1. Climate change at Mars

More than 3.6 billion years ago, Mars had liquid water on its surface. Just like water on Earth today, this water carved the ancient Martian landscape, forming rivers, depositing deltas and collecting in standing lakes. There were even occasional waterfalls. Water was so prevalent then that this period of Martian history is referred to as the "Noachian Period" after the biblical flood.

Modern Mars, however, is exceedingly dry. Gone are the rivers and the lakes. With an average surface temperature of -63^oC, H₂O on Mars now mostly exists as ice in the polar caps and the subsurface. Although the atmosphere is made up of 95% CO₂, the surface pressure of 6 millibars today is too low to produce sufficient greenhouse heating. For Mars to have been warm enough in the past to support liquid water on the surface, hundreds of millibars of CO₂ would have been required, and it is believed that most of this CO₂ has been lost to space.

To understand the past, we must first understand the present.

My research focuses on the processes behind the loss of the Martian atmosphere. One main process is photochemical escape. CO₂ is broken up into the constituent C and O atoms by solar UV radiation, and these C and O fragments can then escape into space if they have sufficient energy. Through my research, I have found, among other results:

• The newly-discovered CO₂ photodissociation into C and O₂ is a major contributor to the escape of C atoms from the Mars atmosphere today, implying that C escape rates today are higher than what was previously expected.
• The Martian orbit around the Sun is quite eccentric, resulting in a seasonal variation of 45% in solar radiation received. This translates to a 70% variation in the C escape rate.
• The Extreme UV radiation (λ < 124 nm) from the Sun can increase by 300% at high solar activity, driving a 135% increase in C escape.

While these results apply to atmospheric loss from modern Mars, our characterization and understanding of the various loss processes form the foundation for further studies investigating atmospheric escape from ancient Mars, which had a thicker and warmer atmosphere, and orbited a younger Sun that emitted more intense UV radiation.

2. Monitoring the Martian Atmosphere

Since its arrival at Mars in 2014, the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft has been providing us with an unprecedented view of Mars through its diverse suite of instruments. Among the instruments, the Imaging UltraViolet Spectrograph (IUVS) plays the role of the "official photographer" for the mission, snapping pictures in the UV of the atmosphere, the surface, and even the moons of Mars. My role in the team is to process and analyze these images to study the structure of the Martian atmosphere and the various processes that occur today.

While other orbiters at Mars have Sun-synchronous orbits that allow them to observe Mars at only two particular local times of the Martian day, MAVEN has a non-Sun-synchronous orbit, giving us a unique opportunity to study how various atmospheric phenomena change over the Martian day. By observing the UV radiation emitted by the atmosphere through IUVS, we are able to determine the temperature and composition of the atmosphere at various altitudes. UV images taken in the day and night tell us different information. In the day, the CO₂⁺ UV doublet emission at 298–299 nm tells us the density of CO₂ and the atmospheric temperature. We found regular fluctuations in atmospheric density and temperature over a Martian day due to thermal tides, in response to the diurnal heating by the Sun. A bright reflection of sunlight would be a cloud, possibly formed from a low temperature layer in the atmosphere, or the cooling of an air parcel when it is pushed up one of the tall mountains on the Tharsis plateau. At night, nitric oxide (NO) emissions allow us to trace atmospheric circulation.