In 2012, NASA landed the rover Curiosity in the Gale crater on Mars because
the crater was thought by many scientists to be the site of an ancient lake
on Mars more than 3 billion years ago. Since that time, the rover has been
driving along, carrying out geological analyses with its suite of
instruments for over 3,190 sols (martian days, equivalent to 3278 earth
days). After analyzing the data, researchers from Department of Earth
Sciences, the Faculty of Science at HKU, have proposed that the sediments
measured by the rover during most of the mission did not actually form in a
lake.
The researcher team suggested that the large mound of sedimentary rocks
explored and analyzed for the last eight years actually represent sand and
silt deposited as air-fall from the atmosphere and reworked by the wind. The
alteration minerals formed by the interaction between water and the sand did
not occur in a lake setting. The “wet” environment, they propose, actually
represents weathering similar to soil formation under rainfall in an ancient
atmosphere that was very different from the present one.
The discovery was published recently in Science Advances in a paper led by
research postgraduate student Jiacheng LIU, his advisor Associate Professor
Dr. Joe MICHALSKI, and co-author Professor Mei Fu ZHOU, all of whom are
affiliated with the Department of Earth Sciences. The researchers used
chemistry measurements and x-ray diffraction (XRD) measurements, in addition
to images of rock textures, to reveal how compositional trends in the rocks
relate to geological processes.
“Jiacheng has demonstrated some very important chemical patterns in the
rocks, which cannot be explained in the context of a lake environment,” said
Dr. Michalski. “The key point is that some elements are mobile, or easy to
dissolve in water, and some elements are immobile, or in other words, they
stay in the rocks. Whether an element is mobile or immobile depends not only
on the type of element but also on the properties of the fluid. Was the
fluid acidic, saline, oxidizing, etc. Jiacheng’s results show that immobile
elements are correlated with each other, and strongly enriched at higher
elevations in the rock profile. This points toward top-down weathering as
you see in soils. Further, he shows that iron is depleted as weathering
increases, which means that the atmosphere at the time was reducing on
ancient Mars, not oxidizing like it is in the modern day, rusted planet.”
Understanding how the Martian atmosphere, and the surface environment as a
whole, evolved is important for the exploration for possible life on Mars,
as well as our understanding of how Earth may have changed during its early
history.
“Obviously, studying Mars is extremely difficult, and the integration of
creative and technologically advanced methodologies are necessary. Liu and
co-authors have made intriguing observations via the utilization of remote
sensing techniques to understand the chemical composition of ancient
sediments that inform on their early development. Their data present
challenges to existing hypotheses for both the depositional environment of
these unique rock formations and the atmospheric conditions that they formed
under – specifically, the authors show evidence for weathering processes
under a reducing atmosphere in a subareal environment similar to a desert,
rather than formation in an aqueous lake environment. Indeed, this work will
inspire new and exciting directions for future research.” Assistant
Professor from Department of Earth Science Dr. Ryan McKenzie added.
China successfully landed its first lander, Zhurong, on Mars in May this
year. Zhurong is currently roving the plains of Utopia Planitia, exploring
mineralogical and chemical clues to recent climate change. China is also
planning a sample return mission likely to occur at the end of this decade.
Reference:
Intense subaerial weathering of eolian sediments in Gale crater, Mars
Science Advances 06 Aug 2021:
Vol. 7, no. 32, eabh2687
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