00:00For millions of years, the volcano had been silent.
00:03No lava, no ash, no sign that anything beneath it was still alive.
00:07Then, in 2018, a spacecraft circling Mars photographed something stretching away from
00:13its summit.
00:14It looked like smoke, not a small cloud drifting above the mountain, but a narrow white trail
00:20reaching hundreds of miles across the planet.
00:22It appeared beside one of the largest volcanoes on Mars, grew with astonishing speed, and
00:27then vanished before the Martian morning was over.
00:30There was just one problem.
00:31Mars cannot produce ordinary smoke without something burning.
00:35And according to scientists, this volcano had been dormant for tens of millions of years.
00:40So what had Mars Express actually photographed?
00:43But before we start, smash the like button and make sure to subscribe if you haven't,
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00:51At first, the answer seemed simple.
00:53But as researchers began watching the plume more closely, they discovered that a
00:57it was following a strange ritual, one that began in darkness, repeated through the Martian
01:02seasons, and had apparently been happening long before anyone noticed.
01:06The story begins in June 2003, when the European Space Agency launched Mars Express toward the
01:12Red Planet.
01:13The spacecraft carried several scientific instruments, but one of its simplest cameras was not originally
01:19designed to solve a major mystery.
01:22Known as the visual monitoring camera, it had a job much closer to that of a basic webcam.
01:27Confirmed that the Beagle 2 lander had separated from the orbiter.
01:31After that task, the camera was switched off.
01:34For years, it remained little more than an old piece of equipment traveling around Mars.
01:38Then engineers brought it back.
01:40Its images were not as sharp as those from the orbiter's more advanced instruments,
01:44but the camera had one important advantage.
01:47It could see a very wide area at once.
01:50Instead of revealing tiny details on the ground, it could show scientists something closer to
01:55the big picture.
01:56And in September 2018, that big picture contained something difficult to ignore.
02:01A white line had appeared near Arsia Mons.
02:04The mountain is enormous.
02:06Its summit rises roughly 12 miles above the surrounding plains, making it far taller than Mount Everest
02:12when measured from the land around it.
02:14At first glance, the location made the photographs look almost obvious.
02:18A plume beside a volcano usually means an eruption.
02:22But Mars is not Earth.
02:24Arsia Mons is an ancient shield volcano, and scientists have found no evidence that it suddenly
02:29erupted in 2018.
02:31There was no fresh lava spreading across the surface, no ash cloud darkening the atmosphere,
02:37and no thermal signal suggesting that the mountain had awakened.
02:40The white formation also behaved strangely.
02:43It did not rise vertically from a crater.
02:46Instead, it stretched westward from the side of the volcano, forming a long, thin trail across
02:51the sky.
02:52And it kept returning.
02:53The more scientists examined the images, the less the plume resembled the aftermath of a
02:58single event.
02:59It seemed to be part of a cycle.
03:01Researchers began comparing images taken at different times of the Martian day.
03:05That was when the real pattern emerged.
03:07The cloud did not simply remain above Arsia Mons.
03:11It began forming before sunrise, while much of the surrounding surface was still in darkness.
03:16Near the western slope of the volcano, a small patch of white would appear high in the atmosphere.
03:22Then it would start moving away from the mountain.
03:24And it moved fast.
03:25For around two and a half hours, the formation could expand westward at more than 370 miles per
03:31hour.
03:32At its largest, it stretched approximately 1,100 miles across Mars, farther than the distance
03:38from Paris to Rome and back again, yet the cloud remained surprisingly narrow, only about
03:4490 miles across.
03:45Then, as the sun climbed higher and the atmosphere warmed, the entire structure began to fade.
03:51By late morning, it was gone.
03:53The next day, under the right seasonal conditions, the process could begin again.
03:57A white trail forming in darkness, racing away from a dead volcano and erasing itself after
04:03sunrise.
04:04The explanation eventually led scientists back to the mountain itself.
04:08Arsia Mons did not need to erupt to create the plume.
04:11As Martian air moved toward the volcano, the enormous slope forced it upward.
04:16Higher in the atmosphere, the air became colder.
04:19A small amount of water vapor it carried could then freeze into tiny ice particles, producing
04:24a cloud.
04:25On Earth, a related process can create clouds near mountains.
04:29Scientists call them orographic clouds, or sometimes lee clouds.
04:33So the object in the photographs was not smoke.
04:36It was water ice.
04:38But solving that part of the mystery only made the scale of the phenomenon more remarkable.
04:43Mars has an atmosphere less than 1% as dense as Earth's.
04:47It is a cold, dry world where liquid water cannot remain stable on the surface for long.
04:53Yet under the right conditions, a single volcano can shape the air above it so powerfully
04:58that it creates the largest known orographic cloud on the planet.
05:02And Arsia Mons appears to do this with extraordinary precision.
05:06The plume is seasonal.
05:08It returns during spring and summer in the southern half of Mars, when the combination
05:12of temperature, wind, and water ice allows the cycle to begin.
05:16Researchers searched through observations from other spacecraft and discovered that the cloud
05:21was not new in 2018.
05:23Earlier missions had photographed parts of it.
05:26Mars Express had recorded similar formations in previous Martian years.
05:30Other orbiters had also captured evidence.
05:32Scientists even found traces of the phenomenon in images taken by Viking 2 during the 1970s.
05:38For decades, spacecraft had been photographing pieces of the same enormous morning cloud.
05:44But most cameras were not looking at Arsia Mons at the correct time.
05:48Many passed over the region later in the day, after the plume had already disappeared.
05:53Others saw only part of it because their field of view was too narrow.
05:56The old Mars webcam, with its wide view and unusual observation times, finally allowed researchers
06:02to watch the formation through its daily life cycle.
06:05The camera created for a brief engineering task had become the instrument that revealed
06:10one of the strangest repeated events in the Martian atmosphere.
06:14Yet one question has never completely disappeared.
06:17Arsia Mons is not the only giant volcano in this part of Mars.
06:21It belongs to the Tharsis region, an enormous volcanic province containing several mountains
06:26capable of influencing the atmosphere.
06:29Nearby peaks rise into the same thin sky and face many of the same seasonal winds.
06:34But at this time of the Martian year, Arsia Mons is the only known low-latitude location that
06:40produces such a spectacular elongated cloud.
06:43Why this volcano?
06:44Its height, shape, and surrounding terrain clearly help create the right atmospheric wave.
06:49But scientists are still working to understand why the effect becomes so large and persistent
06:54here, while neighboring volcanoes do not produce an identical display.
06:57The plume is no longer an unexplained object, its ingredients are known, its daily cycle has
07:03been measured, its speed, altitude, and enormous length can all be calculated.
07:08And yet the mountain has kept part of its secret.
07:11Mars Express is still orbiting the red planet in 2026, more than two decades after it arrived.
07:18The mission has survived far beyond its original timetable, continuing to photograph Mars, study
07:23its atmosphere, and examine the history of water across its surface.
07:27Other missions have come and gone during that time.
07:30Landers have listened to Marsquakes.
07:32Rovers have crossed ancient lake beds.
07:34Orbiters have studied how the planet's atmosphere escapes into space.
07:38But the strange cloud beside Arsia Mons offers a different kind of lesson.
07:42Not every mystery on Mars is buried beneath the ground.
07:45Some of them appear in plain sight, last for only a few hours, and disappear before most spacecraft
07:51are in the right place to see them.
07:53The images from 2018 did not reveal an eruption, an explosion, or evidence that someone was secretly
07:59changing Mars.
08:00They revealed something real, and perhaps more haunting.
08:04Before sunrise, when the surface below is still dark, ice begins forming high beside an ancient
08:10volcano.
08:10The cloud accelerates westward, stretches across nearly half a continent, separates from the
08:16mountain, and dissolves as the morning warms.
08:18Then the sky becomes empty again.
08:21To anyone arriving a few hours later, there would be nothing to suggest that it had ever existed.
08:26But when the Martian season is right, the White Trail returns.
08:29It has likely repeated this ritual for years, perhaps for centuries, with no one there to watch it.
08:35And somewhere above Arsia Mons, before another silent sunrise, the mountain may already be preparing
08:40to draw the line across the sky again.ocumented
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