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All posts for the month August, 2014

Automated pitted cone identification experiment

Posted by admin on August 24, 2014
Posted in: Data Science.
A field of pitted cones in Utopia Planitia on Mars, as observed by the HiRISE instrument onboard the Mars Reconnaissance Orbiter. Taken from http://beautifulmars.tumblr.com/post/82817530060/field-of-cones-in-utopia-planitia.

A field of pitted cones in Utopia Planitia on Mars, as observed by the HiRISE instrument onboard the Mars Reconnaissance Orbiter. Taken from http://beautifulmars.tumblr.com/post/82817530060/field-of-cones-in-utopia-planitia.

A visit from a planetary sciences colleague this weekend got me excited again about automated feature identification for remote-sensing imagery. This colleague and I talked about her work on Martian pitted cones (examples shown at left). These small hills form when a basalt flow passes over volatile (i.e., water) deposit, heating it and producing a steam explosion that uplifts the flow into a cone shape with a crater at the apex.

As for the vast majority of geomorphological studies, pitted cones on Mars are identified by groups of dedicated researchers, sifting by hand through hundreds or thousands of high-resolution images. If, instead, identification could be automated, it would help realize dramatic savings in person-hours and probably significantly increase the number of known features. It could also mitigate potential observational biases introduced by human image processing.

As an experiment, I applied algorithms from the scikit-image python package to find pitted cones in the example image shown at left. Fortunately, the documentation already provides a good example of circular feature detection.

So modifying the example code, I applied it to a small portion of the example image, and the figure below shows the results. Here I’m just showing the top ten most strongly detected circular features.

Unfortunately, the algorithm is not perfect — some of the obvious cones were not picked out, and others were highlighted more than once. But overall, not terrible, considering it took me about an hour and a half to implement (which included upgrading to Python 3 via the Anaconda package because scikit-image wouldn’t work with the Enthought Python version 2.7 — this upgrade will probably adversely affect my ability to use astroml, by the way).

Possible ways to improve things:

  1. The example code only returns the top two most strongly detected features for each circle radius it tries; this restriction would be simple to remove.
  2. The fact that the pitted cones are cone-shaped means you could require that any putative crater be framed by an appropriately shaped shadow. Regardless of the cone’s actual height (probably unknown anyway), the shadow on the cone should darken as the solar incident angle approaches 180 degrees, modulo any nearby morphological features (as evident in the figure below).
My attempt to automatically identify pitted cones using scikit-image.

My attempt to automatically identify pitted cones using scikit-image.

Valsecchi+ (2014) — “From Hot Jupiters to Super-Earths Via Roche Lobe Overflow”

Posted by admin on August 21, 2014
Posted in: Publications.
Artist's conception of tidal disruption of a gas giant planet.

Artist’s conception of tidal disruption of a gas giant planet.

Neat paper today from Francesca Valsecchi and colleagues at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics. They looked at the final fates of gas giant planets that wander too close to their host stars, sometime called “hot Jupiters”.

This unexpected but apparently fairly common class of planet consists of massive planets made mostly of hydrogen and helium, like Jupiter, but, unlike Jupiter, these planets orbit tens or even hundreds of times closer to their host stars than the Earth orbits the Sun. Consequently, many of these hot Jupiters are fated to spiral closer and closer to their stars, eventually getting so close that their host stars’ gravity rips them apart, drawing their atmospheres into thin accretion disk around the star. This process is called “Roche lobe overflow” (RLO).

Once the progenitor hot Jupiter has lost its atmosphere, what’s left behind is probably the rocky/icy core at its center, and following up studies by other groups, Valsecchi and colleagues point out that RLO of hot Jupiters may help explain the puzzling presence of small rocky planets also found orbiting very close to their host stars. These little bodies may indeed be the skeletal remnants of unspeakable astrophysical violence.

MacKenzie+ (2014) — “Evidence of Titan’s Climate History from Evaporite Distribution”

Posted by admin on August 15, 2014
Posted in: Journal Club.
A false-color, infrared map of Titan’s north pole. The black arrow points to an evaporite deposit along the shore of Ligeia Mare.

A false-color, infrared map of Titan’s north pole. The black arrow points to an evaporite deposit along the shore of Ligeia Mare.

I read a recent paper by MacKenzie et al. (2014) about evaporite deposits along the rims of lakes on Saturn’s moon Titan.

The more we learn about Saturn’s enigmatic moon Titan, the more it resembles the Earth: Titan has a thick atmosphere made mostly of nitrogen and a complex “hydrologic” cycle involving big storms, river beds, and even lakes and seas. But, because Titan’s surface temperatures are nearly -300 degrees F, unlike on Earth, the liquids involved in the cycle are methane and ethane.

And recently, observations from the Cassini spacecraft in orbit around Saturn have found evidence for what may be evaporite deposits along the rims of many of Titan’s seas and lakes. Evaporite deposits typically form on Earth when water collects in isolated basins and slowly evaporates away, leaving behind the minerals dissolved in the water. They are common throughout the Southwest in the US.

In this recent paper, MacKenzie and colleagues conducted a thorough mapping of the putative evaporite deposits on Titan to understand their extent and possible connections to climate. The study presents lots of interesting results: in particular, they find evaporites occur at a variety of latitudes, including in places that look a lot like dry lake beds. This result corroborates the suggestion that these places were indeed filled with liquid in the past but climatic changes have since dried them out.

Surprisingly, even though there are many apparent dry lake beds near the south pole on Titan, MacKenzie and colleagues find no evidence for evaporites there. They speculate either the deposits were laid down long ago (> 50,000 years ago) and have since been buried OR conditions were never suitable for evaporite formation, even when the south polar lake beds were filled. Either result could be telling us something very interesting about evolution of Titan’s climate.