This answer to Visible sky features from Mars got me thinking.

On Earth it takes a while for the sky to get dark after sunset, which makes looking for Mercury a challenge because it follows the Sun fairly closely.

As seen from Mars, Mercury will have an even smaller separation from the Sun, but with its much thinner and cleaner and dryer atmosphere I have a hunch it might be easier to spot Mercury after sunset because the sky will darken (sufficiently to easily spot Mercury) much more quickly than it does on Earth.

Question: How quickly does night come on Mars compared to Earth? One can compare equatorial locations at the equinoxes to avoid lengthy discussions of geometry.

I've asked here rather than in Astronomy SE because there is likely to be spacecraft data on this from various Mars landers and rovers.

types of twilight on Earth

types of twilight on Earth


1 Answer 1


This caption on a photo from the Mars Pathfinder mission seems relevant:

[...] The image was taken as part of a twilight study which indicates how the brightness of the sky fades with time after sunset. Scientists found that the sky stays bright for up to two hours after sunset, indicating that Martian dust extends very high into the atmosphere.

And on another photo:

[...] At the time of the last image the sky is only 1% as bright as at the beginning, but the IMP can easily adapt to the darkness and return these pictures. Because there is so much dust extending high into the martian sky, the sky stays bright for more than an hour after sunset.

That does not seem to play well with the maximum angular separation of 19.6° of Mercury from Mars, compared to 28.1° for Earth. A considerable difference, since it cuts out the "nicest" third of the visible time.

Under the best possible conditions, that would put Mercury in the sky for 1 hour 20 minutes, well within the dust-induced twilight period.

  • $\begingroup$ Wow this was unexpected, but in hindsight makes sense. I wonder just how bright "sky stays bright" really means though. It would be great to compare quantitatively (e.g. magnitude per arcsec^2) but I'd failed to ask about that. $\endgroup$
    – uhoh
    Jan 23, 2020 at 3:09
  • $\begingroup$ How about an observatory on Olympus Mons? (22 km altitude) $\endgroup$ Jan 23, 2020 at 15:20
  • $\begingroup$ accepting and fyi just asked How bright is twilight on Mars compared to Earth? $\endgroup$
    – uhoh
    Mar 20, 2020 at 2:58
  • $\begingroup$ @CarlWitthoft if twilight lasts two hours, then the altitude of the dust must be far higher than that. $\endgroup$
    – uhoh
    Mar 20, 2020 at 2:59
  • 1
    $\begingroup$ @uhoh indeed! Found some studies which estimate 70 km during quiet times and 90 km during global dust storms. $\endgroup$ Mar 20, 2020 at 11:49

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