Research
Yangcheng's research covers a variety of topics ranging from methane on Mars and biosignature from exoplanets to physical oceanography. Yangcheng has expertise in atmospheric photochemistry, geofluid dynamics and atmospheric transport, and radiative transfer. Below are three examples of Yangcheng's research.
Locating methane sources on Mars and reconciling inconsistencies in observations
Methane on Mars have been a hot topic in the Mars science community and have drawn attention from the public for almost two decades. The fact that 90–95% of methane in the Earth atmosphere has a biological origin raises a question – Is methane on Mars a biosignature?
The Curiosity rover has been measuring methane concentration at Gale crater on Mars since 2012, and has reported strange results – The background concentration is low, but strong methane signals have been detected from time to time. These randomly occurring, particularly strong methane signals are called "methane spikes", and they obviously contradict with the long lifetime of methane that standard photochemical models tell us. Therefore, we turn to another possibility – the strong methane signals may come from a nearby methane source. In our hypothesis, methane is episodically emitted from a surface emission hot spot that is very close to the Curiosity rover – probably within the 150-km wide Gale crater. The signals are carried by wind to the Curiosity rover, get picked up by the detector, and are then dispersed away by atmospheric turbulence. This hypothesis can also explain why the Trace Gas Orbiter has not detected methane in the middle to high atmosphere – because methane emission is both local and small, methane signals can be easily diluted and then destroyed by photolysis and the reactions with O(1D) and OH, before accumulating to a significant level in the middle to high atmosphere where they are be picked up by the Trace Gas Orbiter.
We have adapted the Stochastic Time-Inverted Lagrangian Transport (STILT) model for the Martian atmosphere, and employed the new model, MarsSTILT, to find out historical trajectories of methane plumes transported by wind. We also mapped out potential emission sites around Gale crater, and evaluated their possibilities. We found that if our understanding of the methane lifetime is correct –more than 300 years, the methane source responsible for the methane spikes must be located in the northwestern part of Gale crater, and very likely only tens of kilometers to the west of the current position of the Curiosity rover, a place that the rover can possibly probe in the future. Our work was published in Earth and Space Science.
Methane-rich air parcels are traveling backwards in time from the detector (the Curiosity rover) to their origins.
Ozone and nitrogen chemistry in the atmospheres of terrestrial exoplanets
How can we know if life has emerged on planets beyond the solar system? One method is to screen the atmospheres of exoplanets using spectroscopy, and look for absorption features of biologically relevant molecules. On Earth, it was photosynthesis that elevated the oxygen abundance in the atmosphere to the present level, so we call the presence of oxygen and ozone (a photolytic product of oxygen) molecules a "biosignature". Looking for oxygen and ozone in the atmospheric spectra of exoplanets could tell us if photosynthetic life could have emerged elsewhere in the universe. Compared to the oxygen biosignature, the ozone biosignature has its advantage in that even if oxygen abundance is as low as that on the Proterozoic Earth, there will still be a significant ozone layer. In order to see whether ozone layers can develop on tidally locked exoplanets orbiting different types of stars, and if yes, their thicknesses and detectability, we use a fully coupled three-dimensional chemistry-radiation-dynamics model to simulate the ozone layers and use radiative transfer tools to simulate their absorption features in the atmospheric transmission spectra of transiting exoplanets.
But detection of oxygen and ozone still cannot confirm the presence of photosynthetic life because water photolysis and hydrogen escape to space can also build up oxygen and produce ozone. A recently discovered, more discriminative biosignature is long-term variability of an ozone layer that is driven by emission of nitrogen oxides from a biosphere. Oscillations in ozone abundance have been found in our simulations, which highlights the importance of time-domain astronomy to characterizing exoplanetary atmospheres and detecting biosignature. This result also updates our conventional thinking that all atmospheric photochemistry systems will ultimately reach a steady state given constant external forcings – spontaneous oscillations are possible.
A snapshot from a 3D photochemical-climate simulation. a) and b) O3 distribution. c) Temperature. d) Pressure at 35 km altitude. e) Zonal wind. f) Brunt-Väisälä frequency.
Vertical exchange between the mixed layer and the thermocline induced by submesoscale dynamics in the upper ocean
Submesoscale turbulence in the upper ocean consists of fronts, filaments, and vortices that have horizontal scales between 100 m and 100 km. High-resolution numerical simulations have suggested that submesoscale turbulence is associated with strong vertical motion that could substantially enhance the vertical exchange between the mixed layer and the thermocline, which may have a profound impact on marine ecosystems and climate. Theoretical, numerical, and observational work indicates that submesoscale turbulence is energized primarily by a baroclinic instability in the mixed layer, which is most vigorous in winter.
We have demonstrated how such mixed-layer baroclinic instabilities induce vertical exchange between the mixed layer and thermocline, and proposed a scaling law for the dependence of the exchange on environmental parameters. From linear stability analysis and nonlinear simulations, we show that the exchange, quantified by how much thermocline water is entrained into the mixed layer, is inversely proportional to the thermocline Richardson number. Our results imply that the tracer exchange between the mixed layer and thermocline is more efficient when the mixed layer is deeper, the lateral buoyancy gradient is larger, and the thermocline stratification is weaker. The scaling suggests vigorous exchange between deep mixed layers and the permanent thermocline in winter.
Simulated submesoscale turbulence in the upper ocean.