Mars’ Atmospheric Escape:

         

                                 

         

Noora Alsaeed, Bruce Jakosky, and Marek Slipski

                                                                                                                                                                                                                                     

         

                                   

                                                       

Evidence of past liquid water suggests that Mars’ atmosphere must have been thicker. The thin atmosphere we see today is thus thought to be a result of atmospheric escape to space. Sputtering, one form of possible escape, is predominantly reliant on the upper layers of the atmosphere, specifically the homopause, the boundary below which the atmosphere is well mixed, and the exobase, the boundary above which molecules are free to escape to space. Our focus is to understand the variability in these layers using values extracted and derived from the MAVEN probe.

Variability in the Upper Atmosphere

Background

 

Future Work

Our approach to studying the variability can be split into: a) short term variability b) long term variability

Acknowledgments Much gratitude to the Mohammed bin Rashid Space Center for funding my REU experience, and to LASP’s Marty and Erin for organizing and making this research experience possible. Special thanks to Bruce and Marek for guiding me and encouraging me to explore my own questions.

__________________References___________________

*Offermann, D., Jarisch, M., Schmidt, H., Oberheide, J., Grossmann, K. U., Gusev, O., ... Mlynczak, M. G. (2007). The "wave turbopause". Journal of Atmospheric and Solar-Terrestrial Physics, 69(17-18), 2139-2158. DOI:10.1016/j.jastp.2007.05.012

As shown in the previous plots, the derived homopause altitude can sometimes vary by up to 40 km from one orbit to the next.

i) Geographical Causes? Since longitude is the only parameter that is varying greatly from one orbit to the next, it could be causing the orbit-to-orbit variations. For a limited range of solar zenith angles where the derived homopause (hp) altitudes were almost the same, we find the mean hp and make the following plot versus longitude:

We have extracted the different location parameters for each orbit. These include: solar zenith angle (SZA), local solar time (LST), latitude, longitude, and periapse altitude. All of which change linearly from one orbit to the next, with the exception of longitude.

Repeating this for other ranges of SZA, we find that there is no obvious ties of variability to a specific geographic location.

ii) Wave Structures?

The method we use to derive the homopause altitudes relies on fitting density profiles and extrapolating downward. It is possible that wave structure in the atmosphere could be causing the variability from one orbit to the next, as it would affect the fit. iii) Variance Analysis: Going back to Fig 1a, we can see that there are some regions with low variability compared to others. To investigate what could be driving this, we’ve binned the data by orbit number, grouping ranges with similar variability together. We then compute the variances of these groups and find the mean values for the different location parameters.

iv) Outbound data:

Fig 1a Fig 1b

Analysis

Short term variability

Long term variability

•  Deep dip orbits have lower variability, possibly since wave structure would have less of an effect given more data points

•  There is a general upward trend with solar zenith angle which implies that there is more variability in the night time compared to the day time.

Aside from the orbit to orbit changes in homopause and exobase altitudes, we see long term changes which are evident in Fig1b. The precession of MAVEN’s orbit around Mars means that we cannot discern what is driving these changes very easily.

One interesting trend we noticed is a dip in the homopause altitudes at mid-latitudes. This region coincides with the anomalous region marked in red in Figure 2 above. Observations and measurements of Earth’s turbopause1 done by Offerman et al reveal a similar trend with latitude. We still do not have a good idea on what physical phenomena might be causing this trend.

1 the turbopause and homopause are essentially the same boundary, they differ only in method of measurement.

Fig 3

There is a general tendency for higher altitudes at lower solar zenith angles (SZA). This is consistent with what we would expect since a lower SZA means more direct sunlight and thus higher temperatures which would expand the atmosphere and push the homopause and exobase up.

So far we’ve only been using i n b o u n d d a t a t o d e r i v e t h e homopause altitudes, so we redid the derivation using outbound data. Although over all they seem to be following the same trend, we can see differences for smaller sections. This might indicate that the variability is driven by changes in the atmosphere rather than the location parameters.

Fig 3* Wave turbopause (homopause) vs latitude on Earth

Conclusions •  The short term variability is likely is due to a combination of

wave structure in the atmosphere (both as real geophysical variations and as it affects our extrapolation) and geographical variations that are sampled during a single orbit.

•  For long term variability we can see that it relies on one or

more of the location parameters though its still difficult to disentangle the direct effects of each one of the parameters.

•  There needs to be more work done to verify if indeed wave structure is causing the orbit to orbit variability we see. This includes communication with other groups such as Terada et al. who are working on measuring wave amplitudes in the atmosphere using MAVEN data.

•  Further work on disentangling the effects of each parameter

such as SZA and latitude would be needed to have a better idea. This would include communication with Offerman et al and other atmospheric scientists.

Fig 1b

Fig 2