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Shape of Antarctic ice surface. Credit: BBC News (http://www.bbc.co.uk/news/science-environment-21692423)

Shape of Antarctic ice surface. Credit: BBC News

A detailed analysis of Antarctic ice data measurements comprised 50 years of exploration show that the White Continent contains about 26.5 million cubic km of ice. These numbers come out of an international project known as Bedmap2, which is a second attempt to reconstruct the ice thickness map of Antarctica. In recent years, satellite based observations provided us with a very good assessment of the height of the ice surface.

I would like to discuss one method that could be adopted for space exploration and could be used to estimate the thickness of ice mantle on extraterrestrial planets, on Ceres and other bodies in the main asteroid belt, and on Jovian moons.

This is the Passive Radio [frequency] Ice Depth Experiment (PRIDE) from JHU APL (Miller, Schaefer, and Sequeira, 2012). PRIDE is a passive receiver of a naturally occurring signal generated by interactions of deep penetrating Extreme High Energy (EHE) cosmic ray neutrinos( > 10^18 eV).

An illustration of the PRIDE concept (Miller, Schaefer and Sequeira, 2012)

An illustration of the PRIDE concept. A high energy cosmic ray neutrino penetrates the ice at a grazing angle. The neutrino initiates a shower of secondary charged particles (the Askaryan effect, 1962) that emit a conical pulse of Cerenkov radiation that can be detected at satellite altitude. The distributions of the characteristics of detected pulses will indicate the thickness of the ice layer. Figure credit (Miller, Schaefer and Sequeira, 2012)

EHE cosmic ray neutrinos is a product of the interaction of cosmic ray protons with cosmic background radiation. These neutrinos could penetrate deep into the ice and interact with hydrogen nuclei. These interactions produce a shower of particles that generates Cherenkov radiation. This radiation is then detected by the PRIDE instrument.

For the given conditions (water ice, T 50-140 K), the spectrum of emitted radiation peaks at 0.2–2 GHz. This radiation could be detected from orbit.

This concept is especially useful for long missions going to Jupiter moons or to the asteroid belt because it employs a passive remote sensor. The instrument registers event rate, amplitude and direction of each event for EHE cosmic ray neutrinos with energies 10^18 – 10^19 eV. At this energies, the detectable signals with SNR>5 could be registered at satellite altitude 100-500 km and be able to measure ice thickness ranging from 10 to 100 km.

A strawman PRIDE antenna. Credit -Miller, Schaefer, and Sequeira (2012)

a strawman PRIDE antenna array for full 360-degree azimuthal coverage. The two rings of antennas are offset both vertically and horizontally to enable reconstruction of event direction via timing differences in neighboring receivers; b – angular acceptance. Credit -Miller, Schaefer, and Sequeira (2012).

The antenna design is based on commercially available flanged horn antenna. For using on a spacecraft some modifications need to be made to reduce its thickness and also to make it shorter and wider. Ideally, the array of 8 antennas collected in 2 rows is needed for 360-degree azimuthal coverage. This allows:
– measuring both zenith and azimuth reconstructing with 1-2° zenith angle accuracy;
– each event to be registered by two or three antennas.

There are some concerns regarding the noise that could bias the measurements. The receiver temperature is expected to be ~ 140 K, so it will contribute to thermal noise. However, authors expect “that the receiver noise can be reduced so that the local ice will again be the limiting factor”. The estimated signal-to-noise ratio (SNR) due to ice noise is expected to be ~10 at 100K.

Local RF noise from Jupiter environment comes in forms of bursts at frequencies of tens MHz. Also there is a thermal emission from Jupiter and noise due to synchrotron emission from electrons in Jupiter’s magnetosphere. According to authors, synchrotron emission “produces much less noise than the burst or thermal emission sources do at higher and lower frequencies, and matches the same range of 0.2–2 GHz that is optimal for both Cerenkov emission and ice transparency”.

The short radio bursts might also turn false triggers, but they have the spectral characteristics different from Askaryan-Cerenkov bursts.

Because PRIDE is a passive receiver it doesn’t require additional resources such as power, cooling, etc. The instrument also doesn’t have moving parts. Antennas could be placed on the spacecraft body and any open locations. I believe that makes it a perfect instrument candidate for deep-space mission to explore the icy moons of Jupiter, like Jupiter Icy moons Explorer (JUICE). I hope they hear me at ESA.

References:

Antarctic ice volume measured –http://www.bbc.co.uk/news/science-environment-21692423

Askaryan, G.A., 1962. Excess negative charge of an electron–photon shower and its coherent radio emission. Sov. Phys. J. Exp. Theor. Phys. 14, 441–443.

Barwick, S.W. et al., 2006. Constraints on cosmic neutrino fluxes from the ANITA experiment. Phys. Rev. Lett. 96 (171101), 1–4.

Carry, Benoit; et al. (November 2007). “Near-Infrared Mapping and Physical Properties of the Dwarf-Planet Ceres”(PDF). Astronomy & Astrophysics 478 (1): 235–244.arXiv:0711.1152Bibcode 2008A&A…478..235C.doi:10.1051/0004-6361:20078166.

Miller, Schaefer, Sequeira, PRIDE (Passive Radio [frequency] Ice Depth Experiment): An instrument to passively measure ice depth from a Europan orbiter using neutrinos, journal homepage: www.elsevier.com/locate/icarus, 2012

ResearchBlogging.org
Miller, T., Schaefer, R., & Brian Sequeira, H. (2012). PRIDE (Passive Radio [frequency] Ice Depth Experiment): An instrument to passively measure ice depth from a Europan orbiter using neutrinos Icarus, 220 (2), 877-888 DOI: 10.1016/j.icarus.2012.05.028

Varner, G., 2003. Self-Triggered Recorder for Analog Waveforms v. 3 (STRAW3) Data Sheet. Dept. Of Physics, University of Hawaii.