Astrobiophysics is concerned with the effect of astrophysical processes on life on Earth, as well as effects on possible life elsewhere, and the detection of such life. A wide variety of research areas meet here, including astrophysics, astronomy, biochemistry, evolutionary biology, paleontology, atmospheric science , and a host of others.
Alexander J. Krejci (pronounced CRAY-chee) from Lawrence is one of five finalists for the 2008 Vanderbilt Prize for Undergraduate Research in Physics and Astronomy. The finalists and the prize winner will receive their awards Aug. 15 in a ceremony at Vanderbilt University in Nashville, Tenn.
Krejci, majoring in physics and geology at KU, was selected for his project “A Code to Compute High Energy Cosmic Ray Effects on Terrestrial Atmospheric Chemistry.”
Adrian Melott, professor of physics and astronomy, who has advised Krejci’s undergraduate projects, noted that Krejci has worked specifically on computations to help determine the effects of increases in the high-energy cosmic ray flux on the Earth’s atmosphere and ecology.
Krejci’s research contributions are included in two papers on astrobiophysics submitted to scientific journals. He is the first author on one paper and a contributing author on the other.
He will receive a $500 cash prize from Vanderbilt University.
GRBs are the most powerful known explosions in the Universe, sending out beams of radiation that have been an active research interest for more than 30 years. Only in the 90's was it firmly understood that they are at huge, cosmological distances, and therefore correspondingly powerful. They are probably created by the energy released when a star collapses to form a black hole. For more background you can read this public discussion produced by NASA, or a slightly more technical discussion with an entry into the scientific literature here.
Knowing the power of these bursts and their average rate throughout the Universe, it is possible to calculate the likely rate of events in our "neighborhood". In this case, any burst on our side of the Milky Way galaxy is likely to be dangerous, if it is pointed at the Earth. Several groups have independently estimated that one or more GRBs is likely to have "blasted" the Earth from a distance of 6,000 light years, or even less, within the last 500 million years or so. Some evidence to support this rate has come from the observation of a GRB remnant in our own galaxy. It appears to be young, but not pointed at our solar system.
Possible effects on the Earth include sky-darkening levels of "smog", nitric acid rain, and especially destruction of the ozone layer, which protects us from the Sun's ultraviolet radiation. We have been computing just how severe these effects are likely to have been given various possible GRB "events" affecting our planet.
We are also examining the fossil record for signs of GRB effects. Given
the severity of the effects, we think it is reasonable that some of the
dieoffs known as "mass extinctions" may have been induced by GRBs.
The end
Ordovician extinction, the second-largest in the fossil record,
seems to be a good candidate.
Here is an image of Ordovician life provided by William Berry, University
of California Museum of Paleontology.
We have written a paper (click here for pdf file) on the possible connection of a GRB event with the end Ordovician extinction. We hypothesize that this extinction is linked to the effects of a gamma-ray burst, resulting in a damaged ozone layer and a decline in global temperatures, both as side effects of changes in atmospheric chemistry. Patterns from the fossil record suggest that organisms whose lifestyle left them little shielded from the Sun's UV radiation may have been most susceptible to extinction. Our work may have some practical value in understanding the dangers we face today from our adverse impact on the ozone layer. Here are links to news stories about our work in Nature, New Scientist , and CNN.
Over the last few years, we have begun to explore the details of how such an event would impact the Earth's atmosphere. We have simulated the evolution of the chemistry and the resultant loss of ozone protection from solar UVB. It is possible to combine this information with known DNA damage at various wavelengths to construct a time-dependent map of the DNA damage. You can view a movie of the DNA damage from a burst occuring at noon over the equator in March. In the film, you first see a "normal" year prior to the burst at day 0. White is coded to the global annual average rate of DNA damage, blue below that, and increasingly darker red to higher damage rates. The effects are extremely intense for a few months and persist for years. To view the movie in gif format click here . A screen refresh will rerun the movie. To view in mpeg (better image, but may cause problems in some viewers) click here. Anything darker than pink corresponds to likely widespread lethal UV for organisms living near the surface of the water. Bursts at other latitudes, times of year, times of day have somewhat different effects, but because they get much more sunlight than polar regions, equatorial and mid-latitudes have the biggest integrated damage. In April 2005, NASA issued a press release on this extinction hypothesis which contains additional narrative and animations.
The following papers (linked in a variety of ways) contain details of the atmospheric computations, with a focus on ozone depletion and UVB transmission:
In addition to damage from solar UV due to ozone depletion, two other effects may be biologically important. Nitrogen dioxide, a brown gas, may cut off sunlight and lower temperatures. We are interested in the possibility that this may in particular have triggered the end-Ordovician glaciation. In the years after the burst, the atmosphere is cleaned by rainout, largely in the form of dilute nitric acid--which may paradoxically act as a fertilizer for plants. We have shown results (in Geophysical Research Letters) of computations of these effects for our fiducial gamma ray burst.
The discoverers of the cycle considered but could not construct a plausible mechanism to drive this peridocity. We have proposed a definite theory to explain this. Descriptive news stories from New Scientist, National Geographic.com, and Space.com are available.
Our galaxy is a thin disk. The whole solar system moves around the galaxy
in a complicated motion, an eccentric orbit, also bobbing up and down
within the disk as it goes. We noticed that times of major fossil biodiversity drop
seemed to coincide with the times when the Sun has bobbed "up", or more specifically
galactic north, perpendicular to the plane of the disk. It turns out that they
agree to better than one chance in ten million. Now our galaxy's "north"
happens to lie very close to the direction of the
Virgo Cluster of galaxies, the biggest mass concentration in our
galaxy's neighborhood. Our galaxy naturally is falling toward this cluster,
it turns out at about 200 kilometers per second through the hot gas that
surrounds galaxies. Such motion should produce a shock wave, such as
the one
imaged here from a laboratory experiment . In fact, our Sun and solar
system have a
shock structure
surrounding them as the Sun moves through the hot gas in our galaxy.
We expect that just as the shock at the boundary of our solar system produces
cosmic rays, the shock at the north side of our galaxy should too. The galaxy has
one extra feature that makes it different: a turbulent magnetic field, strongest
inside the disk of the galaxy. This magnetic field shields us from the cosmic
rays except when the sun bobs up out of the disk on the north side, and the
shielding distance is thin. Many more high-energy cosmic rays can reach us,
many of them energetic enough to get past the Earths protective magnetic field
and reach the ground. This can cause all kinds of problems with the climate
as well as directly damage the molecules of life.
This model does an excellent job of explaining the diversity cycle; but of course
it was constructed to do so. It can also predict something new.
Because the galaxy is not perfectly smooth, the Sun bobs up to different
heights each time, and this motion has been calculated back into the past.
When the Sun bobs up further, the damage can be worse. We define the damage
at each event as the biodiversity minimum subtracted from the preceeding peak,
to get a drop. Let's
plot this "extinction strength" along with the cosmic ray flux predicted by
our model for each of the upward excursions in the last 500 million years:
as you can see, the fit predicted by the model is in amazing agreement:
extinction strength goes in a one-to-one correspondence with the increase in
cosmic rays.
This work was recently published in Astrophysical Journal. A story on it had the highest-hit count on the website Science Now, a publication of Science magazine, in August 2007.
Opportunities exist for students to be involved in research at both the graduate and undergraduate level. We have a block of supercomputer time at the National Center for Supercomputer Applications for those computations too large for local workstations. We maintain an active net of collaborators especially including NASA Goddard Space Flight Center, in their Astroparticle Physics Laboratory and their Laboratory for Atmospheres. The SWIFT mission will provide new information about Gamma-ray bursts, including much-needed data on their rate in the "recent" Universe (since the Earth formed).
Our recent research publications can usually be found in the ArXiv.
Last Updated: July 3, 2008
