SHARAD: Delving Deep at Mars
Posted By Bill Dunford
22-09-2014 9:41 CDT
With MAVEN's arrival at Mars, four active robotic spacecraft are now orbiting the Red Planet (with a fifth set to arrive this week). Most of the instruments deployed in these missions are examining the outer layers of Mars' mysteries, from the upper atmosphere down to the dunes and canyons visible on the surface.
But some go deeper. One of the most important of these is SHARAD (SHAllow-RADar) on board the Mars Reconnaissance Orbiter (MRO). This instrument sends waves that penetrate the surface, return, and are captured by the SHARAD antenna. Rock, sand, and water all return different, recognizable signals. Water, for example, has a very strong radar return. The data can be represented by radargrams that form a picture of subsurface structures to a depth of several hundred meters.
The radargrams reveal surprisingly lovely visual echoes from a world's hidden realms.
SHARAD is a joint Italian-American effort led by the Italian Space Agency (ASI). On the US side, Dr. Roger Phillips is the deputy team leader, working with the Southwest Research Institute and Washington University in St. Louis. I had the chance to ask him a few questions about what it's like to explore Mars using this powerful tool.
What are the best examples of scientific discoveries at Mars that came from or were helped by SHARAD observations?
I will give three examples:
We have nailed the stratigraphy of the north polar ice cap (North Polar Layered Deposits, NPLD). For the NPLD, we have constructed a complete 3-D volume that can be sliced in any direction to look at internal structure, have constrained the NPLD to be no older than about 5 million years (as predicted by climate models), have found buried canyons and possibly buried impact craters, and have solved the problem of the origin of the mysterious spiral troughs.
At mid-latitudes on Mars there exist features known as a lobate debris aprons (LDA). The surfaces of these features show evidence of flow. However, it had been unclear for any specific LDA as to whether the subsurface volume is mostly pure ice beneath a veneer of rocks (i.e., a debris-covered glacier) or instead is mostly rock, but lubricated by ice in void spaces (i.e., a rock glacier). SHARAD showed without a doubt that for many LDAs, the material is nearly pure ice, representing the last gasp of ice deposited at times of high obliquity (rotation axis tilt).
Finally, SHARAD discovered a massive deposit of CO2 ice buried in the south polar layered deposits (SPLD). At the time we published our result (2011), we thought the buried mass almost equaled the mass of the present atmosphere. Further work has now revealed that the buried CO2 mass very likely exceeds the present atmospheric mass, and, furthermore, wherever the top of the deposit is exposed at the surface there are obvious sublimation features seen in HiRISE and CTX images. At high obliquities, a substantial fraction of the CO2 deposit sublimates into the atmosphere. The increased surface pressure would lead to a greater persistence of liquid water at the surface (though ultimately water must change phase). This result may have big implications for the more recent water-sourced features on Mars, and SHARAD may help in unraveling the existing puzzle about how they form. Stay tuned.
What's the most surprising observation or finding so far?
I would say that it is the buried CO2 deposit.
In second place is the lack of downward deflection of the surface beneath the mass of the NPLD. Tthere was a good hint of this in MARSIS data (MARSIS is the sounding radar on board the Mars Express orbiter), but it took the better vertical resolution of SHARAD to really confirm this and provide tight quantitative constraints on the result. The surface doesn’t deflect? So what? Well, so, don’t think about the surface but rather the outer rigid shell of the planet, known as the “lithosphere.” No (or extremely little) deflection of the lithosphere means that it must be very thick, in this case at least 300 km. The thickness is determined by the amount of heat escaping from the interior of Mars. So either Mars has much smaller amounts of heat-producing elements than anybody had thought or the heat coming out of the interior is quite variable from place to pace on Mars. The amount of heat-producing elements is a major control on tectonics and volcanism on Mars, as well volatile release from the planet’s interior.
Can you give me an example of an observation that was particularly challenging to obtain? What were the difficulties and how did you overcome them?
Observations by themselves are not difficult to obtain. Trying to tie together multiple observations into a 3-D volume is extremely challenging. Each SHARAD radargram gives a two-dimensional profile of along-track distance and time delay showing the surface and subsurface reflectors that are nominally directly beneath the spacecraft (i.e., at nadir). But surface returns off to the side of the ground track (known as "clutter") arrive with time delays competing with subsurface reflectors. We routinely use simulations of surface clutter made from digital elevation models (DEMs) to distinguish between real subsurface reflectors and the clutter. However, off-nadir subsurface reflectors also appear in the radargrams but not in the simulations, and thus can be quite a nuisance. But there is lemonade to be made from lemons here. The solution is to consider multiple radargrams simultaneously and invert for the correct 3-D position of the surface and subsurface reflectors. While this is a significant problem computationally, the real challenge, since you asked, is to get the timing exactly right between the different radargrams. Subtle errors in timing, such as variable delays in the radar signal as it passes through the changing Martian ionosphere, do not greatly affect 2-D radargram interpretation but wreak havoc when trying to assemble a 3-D volume. We have overcome this problem by digging very deeply into the subtlest aspects of the data processing and have now produced the first viable 3-D model of the NPLD. Nathaniel Putzig presented this work at the recent Mars conference and it generated considerable excitement in the Mars polar bear community; one unexpected result was the appearance in 3D of what may be impact craters buried within the ice.
Are there any firsts with this instrument in terms of technology or capabilities as opposed to what has flown on other missions?
There have been four radar sounders in orbit about bodies other than the Earth. The first was the Apollo Lunar Sounder Experiment flown on Apollo 17, the final mission of that program. SHARAD is the most recent. The design of that radar was rather straightforward and it of course had to deal with the usual constraints of weight, power and volume imposed on any instrument on a planetary spacecraft. SHARAD really benefits from the high data rate capability of MRO, which allows dense spatial coverage and innovative data processing on the ground (e.g., 3-D volumes).
Does your instrument have any role to play in the Siding Spring observation campaign?
As Mars passes through the coma of Siding Spring, the electron density of Mars’ ionosphere should be perturbed. In another lemons-in-to-lemonade fable, the correction to the ionospheric distortion of the radar signal yields the vertically integrated total electron content (TEC) of the ionosphere in the vicinity of the MRO spacecraft. We plan to monitor the TEC before, during, and after Siding Springs’ passage of Mars. Our measurements will mostly be on the nightside to avoid the overwhelming effects of the solar-generated ionosphere.
Are there other upcoming events in the next year or so that are especially interesting?
Perhaps the most interesting upcoming event is summer at the South Pole of Mars, when it is fully illuminated and frost-free. This is not so much a SHARAD event, but more of an opportunity for HiRISE (high-resolution camera) and CRISM (imaging spectrometer), in addition to their own specific interests, to provide the kinds of data that we need to further our understanding of the massive CO2 deposit that SHARAD discovered. SHARAD acquires subsurface data essentially from the nadir direction only and because of the inclination of the MRO orbit, radar data from 87°S to the pole is not obtained. Yet we know from HiRISE and CTX (context camera) images that the CO2 deposit extends to the South Pole, and our volume estimate for the SHARAD exclusion zone is based on a very conservative extrapolation into that region. Thus our estimate of the CO2 mass is a conservative lower bound and, who knows, there could be twice as much CO2 buried in the total deposit as there is in the present atmosphere. Enter HiRISE. When MRO is in a rolled position, HiRISE can acquire images poleward of 87°S. We are looking to HiRISE to provide better thickness estimates of the CO2 deposit where it is exposed in troughs by acquiring stereo pairs and constructing DEMs. Half a dozen DEMs would go a long way towards pinning down the volume of the CO2 deposit, a number that will undoubtedly increase from our current estimate. We are also anticipating a CRISM campaign focusing on the CO2 deposit where it exposed at the surface. Current data indicate that the deposit has a thin surface veneer of a dust-water-ice mixture, and the stability of the CO2 depends on the relative fractions of the dust and water ice in the veneer. We expect CRISM data from this campaign to help us understand the composition of this thin deposit and thus the stability of the underlying CO2.
Does your team get a say in spacecraft targeting, or is it strictly ride-along?
We are very actively involved in spacecraft targeting. Targeting requests pour into the SHARAD operations center at Southwest Research Institute in Boulder, Colorado. This information is prioritized and passed on to SHARAD operations in Rome, where formal command files are generated and passed on to the MRO project at JPL for eventual upload to the spacecraft. Targeting is often a complicated procedure because there are multiple acquisition environments for SHARAD that have continuously changed throughout the mission.
What areas are you or the science community still hoping that SHARAD can investigate further? In other words, are there certain places on the planet or certain kinds of features that are calling out for more imaging than what you've been able to get to so far?
We have ambitious plans for the third extended mission of MRO. In addition to work on the SPLD (including the search for new CO2 deposits), we will carry out an intense campaign for the LDA deposits. They are not all the same, in terms of both the SHARAD signals and the nature of their surfaces. Like the SPLD efforts, this will be a multi-instrument activity involving SHARAD, HiRISE, CRISM, and CTX. We plan to test two end-member hypotheses for their origin: (i) The current ice is a remnant of a previously thicker and more extensive regional ice sheet. (ii) The current ice represents the maximum extent of advancing and coalescing individual glaciers. Finally, we've recently discovered subsurface reflectors in the Valles Marineris region, and we will follow up with more observations. These SHARAD observations may have implications for the deposition and stability of volatiles at equatorial latitudes.
Any major disappointments in terms of what you’ve been able to see or do with the instrument?
My favorite major disappointment is the failure to detect subsurface reflectors in the Meridiani Planum region. Given that this area is extremely flat with little surface clutter and that there is a well-documented stratigraphy in this region (thanks to the Opportunity rover and intense scrutiny from overhead), we expected a plethora of subsurface reflectors. But despite an intense observing campaign, we found nothing, zilch, zip, nada. Every once in a while I say: “Gee, maybe we ought to try Meridiani again with a different set up on the radar or with newer processing techniques.” And somebody else says: “Yeah, right.”
Are there lessons from the work with this instrument that could be important in exploring Europa or other targets in the future?
Certainly people have thought about radar sounders elsewhere in the solar system. Sending one to Europa has been contemplated for nearly 20 years. Here are the takeaways from SHARAD: Go for all the power you can possibly get, whether it is from the transmitter, range and Doppler compression, pre-summing, etc. You will NEVER have enough effective power. You will need good DEMs (digital elevation models) to deal with surface clutter; they need to be at better resolution than the radar. At Gale crater, we were fooled with MOLA (laser altimeter) DEMs, but became un-fooled with higher-resolution HRSC DEMs.
Anything else you'd like to mention or make people aware of? Are there common misconceptions about MRO or observing Mars among space enthusiasts that you think could be cleared up?
Well, my rant is going to be about how unloved the orbiters seem to be relative to the landers. They should be equally loved. MGS showed us that Mars is a sedimentary planet in addition to being an igneous planet. Mike Malin’s MOC camera first showed us that Gale crater has a sedimentary mound (now known as Mount Sharp) in the center that is higher than the crater rim. How did that happen? What a scientifically neat place that would be to visit! Then came three more orbiters, culminating in MRO. That mission has provided immense detail about the red planet and has opened up yet another new Mars to behold.
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Want to know more about SHARAD and how it operates? You can download presentations from a workshop held this year in conjunction with the Lunar and Planetary Science Conference.