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Recent photos from the Mars Express probe show that a dark spot on Mars, in the Utopia Planitia plain, recorded by the Viking probes in 1976, has moved at least 320 km south at a rate of 6.5 km per year, a phenomenon that scientists possibly attribute to Martian winds but still lacks definitive explanation.
A dark formation on the surface of Mars that has intrigued scientists for half a century is growing, and the latest images confirm that the advance has shown no signs of slowing down. The spot was first identified in 1976 when NASA’s Viking probes landed on the red planet and recorded an area of soil with a darker coloration than the surrounding area, located within the vast plain known as Utopia Planitia, in the northern hemisphere of Mars. Photographs taken in 2024 by Mars Express, the European orbital vehicle, and released in April show that the edges of the formation have advanced at least 320 kilometers to the south, a pace suggesting an expansion of approximately 6.5 kilometers each year.
The composition of the spot is already known: it consists of soil covered by ash and volcanic rocks deposited by eruptions that occurred millions of years ago in Mars’ geological history. What remains unanswered is why this dark layer is progressively spreading across the planet’s surface. ESA has presented two hypotheses: either Martian winds are lifting and transporting the ash to neighboring areas, or these same atmospheric currents are removing the orange dust layer that previously concealed the dark volcanic material underneath. Neither of the two explanations has been conclusively confirmed.
Where is the spot that is spreading across the surface of Mars
The formation is located in Utopia Planitia, a plain approximately 3,300 kilometers wide that occupies a large part of the northern hemisphere of Mars. The region is one of the most studied on the planet: in 1976, the Viking 2 probe landed there to conduct biological experiments and operated until 1980, and in 2021 the Chinese rover Zhurong also landed in the same plain, exploring the surface before ending its activities in 2023. Data collected by Zhurong allowed Chinese scientists to argue that Utopia Planitia was covered in the past by one of the largest bodies of liquid water in Mars’ history, and the team even mapped what would be an ancient coastal island.
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The plain exhibits geological characteristics that make it especially interesting for researchers. The surface of Mars in this region is marked by grabens, fissures in the ground that provide clues about tectonic activities that may have shaken the planet in past eras. Additionally, there is evidence of subsurface ice in the area, a factor that complicates any attempt to search for traces of microbial life beneath the surface, but at the same time makes Utopia Planitia a candidate for future exploration missions.
Why the dark spot on Mars continues to grow
The rate of expansion identified in the images is consistent over the decades, which rules out a one-time event as the cause. By comparing the records from the Viking probes in 1976 with photographs from Mars Express in 2024, researchers concluded that the southern boundary of the spot has advanced at least 320 kilometers, a movement consistent with a continuous progression of about 6.5 kilometers annually across the surface of Mars. The regularity of the advance suggests that the phenomenon is driven by a persistent atmospheric process, rather than an sporadic geological cause.
The winds of Mars are the main candidate for the engine of this expansion. The Martian atmosphere, although thin compared to Earth’s, produces currents capable of moving fine particles of dust and ash over considerable distances. The ESA’s hypothesis is that these winds are either carrying volcanic ash to new areas, covering the orange soil with dark material, or sweeping away the light dust that previously concealed the volcanic substrate. In both scenarios, the visual result is the same: the dark spot appears to grow because more dark soil is exposed with each season of winds on Mars.
What the Viking probes discovered when they arrived on Mars in 1976
The Viking missions represented humanity’s first direct contact with the surface of Mars. Viking 2 landed in Utopia Planitia with the goal of conducting experiments that could detect signs of microbial life in the soil, and during four years of operation collected data that shaped scientific understanding of the composition and conditions of the Martian surface. It was in this context that the first images of the dark spot were recorded, although at the time the phenomenon did not receive the attention it is gaining now.
Fifty years later, the scale of change is visible. What appeared in 1976 as a dark area confined within a crater on the surface of Mars now spreads for hundreds of kilometers beyond its original boundaries. The direct comparison between the Viking photos and those from Mars Express shows that the spot not only remained but expanded measurably, raising questions that the instruments of the 1970s were not capable of answering and that even current technology addresses only with hypotheses.
What the spot may reveal about Mars’ geological past
The existence of volcanic ash spread over such a vast area confirms that Mars had intense volcanic activity in remote periods. The dark material that makes up the spot was deposited by volcanic activity dated millions of years in the past, and its preservation on the surface indicates that erosive processes on Mars are slow enough for ancient geological traces to survive for immense periods. This durability is one of the reasons why scientists consider the red planet an exceptionally well-preserved geological archive.
The interaction between the winds and this volcanic material also provides data on the current atmospheric dynamics of Mars. If the spot is indeed growing because the winds are redistributing ash or removing surface dust, each centimeter of advance acts as an indirect record of the planet’s atmospheric circulation patterns over decades. For researchers, monitoring the expansion of this formation is a way to study the Martian climate in real-time, using Mars’ own surface as a measuring instrument. The phenomenon that began as a visual curiosity in the 1976 photos has transformed into a scientific tool that may help understand how the planet functions today.
And you, do you think the spot on Mars may have an explanation that science has not yet considered? Do you believe future missions will be able to decipher the mystery? Leave your opinion in the comments.
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