Magnetic fields in the early solar system may have arisen from many sources, including the young Sun, the protoplanetary gas disk, and large planetary bodies like the asteroid Vesta (at left). The signatures of these magnetic fields were imprinted on meteorites present at this time, exactly the way molten rocks on the Earth can record the Earth’s magnetic field. These ancient magnetic fields are called paleomagnetic fields.
By studying the paleomagnetic signatures imprinted on meteoritic materials, Fu and his colleagues are able to measure the strengths and even directions of magnetic fields in the early solar system. Understanding this early magnetism can teach us about the formation of planets, the evolution of the protoplanetary disk from which they formed, and even about how planetary magnetic fields themselves are generated.
Good talk today from one of our own, Dr. Jackie Faherty, a Hubble fellow here at Carnegie.
Artist’s impression of the disc of dust and gas around a brown dwarf. From http://en.wikipedia.org/wiki/File:Artist%E2%80%99s_impression_of_the_disc_of_dust_and_gas_around_a_brown_dwarf.jpg.
Faherty talked about brown dwarfs (BD), a relatively new class of astronomical object that straddles the border between planets and stars. Brown dwarfs typically have masses between 13 and 75 times that mass of Jupiter and are made mostly of hydrogen gas, with lots of complex and interesting molecules mixed into their atmospheres.
Faherty studies the motion and distances of BDs using an age-old technique called parallax determination. It is very important to accurately estimate the distance for a BD because combining that estimate with measurements of a BD’s brightness and temperature gives a sense of the BD’s age — critical for understanding how BDs evolve over time.
BDs exhibit a bewildering variety of compositions and evolutionary behavior — some have variable clouds and weather — and the field is moving very rapidly. For example, astronomer Ian Crossfield just this year produced the first image of a BD’s atmosphere. And, in her talk, Faherty highlighted important similarities and differences between BDs and gas giant planets and showed how they may help unravel the mysteries of planet formation.
One of the neatest things he showed was a video of GPS displacements during the huge Tōhoku earthquake in Japan three years ago (above). The left panel shows lateral displacement (the largest arrows reflect only a few meters of displacement, not hundreds of kilometers, as it may seem), while the right panel shows vertical displacement. The waves associated with the earthquake and an aftershock are shown pretty dramatically, as they propagate through the Earth’s surface away from the earthquake source.
The Earth is very old, and figuring out the ages of rocks can be tricky. Luckily, the Earth provides some help, notably in the form of small crystals called zircons (one shown at left). Zircons are hardy little crystals that contain trace amounts of uranium (U) and lead (Pb). When zircons crystallize, they can incorporate some U into the crystal, but Pb is chemically excluded.
However, U radioactively decays into Pb at a well-known rate, and so if you find Pb in an old zircon, you know that it used to be U and formed by decay. By counting up the amount of Pb, you can estimate the zircon’s age.
Prof. Schoene and his research group use this fact to estimate the ages of rocks on the Earth’s surface. This work can tell us, for example, whether the chemistry of lavas erupted onto the surface have changed over time.
In fact, work by Schoene’s group has found preliminary indications that there may have been a big shift in the chemistry of lava flows right about the time that oxygen appeared in abundance in the Earth’s atmosphere, 2.5 billion years ago during the Great Oxygenation Event (GOE). The GOE is suspected to have devastated Earth’s biosphere — oxygen was toxic to most life at that time — and now it seems that eruptions from the Earth’s interior were affected, too.
Banded iron formation with tiger-eye, Mount Brockman, Australia. From http://www.pbase.com/image/94666060.
Gaschnig described detailed analyses of transition metals (TMs) to understand how the composition of minerals at the Earth’s surface might have changed over billions of years. These elements can be especially sensitive to their chemical environment, and so the TM content of ancient rocks tells us what things were like chemically as the rocks were formed.
A particularly interesting result emerged from Gaschnig’s work: the TM signatures he’s measured for some ancient rocks show evidence for initially low levels of oxygen immediately following the beginning of the Great Oxygenation Event (GOE), the phase in Earth’s history when ancient cyanobacteria became so prominent that they produced vast quantities of oxygen.
All this oxygen dramatically affected the chemical environment on the Earth, and its relatively sudden appearance in the atmosphere is reflected the mineralogy of some rocks, such as the banded iron formations shown at the right. Of course, without oxygen, most life as we know it would be impossible, but when it first appeared, the oxygen probably devastated the biosphere.
Some of Gaschnig’s rock samples dated to before and after the GOE show chemical changes in the TM content that may be consistent with the presence of oxygen but only at low levels (at least immediately after the GOE began) and so can be used to fill in some of the missing details about this dramatic chapter in Earth’s history.
At a subduction zone, one plate of oceanic lithosphere dives under another plate, which ‘dewaters’ to plate (blue arrows) into the overlying mantle wedge and produces arc volcanism at the surface. Part of the hydrated mantle wedge frees itself and mixes into surrounding depleted mantle. From Widom, Nature 443, 516-517 (2006).
One element that is particularly important for her studies is lithium (Li). Dr. Tian described how Li is thought to behave chemically within the mantle in a way that allows her to trace the subduction of material into the mantle and show that it is later erupted at a mid-ocean ridge (illustrated at right).
Artist’s conception of a large planetary impact of the kind that occurred during planet formation. From http://www.hdwallpapersinn.com/planet-impact-wallpapers.html.
Among the key results from Kepler are discoveries of a wide variety of orbital architectures (the arrangements of planetary orbits). The processes that gave rise to the planets determined, for example, the orbital periods of the planets.
Many Kepler planets reside in systems with multiple planets, and many members of these multiplanet systems have orbital periods that are very nearly integer multiples of one another. That is, the planets are near a mean-motion resonance, which means the planets interact strongly gravitationally.
Prof. Schlichting described one explanation for these near resonances: while the planetary systems were still very young, interactions between the nascent planets and the protoplanetary gas disks from which the planets form gently tuned the gravitational interactions between the planets, keeping them slightly out of resonance.
There has been some debate about whether the near resonances for many Kepler planetary systems mean that the planets did or did not undergone strong gas disk migration. In the simplest picture, this migration should drive planets into resonances, inconsistent with the observations of near-resonances.
But Prof. Schlichting’s modification to that picture means that the planets could have undergone migration after all. Turns out planetary systems were pretty complicated, dynamic places early on.
Flow structure of the convection cell in a model of the Earth’s interior. Figure 3 from Crowley & O’Connell (2012) — http://adsabs.harvard.edu/abs/2012GeoJI.188…61C.
New models from Prof. O’Connell and his student John Crowley suggest that the Earth may have undergone different stages of tectonic evolution, with the tectonic plates moving quickly at some times in the Earth’s history but much more slowly at others.
The evolution between different geophysical modes may help explain a longstanding puzzle in Earth science: the amount of heat coming out of the Earth is much greater than expected and has been thought to require much more heating from radioactive isotopes than geochemical analyses allow.
If, instead, O’Connell and Crowley are right, then this large heat flow is really just a symptom of Earth’s geophysical fickleness: sometimes lots of heat comes out, other times less.
So why did the Moon have a magnetic field long ago and why doesn’t it anymore? One exotic idea Dr. Wieczorek talked about was the idea that large asteroidal or cometary impacts could disrupt the rotation of the Moon’s mantle.
As a result, the mantle and core would rotate at different rates in different directions, which could stir up and heat fluid in the Moon’s interior. This heating could drive internal convection and produce a magnetic field, similar to the way the magnetic field in the Earth is generated.
This image of the brown dwarf binary CFBDSIR 1458+10 was obtained using the Laser Guide Star (LGS) Adaptive Optics system on the Keck II Telescope in Hawaii. Taken from wikipedia.org.
Good DTM seminar today by Prof. Chris Tinney, presenting a long and fascinating list of results from exoplanetary astronomy.
Among many projects his group is working on at the University of New South Wales, they are observing potential planet-hosting stars for the radial velocity signals of gas giant planets in orbits with long periods, comparable to Jupiter’s period of 12 years. Such planets require long observational baselines to find because you must observe the planet-hosting star for at least one orbital period, 12 years, to conclusively say that you’ve found such a planet. As a result, there aren’t too many long-period exoplanets known.
His group is also looking for brown dwarfs, exotic objects that aren’t quite stars but aren’t quite planets either. Brown dwarfs have masses large enough (probably greater than 13 times Jupiter’s mass) that they can fuse deuterium in their interiors — unlike planets — but small enough that they don’t fuse the lighter isotope, hydrogen — unlike stars.