 |
| MSL Curiosity rover at "John Klein" drill site This self-portrait of NASA's Mars rover Curiosity combines dozens of exposures taken by the rover's Mars Hand Lens Imager on Feb. 3, 2013 plus three exposures taken May 10, 2013 to show two holes (in lower left quadrant) where Curiosity used its drill on the rock target "John Klein". |
The team responsible for the Sample Analysis at Mars (SAM) instrument
suite on NASA's Curiosity rover has made the first definitive detection
of organic molecules at Mars. Organic molecules are the building blocks
of all known forms of terrestrial life, and consist of a wide variety of
molecules made primarily of carbon, hydrogen, and oxygen atoms.
However, organic molecules can also be made by chemical reactions that
don't involve life, and there is not enough evidence to tell if the
matter found by the team came from ancient Martian life or from a
non-biological process. Examples of non-biological sources include
chemical reactions in water at ancient Martian hot springs or delivery
of organic material to Mars by interplanetary dust or fragments of
asteroids and comets.
The surface of Mars is currently inhospitable to life as we know it,
but there is evidence that the Red Planet once had a climate that could
have supported life billions of years ago. For example, features
resembling dry riverbeds and minerals that only form in the presence of
liquid water have been discovered on the Martian surface. The Curiosity
rover with its suite of instruments including SAM was sent to Mars in
2011 to discover more about the ancient habitable Martian environment by
examining clues in the chemistry of rocks and the atmosphere.
The organic molecules found by the team were in a drilled sample of
the Sheepbed mudstone in Gale crater, the landing site for the Curiosity
rover. Scientists think the crater was once the site of a lake billions
of years ago, and rocks like mudstone formed from sediment in the lake.
Moreover, this mudstone was found to contain 20 percent smectite clays.
On Earth, such clays are known to provide high surface area and optimal
interlayer sites for the concentration and preservation of organic
compounds when rapidly deposited under reducing chemical conditions.
While the team can't conclude that there was life at Gale crater, the
discovery shows that the ancient environment offered a supply of
reduced organic molecules for use as building blocks for life and an
energy source for life. Curiosity's earlier analysis of this same
mudstone revealed that the environment offered water and chemical
elements essential for life and a different chemical energy source.
"We think life began on Earth around 3.8 billion years ago, and our
result shows that places on Mars had the same conditions at that time --
liquid water, a warm environment, and organic matter," said Caroline
Freissinet of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
"So if life emerged on Earth in these conditions, why not on Mars as
well?" Freissinet is lead author of a paper on this research submitted
to the Journal of Geophysical Research-Planets.
The organic molecules found by the team also have chlorine atoms, and
include chlorobenzene and several dichloroalkanes, such as
dichloroethane, dichloropropane and dichlorobutane. Chlorobenzene is the
most abundant with concentrations between 150 and 300
parts-per-billion. Chlorobenzene is not a naturally occurring compound
on Earth. It is used in the manufacturing process for pesticides
(insecticide DDT), herbicides, adhesives, paints and rubber.
Dichloropropane is used as an industrial solvent to make paint
strippers, varnishes and furniture finish removers, and is classified as
a carcinogen.
It's possible that these chlorine-containing organic molecules were
present as such in the mudstone. However, according to the team, it's
more likely that a different suite of precursor organic molecules was in
the mudstone, and that the chlorinated organics formed from reactions
inside the SAM instrument as the sample was heated for analysis.
Perchlorates (a chlorine atom bound to four oxygen atoms) are abundant
on the surface of Mars. It's possible that as the sample was heated,
chlorine from perchlorate combined with fragments from precursor organic
molecules in the mudstone to produce the chlorinated organic molecules
detected by SAM.
In 1976, the Gas Chromatograph Mass Spectrometer instrument on NASA's
Viking landers detected two simple chlorinated hydrocarbons after
heating Martian soils for analysis (chloromethane and dichloromethane).
However they were not able to rule out that the compounds were derived
from the instrument itself, according to the team. While sources within
the SAM instrument also produce chlorinated hydrocarbons, they don't
make more than 22 parts-per-billion of chlorobenzene, far below the
amounts detected in the mudstone sample, giving the team confidence that
organic molecules really are present on Mars.
The SAM instrument suite was built at NASA Goddard with significant
elements provided by industry, university, and national and
international NASA partners.
For this analysis, the Curiosity rover sample acquisition system
drilled into a mudstone and filtered fine particles of it through a
sieve, then delivered a portion of the sample to the SAM laboratory. SAM
detected the compounds using its Evolved Gas Analysis (EGA) mode by
heating the sample up to about 875 degrees Celsius (around 1,600 degrees
Fahrenheit) and then monitoring the volatiles released from the sample
using a quadrupole mass spectrometer, which identifies molecules by
their mass using electric fields. SAM also detected and identified the
compounds using its Gas Chromatograph Mass Spectrometer (GCMS) mode. In
this mode, volatiles are separated by the amount of time they take to
travel through a narrow tube (gas chromatography -- certain molecules
interact with the sides of the tube more readily and thus travel more
slowly) and then identified by their signature mass fragments in the
mass spectrometer.
The first evidence for elevated levels of chlorobenzene and
dichloroalkanes released from the mudstone was obtained on Curiosity Sol
290 (May 30, 2013) with the third analysis of the Cumberland sample at
Sheepbed. The team spent over a year carefully analyzing the result,
including conducting laboratory experiments with instruments and methods
similar to SAM, to be sure that SAM could not be producing the amount
of organic material detected.
"The search for organics on Mars has been extremely challenging for
the team," said Daniel Glavin of NASA Goddard, a co-author on the paper.
"First, we need to identify environments in Gale crater that would have
enabled the concentration of organics in sediments. Then they need to
survive the conversion of sediment to rock, where pore fluids and
dissolved substances may oxidize and destroy organics. Organics can then
be destroyed during exposure of rocks at the surface of Mars to intense
ionizing radiation and oxidants. Finally, to identify any organic
compounds that have survived, we have to deal with oxychlorine compounds
and possibly other strong oxidants in the sample which will react with
and combust organic compounds to carbon dioxide and chlorinated
hydrocarbons when the samples are heated by SAM."
As part of Curiosity's plan for exploration, an important strategic
goal was to sample rocks that represent different combinations of the
variables thought to control organic preservation. "The SAM and Mars
Science Laboratory teams have worked very hard to achieve this result,"
said John Grotzinger of Caltech, Mars Science Laboratory's Project
Scientist. "Only by drilling additional rock samples in different
locations, and representing different geologic histories were we able to
tease out this result. At the time we first saw evidence of these
organic molecules in the Cumberland sample it was uncertain if they were
derived from Mars, however, additional drilling has not produced the
same compounds as might be predicted for contamination, indicating that
the carbon in the detected organic molecules is very likely of Martian
origin."
No comments:
Post a Comment