Mars’ Black Beauty
Recently, scientists studying samples from the martian meteorite Black Beauty (aka NWA 7533/7034 ) uncovered new details about the ancient climate of Mars. Their results will shape astrobiologists’ understanding of potentially habitable environments in Mars’ past.
The study focused on minerals known as zircons found in meteorite samples. Zircons are also found on Earth and form when lava cools. Using these minerals, and the geological information they contain, the international team of researchers was able to construct a timeline of Mars’ climate history.
To read more about zircons and the team’s results, check out the press release from Florida State University.
In order to learn a bit more about Black Beauty and the lessons it has to teach astrobiologists, astrobio.net spoke with Dr. Munir Humayun, a co-author of the study and Professor in the Department of Earth, Ocean, and Atmospheric Science & the National High Magnetic Field Laboratory at Florida State University.
Astrobiology Magazine (AM): Where and when was Black Beauty found and how did you get involved in studying samples from the meteorite?
Munir Humayun:Black Beauty was recovered from the desert of southern Morocco and ended up with a meteorite dealer, Aziz Habibi, in Erfoud, who then sold a stone from it to US meteorite collector Jay Piatek. Separate stones (there are at least 8 stones known) were sold to French collector Luc Labenne, and to others. Piatek sent his sample to the University of New Mexico for identification, where a team led by Dr. Carl Agee identified it as a unique Martian meteorite. Their results were reported in the journal Science. Carl also gave it the popular nickname Black Beauty.
A sample brought by Luc Labenne to the MNHN (Paris) was examined by Dr. Brigitte Zanda and colleagues. That sample was brought to my lab where we identified it as a highlands breccia on the basis of its high abundance of siderophile elements – elements that are abundant in meteoritic impactors but very low in martian crustal rocks. Very importantly, Dr. Alexander Nemchin and colleagues analyzed the radiometric ages of the mineral zircon in this meteorite and obtained ancient ages that confirmed its highlands origin.
AM: How does this meteorite differ from other Mars meteorites that have been found on Earth?
Munir Humayun: With one exception, the other Martian meteorites are younger igneous rocks ranging in age from 1.4 billion years old to younger ages of 150 million years.
(The exception is the breccia, ALH 84001, which became notorious in 1996 when a team of NASA scientists announced that there were nanofossils preserved within it. That rock is about 4 billion years old, and consists mostly of a single igneous mineral, orthopyroxene, but has carbonate and magnetite minerals in fractures or veins that contain clues to the ancient martian environment.)
The surface of Mars is covered with ancient cratered terrains, particularly in the southern highlands, while younger volcanic rocks make up only about 15% of its surface. It has been very surprising that the recovered martian meteorites to-date have largely been associated with this minor surface exposure while the majority of the surface went unsampled. Likely, we hadn’t learned to recognize these highlands breccias before, and I expect that there may be more such meteorites.
Nonetheless, Black Beauty is the first highlands breccia to be recognized and it is providing important clues to the most common surface rocks of Mars. This new information comes at a fortuitous time since the Mars rovers, and the Mars Science Laboratory (MSL) Curiosity are actively exploring Mars and observing rocks of very similar composition to Black Beauty.
AM: Have any constraints been placed on when Black Beauty was ejected from Mars and how long it spent in space before landing on Earth?
Munir Humayun:Black Beauty was ejected about 5 million years ago from the surface of Mars. We don’t know yet how long it has resided on Earth, but the fresh appearance of Black Beauty including the lack of desert weathering products common in meteorites with a long residence time on Earth indicates it has not spent long on Earth.
AM: The paper reports that variations were found in the oxygen isotopic composition of four zircon grains found in samples from NWA 7533. Is there a simple way of explaining why different isotopes would be found in the same meteorite?
Munir Humayun: Yes. In a rock that’s a breccia, isotopic homogenization is not achieved because bits and pieces of different portions of the Martian crust get incorporated in it. The specific oxygen isotope signature that we observed is produced in the atmosphere of Mars by ultraviolet light forming ozone. That isotopic signature is then transmitted to hydrogen peroxide and water. Isotopically labeled water enters the rock and reacts with the zircon minerals in several ways leaving its distinct isotope signature in the mineral structure of the zircon (ZrSiO4), an oxygen-bearing mineral.
One of the simplest ways it does this in is that the crystal structure of zircon is damaged by the radioactive decay of uranium and thorium contained within it. This radioactive decay forms the basis of dating the mineral grains, but when the radioactivity is high in portions of the grains it turns the crystalline zircon into an amorphous (glassy) mess that reacts with water.
(Crystalline zircon survives weathering on Earth extremely well, but the amorphous variety does not.)
The interaction between water and damaged zircons can continue until a heating event on Mars (perhaps an impact, or local volcanic activity) anneals (recrystallizes) the zircon crystal structure. At this point, the newly formed zircon crystal re-starts accumulating the daughter products of U radioactive decay, i.e. it re-starts the clock; and it locks in the oxygen isotope composition of the last fluids to interact with it.
Thus, our team was able to show that the youngest zircon to lock in its radiometric clocks at about 1.4 billion years ago had the record highest oxygen isotope signal implying that the Martian atmosphere appears to have gotten thinner with time. The reasoning behind this inference is that in a dense (warm) early atmosphere the ultraviolet radiation from the Sun cannot reach deep into the atmosphere – this is what happens on Venus and Earth.
AM: You’ve built an isotopic record of how Mars’ atmosphere changed (with dates) – what period of time does this record cover?
Munir Humayun: The record covers the period from nearly the birth of the martian crust at 4.43 billion years ago to about 1.4 billion years ago.
AM: In that isotopic record, what would be your ‘highlights’ or key moments on the timeline?
Munir Humayun: We basically have two control points: one at about 4.4 billion years ago, and another at 1.4 billion years ago. MSL is measuring the oxygen isotopic composition of modern Martian atmosphere, and younger shergottite meteorites contain weathering products that must have been incorporated into the rock after their igneous crystallization age at 150 million years. Thus, we have added two control points to the record of time that significantly precedes that which can be obtained from the other martian meteorites, all the way back to the birth of Mars.
AM: Will you be continuing your study using samples from other Mars meteorites?
Munir Humayun: Absolutely. Black Beauty contains a treasure trove of clues to the history of Mars.
The analytical work for the study was largely accomplished at the NordSIMS facility at the Natural History Museum in Sweden by Dr. Alexander Nemchin and Prof. Martin Whitehouse.
Funding sources for this work included the NASA Cosmochemistry program, the Knut and Alice Wallenberg Foundation, and the Swedish Research Council.
The study was published in the journal Nature Geoscience.