Clearing the Air: Solar System Bodies as Windows into the Impact of Aerosols on Exoplanet Atmospheric Retrievals

Introduction: As we enter the next chapter in our characterization of exoplanet atmospheres with state-of-the-art telescopes such as the James Webb Space Telescope, the efficacy of our atmospheric retrieval pipelines is more important than ever. At present, these models share a major challenge: the parameterizations of aerosols such as clouds, dusts, and hazes. Aerosols are ubiquitous in the atmospheres of both Solar System bodies and exoplanets, but, by necessity, must be heavily simplified in exoplanet inference models. Understanding the impact of these aerosols on atmospheric spectra is key to deriving accurate compositional information from exoplanet atmospheric retrievals. Fortunately, we have the opportunity to use pre-existing Solar System observations to validate and improve exoplanet-focused approaches to representing aerosol structures. We derive aerosol profiles from occultation data of Solar System worlds with known atmospheric composition, such as Mars and Titan. These profiles provide an opportunity for ground-truth verification of exoplanet atmospheric characterization tools and allow us to improve our retrieval pipelines. 

We will be presenting aerosol profiles derived from occultation observations of Mars and Titan, as well as comparisons of these with parameterizations of aerosols in various exoplanet atmospheric retrieval models. We aim to understand if simplified model representations produce results that resemble real clouds and hazes and, if not, where we can improve, as well as determine what impact these simplifications have on retrievals.

Methods: This work involves two major stages. The first is to use the large collection of pre-existing Solar System occultation observations to create an empirically-driven database of aerosol structures. In the second stage, we  will use this ground truth to explore parameterizations of aerosols in exoplanet atmospheres and validate approaches to representing clouds/hazes in models.

For the first stage, our highest priorities are Mars and Titan, and we began with Mars. The Mars Atmosphere and Volatile Evolution (MAVEN) mission's Imaging Ultraviolet Spectrograph (IUVS) has taken 1719 occultation observations of Mars. The data are readily available in the Planetary Data System (PDS) in a derived format, which provides the aerosol optical depths at 1000 nm. There have been 48 occultation campaigns since the beginning of the mission, with campaigns occurring approximately every two months and each campaign consisting of order 10-100 individual occultation observations.

Concurrent with our analysis of the MAVEN IUVS data, we have also begun to explore occultation observations of Titan from Cassini’s UVIS instrument. Additionally, we are compiling exoplanet cloud parameterizations from different atmospheric retrieval pipelines which we will compare to our aerosol profiles.

World	Mission	Instrument	Band (μm)	Resolution	Date Range	No.
Venus	Venus Exp.	SPICAV-SOIR	2.3 – 4.2	0.2 cm−1	2007 – 2013	337
Earth	SCISAT -1	ACE-FTS	2 – 100	0.0025 cm−1	2004 –	10k+
Mars	MAVEN	IUVS	0.18 – 0.34	400 (λ/∆λ)	2015–	1719
Mars	TGO	NOMAD	0.2 – 4.3	> 0.15 cm−1	2018 –	10k+
Saturn	Cassini	VIMS-IR	0.85 – 5.1	16.6 nm	2005 – 2017	172
Saturn	Cassini	UVIS	0.11 – 0.19	0.28 nm	2006 – 2016	101
Titan	Cassini	VIMS-IR	0.85 – 5.1	16.6 nm	2004 – 2016	38
Titan	Cassini	UVIS	0.11 – 0.19	0.28 nm	2006 – 2016	15
Pluto	New Hor.	Alice	0.052 – 0.19	0.3–0.6 nm	2015	2

Table 1: A selection of the mission data being considered in this work.

Results: We have derived slant aerosol profiles for 45 Maven IUVS occultation campaigns, two of which are shown below.

Figure 1: Aerosol profiles from MAVEN’s campaign 24, taken from 9/12/2018-9/13/2018 (left) and 30, taken from 11/11/2019-11/12/2019 (right).

These plots extend to an altitude of 90 km, corresponding to the MUV range of the IUVS instrument. Aerosol extinction above this altitude, corresponding to the FUV range of the instrument, is unable to be confidently distinguished from the CO2 signal, so we have not included this range. The data become noisy around 60 km due to the transition from the MUV channel to the FUV channel. These profiles show a consistent structure of  higher optical depths at low altitudes, in accordance with opacity due to dust at low altitudes on Mars.

Future Work: Our next step will be to construct aerosol profiles for Titan with Cassini’s UVIS occultation observations. We will investigate aerosol parameterizations in a variety of exoplanet models and retrieval pipelines to compare to the Martian aerosol profiles as well as those we derive for Titan. We will then continue compiling aerosol profiles using occultation data of Venus, Earth, Saturn and Pluto to expand our catalog of ground-truth calibrations. We will subsequently compare these to the exoplanet parameterizations, with the ultimate goal of improving the efficacy of retrieval pipelines and deriving more accurate atmospheric composition information from transit observations of exoplanet atmospheres.