Researchers Trace Mercury's Origins to Rare Enstatite Chondrite Meteorite

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http://news.mit.edu/2016/mercury-origins-rare-meteorite-0627

Researchers trace Mercury's origins to rare meteorite
Experiments show planet cooled dramatically in half a billion years.
Jennifer Chu | MIT News Office
June 27, 2016

Around 4.6 billion years ago, the universe was a chaos of collapsing gas
and spinning debris. Small particles of gas and dust clumped together
into larger and more massive meteoroids that in turn smashed together
to form planets. Scientists believe that shortly after their formation,
these planets - and particularly Mercury - were fiery spheres of molten
material, which cooled over millions of years.

Now, geologists at MIT have traced part of Mercury's cooling history
and found that between 4.2 and 3.7 billion years ago, soon after the planet
formed, its interior temperatures plummeted by 240 degrees Celsius, or
464 degrees Fahrenheit.

They also determined, based on this rapid cooling rate and the composition
of lava deposits on Mercury's surface, that the planet likely has the
composition of an enstatite chondrite - a type of meteorite that is
extremely rare here on Earth.

Timothy Grove, the Cecil and Ida Green Professor of Geology in MIT's
Department of Earth, Atmospheric, and Planetary Sciences, says new information
on Mercury's past is of interest for tracing Earth's early formation.

"Here we are today, with 4.5 billion years of planetary evolution, and
because the Earth has such a dynamic interior, because of the water we've
preserved on the planet, [volcanism] just wipes out its past," Grove
says. "On planets like Mercury, early volcanism is much more dramatic,
and [once] they cooled down there were no later volcanic processes to
wipe out the early history. This is the first place where we actually
have an estimate of how fast the interior cooled during an early part
of a planet's history."

Grove and his colleagues, including researchers from the University of
Hanover, in Germany; the University of Liege, in Belgium; and the University
of Bayreuth, in Germany, have published their results in Earth and Planetary
Science Letters.

Compositions in craters

For their analysis, the team utilized data collected by NASA's MESSENGER
spacecraft. The MErcury Surface, Space ENvironment, GEochemistry, and
Ranging (MESSENGER) probe orbited Mercury between 2011 and 2015, collecting
measurements of the planet's chemical composition with each flyby. During
its mission, MESSENGER produced images that revealed kilometer-thick lava
deposits covering the entire planet's surface.

An X-ray spectrometer onboard the spacecraft measured the X-ray radiation
from the planet's surface, produced by solar flares on the sun, to determine
the chemical composition of more than 5,800 lava deposits on Mercury's
surface.

Grove's co-author, Olivier Namur of the University of Hanover, recalculated
the surface compositions of all 5,800 locations, and correlated each composition
with the type of terrain in which it was found, from heavily cratered
regions to those that were less impacted. The density of a region's
craters can tell something about that region's age: The more craters
there are, the older the surface is, and vice versa. The researchers were
able to correlate Mercury's lava composition with age and found that
older deposits, around 4.2 billion years old, contained elements that
were very different from younger deposits that were estimated to be 3.7
billion years old.

"It's true of all planets that different age terrains have different
chemical compositions because things are changing inside the planet,"
Grove says. "Why are they so different? That's what we're trying
to figure out."

A rare rock, 10 standard deviations away

To answer that question, Grove attempted to retrace a lava deposit's
path, from the time it melted inside the planet to the time it ultimately
erupted onto Mercury's surface.

To do this, he started by recreating Mercury's lava deposits in the
lab. From MESSENGER's 5,800 compositional data points, Grove selected
two extremes: one representing the older lava deposits and one from the
younger deposits. He and his team converted the lava deposits' element
ratios into the chemical building blocks that make up rock, then followed
this recipe to create synthetic rocks representing each lava deposit.

The team melted the synthetic rocks in a furnace to simulate the point
in time when the deposits were lava, and not yet solidified as rock. Then,
the researchers dialed the temperature and pressure of the furnace up
and down to effectively turn back the clock, simulating the lava's eruption
from deep within the planet to the surface, in reverse.

Throughout these experiments, the team looked for tiny crystals forming
in each molten sample, representing the point at which the sample turns
from lava to rock. This represents the stage at which the planet's solid
rocky core begins to melt, creating a molten material that sloshes around
in Mercury's mantle before erupting onto the surface.

The team found a surprising disparity in the two samples: The older rock
melted deeper in the planet, at 360 kilometers, and at higher temperatures
of 1,650 C, while the younger rock melted at shallower depths, at 160
kilometers, and 1,410 C. The experiments indicate that the planet's
interior cooled dramatically, over 240 degrees Celsius between 4.2 and
3.7 billion years ago - a geologically short span of 500 million years.

"Mercury has had a huge variation in temperature over a fairly short
period of time, that records a really amazing melting process," Grove
says.

The researchers determined the chemical compositions of the tiny crystals
that formed in each sample, in order to identify the original material
that may have made up Mercury's interior before it melted and erupted
onto the surface. They found the closest match to be an enstatite chondrite,
an extremely rare form of meteorite that is thought to make up only about
2 percent of the meteorites that fall to Earth.

"We now know something like an enstatite chondrite was the starting
material for Mercury, which is surprising, because they are about 10 standard
deviations away from all other chondrites," Grove says.

Grove cautions that the group's results are not set in stone and that
Mercury may have been an accumulation of other types of starting materials.
To know this would require an actual sample from the planet's surface.

"The next thing that would really help us move our understanding of
Mercury way forward is to actually have a meteorite from Mercury that
we could study," Grove says. "That would be lovely."

This research was funded, in part, by NASA.