Day 1: The Grand Tour
by Peter Gao
The first class of The Science of the Solar System saw a Grand Tour through our cosmic backyard, starting off with the planet closest to the Sun: Mercury. Mercury’s pockmarked surface gave it a similar appearance to the Moon, but upon closer inspection there were details that the Moon does not have. Images from the orbiting MESSENGER spacecraft have shown geologic features that suggest compression—ridges and thrust faults that indicate folding of surface material, and the subduction of one surface slab under another. The distribution of these features point to a shrinking of Mercury on a global scale, and calculations show that Mercury’s radius has likely decreased by up to 11 km since its formation.
How can a planet just shrink? In addition, why do we not see this elsewhere in the Solar System? The answer lies in Mercury’s unique interior structure. Unlike the other rocky planets, Mercury is dominated by a iron-nickel core with a radius 80% that of the total planetary radius, which is then surrounded by a thin silicate mantle. By comparison, the core of Earth, and likely that of Venus and Mars, are only about half as large as the planets they lie within; the Moon’s core is even smaller, clocking in at only a few hundred kilometers in radius. The large core surrounded by a thin skin means that core dynamics will affect surface features much more on Mercury than on the other rocky planets, and indeed core dynamics is crucial here. Current observations suggest that Mercury’s core is at least partially liquid. As this liquid cools, it may solidify and form a solid inner core like that of Earth. Like most substances, solidification decreases the volume of a substance—within Mercury, a pure liquid core will have a greater volume than that which has cooled and has formed a solid center. Therefore, we can construct the following hypothesis: Mercury formed with a liquid iron-nickel core, which then cooled enough to form a solid inner core. The formation of this inner solid core decreased the total core volume, which the overlying silicate mantle attempted to compensate by “shrinking” as well, resulting in thrust faulting, folding, and other features of compression on the surface.
And there you have it: An Incredible Shrinking Planet!
by Danielle Piskorz
Titan is a moon of Saturn orbiting the Sun ~10 AU, yet it is astonishingly similar to Earth. Presently, each has a nitrogen-dominated atmosphere, clouds in the sky, and a surface that features standing bodies of liquid, mountains, dunes, and winds.
Both Titan and Earth are subject to the effects of Milankovitch cycles. Really, all planets are subject to Milankovitch cycles, but there is bona fide evidence for previous Milankovitch cycles on both bodies. Orbital forcing changes Earth’s eccentricity, axial tilt, and orbit precession, which in turn has a profound effect on our planet’s climate on timescales of tens of thousands of years. For example, it has been shown that oxygen and nitrogen air bubbles in Antarctic ice cores responded to varying insolation levels on Milankovitch timescales.
When we observe methane/ethane lakes on Titan, we find that they exist mostly in the northern hemisphere. Why? Because of Milankovitch cycles! Due to its changing orbital properties, Titan has high solar insolation in the northern hemisphere on 32,000-year timescales. This means that the lakes on Titan would move between the northern and southern hemispheres every 32,000 years!
Titan is home to many mysteries to be solved for sure, but it’s always encouraging to see another place in the Solar System so similar to and as dynamic as our planet.
by Michael L. Wong
Ed Stone, project scientist for the NASA Voyager missions, has been running the show for a solid 37 years. “Talk about job security,” Peter quipped.
Of all the moments from the first day of #SciSolSys, Peter’s excitement about the Voyagers’ longevity stood out the most. Earlier, during my opening remarks, I asserted that Planetary Science is distinctly recent human venture. Compared to the centuries-old disciplines of Physics, Mathematics, and Biology, a hardcore scientific study of the planets has only been around for the past few decades. Planetary Science is, by all measures, still in its infancy. But Peter reminded me that we’ve had a spacecraft alive and operating continuously for the past 37 years. Thirty-seven! Compared to my age of 23, that suddenly seemed like an eternity.
So what has Voyager done in nearly four decades of sailing through the void? “It’s the first manmade object to leave the Solar System!” said Peter, arms flailing. This awesome statement begged the question: What does it mean to leave the Solar System? What is the boundary of our little cosmic nest? Is it the orbit of the last planet, Neptune? Is it the edge of the large ring of icy debris known as the Kuiper Belt?
NASA scientists decided to define the boundary between the Solar System and interstellar space as the heliopause—the place were the Sun’s solar wind is no longer stronger than that of surrounding stars. Stellar wind, a plasma stream of ionized particles, is carried by the prevailing magnetic field lines in the region. Hence, when Voyager 1 detected abrupt and lasting changes in the magnetic field direction and plasma intensity, project scientist Ed Stone decided it was time to declare that the little spacecraft that could had reached interstellar space.
So, at 37-years old, Voyager has finally left the Solar System. And, at 37-years young, Voyager has just begun to traverse a brand new region of the cosmos.
Day 2: Thermal Equilibrium
by Danielle Piskorz
Prof Brown returned for the second class period of Science of the Solar System on Thursday. He had decided that the majority of the 90-minute class was going to be spent thinking about temperatures on Mars. The overall goal was to take the first steps toward understanding the seminal 1966 Leighton & Murray calculation. This calculation aimed to explain the Martian ice caps by considering the Martian orbit, subsurface heat flow, and surface volatiles while applying the concept of energy balance.
My group began with a thought experiment: What would the temperature of a planet be as it went from day to night if it were in instantaneous thermal equilibrium? This question requires balancing the power in and the power out. The power in is given by the solar luminosity at the location of the planet. It’s also important to think about albedo. Albedo is the fraction of light reflected off of a surface. We assumed the planet is a blackbody, and so its power out is given by the power radiated by a blackbody. To deduce the behavior of the temperature over time, we must recall the instantaneous equilibrium caveat: at sunset, the temperature would instantly go from the effective temperature to ~0K. My students quickly realized that this was unphysical and that the problem is much more complicated.
In reality, a planet would store heat during the day and re-radiate it at night. The power in (or flux in) will be adjusted by a factor encompassing the solar insolation angle throughout the day, or lack of sunlight at all at night. The power out (or flux out) is adjusted by a term encompassing the specific heat of the rock and the transfer of heat into and out of the regolith.
By the end of the class, the students had a good framework for starting their problem sets, and by Monday they will have reproduced the framework of one of the most groundbreaking calculations in planetary science.
by Peter Gao
Prof. Mike Brown is back, which means it's time to dive in to the science and be a bit more quantitative than we were in our introduction class. Today's mission: to do what Leighton and Murray did in their groundbreaking 1966 paper and calculate the surface temperature of Mars as a function of time on a time scale of a Martian year. The class was split into groups, and I began work with Alec, Junjie, and Jiabin on working out what those two great minds did way back when.
The temperature of a system is a function of the energy input and output, so what is the energy input and output of the Martian surface? The students answered fairly quickly: solar radiation for the former, and thermal radiation for the latter. Indeed, to 1st order the heating is caused by the Sun, and that heat is lost through thermal radiation of the surface. I will forgo the equations here for simplicity's sake, and also I don't like typing them out. Just remember that temperature is raised to the 4th power.
On a planet that reach thermal equilibrium instantaneously, the temperature vs. time plot for a single Martian day is a simple step function, with lows of 0 K (or 3 K, if you want to take into account the temperature of the Universe), and highs of ~259 K if the planet were at 1 AU, like the Earth. For Mars, this is lower. However, there are more factors to consider. The students had already realized this when I told them to draw the initial temperature vs. time curve, and they returned gradual curves instead of step functions. Of course, real systems don't instantaneously do much of anything, much less exchange energy. We must now deal with two extra factors: the planet's rotation, and the finite heat capacity of the ground.
Time was running out, so we simplified these steps. The rotation effect was simple to consider: it is a sinusoidal modulation on the constant solar flux, so we stuck a sin(t) to the energy input, with no regard for constants and factors of a few. For the heat capacity of the ground, we remembered that the heat capacity times the difference in temperature equaled the energy flow between two systems, which we later replaced with the correct heat conduction equation. In this way, we were able to account for energy loss of the surface to the regolith beneath.
This was good enough! It is now up to the students to tease out the details, add more layers to the underground portion, and account for the energy effects of sublimating CO2 and the obliquity and orbit of Mars, which are all necessary to complete their first homework assignment.