For space watchers, Mars is like a second home. Astronomers have been studying the Red Planet for centuries – the first map of the Martian surface was sketched 500 years ago. Since then, it has become the most surveyed planet in the solar system, besides Earth. We have sent over 50 robot explorers to patrol its surface and watch it from orbit, seven of which are operational on and around the Red Planet right this minute. It's not for nothing that space aficionados quip that Mars is the only planet known to be inhabited solely by robots.
So it was that much more surprising when, on 12 March 2012, amateur astronomers around the world noticed a strange blob rising out of the planet's southern hemisphere, soaring to 250 kilometres above the surface.
They watched for 11 days as it grew to around 1000 kilometres across, even stretching a "finger" out into space. "I was really quite amazed that it was sticking out the side of the planet quite prominently," says Damian Peach, who lives in Selsey, UK, and was one of the first to spot it.
Poor weather and other issues meant no one had their eye on Mars the following week, and by 2 April it seemed to have disappeared. Then on 6 April a second object of the same type emerged from the same spot and lasted another 10 days. It, too, has not been seen since.
Nearly three years later, the sighting still defies explanation. In an attempt to pin down the blobs' origins, Agustin Sánchez-Lavega of the University of the Basque Country, Spain, and colleagues, including Peach, sought out images of Mars from that period. They wound up collecting pictures from 18 observers equipped with a variety of small telescopes. The team also searched through old images taken by the Hubble Space Telescope and identified a similar object in 1997.
Exotic theories
The team considered several possible explanations, each more exotic than the last. Despite their best efforts, though, they couldn't come up with any that were consistent with known processes – and neither can anyone else. "Frankly, I'm puzzled by the observations," says Bruce Jakosky of the University of Colorado, Boulder, who leads NASA's Mars-atmosphere-observing MAVEN mission. "I don't understand how material can get that high and stay there for so long."
One clue is that the blobs seemed to appear at Mars's terminator, the fuzzy line where night turns into day. That suggests a change in atmospheric temperature due to the morning sun may be responsible. The team's best guess is that the object is a cloud of frozen carbon dioxide and water particles condensing in the upper atmosphere.
If that's the case, it would be unlike any cloud seen anywhere on Mars, or on Earth. Clouds on both planets are only ever seen at altitudes below 100 kilometres, which on Earth is the accepted height for the beginning of outer space. "If the phenomenon is a cloud, then the most similar phenomena on Earth will be the mesospheric clouds that form at 80 kilometres altitude on polar regions," says Sánchez-Lavega.
Auroras
With that in mind, thoughts have turned to other potential explanations. Charged particles from the sun interact with the Earth's magnetic field to make the upper atmosphere glow – the phenomenon we call auroras. Mars's magnetic field is weak and patchy in comparison, meaning auroras were only seen there for the first time in 2005. That sighting was over a region tantalising close to the unexplained blobs.
The team calculate that the blobs could be auroras, but only if it is more than 1000 times brighter than Earth's. That seems unlikely, especially since the sun wasn't particularly active in March 2012. "The fact that you might see a visible aurora is not completely out of the realm of possibility," says Nicholas Heavensof Hampton University in Virginia. "But they don't really come to visible brightness anywhere near what this thing would be."
What about something like a massive volcano pumping material into the atmosphere? The blobs seemed to extend upwards from the surface of Mars, though it's hard to determine this exactly given the quality of the images we have. But that wouldn't explain why the blobs have only appeared in the morning says Sánchez-Lavega, and in any case we don't know of any active volcanoes on Mars.
"You would think that something large enough to dump that much vapour in the atmosphere would be picked up," says Heavens. A massive dust storm is also ruled out, as they normally don't reach above 60 kilometres and the blob doesn't carry Mars's signature dusty red.
Aliens at work?
OK, now we're getting desperate. Could the explanation be biological? Whether there is life on Mars is one of the planet's major mysteries (see box), but any alien hunters excited by the blobs should calm down, says Sánchez-Lavega: "No life past or present [has been] detected so far on Mars, so it cannot be." Heavens says there isn't really enough data to rule either way, but it's better to be cautious. "If there is no positive evidence, you should probably exclude something biological."
It seems that all we can do is wait and hope the object turns up again, although Peach thinks that the favourable conditions that occurred in 2012 may not happen for some time. "The season on Mars is very cloudy around that time of year, and that just happened to occur at opposition," he says, referring to when Mars and Earth are aligned on the same side of the sun. That particular combination won't happen again for over a decade, he says.
Perhaps instead it will be observed by one of our robot minions. MAVEN is on an orbit that would have allowed it to fly through the blob, but the probe only reached Mars last year
. "MAVEN should see something like this very easily if it occurred again, if we were at the right place at the right time," says Jakosky. "I've given a heads-up to our science team, so they'll be keeping an eye out for it."
. "MAVEN should see something like this very easily if it occurred again, if we were at the right place at the right time," says Jakosky. "I've given a heads-up to our science team, so they'll be keeping an eye out for it."
Journal reference: Nature, DOI: 10.1038/nature14162
Mars's other mysteries
Anyone there?
You can't talk about Mars without talking about the possibility of life. The latest view, thanks to NASA's Curiosity rover, is that Mars was definitely capable of hosting life in the past – but the jury is still out on whether microbes once thrived there. The ExoMars rover, launching in 2019, might be equipped to give a definitive answer.
Who dealt it?
A related mystery is figuring out what is producing short-lived bursts of methane on Mars. Burping microbes are one possibility, but geological processes can also produce pockets of underground methane that might gradually seep out to the surface. The ExoMars Trace Gas Orbiter will be on the case when it launches in 2016.
Call that a moon?
Mars has the weirdest moons in the whole solar system. Phobos and Deimos are lumpy, misshapen rocks, tiny compared to other planets' spherical satellites. They might be captured asteroids, shot out of the asteroid belt towards Mars by Jupiter's gravity, or they could be chunks of Mars's crust blasted off the surface by an ancient impact. A Russian mission to land on Phobos in 2011 was meant to bring samples, and answers, back to Earth, but a botched launch means we remain in the dark for now.