APOD: Two Worlds, One Sun (2020 Aug 02)

[APOD Robot]

Two Worlds, One Sun

Explanation: How different does sunset appear from Mars than from Earth? For comparison, two images of our common star were taken at sunset, one from Earth and one from Mars. These images were scaled to have same angular width and featured here side-by-side. A quick inspection will reveal that the Sun appears slightly smaller from Mars than from Earth. This makes sense since Mars is 50% further from the Sun than Earth. More striking, perhaps, is that the Martian sunset is noticeably bluer near the Sun than the typically orange colors near the setting Sun from Earth. The reason for the blue hues from Mars is not fully understood, but thought to be related to forward scattering properties of Martian dust. The terrestrial sunset was taken in 2012 March from Marseille, France, while the Martian sunset was captured in 2015 by NASA's robotic Curiosity rover from Gale crater on Mars. Last week a new rover and a helicopter -- onboard Mars 2020 -- launched for Mars.

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[rj rl]

Interesting, I thought the mechanism was well understood.

[David Cooper]

Note that the disc of the sun on Mars is not distorted by atmospheric refraction and remains undistorted.

[orin stepanek]

TwoWorldsOneSun_Bouic_1080.jpg

Interesting how the Earth sunset is orange; and the Mars sunset is blue!

[ozoneman@comcast.net]

My theory on the blue sky is Earth has clouds, water and with our atmosphere is orange while Mars has no clouds and no water (moisture)?

[Ann]

Blue planet Earth and red planet Mars. Source: http://photojournal.jpl.nasa.gov/catalog/PIA02570

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Let's not forget that Earth is blue and Mars is red!

As Ethan Siegel of Forbes wrote (and you may not necessarily agree):

Ethan Siegel wrote:

With our magnetic field intact, our planet will remain blue and alive for the foreseeable future.

Ann

[neufer]

Rayleigh scattering scatters out blue light.
Martian dust scatters out red light.

Comparison of midsol vs sunset sky horizon colors, sol 956 NASA / JPL-Caltech / MSSS / Damia Bouic

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[Chris Peterson]

Interesting, I thought the mechanism was well understood.

Yes and no. It is entirely a function of the scattering agents in the atmosphere- gas molecules and dust particles. But since the details of the particles in the martian atmosphere are not well known, the details of the scattering are also unknown.

The principles of what's going on are extremely well described by current understanding; we're simply missing some of the inputs to the equations.

[skyping]

Is "Two Worlds, One Sun" available for purchase as a wall poster? Very cool. Thanks David.

[Cousin Ricky]

My theory on the blue sky is Earth has clouds, water and with our atmosphere is orange while Mars has no clouds and no water (moisture)?

The reason for our blue sky on earth is Rayleigh scattering. It has nothing to do with water or clouds. Our sky turns orange at sunset because, with a longer path for sunlight to travel through, the blue gets scattered away.

The Martian sky is beige because of all the dust in the atmosphere. But there is not enough atmosphere to scatter the sunlight until sunset. BTW, Mars does have clouds, just not as much as Earth.

[Cousin Ricky]

Earth: blue sky and orange sunsets.
Mars: orange sky and blue sunsets.

How poetic! Or is it contrarian?

[Hmm]

Imagine if it were possible to see what the sunset looks like from one of the "habitable zone" planets some day? Kepler 452b, for instance.

[Boomer12k]

Eight worlds... one Sun...

Nice comparison ....

I was out looking at the sky about 1am... I had gotten an earlier shot of the Moon with Saturn and Jupiter on either side... but around 1am went out again... and in the east was startled to see a big red star... and I wondered and wondered what it could be... so checked with software and it was MARS!!!! LOL... I had not seen it for some time...and did not expect it to be so bright... Hope to get some shots soon...

:---[===] *

[Chris Peterson]

Eight worlds... one Sun...

Nice comparison ....

I was out looking at the sky about 1am... I had gotten an earlier shot of the Moon with Saturn and Jupiter on either side... but around 1am went out again... and in the east was startled to see a big red star... and I wondered and wondered what it could be... so checked with software and it was MARS!!!! LOL... I had not seen it for some time...and did not expect it to be so bright... Hope to get some shots soon...

:---[===] *

Same thing happened to me a few nights ago. Was walking from my observatory back up to the house and noticed the bright red object low in the east. Wasn't expecting it, because I haven't been paying attention to Mars lately, but didn't need to look it up. There are no red stars that bright, and none in that part of the sky. Nothing else it could be.

[Cousin Ricky]

I was out looking at the sky about 1am... I had gotten an earlier shot of the Moon with Saturn and Jupiter on either side... but around 1am went out again... and in the east was startled to see a big red star... and I wondered and wondered what it could be... so checked with software and it was MARS!!!!

Saturn does this to me all the time, though once I have a fix on it for the season, it doesn’t fool me again... until the next year.