Non PSI Personnel: Georgina Miles (Co-Investigator, Southwest Research Institute), Francis Nimmo (Co-Investigator, University of California, Santa Cruz), John Spencer (Co-Investigator, Southwest Research Institute)
Project Description
In 2005 data returned by the Cassini spacecraft showed that Enceladus’ southern polar region is highly active. The activity is concentrated along four fractures located close to Enceladus’ south pole commonly known as tiger stripes, which are also the source of Enceladus’ plumes. Accurately constraining the endogenic heat flow from this active region (often call South Polar Terrain, SPT) has proved challenging, and estimates of the SPT total heat flow vary from 5.8 ± 1.9 GW to 15.8± 3.1 GW. The reason for these discrepancies is that the SPT has been in sunlight, and thus its emission is a combination of passive (reradiated sunlight) and endogenic. Isolating the endogenic component is non-trivial and relies on either modeling (and removing) the passive component (as per the 15.8 GW estimate), or only quantifying the endogenic emission greater than passive emission temperatures (as per the 5.8 GW, so this doesn’t include any low temperature, <80 K,
endogenic emission).
Intriguingly another estimate of the heat flow from (only) Enceladus’ tiger stripes estimate theirheat flow to be 4.2 GW. This means that if the total heat flow is close to 15.8 GW then a lot of the heat flow must be coming from somewhere else. The most logical place for this extra emission is the region between the stripes, called interstitial (or funiscular) terrain. Models of how this region formed require high heat flows: 200 and 500 mW m-2 (equivalent to 2.1 and 5.3 GW of emission). This is an enormous amount of heat, which has never been quantified, and has huge implications for Enceladus’ total heat emission rate, and hence long-term thermal and orbital evolution. Quantifying this heat flow is the goal of this proposal.
Since 2009 Enceladus’ SPT has been in winter, which is important in this study for two reasons:
1) it reduces the magnitude of the passive emission and
2) it reduces the difference in passive emission predicted by thermophysical models using a variety of feasible surface parameters.
This results in the uncertainty of the passive emission prediction dramatically reducing, from ~4.0 GW in fall, to 0.6 GW in winter. During winter and late-fall Cassini’s Composite Infrared Spectrometer (CIRS) took 5 observations of the SPT at a spatial resolution high-enough (<40 km) to put its fields of view between the stripes. These data, we propose to analyze, provide a “game-changing” opportunity to measure Enceladus’ heat flow, as the passive emission in them is minimized.
We performed a preliminary study to compare the surface temperatures derived from one swath from one of these CIRS observations (taken in 2015, chosen because it covered all four tiger stripes) to that predicted by a small number of passive models. The results show the observed temperatures are much greater than ones the models predict. Depending on the model, heat flows between 279±48 to 380±36 mW m-2 are required to explain the difference (2.9±0.5 to 4.0±0.4 GW). These are huge heat flows (!), but are well within those predicted by terrain formation models, and would explain the discrepancy between the different endogenic emissions determined. These results show Enceladus’ interstitial region have endogenic emission and more detailed analysis is required to constrain it.
The work we propose will provide the first estimate of the heat flow from Enceladus interstitial regions (and arguably the most accurate estimate of the heat flow from the tiger stripes too), by using all the CIRS FP1 data with high enough spatial resolution to fit between the stripes (13 swaths), and by increasing the scope of our passive modeling effort to ensure that possible parameter space is explored. The analysis is based upon work already conducted (e.g. Spencer et al., 2006; Howett et al., 2011; Spencer et al., 2018), and directly contributes to the goals CDAP, by seeking to better understand the history and evolution of Enceladus’ activity using Cassini data.
All data we use are in the PDS.