Tiny Crystal Shapes Get Close Look From Mars Rover

Star-shaped and swallowtail-shaped tiny, dark bumps in
fine-layered
bright bedrock of a Martian ridge are drawing
close inspection by NASA’s
Curiosity Mars rover.

This set of shapes looks familiar to geologists who have
studied gypsum crystals
formed in drying lakes on Earth, but
Curiosity’s science team is considering
multiple possibilities
for the origin of these features on “Vera Rubin
Ridge” on
Mars.

One uncertainty the rover’s inspection may resolve is the
timing of when
the crystal-shaped features formed, relative to
when layers of sediment accumulated
around them. Another is
whether the original mineral that crystallized into
these
shapes remains in them or was subsequently dissolved away and
replaced by
something else. Answers may point to evidence of a
drying lake or to
groundwater that flowed through the sediment
after it became cemented into
rock.

The rover team also is investigating other clues on the same
area to
learn more about the Red Planet’s history. These
include stick-shaped features the
size of rice grains, mineral
veins with both bright and dark zones, color
variations in the
bedrock, smoothly horizontal laminations that vary more than
tenfold in thickness of individual layers, and more than
fourfold variation in
the iron content of local rock targets
examined by the rover.

“There’s just a treasure trove of interesting targets
concentrated
in this one area,” said Curiosity Project
Scientist Ashwin Vasavada of
NASA’s Jet Propulsion Laboratory,
Pasadena, California. “Each is a clue,
and the more clues, the
better. It’s going to be fun figuring out what it all
means.”

Vera Rubin Ridge stands out as an erosion-resistant band on the
north
slope of lower Mount Sharp inside Gale Crater. It was a
planned destination for
Curiosity even before the rover’s 2012
landing on the crater floor near the
mountain. The rover began
climbing the ridge about five months ago and has now reached
the uphill, southern edge. Some features here might be related
to a transition
to the next destination area uphill, which is
called the “Clay Unit”
because of clay minerals detected from
orbit.

The team drove the rover to a site called “Jura” in
mid-January
to examine an area where — even in images from
orbit — the bedrock is
noticeably pale and gray, compared to
the red, hematite-bearing bedrock forming
most of Vera Rubin
Ridge.

“These tiny ‘V’ shapes really caught our attention, but they
were
not at all the reason we went to that rock,” said
Curiosity science-team
member Abigail Fraeman of JPL. “We were
looking at the color change from
one area to another. We were
lucky to see the crystals. They’re so tiny, you
don’t see them
until you’re right on them.”

The features are about the size of a sesame seed. Some are
single elongated
crystals. Commonly, two or more coalesce into
V-shaped “swallowtails”
or more complex “lark’s foot” or star
configurations. “These shapes
are characteristic of gypsum
crystals,” said Sanjeev Gupta, a Curiosity
science-team member
at Imperial College, London, who has studied such crystals
in
rocks of Scotland. Gypsum is a form of calcium sulfate. “These
can form
when salts become concentrated in water, such as in
an evaporating lake.”

The finely laminated bedrock at Jura is thought to result from
lakebed
sedimentation, as has been true in several lower,
older geological layers Curiosity
has examined. However, an
alternative to the crystals forming in an evaporating
lake is
that they formed much later from salty fluids moving through
the rock.
That is also a type of evidence Curiosity has
documented in multiple geological
layers, where subsurface
fluids deposited features such as mineral veins.

Some rock targets examined in the Jura area have two-toned
mineral veins
that formed after the lake sediments had
hardened into rock. Brighter portions
contain calcium sulfate;
darker portions contain more iron. Some of the
features shaped
like gypsum crystals appear darker than gypsum, are enriched
in
iron, or are empty. These are clues that the original
crystallizing material
may have been replaced or removed by
later effects of underground water.

The small, stick-shaped features were first seen two days
before
Curiosity reached Jura. All raw images from Mars rovers
are quickly posted
online, and some showing the “sticks” drew
news-media attention comparing
them to fossils. Among the
alternative possibilities is that they are bits of
the dark
vein material. Rover science team members have been more
excited about
the swallowtails than the sticks.

“So far on this mission, most of the evidence we’ve seen about
ancient lakes in Gale Crater has been for relatively fresh,
non-salty water,”
Vasavada said. “If we start seeing lakes
becoming saltier with time, that
would help us understand how
the environment changed in Gale Crater, and it’s
consistent
with an overall pattern that water on Mars became more scarce
over
time.”

Such a change could be like the difference between freshwater
mountain
lakes, resupplied often with snowmelt that keeps
salts diluted, and salty lakes
in deserts, where water
evaporates faster than it is replaced.

If the crystals formed inside hardened rock much later, rather
than in an
evaporating lake, they offer evidence about the
chemistry of a wet underground
environment.

“In either scenario, these crystals are a new type of evidence
that
builds the story of persistent water and a long-lived
habitable environment on
Mars,” Vasavada said.

Variations in iron content in the veins, smaller features and
surrounding
bedrock might provide clues about conditions
favorable for microbial life. Iron
oxides vary in their
solubility in water, with more-oxidized types generally
less
likely to be dissolved and transported. An environment with a
range of
oxidation states can provide a battery-like energy
gradient exploitable by some
types of microbes.

“In upper Vera Rubin Ridge, we see clues that there were
fluids
carrying iron and, through some mechanism, the iron
precipitated out,”
Fraeman said. “There was a change in fluid
chemistry that could be
significant for habitability.”

For more about NASA’s Curiosity Mars rover
mission, visit:

https://mars.jpl.nasa.gov/msl

News Media Contact

Guy Webster / Andrew Good
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278 / 818-393-2433
guy.webster@jpl.nasa.gov / andrew.c.good@jpl.nasa.gov

Laurie Cantillo / Dwayne Brown
NASA Headquarters, Washington
202-358-1077 / 202-358-1726
laura.l.cantillo@nasa.gov / dwayne.c.brown@nasa.gov

2018-026

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