Matt Siegler gives talk on planetary heat flow

Photo by Casey Gomez

Last Thursday, February 28, 2019, Matt Siegler visited Tech from NASA’s Jet Propulsion Laboratory (JPL) to talk about his work on planetary science. Siegler is interested in building a geothermal lab at Tech and in partnering with JPL via the GT-JPL Strategic University Research Partnerships (SURP) program.

“I would love to expand the mission and instrument presence of Georgia Tech, develop missions and instruments of my own, have years of experience working at JPL and aid in the further development of C-STAR and Georgia Tech’s strategic partnership with JPL,” he said. 

Stepping up to the stage, Siegler had an engaging smile on his face. As the co-creator of the webseries Ph.D. TV — an offshoot of Ph.D. Comics ­— he peppered his talk with amusing jokes. 

Siegler obtained his undergraduate degree in physics and film at Cornell University, spent a year at CERN and went on to obtain his Ph.D. at UCLA in 2011. He worked as a science advisor on the 2014 Cosmos sequel and has a passion for bringing science to non-scientists. His current avenues of research span geology, astronomy, climate science, chemistry and engineering, but are tied together by planetary science. 

“I look at (nearly) everything involving thermal processes from the [planet’s] surface downward,” he said. 

The talk concentrated on three of his prominent research areas: the InSight Mission to Mars, heat flow on the Moon and the Diviner project and microwave lunar heat flux from lunar probe Chang’e, Arecibo Observatory and the Very Large Array (VLA). 

NASA’s InSight Mars Lander reached the surface of Mars on November 26, 2018. InSight will remain stationary on the Martian surface, collecting data from a single location.

Heat flow is the mechanism behind geothermal activity like volcanoes and plate tectonics. Siegler hopes to use InSight’s measurements of heat flux to understand Mars’ geology, interior activity and chemical and thermal evolution. 

Unlike previous missions, which characterized the surface of Mars, InSight will look deep into the interior of the planet. Its goal is to determine the planet core’s size, composition and state of matter, the crust’s thickness and structure, the mantle’s structure and composition and the temperature and heat flow of the interior. 

Siegler’s focus is on InSight’s Heat Flow and Physical Properties Package (HP3), designed to take the first in situ heat flux measurement on Mars. Surface disturbances on Mars prohibit heat flow measurements taken at a shallow depth. 

InSight plans to execute the deepest dig that humans have ever achieved on any object besides Earth. The InSight lander will use a drill to hammer into the Mars’ surface at a slow and methodical rate. The HP3 instrument will subsequently deploy a heat flow probe 5 meters deep below the surface of the planet to measure the thermal gradient and thermal conductivity which determine heat flow. At the time of the talk, drilling was scheduled to begin the next day, March 1, 2019.

In his role as co-investigator, Siegler made multi-scale 3D models of non-geothermal sources of heat flux to better understand the measurements that HP3 will make. Siegler looked at heat disturbances and transient phenomena, like seasonal shadows, which could affect the data. He also worked on large scale models of Mars’ heat flux, including mantle heat production, surface topography, surface temperature and lateral heat conduction. 

Mars’ orbit is so elliptical that, unlike Earth, its orbital position with respect to the sun has more effect on the planet’s temperature than the direction of its tilt angle. The planet’s orbit and long term climate variations can cause geothermal changes that imitate heat flux and act as confounding variables. 

Fortunately, the current climate conditions have minimal climate effects: Mars’ current temperature is not very different from the long term average over the past 900,000 years, resulting in only a small decrease in heat flux at InSight’s location. In addition, the location and timing of InSight’s landing optimized the heat flux measurement conditions. 

InSight was lucky in many  ways; future follow-up expeditions may primarily be a measure of past climate, rather than geothermal heat alone.

“InSight will measure a lot more than just geothermal heat, but luckily not that much more,” Siegler said. 

The second chapter of Siegler’s research talk covered the Diviner experiment and lunar heat flux. There have been two previous measurements of heat flux on the surface of the moon at the Apollo landing sites, but a measurement of heat flux from orbit demonstrated that these two samples were taken near the boundary of unusually radiogenic terrain. As a result, the moon was thought to be much more active than it actually is. 

The Diviner Lunar Radiometer Experiment, an infrared radiometer on NASA’s Lunar Reconnaissance Orbiter, was introduced to combat this undersampling. Diviner took measurements in polar cold traps, which are craters within craters on the moon that never see even a hint of sunlight. The minimum temperature that Diviner recorded was a surprising 18 Kelvin, which is not far from absolute zero, the temperature at which fundamental particles have little to no motion. This temperature is far too low to be consistent with geothermal activity. 

Diviner’s data provide a constraint on the reduced heat flow of the Moon, which in turn gives information about the composition of the mantle and lower crust. Siegler and his group constructed global models of geothermal heat flux to apply to future missions. 

“From Diviner, we have a new and important constraint on geothermal heat production of the lunar crust and mantle to add to our reanalysis of Apollo,” concluded Siegler. 

In the third and final chapter, Siegler covered microwave heat flux. “Aka: wouldn’t it be great if we could just map heat flux from orbit?” he said.

Since microwave radiation should be sensitive to physical temperature, Siegler hopes to use microwave observatories to make measurements of lunar heat flux. The VLA and Arecibo Observatory are obvious choices, but so far he has most extensively studied data from lunar probe Chang’e, which created the highest resolution 3D map of the lunar surface in existence by using microwave remote sensing. 

His most recent work in this area translates microwave measurements to physical temperature using a weighting function that is determined by a quantity called the lunar “loss tangent,” which in turn is highly dependent on the amount of titanium in the region of interest. With Chang’e 1’s data, he was able to obtain a best fit constraint on the loss tangent’s dependence on titanium. His group used this titanium fit to model the subsurface temperature of the moon for various geothermal heat fluxes and, coincidentally, to obtain the rock abundance on the surface. 

“The Chang’e 1 and 2 [microwave radiometer (MRM)] instrument is a great way to map titanium bearing ilmenite, may be able to map buried rocks and hints at global heat flux variations. Orbital microwave measurement could provide a precise relative heat flux measurement technique, but absolute heat flux may not be possible — at least for MRM — and ground truth missions are likely required,” Siegler said.

As for future work in lunar microwave measurements, Siegler looks to longer wavelengths, and in particular to some early lunar microwave data taken by the VLA. “A longer wavelength would see even deeper and therefore be more influenced by geothermal heat. … With this preliminary [VLA] data as motivation, we are actively proposing longer wavelength instruments with U. Michigan and JPL,” Siegler said.  

“In five years, I plan to have a new thermophysical properties and instrumentation lab, some successful instrument development (microwave, thermal and in-situ geophysics) projects going, be a popular teacher (not just for my grade inflation), build many strong collaborations with the people I am meeting today, and have a happy and productive research group. In 10-15 years, I plan to be leading spacecraft missions and instruments from Georgia Tech, have an even happier and larger research group, and be surrounded by some great friends that I just met today,” he said.