A giant floating raft of volcanic rock called pumice could have drifted across an ancient Martian ocean and created one of the most mysterious features we can see on the planet鈥檚 surface today.
The Medusae Fossae Formation (MFF), found near the Martian equator, spans about 5000 kilometres. Comprising rolling hills and mounds, scientists have previously suggested its origin was volcanic ash from the nearby Olympus Mons or Elysium Mons volcanoes.
However, the size of the MFF poses some problems for this hypothesis. 鈥淚t鈥檚 enormous, over a million square kilometres of land area covered by [an estimated] million cubic kilometres of ash,鈥� says James Zimbelman at the Smithsonian Institution in Washington DC. 鈥淭hat鈥檚 many orders of magnitude larger than the largest volcanic eruptions we know here on Earth.鈥�
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Instead, Zimbelman and Peter Mouginis-Mark at the University of Hawaii looked at whether a raft of pumice could have been the origin of this mysterious feature. Mouginis-Mark first proposed the idea in 1993, but subsequent images from Mars-orbiting spacecraft have provided new evidence.
Pumice is a light and porous volcanic rock produced when lava interacts with water and rapidly cools, leaving behind trapped bubbles of gas. On Earth, it can build up to produce vast floating rafts, such as one spanning 150 square kilometres seen on the Pacific Ocean in 2019.
Evidence of landslides on Olympus Mons suggest a similar process could have happened on Mars, say the researchers. The pumice would have built up before breaking away, drifting across a theorised ancient Martian ocean and eventually coming to rest at a shoreline 鈥� the current location of the MFF.
鈥淭he main problem is there isn鈥檛 a volcano nearby that appears to have exploded with the volume of material [needed for the volcanic ash idea],鈥� says Mouginis-Mark. 鈥淪o we started to talk about, well, if you鈥檝e got these rafts of pumice floating around on a palaeo-Martian ocean, where would they go?鈥�
Evidence for such an ocean is debated, but if it did exist, it would probably have been at least a few hundred metres deep, spanning the planet鈥檚 northern hemisphere. A southwesterly Martian wind, made possible by the thicker atmosphere at the time, could then have transported a pumice raft on the ocean 4000 kilometres from Olympus Mons to where the MFF is found today.
Kevin Lewis at Johns Hopkins University in Maryland, who wasn鈥檛 involved in this research but previously worked on the volcanic ash origin of the MFF, said the pumice raft idea was 鈥渘ot totally outlandish鈥� and worthy of further investigation. 鈥淚t鈥檚 certainly an interesting and creative new hypothesis,鈥� he says.
If true, it could sway the debate on the presence of ancient large oceans on Mars, and potentially its previous suitability for life.
鈥淭he rafted pumice proposal really requires an open, unfrozen body of water,鈥� says Lewis. 鈥淚t would certainly provide some pretty interesting information about the Martian palaeoclimate in terms of water abundance and maybe habitability.鈥�
Unfortunately, without sampling the MFF directly, it will be difficult to make a conclusive statement on its origin, and that might not happen any time soon.
鈥淚t really isn鈥檛 high on most people鈥檚 priority lists,鈥� says Mouginis-Mark. 鈥淏ut we鈥檙e now at a point where we can at least put together some reasonable models.鈥�
Icarus
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