Which Is The Brightest Planet, As Seen From Mars?

What determines a planet’s brightness from another?

How bright a planet looks from another world depends on several factors. Size, distance, and how much of the light that falls on it is reflected are clearly important. Less obvious factors also matter. One is how far the planet is from the Sun – Saturn is much fainter than Venus from our perspective not only because it is so much farther from us, but because it’s also much farther from the Sun, so it receives much less light to reflect.

Another factor to consider is how much of the planet is illuminated when seen from the relevant location. We all know the full Moon is much brighter than a lunar crescent. Things get a bit more complex with Venus and Mercury, however. We see them as close to “full” when they are almost on the other side of the Sun from us, so the distance between us is very great. On the other hand, when they are close to passing inside us, only a crescent is lit up – a much smaller proportion of the sunlight the planet receives is sent our way, but the smaller distance means we see more of it. 

Partial illumination has a modest effect when looking at planets farther away from the Sun – at times a small telescope will show Mars as some way off full – but the difference is relatively minor. 

All of this makes calculating the brightness of planets in another planet’s sky quite complex, even if you’re just worried about peak brightness, not the variation.

The inner planets from Mars

The stars from Mars will look quite similar to from Earth – brighter because there is less atmosphere in the way, when there isn’t a sandstorm, but spacesuit faceplates may balance that out. Their locations will look a bit different, rotating around different pole stars, but otherwise the Martian sky will be quite familiar, aside from the replacement of one big Moon with two small ones.

As Curiosity has shown, Earth is a bright object in the Martian sky, at least when not lost in the Sun’s glare. That’s unsurprising. Earth is slightly larger than Venus, and while being farther from the Sun we don’t get as much light to reflect as the next planet in, we also get a lot closer to Mars.

Overall, the Earth never gets as bright in the Martian sky as Venus does, mainly because the Venusian clouds are so reflective, but the two are comparable. Given that the brightness of each varies with their phases, it’s not unusual for the Earth to be the brighter of the two on a specific night.

The NASA Global Surveyor photographed Earth from Martian orbit when it was close to its ideal orientation, appearing around half full, and scientists at JPL calculated the brightness as magnitude -2.5, about its peak. It’s also about a 10th as bright as Venus gets in our skies.

Of course, Venus will never get as bright to a Martian observer as it does to us. It’s substantially farther away (the closest Mars-Venus approach is almost three times as distant as the closest one between Earth and Venus). 

For his book Astronomy for Entertainment, Yakov Perelman calculated the brightness of various planets as seen from other planets. Perelman calculated Venus’s peak brightness from Mars as magnitude -3.2, so when both planets are at their brightest Venus looks about twice as bright. However, Earth can get a lot higher in the Martian sky at sunset than Venus ever does, which has to count for something.

Looking out

Jupiter and Saturn would both look quite similar from Mars to how they do from Earth – there are no significant phases to worry about. At their brightest each will be a bit brighter from Mars than Earth because the distance is less, but the difference is small. Moreover, most of the time, when the outer planets are not close to opposition, their distance to Mars and to Earth is more similar.

Consequently, Jupiter reaches -3 and Saturn -0.6, compared to -2.5 and -0.4 from Earth.

What about the moons?

Earth’s giant Moon completely overshadows any planets, but the Martian moons are tiny by comparison. If they were as far from the planet as our Moon is, they’d be very faint, but they orbit much closer. Deimos is only a little brighter than Venus is from Earth (around magnitude -5), although the contrast with the fainter Venus in the Martian skies would make it stand out more.

Phobos, however, can get quite bright – as much as -9 or -10. That’s about 16 times fainter than the full Moon from Earth. There’s more variation for the Martian moons between peak brightness when overhead and how they look on the horizon than for our own moon.

Earth from Mars vs. Mars from Earth

One comparison that may be of interest is how the Earth would look from Mars compared to how Mars looks to us, color aside.

The Earth has almost seven times the surface area of Mars to reflect light from, and it’s much less reflective. Being closer to the Sun, the Earth also gets considerably more light to reflect.

That’s balanced by the fact that the only time the Earth looks anything close to full, it’s also close to the farthest away it can get from Mars, whereas Mars is always close to full to us.

Putting all that together, Martian peak brightness is -2.94 to us, a little brighter than Earth looks from it, but most of the time Mars is further from that peak.

Summary

Earth is the fifth brightest object in the Martian night sky, if each is taken at its peak. It’s easily beaten by Phobos and Deimos. Venus is third, about five times fainter than Deimos’s peak. Jupiter is slightly fainter than peak Venus, but its greater consistency means it would usually outshine Venus. Earth is just a little fainter still, while Mercury and Saturn would each be quite a long way behind, with Earth’s Moon fainter still, but still brighter than all but the dozen or so brightest stars.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.