to be roughly a monthly periodicity. But looking more closely suggests that my first guess was wrong. The "jaggedness" doesn't match the length of a month.

Looking more carefully, this appears to be a result of the different albedo of Mars on different sides, and the fact that Mars's rotation is quite close to 24 hours.

The graph is plotted once every two days (probably at a 00:00 UTC), And each 48 hours, Mars rotates a bit less than 720 degrees. So, just like a strobe effect, it seems that Mars is rotating very slowly. As it rotates, different parts of the surface are visible from Earth, and some parts are darker than others.

I tested this by getting Nasa Horizons to generate the data for Mars in July -Jan 2023. The "jaggedness" was visible. I noted that on 19 July, Mars was dim. I then asked Horizons to generate the data just for that day, and noted that over the course of one day, Mars shows the full range of brightness variations that are seen over the month. I conclude that this is a result of Mars rotating at just less than once every 24 hours, and having different brightness on different parts of its surface.

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    $\begingroup$ What a wonderful example of "aliasing"! $\endgroup$
    – kruemi
    Commented Jun 10 at 10:02
  • $\begingroup$ Why does the same pattern appear on the predicted data. $\endgroup$
    – Harrychink
    Commented Jun 10 at 11:22
  • $\begingroup$ What predicted data are you referring to? But in any case, this seems like a very regular and explainable cycle, so I'd probably expect any competent prediction to take it into account. $\endgroup$ Commented Jun 10 at 13:26
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    $\begingroup$ Because the predicted data uses the known variation in albedo of Mars to make its prediction. $\endgroup$
    – James K
    Commented Jun 10 at 17:10
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To follow up on @JamesK's excellent answer and observations we can calculate the period of apparent oscillations due to the aliasing that arises from sampling at a frequency close to or below the actual oscillations.

If we assert that a "day" in this context is the recognized 24 hours or 86400 seconds, and that Mars' rotational period is 1.025957176 days, then if we observed Mars from a fixed distant point in space, the number of oscillations per year due to oscillations of the effective albedo of the side presented would be given by

$$\text{ 365.25 × (1 - 1/1.025957176) = 9.24}$$

You can think of this as the difference in frequencies, cf. moiré pattern and heterodyne.

But we're observing from Earth, not from infinity.

Since from Mars' perspective the Earth oscillates left and right with respect to the Sun, we can ignore that phase modulation and consider the frequency presented to Earth's average position - the Sun.

That means we need to use the average solar day of Mars instead.

$$\text{ 365.25 × (1 - 1/1.02748843) = 9.77}$$

That's a good match to the apparent ~10 cycles per year in the plot.

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  • $\begingroup$ Why doesn’t the same happen for say, ceres $\endgroup$
    – Harrychink
    Commented Jun 10 at 5:44
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    $\begingroup$ @Harrychink That's a good question! I think that "Why does Mars have such a large variation in albedo compared to other spherical solar system bodies, besides of course Earth?" could be a stand-alone question. I'll bet that the answer has to do with it being the only other rocky planet that's had an ocean, but that's just a guess. Of course you have to rule out asteroids that are non-spherical, and unusual moons like Ganymede and Enceladus which have nonuniform albedos for other reasons, and our Moon - unless of course you observe it from Mars. $\endgroup$
    – uhoh
    Commented Jun 10 at 5:54
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    $\begingroup$ Of the spherical bodies in the solar system, with a visible surface, having regions of different albedo is fairly common. Earth. Mars, Moon, Pluto, Ganemede, Io ... Mercury, Calisto, Europa don't.... but since its pretty common both ways, I'm not sure a special explanation is needed except "that's how it is". Geology tends to create regions, and Mars has been geologically active fairly recently. $\endgroup$
    – James K
    Commented Jun 10 at 20:11

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