Meteorites on Ice

Left: map of Antarctica, where the highlighted area is blown up in a big map showing Transantarctic Mountain range and the ice fields where meteorites have been found. Image: NASA. Right: Sketch showing how meteorites falling on the ice get transported and concentrated next to mountain ranges. Image: NASA.

Although not exactly Moon focused, I hope that this story is of interest to Moon Zoo Forum members as this is the way many lunar meteorites are found here on Earth (see previous Moon Zoo blog ).This past winter (2011-2012) I was lucky enough to join the Antarctic Search for Meteorites (ANSMET) team to hunt for meteorites in Antarctica. The ANSMET programme, funded by NASA and the US National Science Foundation and run between Case Western Reserve University, NASA JSC and the Smithsonian, has been running since 1976 exploring the ice of Antarctica for meteorites. So far about 20,000 meteorites have been collected and made available to the scientific community to study to understand planetary processes. Most of these meteorites originate from the asteroid belt, but some very rare ones have come from Mars and the Moon.Why Antarctica? The team collects meteorites in Antarctica because they are well preserved in the cold dry icy environment. They are also easy to spot as dark rocks on the white ice. Most meteorites are found in icefields close to the Transantarctic Mountain range – you can see a map where all the yellow labels mark places that meteorites have been collected. These localities are really great for concentrating lots meteorites that have travelled from the South Pole ice plateau, and are brought up to the ice sheet surface near mountain ranges

Arriving and life on the ice: Images: Antarctic Search for Meteorites Program / Katie Joy.

How do we get to Antarctica and what is life like on the ice? Our team of six guys and two girls flew to Christchurch, New Zealand, and then were flown down to the US McMurdo base in Antarctica. We spent a week or so in McMurdo preparing for our expedition. We packed up our gear and selected food supplies to last us for six weeks camping on the ice. When we were trained and prepared we flewout to the Transantarctic Mountains on a military Hercules plane with skis and then a smaller Twin Otter plane. We set up camp in the Miller Range – a stunning area with mountains and glaciers. Our camp consisted of four tents, where we lived two people to a tent, and a tent with a toilet (a glamorous bucket!) and one that we used to all gather in the evenings (the party tent). Temperatures varied from about -10°C down to -30°C outside, but when the wind was blowing from the South Pole plateau, it felt a lot colder! You have to wrap up in many layers to stay warm – I typically wore four layers on my legs and between five and seven on top, including my big orange down jacket. I also wore a full face mask to protect my face and eyes from the cold and glare from the sun.

Collecting meteorites on the ice (the meteorite are the brown/black rocks we are all gathered around!): Images: Antarctic Search for Meteorites Program / Katie Joy.

How does meteorite hunting work? We had a surprisingly large amount of snow during our field season – which is rather unusual for Antarctica as it is supposed to be a cold dry desert. The snow caused us lots of problems trying to find the meteorites, so we spent a lot of time stuck in our tents rather than looking for meteorites. When the weather was good enough to allow us to look we would get on our snowmobiles (skidoos) and drive out to a new area of ice. We lined up and drove up and down in straight line formation with about ten metres between each team member. When someone spotted a black rock on the ice they would jump off their skidoo, check it was a meteorite, and then wave their arms madly in the air to call the other people over to come and help collect it. We photographed the meteorite, logged its location and carefully put it into a special collection bag ready to send back to NASA. Sometimes we would walk across the ice to search, and other times we would look in rocky areas called moraines to see if we could spot a meteorite. It was a frustrating process when you didn’t find a meteorite, but great fun and satisfying when you did. Our team found 302 stones this year, which considering the bad weather, wasn’t a bad total at all. In fact we were lucky enough to collect the 20,000th sample ANSMET have collected, which was cause for a big celebration.

The samples are all now back at NASA Johnson Space Center ready to be classified and studied by scientists all over the world. Initial identification of the meteorites was recently announced at http://curator.jsc.nasa.gov/antmet/amn/amn.cfm#352 and the meteorites our team collected have been given the name Miller Range 11XXX as they were collected in the 2011 ANSMET season. So far (the curation staff are still working hard to classify all the samples!) it doesn’t look like we found any lunars or martian meteorites this year, but did find a large Howardite and several Diogenite meteorites, which may have originated from the asteroid Vesta.

ANSMET will be taking place this year, the team sets off to the ice at the beginning of December 2012, and you can follow the blog charting the expedition via http://geology.cwru.edu/~ansmet/

More information about the ANSMET programme can be found at http://curator.jsc.nasa.gov/antmet/program.cfm and http://geology.cwru.edu/~ansmet/

This is modified from a blog that first appeared on http://earthandsolarsystem.wordpress.com/2012/07/30/meteorites-on-ice/

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Big Bangs in the Solar System

Hello all. I have written a few Moon Zoo blogs about different aspects of lunar geology and the geological history of the Moon, here, I thought I would share with you the lunar research topic I am currently working on. I hope that it is interesting and if you have any questions please do let me know. Katie Joy

The Moon’s surface is absolutely covered in impact craters (Figure 1). These range in size from the behemoth South Pole-Aitkin basin, which is a staggeringly ~2500 km in diameter and 13 km deep, all the way down to microscopic impact craters on glass beads that are less than a millimetre in diameter. No matter the size, the one thing all these impact craters have in common is that that they were created when something smashed into the Moon at high speed on the order of 15 km/sec or more (that’s pretty fast when you consider a bullet out of shotgun travels at about 0.9 km/sec).

Impacts have changed the surfaces of all rock and ice planetary bodies in the Solar System, including the Earth. However, they are so well preserved on the Moon that we can use lunar impact craters and lunar impact rocks that were made when these craters formed, to reveal the history of impact bombardment in the whole of the inner Solar System through the past 4.5 billion years.

Figure 1. Impact craters and basins on the Moon are clearly seen in topography maps of the Moon. The gigantic South Pole-Aitkin basin (labelled SPA) is located on the southern part of the far side of the Moon. Credit: Clementine mission/NASA.

When did the Moon get bombarded?
We know that a lot of large projectiles hit the Moon early in its history, and that the rate of bombardment rapidly declined after ~3.5 billion years ago. But there are many gaps in our knowledge. Our understanding of the Moon’s impact history comes from (i) absolute age dates of rocks and soils brought back by the Apollo and Luna missions and (ii) understanding the relative age of when one impact crater formed compared with another (e.g., seeing when one crater occurred on top of another crater to build up a scale of what is older and what is younger).

Data from both sources have been combined to develop a lunar cratering curve – where the number of craters on a region of the Moon’s surface (as measured by crater counting like you are doing in the Moon Zoo crater survey task) is compared with the absolute rock age of that surface. A graphical representation of this curve is shown in Figure 2. As we have rocks from only nine known localities on the Moon, the scientific calibration of this curve is poor before ~4 billion years ago and for more recently after 3 Ga (see Figure 2). So, there are still some big questions about how many impacts have been hitting the Moon through time and if there has been one or more ‘spikes’ in the impact rate; when the Moon was pummelled by impacting projectiles in a very short period of time. One of these spikes is thought to have happened at ~3.9 Ga and is known as the lunar cataclysm (Figure 2). Other smaller spikes may also have occurred at other times in the Moon’s past.

Figure 2. The grey line indicates a possible lunar cratering chronology (number of impact craters ?1 km per km2 that have formed on the Moon’s surface at different times, based on a set ‘Size-Frequency Distribution’ for the projectiles). The black calibration points are derived from age dating samples of impact basins and lava flows (A = Apollo mission sampled lava flow, L = Luna mission sampled lava flows). The blue points are best guess calibration ages of impact craters and basin based on rock ages of material found at Apollo landing sites rather than in the crater itself. The red question marks refer to questions about the parts of the lunar impact curve where we have no calibration data at all. Credit: K. Joy adapted from the Stoffler et al. (2006) chapter in the New Views of the Moon book and from David Kring/LPI’s lunar impact flux diagram <http://www.lpi.usra.edu/nlsi/science/>

 

What has been causing such damage to the Moon and why do we care?
There are several different types of projectiles that have been hitting the Moon through time. These include:

• ‘Primordial’ material left over from the formation of the Solar System – dust and small bodies that impacted the Moon very early in its history from 4.5 to 4 billion years ago (Ga).
• Asteroids – Asteroidal projectiles originate from material that has migrated in from the asteroid belt and beyond. These types of projectiles are large enough to create cm to kilometre size impact craters. Asteroids include a range of varieties including types that have differentiated (i.e., formed a crust, a mantle and a core) and those that have never been melted and represent primitive (very ancient) Solar System material. If a fragment of an asteroid survives the impact event, this material is termed a meteorite. Meteorites that have come from primitive parent bodies are known as chondritic, and those from differentiated parent bodies are known as achondritic. Achondritic meteorites include a wide range of different types of stones including iron meteorites, martian meteorites, meteorites from the crusts of objects like the asteroid Vesta.
• Comets – small icy rocky dusty bodies that orbit around the Sun and travel on orbits from the inner Solar System out into outer Solar System (known as short period comets). Some even travel outside of the Solar System’s elliptical plane (known as long period comets). Cometary projectiles are large enough to create cm to kilometre size impact craters on the Moon, but are not thought to easily survive entry through the Earth’s atmosphere.
• Dust – the Solar System is full of dust sized particles left over from impacts between planets and asteroids. When dust and micrometeorids hit the Earth’s atmosphere they burn up and form bright streaks of light we call shooting stars. However, as the Moon does not have a protective atmosphere, small dust particles rain down straight onto the lunar surface – most are instantly melted and form glassy constructs in the lunar regolith, while others form small millimetre to micrometer sized impact crater pot holes on rocks and soil particles.

It is important to understand what sort of projectiles have been striking the Moon back through time as this provides us with information about dynamical processes in the Solar System. If we can understand what types of impactors caused a big spike in the lunar impact record at 3.9 Ga (e.g. the lunar cataclysm), then this knowledge might reveal information about what types of Solar System processes were occurring at this time. For example, some researchers have suggested that the lunar cataclysm was caused when a small planet or large asteroid broke up and its debris was thrown into an Earth-Moon crossing orbit. Other researchers have suggested that the cataclysm was caused when the orbits of the gas giants (Jupiter, Saturn etc.,) were suddenly altered, and that a dramatic change in orbit meant that large amounts of outer Solar System asteroids and comets were thrown into the inner Solar System, causing widespread damage to the asteroid belt and all inner Solar System planets. This hypothesis is known as the ‘Nice Model’ (named after Nice the city, not the term of affection!).  However, these hypotheses are at the moment just good guess work – and now we need further evidence to support, or refute such ideas.

How do we know what types of projectiles have been hitting the Moon in the past?
Well, we know what types of meteorites we find on Earth at the present day. We can classify collected stones and use this information to tell us about what types of asteroid parent bodies they come from. However, it is possible that the type of meteorites we find at the present day are not the same types that were hitting the Earth and Moon in the past.

Therefore, to understand the timing and sources of ancient bombardment on the Moon we must look at lunar samples returned by the Apollo and Luna missions, or that have been collected here on Earth as lunar meteorites (see blog here). Some of these rocks were formed in impact craters, and dating these impact rocks using radiometric dating techniques tells us when impacts to the Moon took place. There are then two methods used to work out what types of projectiles were hitting the Moon at these times:
(1)   To analyse the chemistry of the rock. There are two main groups of elements that help to reveal impactor source. Siderophile elements (e.g., Ni, Fe, Co, Ir, Au, Pt, Ru, Os, etc.,) can be used to trace if the projectile was an asteroid, and if so what type of asteroid it was. Light volatile elements like hydrogen and carbon species (e.g., H2O, CO, CO2, H2, CH4, HCN, N2, etc.,) help to reveal if the sample was formed, or affected, by a volatile-rich cometary impact (see Figure 3).
(2)   To locate and classify fragments of meteorites in the rocks themselves. Fragments that survive impact onto the Moon are very rare (Figure 4). Yet, several have been found in lunar rocks and soils. These samples are very important as they can be analysed and classified as originating from certain types of asteroid parent bodies.

Figure 3. Astronaut photograph of lunar soil sample from the Apollo 16 mission to the Moon. This soil, named 61220, has high levels of carbon and other volatile species and is thought by researchers at NASA’s Johnson Space Center to have been impacted by a comet. Credit: Image and further information about soil 61220 taken from LPI.

Figure 4. Photograph of a meteorite fragment that has been found on the Moon. This is Bench Crater meteorite that is from a carbonaceous chondrite parent asteroid body. The fragment is very small – about 5 × 3 mm in size. The dark opaque areas represent a fine grained matrix. The large brown mineral grains are attributed to clays (phyllosilicates) that have been converted to a ferromagnesian phase that is intermediate in composition to the minerals olivine and pyroxene (information credit Dr. Mike Zolensky). Credit: photograph of Bench Crater meteorite by K. Joy.

Different groups of researchers around the world are examining lunar rocks to search for evidence of the timing and sources of lunar impacts. We hope that these data will help us to not only better understand the geological history of the Moon, but to understand wider processes in the Solar System that have likely effected all rocky planets like the Earth, Mars, Venus and Mercury. This information will help us to understand the history of our own planet, and address how impacts may have affected the development and proliferation of life here on Earth throughout the past 4.5 billion years.

Few things to remember:

• A projectile or an impactor is a term given to an object that strikes the Moon. This object may be a comet, an asteroid, or even dust
• If a projectile is an asteroid, surviving fragments are called meteorites
• If a projectile is smaller than ~50 m the object is often called a meteoroid and surviving fragments are also called meteorites
• If a projectile is smaller than ~1 cm the object is called a micrometeoroid and surviving fragments are called micrometeorites
• Is a projectile is dust sized, surviving fragments are called interplanetary or interstellar dust particles

Resources:

Types of impact crater shapes – the shape and structure of impact craters reveals what sizes meteorite or comet formed the crater

Understanding the number and size of impact events on the Moon at the present day – something that is very important for planning safe future lunar habitats and bases

General info about the postulated lunar cataclysm

Lunar Meteorites – bits of the Moon found here on Earth

This post is a little off track from Moon Zoo images of the lunar surface, but I want to share some information about my particular favourite Moon related subject — lunar meteorites.

What are lunar meteorites?
Thousands of meteorites have been found all over the Earth — the vast majority are thought to have originated from the asteroid belt — but very rarely some are identified that come from bodies like Mars, the Moon, and large asteroids.

Lunar meteorites are chunks of the Moon that were blasted off its surface by meteorite impacts, that then flew through space from the Moon and were captured by Earth’s gravity. The rocks then fell through the Earth’s atmosphere — often losing some mass on the way — and then land here on Earth.

Allan Hills 81005 - the first lunar meteorite to be recognised. This rock was picked up in Antarctica in 1981. Credit: NASA

How many have been collected so far?
So far (as of 2010) Dr. Randy Korotev, a lunar meteorite researcher from Washington University in St. Louis, states that 136 individual lunar meteorite stones have been collected. As some fell through the atmosphere at the same time, these samples actually represent only 66 meteorites! It is likely that many many more lunar meteorites than this have fallen onto the Earth through time, but because of high weathering rates (wind, rain, organic disaggregation, etc.) they have most likely been weathered away and could no longer be identified as having a lunar origin. We are lucky to have the ones that we have!

How do we know they have come from the Moon (and are not just chunks of grey concrete!)?
Proof of lunar origin can be taken from several lines of evidence that often requires analysis in a laboratory.

1. The presence of a glassy crust suggests that the sample is of meteoritic origin and has passed through the Earth’s atmosphere causing frictional melting of the surface.
2. The rock does not have chondrules and therefore can be classified as an anchondrite.
3. The rock is poor in metal and therefore is classified as stony.
4. Typical bulk rock elemental ratios and mineral chemistries that are similar to those of the Apollo and Luna Moon samples.
5. An oxygen isotope ratio corresponding that of the Earth-Moon trend.
6. Typical mafic mineral Mn/Fe ratios are indicative of the volatile-poor nature of lunar samples that trend on a ‘lunar line’ distinct from terrestrial rocks and samples from Mars and asteroids.

North West Africa 4472 - a lunar meteorite collected in Africa in 2006. I have been studying this sample and you can find out more information about my study in this document. Image: K. Joy.

When did they land on Earth?
All lunar meteorites are thought to have been launched from the Moon in the last 20 million years. As there are believed to have been no large craters generated on the Moon during this period, it is assumed that all lunar meteorites are launched from craters only a few kilometres in diameter (all craters in the last 1 million years or so are thought to be <3.6 km in diameter). This implies that lunar meteorites have been ejected from relatively shallow depths, and so represent the upper layer of lunar crustal material.

What do they tell us about the Moon?
Manned (Apollo) and unmanned (Luna) missions to the Moon have returned about 382 kg of lunar rocks and soils. These were all collected from rather atypical regions on the lunar near-side, within and around the central lunar nearside, or from equatorial latitudes on the eastern limb. Therefore interpretations of the nature of the lunar crust and mantle have been made from a dataset from geographically restricted areas of the Moon. Lunar meteorites, in contrast, are derived from random sample sites on the surface, and thus provide a wealth of new information about the nature of the Moon, even though their precise provenance is as yet unknown.

Some cool lunar meteorite findings:

• Lunar meteorite Kalahari 009 provides a sample of the oldest basalt (volcanic lava) found on the Moon — it is 4.35 billion years old.
• Lunar meteorite NWA 032 is the youngest basalt (volcanic lava) found on the Moon — it is 2.9 billion years old.
• Many lunar meteorites that are rich in a mineral called plagioclase are believed to have originated from the farside of the Moon. These important stones provide information about how the lunar crust formed.

Some good lunar meteorite resources you might like to check out:

• Dr. Randy Korotev has an excellent website.
• Dr. Kevin Righter at NASA keeps track of all the samples found and prepares some nice summaries about each stone collected.
• There is a lunar meteorite 101 video you can watch to learn more about these exciting bits of Moon rock.