Today we had a great DTM seminar by Rachel Osten from the Space Telescope Science Institute (STScI). She talked about stellar activity and coronal mass ejections (CMEs).
A coronal mass ejection in time-lapse imagery obtained with the LASCO instrument. The Sun (center) is obscured by the coronagraph’s mask.
Osten pointed out that these huge eruptions from stars are important for several reasons. For example, the energy and frequency of CMEs depend on several properties of stars, including their age, rotation rates, and magnetic fields. In general, as stars age, their rotation rates drop, usually reducing the strength of their magnetic fields and the amount of CME activity. And so learning about CMEs can tell us about stellar evolution.
When a star fires off a CME, it typically flares or brightens a bit (see figure at right), and since we can’t see stars other than our Sun up close, we can use the temporary brightening of those stars to study their flare activity. Osten talked about one of her projects to use data from the Hubble Space Telescope to look for flare activity for many stars near the constellation Sagittarius.
That project had a surprising result: many stars that were thought to be older than the Sun actually showed MORE flare activity than they should have. This result might mean these stars actually have previously unknown binary companions that kept the stars spinning quickly and thereby keeping their flare activity up.
Stellar flares may also be important for planetary habitability. For example, a big stellar flare can actually disrupt the atmosphere of an Earth-like planet, perhaps even removing all of the life-protecting ozone.
Comparison of Venus’ and Earth’s topography. For Venus, orange represents the topographic highs, blue the lows. For the Earth, red represents the highs and blue the lows.
Today I saw an interesting talk about Venusian geophysics by Dr. Steve Mackwell.
Dr. Mackwell talked about how mountains, volcanoes, craters, and fractures in Venus’ surface allow scientists to infer the tectonic history of Venus.
For example, unlike the Moon, there aren’t that many craters on the surface of Venus, which should have accumulated with time as more and more asteroids and comets struck the surface. This lack of craters (along with widespread, large-scale volcanic features) suggest that Venus underwent a tremendous volcanic upheaval about a half billion years ago, during which global volcanic eruptions almost completely re-surfaced the planet.
Dr. Mackwell also discussed how his lab experiments in rock mechanics (in which he squeezes and heats rocks to determine their physical properties) have contributed to our understanding of Venus’ history.
For example, the flow of heat from Venus’ interior drives its geophysical activity and plays a key role in determining the strength of the rocks underlying the surface topography. Understanding the relationship between internal heat flow and rock strength, however, requires lab experiments such as Dr. Mackwell’s.
Today’s DTM Seminar speaker was Prof. Zhigang Peng from GA Tech‘s Earth and Atmospheric Sciences Dept. He talked about a kind of earthquake (he called “tremors”) that is triggered when the seismic waves from another larger earthquake pass by. The relationship between earthquake duration and strength (as measured by the seismic moment) for these tremor quakes is apparently much different than for normal earthquakes, suggesting they result from different geophysical mechanisms.
The part of the talk that stuck out most for me was when Prof. Peng played the “sound” of an earthquake, generated by speeding up the seismic vibrations from measured earthquakes. This “earthquake music” is available on his website, but I have posted one such recording below. Apparently, different kinds of earthquakes make different sounds, and so these recordings can be used to tease out information about the quakes.
Sound of the 2004 Parkfield CA earthquake
Figure 2 from Cuk & Stewart (2012) showing the range of model outcomes for different impact conditions.
In Friday’s DTM seminar, Sarah Stewart-Mukhopadhyay of Harvard gave a fascinating talk on the origin of the Moon and the effects of large impacts on the Earth’s volatile inventory.
Lots of evidence (geochemical and astronomical) say that the the Moon originated in a large impact between the proto-Earth and a roughly Mars-sized rogue planet (often called Theia). This explanation accounts for many of peculiarities of the Earth-Moon system, EXCEPT that geochemical data says that Earth and Moon probably formed out of the same material. Old models of the Moon’s formation showed that Moon should have formed mostly out of the impactor, Theia, which (like all other solar system planets) was probably compositionally distinct from the Earth. So the Moon SHOULDN’T be Earth’s compositional twin.
Instead, Stewart and Cuk suggested that the pre-impact Earth was spinning much faster than previously assumed, with a day that lasted about 2 hours, and so enough material from the proto-Earth could have been launched by the impact to make the Moon mostly out of the proto-Earth. Once the Moon formed, Stewart and Cuk suggested it could interact gravitationally with the Earth and the Sun to reduce the Earth-Moon angular momentum and bring it into line with what we see today, removing the one big objection to their fast-spinning proto-Earth idea.
In the last part of her talk, Stewart discussed what such large impacts might do to the gases trapped inside the young Earth. Contrary to popular thought, she suggested that these large impacts actually might NOT melt most of the Earth’s outermost layers, and so the Earth could retain in its interior much of the nebular gas that it was born with.
This idea could help explain geochemical data that suggest gas trapped in different parts of the Earth’s interior currently have different isotopic compositions and would mean that the Earth retains geochemical scars from its adolescence once thought completely erased by the violence of the early solar system.