Live Fast, Die Young: Evolutionary Outcomes of an Asteroid Impact

Figure 1: (A) "Dryophyllum" subfalcatum, (B) unknown nonmonocot,
(C) "Ficus" planicostata, (D) "Populus" nebrascensis

A new semester has started and with it an influx of new students into the lab has begun. Busy has become my middle name. So when I was looking around for a paper to write about I wanted something different and cool. Not exactly hard to find in science. The asteroid known as 2012 DA14 will narrowly miss Earth this Friday, the closest known asteroid flyby on record. And by close we’re talking within the orbits of many communications satellites. This got me thinking about and looking for recent papers about asteroids. It didn't take me long to come by an interesting new paper about the dino-killing Chicxulub bolide impact.

As of now, it is widely accepted that an epic asteroid collision ended the 135 million year reign of the dinosaurs. The Cretaceous-Paleogene boundary (KPB) extinction event is marked by the Chicxulub (CHEEK-sheh-loob) impact on the Yucatán Peninsula in Mexico. This asteroid or comet is estimated to have been about 6 miles (10 km), releasing as much energy as 100 trillion tons of TNT that caused a crater more can 110 miles (180 km) across! This impact coincides with a mass extinction event that includes the dinosaurs. Dramatic climate swings caused by the dust kicked up into the atmosphere were likely the culprit behind many of these extinctions. Before we go further, take a second to think about what you know about this extinction event. You probably think of the mass die-off of the dinosaurs and the subsequent rise of the mammals, right? But, as I have in the past, I’ll now pose a question: What about the plants?

A new paper published yesterday in PLOS Biology asks just that question. We know that in temperate North America the Chicxulub impact resulted in the extinction of over 50 percent of the plant species. From an evolutionary and ecological stand-point, that’s a lot of competitors that were taken out of the game. However, the environment was dramatically altered as well, changing to a cold and dark “impact winter.” Combined, these factors created a unique selection scenario for certain ecological strategies. The new paper takes a close look at the functional traits associated with these strategies.

The researchers measured fossil leaf assemblages spanning a 2.2 million year interval across the KPB, assessing four differing selection scenarios for functional traits. First, wrap your head around the concept of “functional traits.” These are characteristics that define species in terms of their ecological roles. In the case of leaves, these include leaf mass per area (LMA; Do you make a big, expensive leaf or a light, cheap one?) and leaf minor vein density (VD; Do you have more veins to transport lots of water?), among many others. Because leaves are the food producers, these traits are linked to plant growth and fitness. Next, you can relate these traits to the “leaf economic spectrum” (LES) that contrasts species with inexpensive short-lived leaves with fast returns on carbon and nutrients (deciduous, angiosperm, broadleaf) to costly long-lived leaves with slow returns (coniferous, gymnosperm, evergreen). The former is typically selected for in a less resource variable environment and vice versa. From this, you can get a more global perspective on changes in species composition.

The researchers measured LMA and VD for fossil leaf assemblages spanning the KPB. To do this they digitally photographed specimens that could be measured and confidently reconstructed. Then they used Photoshop to digitally separate the leaf from its rock matrix. For LMA they used ImageJ to calculate leaf area and petiole width, and then ran these numbers through empirical scaling functions (a.k.a. equations). For VD, they used a MATLAB line-counting program to isolate the veins and then manually counted the number of vein-line intersections, computing the mean distance between veins  as the sum of all line counts divided by the sum of all distances (a.k.a. a slightly less complicated equation). They ran a few scenarios to account for site and region plant specificity as well.

They found LMA to decrease and VD to increase across this time period. Even changes just these two traits reflect large physiological and biological shifts in plant functioning over a relatively short period of time. According to their data, the Chicxulub impact led to the selective extinction of species with slow strategies. This caused a directional selection away from evergreen species along with a stabilizing selection of deciduous angiosperms. The authors pose a few hypotheses in their discussion that are worth mentioning. The higher observed VD in angiosperms, and their ensuing selection, could have been driven by declining atmospheric carbon dioxide (CO2), which selects for higher hydraulic capacity. This CO2 hypothesis would, of course, not really hold water (no pun intended) for nonangiosperms and shade species, but the authors suggest that the observed increase in VD is more likely to be a direct consequence of the impact selecting for specific leaf economic strategies rather than ongoing-longer term climate change.

In this case, slow and steady did not win the race.


  Blonder B, Royer DL, Johnson KR, Miller I, & Enquist BJ (2014). Plant Ecological Strategies Shift Across the Cretaceous-Paleogene Boundary. PLoS biology, 12 (9) PMID: 25225914




(image via above citation)

Rosetta Meets a Stone

Over the weekend, on July 10th at 16:10 GMT, ESA's Rosetta spacecraft flew by 21 Lutetia, an asteroid 454 million km from Earth, the largest asteroid ever visited by a satellite. The spacecraft is on its way to a 2014 rendezvous with comet 67P/Churyumov-Gerasimenko, and on its way it was perfectly lined up to skim by the asteroid. By 'skim' we're talking it passed by at only 3,162 km (1964 mi) away, this at a speed of 54,000 km/hr. During its flyby, Rosetta took data on 21 Lutetia's surface, dust environment, exosphere, magnetic field, mass, and density. It also took some spectacular close-up images (details down to a scale of 60m) using its OSIRIS instrument, a camera that combines wide and narrow angles. The asteroid is shown to have a battered, cratered surface which includes a giant bowl-shaped depression stretching across much of one side of the asteroid. The photos suggest it is a very old object, likely left over from the formation of the solar system. These images also confirm that Lutetia is an elongated body, with its longest side approximately 130 km (80 mi) long. After its minute-long flyby, Rosetta began transmitting data to Earth for analysis, so expect more interesting findings from one of the oldest, close objects we've observed.

http://www.universetoday.com/2010/07/12/rosetta-meets-asteroid-lutetia/

and
http://www.universetoday.com/2010/07/09/watch-live-webcast-of-rosetta-flyby-of-asteroid-lutetia-july-10/

(image credit ESA 2010 MPS for OSIRIS Team)

Icy Asteroid

Evidence of water ice and organic materials has, for the first time, been discovered on an asteroid. Josh Emery, from the University of Tennessee Knoxville using NASA's Infrared Telescope Facility on Hawaii's Mauna Kea, found that the spectrum of infrared sunlight reflected by the object was consistent with frozen water. The discovery was confirmed by a team lead by Humberto Campins at the University of Central Florida. Organic materials in the form of complex, long-chained molecules were also discovered. The asteroid 24 Themis is approximately 200 kilometers wide and orbits half-way between Mars and Jupiter in the asteroid belt. As this short distance from the Sun (only about 479 million kilometres) is generally too warm for ice to exist for long, it is theorized that the asteroid may contain an abundance of ice in its interior which is outgassed to replenish the surface supply. This discovery suggests that asteroids may be responsible for bringing some of the water and organic molecules to the barren, developing Earth.

Why the water is there, or how it got there in the first place, is still unclear. Think about an asteroid for a second. Picturing a dry, rocky object in space? You aren't alone. Asteroids are widely viewed as being very dry, especially compared to, let's say, comets. Long-range comets (those coming in from the Kuiper Belt in extremely long and often elliptical orbits around the Sun) contain a lot of water but with a different isotropic signature than water found on Earth, and so they are an unlikely source of Earth's water. In 2006, small comet-like objects with tails were discovered in the asteroid belt and dubbed "main-belt comets," and its this set of objects to which 24 Themis belongs. This asteroid, or main-belt comet, belongs to a set of asteroids that is thought to have formed as a result of a large impact from a larger body, suggesting that the original object contained ice. This has implications for our theories on solar system evolution and planet formation. If this ice-on-asteroid phenomenon is relatively widespread then it is likely that ice was more prevalent in the Main Belt region of the early solar system than previously thought. If you expand this idea to other solar systems and extrasolar planets then water/ice may be more abundant than we thought. And, although that might be a stretch based on a sample size of one, it is an interesting direction to look in. Also, we can't ignore the idea that the asteroid may be from Kuiper belt and was knocked into the inner solar system or arrived there when the gas and ice giants migrated their orbits. Future observations and research will work to determine whether the composition of 24 Themis is typical of the asteroids in this region and if the water on these rocky objects is the same as that found on Earth.

Read more on this discovery at:
http://www.physorg.com/news191665880.html
and http://www.nature.com/news/2010/100428/full/news.2010.207.html

(image from www.dlr.de -- credit NASA/JPL -Caltech)

Big Badda-Boom

(images from news.nationalgeographic.com, phy.olemiss.edu, popsci.com, nasa.gov)

To get the gist of this next story a short history of your planet-moon system is in order.

During our solar system's formation it was a violent place. Everything running into everything else. Planets sucking up material and clearing out their orbits. During this time moons were forming around planets and planets were capturing objects and making them moons. Not so with the Earth's moon. The current, accepted theory is that a Mars-sized object collided with the Earth knocking off a massive amount of material. This material stayed in orbit around the planet (think big cloud then ring), eventually collapsing together and cooling to form our Moon. Samples brought back from Moon missions were chemically analysed and found to be very similar to mineral concentrations and isotopes found on Earth, strengthening this theory.

A recent study of the Moon has showed that an asteroid hit the southern hemisphere of the Moon. An enormous crater, approximately 1,500 miles across and five miles deep, resulted from the impact - the biggest, deepest crater on the Moon, the South Pole-Aitken basin.

So why, with all of the craters on the Moon, is this so important? Well, the asteroid impact punched through many layers of the Moon's crust, scattering it across the Moon and even into space. A 300 mile diameter crater - the Apollo Basin - on the edge of this basin provides a cross-sectional view of the lunar crust. It is rare that such glimpses into the surface of planets/moons occur. These deep views of the surface can provide much information about early lunar history.

Results from this study show that as you go deeper into the Moon, the crust contains larger amounts of iron. Think back to the story above and what you know of the concentrations of minerals on and inside the Earth. When the Moon was first forming it was largely molten - big collisions cause big amounts of heat. This fluid-like state allowed heavier elements (such as iron) to sink towards the core, leaving light elements (silicon, potassium, sodium, etc) to 'float' near the surface. So it makes sense that deeper impact craters expose more of these heavier elements, and that is what the researchers are finding.

Additionally, these craters are relatively old. How do you know if a crater is young or old? As time passes and the surface continues to get hit my objects, old craters get covered up by new craters. This provides a relative age. And although the Earth and the Moon were formed from the same pool of materials, you do not see the same cratering on the Earth that you see on the Moon. Why? It isn't that the Earth hasn't been impacted, it has and repeatedly. Its just that on the Earth the crust is constantly recycled by plate tectonics and worn away by weathering events. So the old, deep craters on the Moon actually give us a window into not only the Moon's history but the Earth's as well.

Read more by following this link: http://www.sciencedaily.com/releases/2010/03/100304165918.htm

Update as of 03-10-10: Just published in Nature, a story about the Moon origin theory:

http://www.nature.com/news/2010/100309/full/464150a.html

Rock Solid Agreement

(image from pnas.org)

If there is one thing that science does well its agree on things. Ok, so maybe not, but it is known to happen occasionally. Or at least there is strong evidence in support of these events.

Recently a panel of 41 scientists reviewed 20 years of evidence regarding the cause of the Cretaceous-Tertiary (KT) extinction that occurred approximately 65 million years ago - the extinction event that wiped out the dinosaurs long with many other animal and plant species. One explanation put forth for the cause of this extinction is the eruption of massive volcanoes - or multiple impacts combined with volcanic eruptions. Alternatively, and perhaps more widely known, is the suggestion that a single, large asteroid hit the Earth somewhere around Mexico's Yucatan peninsula.

The conclusion? The big rock from space did it. The panel concluded that while large scale volcanism did occur it was too early in Earth's history to account for the extinctions, and that the evidence is strong in the support of the single impact theory.

Check out the article here: http://www.usatoday.com/tech/science/discoveries/2010-03-04-dinos05_N.htm