Mars Near Opposition 2016: Hubble's Portrait from 50 Million Miles

On May 12, 2016, ten days before Mars reached opposition, Hubble photographed the planet from about 50 million miles away. At that range its camera resolved features only 20 to 30 miles across, enough to show polar ice, weather and the major surface regions in one frame. The portrait was released a week later, on May 19.

Reading the map

The large dark region at the right edge of the disk is Syrtis Major Planitia. It was among the first features on Mars that seventeenth-century observers could make out, and Christiaan Huygens used it to measure how fast the planet turns. A Martian day lasts about 24 hours and 37 minutes. Syrtis Major is now known to be an ancient shield volcano, long inactive, and in this view late-afternoon clouds hang around its summit.

South of it lies the bright oval of Hellas Planitia, an impact basin about 1,100 miles across and almost five miles deep. An asteroid strike carved it some 3.5 billion years ago.

The orange expanse in the middle of the disk is Arabia Terra, a vast upland in the northern hemisphere stretching about 2,800 miles. Its heavy cratering and erosion suggest it is among the oldest surfaces on the planet. Dried river channels, too small to resolve here, wind across it and drain into the northern lowlands.

Along the equator to the south run two long dark bands: Sinus Meridiani on the western end and Sinus Sabaeus on the eastern. Their darkness comes from exposed bedrock and from sand ground out of old lava flows and other volcanic rock. That sand is coarser and reflects less light than the fine dust coating the paler, ruddier regions. Why the planet looks the shade it does, and how much that depends on processing, is the topic of What Color Is Mars?

Weather on the day of the picture

Clouds and ice are as prominent as the ground. A wide blanket of cloud covers the southern polar cap. The northern cap, by contrast, has shrunk to a small patch, because it was late summer in the northern hemisphere. Hubble also caught a thin, wispy afternoon cloud reaching at least 1,000 miles across the mid-northern latitudes, and morning clouds and haze along the western limb of the planet.

This face of Mars holds several landing sites: Viking 1 in 1976, Mars Pathfinder in 1997, and the Opportunity rover, still working when the picture was taken. Spirit and Curiosity landed on the far side.

The camera and the filters

The Heritage team observed Mars under proposal 14499, with planetary scientists J. Bell of Arizona State University and M. Wolff of the Space Science Institute, using the UVIS channel of Wide Field Camera 3. Four filters spanned the range from ultraviolet to red: F275W at 275 nanometers, F410M at 410, F502N at 502 and F673N at 673. Spreading the set from ultraviolet to red gives each part of the scene, ice clouds as well as the darker and brighter ground, a band in which it stands out.

During the single Hubble orbit the team cycled through those filters several times, adjusting exposure lengths so that bright clouds or dust would not saturate frames that were already very short. Mars rotates noticeably even in a few minutes, a complication explained step by step in Phobos Photobombs Mars, along with what turned up when the frames were stretched far beyond normal brightness: the moon Phobos, peeking out from behind the planet's limb in about half of them.

2001 and 2016 compared

Hubble photographed Mars near the 2001 close approach as well, when the planet was about 43 million miles away and closer than at any time since 1988. Placed beside the 2016 portrait the two look quite different, for several reasons. The distance was smaller in 2001, and the planet was tilted more face-on; in May 2016 the north pole was tipped slightly toward Earth. The 2001 image also caught a season of large dust storms, including one spilling out of Hellas. Clouds come and go daily, just as on Earth. And the two pictures used different cameras and filters and were lit differently by the Sun.

Why oppositions come every 26 months

Opposition is the moment when Earth passes between the Sun and Mars, so the two sit on exactly opposite sides of Earth. It happens because Earth's orbit is shorter: Earth circles the Sun in 365 days and Mars in 687, so Earth laps Mars roughly every two years, and oppositions repeat about every 26 months. Around opposition Mars is close, bright and fully lit as seen from Earth, which makes it the best time to photograph.

Not every opposition is equal. The orbit of Mars is noticeably elliptical, so the closest approaches range from about 35 million to 63 million miles. In 2016 opposition fell on May 22 at 47.4 million miles, and on May 30 Mars came closer to Earth than at any time in the previous 11 years, at 46.8 million miles.

Questions readers ask

How close was Mars when Hubble took the 2016 picture?

Hubble took the picture on May 12, 2016, with Mars 0.53 astronomical units from Earth, about 50 million miles or 80 million kilometers. It reached opposition on May 22 and its closest approach on May 30, when the distance shrank to 46.8 million miles.

What is the dark region on the right side of the image?

It is Syrtis Major Planitia, an ancient and inactive shield volcano. It was one of the first Martian features seen through telescopes in the seventeenth century, and Christiaan Huygens used it to time the planet's rotation. In the 2016 view clouds surround its summit.

Why does the northern polar cap look so small?

When Hubble took the picture it was late summer in the Martian northern hemisphere. The northern cap had shrunk to a comparatively small size, while the southern polar region was covered by an extended blanket of cloud.

How often does Mars come to opposition?

About every 26 months. Earth takes 365 days to orbit the Sun and Mars takes 687, so Earth catches up with Mars roughly every two years. Because the Martian orbit is elliptical, the distance at each close approach varies from about 35 to 63 million miles.