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On Monday I asked you to identify this “evening star”. This is a term that is generally associated with the planet Venus, and has been since Roman times. However this particular object isn’t Venus, although it looks very similar to it when seen in this way. I also mentioned that there was another, smaller, body in this image. This other solar system object is just below the main planet, although it is hard to see without enhancing the image. These two objects appear close together in the sky, which doesn’t necessarily mean that they are close together in space. However in this case they actually are. This is Earth and its Moon as seen from Mars. The picture was taken by NASA’s Curiosity Rover. The annotated image below was released by the Curiosity Rover team, and shows the positions of Earth and the Moon. 

It is no surprise that Earth appears as a bright planet in the martian sky. It is the largest of the terrestrial planets, and at closest approach Earth and Mars are only 54.6 million kilometres apart. None of the planets emit their own light, so how easy they are to see when we look up at the night sky is entirely down to how much sunlight they reflect. This is controlled by a number of factors. First it is fairly intuitive that larger planets will reflect more light, and that the closer you are to the sun the more light there will be to reflect. If Jupiter and Saturn were the size of Earth they would be much harder to see, but as they are vastly larger they more than make up for the reduced sunlight in their distant part of the solar system. Venus is very bright because it is relatively large, close to the sun and close to the Earth. However Mercury, which is closer to the Sun, is much darker. This tells us that there is another factor in play, and that it must be to do with how reflective Mercury and Venus are. 

Venus

Mercury

If we look at images of these two planets we can see that they are very different. Venus is covered with bright clouds, while the surface of mercury is much darker, covered with solidified lava and potted by meteorites. The reflectivity of the surface is called its “albedo” and there is a sharp contrast between that of Mercury and Venus. Every type of surface has an Albedo, so a planet’s overall reflectivity is the average of the different terrain types that cover its surface. The clouds of Venus reflect most of the sunlight which hits them, 75 percent in total. Mercury on the other hand reflects only 6 percent of incident light. 

The albedos of Earth and Mars are actually very similar, although for different reasons. Mars has a lot of bare, redish rock, but also the bright polar caps, and wisps of cloud in its thin atmosphere. Earth has much more cloud and ice, but is also covered with a large ocean. The seas are dark, and so reduce the overall albedo of our planet. Even so Earth comes out slightly brighter, with an albedo of around 30 percent, compared to 29 percent for Mars. 

From this information we can infer that Earth will look somewhat brighter in the martian sky than Mars does to us. Their albedos are similar, and they are obviously the same distance apart. However the earth is closer to the sun, and substantially larger.  

Image Credit:

Earth from Mars: NASA/JPL-Caltech/MSSS/TAMU

https://photojournal.jpl.nasa.gov/catalog/PIA17936

Mercury: https://upload.wikimedia.org/wikipedia/commons/d/d9/Mercury_in_color_-_Prockter07-edit1.jpg

Venus: https://upload.wikimedia.org/wikipedia/commons/e/e5/Venus-real_color.jpg

Further reading:

More about this image here: https://www.space.com/24593-mars-rover-curiosity-sees-earth-photos.html

Brightness of the planets: http://magicvalley.com/lifestyles/recreation/column-a-planet-s-brightness-depends-on-several-factors/article_4bce554f-c310-5071-88d0-e39e68a54da4.html

Albedo of solar system bodies: http://sciencing.com/albedo-planets-5203.html