David Polishook - Research Projects


A Martian Origin for the Mars Trojan Asteroids

   We show evidence that the progenitor of the Trojan Eureka cluster of Mars could have originated as impact debris excavated from the Martian mantle. We present new near-infrared observations of two Trojans (311999 2007 NS2 and 385250 2001 DH47) and find that both exhibit an olivine-rich reflectance spectrum similar to Eureka's. Olivine-rich reflectance spectra are rare amongst asteroids but are seen around the largest basins on Mars. They are also consistent with some Martian meteorites (e.g. Chassigny), and with the material comprising much of the Martian mantle. Using numerical simulations, we show that the Mars Trojans are more likely to be impact ejecta from Mars than captured olivine-rich asteroids transported from the main belt. For the first time, this result links directly specific asteroids to debris from the forming planets.


Image: An artist impression of a piece of Martian impact ejecta, that will be caught as a Mars Trojans object, and disrupt into a cluster of bodies.

Polishook et al. 2017, New Astronomy.




Shape modeling to constrain asteroid structure

   In the eyepiece of the telescope, asteroids are no more than a point of light. But in order to reveal their internal parameters (such as density, structure, cohesion) and the physical processes acting on them, their geoid shapes must be exposed. To this end, I derive asteroid shape models using inverse algorithms that match the variability in their brightness, at different geometric locations, to a projected asymmetric geoid.

   Using the derived shape models, we calculate the gravity field on the asteroid surface in order to compute the slope distribution of disintegrated asteroids. This allows us to estimate the rotation period at which the disintegration occurs, by that constraining the cohesive forces between the body components. Moreover, we are able to point to specific places on the surface where it is likely the asteroids would disrupt, opening a path to study their geo-mechanical behavior.


Image: shape models of an asteroid and its ejected component, with the slope distribution on its surface.

Polishook & Aharonson in preparations.

Polishook et al. 2016. Icarus 267 - A large asteroid beyond the rubble-pile spin barrier - a case for cohesion.

Binzel et al. including Polishook 2015. Icarus 256 - Spectral slope variations for OSIRIS-REx target Asteroid (101955) Bennu: Possible evidence for a fine-grained regolith equatorial ridge.






Flybys of Near-Earth Asteroids

   Applying tidal forces, the Earth can modify asteroids that pass very close to it: the asteroid can spin-up, shakes can roll boulders and rocks, sub-surface material can be exposed, and its entire shape can change.

   Only by observing asteroids before and after their flybys, and measure the change (or non-change) in their parameters we can study the strength and elasticity of the internal structure. This allows us to predict what will happen to a future impactor before colliding with the Earth and what measures should be taken in order to destroy or deflect it.


Image: The changed orbit of 2012 DA14, during its flyby on February 15, 2013.


Moskovitz et al. in preparation (cool results from the 2012 DA14's flyby).

Thirouin et al. including Polishook 2016. AJ 152 - First photometric results from the Mission Accessible Near-Earth Objects Survey.

Polishook et al. 2012. Icarus 221 - Spectral and spin parameters of two Earth-grazing near-Earth asteroids.






Asteroids disintegration by rotational fission

   The newly discovered category of asteroid pairs consists of gravitationally unbound pairs that once belonged to a single body. Studies showed that asteroid pairs' progenitors were spun-up by the YORP effect, until they gain sufficient angular momentum to cross the breakup limit for a strength-less object, known as the 'rubble pile spin barrier', and these asteroids split into two components.

   The study of asteroid pairs and the way asteroids can disintegrate has manifold applications: while it teaches us on their internal structure, it can also demonstrate the strong forces that shaped the early solar system at the early stages of planet formation.


Image: Rotation periods of asteroid pairs is correlated to the size ratio between the small and large members of each pair.


Polishook 2014. Icarus 241 - Spin axes of asteroid pairs were modified by the YORP effect suggesting the YORP effect spin them up and not collisions. Also - the first linkage found that asteroid pairs have low density values as expected from 'rubble-pile' asteroids.

Polishook et al. 2014. Icarus 233 - The secondary member of a pair might split due to  a secondary fission.

Pravec, Vokrouhlicky, Polishook, et al. 2010. Nature 466, and its Press Release - Asteroid pairs are formed by the rotational-fission mechanism.

Polishook et al. 2011. Icarus 212 - Binary asteroids with high separation were formed by the YORP effect.

Vokrouhlicky, Durech, Polishook, et al., 2011, AJ 142 - The spin state of the youngest asteroid pair 6070 Rheinland.



Thermal forces modify asteroids

   Re-emission of sunlight from atmosphere-less bodies can modify their orbit (the Yarkovsky effect), impose a torque on their spin, and change their rotational axis. This mechanism has significant role in transporting small bodies in the Solar System, forming near-Earth asteroids; splitting fractured bodies by spinning them up; determining the special and size distribution of small bodies in the Solar System through its history.

We study the parameters relevant for this mechanism, such as size and spin, and its effects on asteroids such as spin distribution and formation of binaries.


Image: The effect of asteroid's spin on the YarkovskyŐs efficiency.


Moskovitz, Polishook et al. 2017. Icarus 284 - Aspect-dependent variability of thermal emission from near-Earth asteroids.

Scheirich et al. including Polishook, 2015. Icarus 245 - An observational constraint on the orbital evolution due to the thermal BYORP effect of near-Earth asteroid (175706) 1996 FG3.

Durech et al. including Polishook. 2012. A&A 547 - Constraints on the YORP effects of three asteroids.

Polishook et al. 2011. Icarus 212 - Binary asteroids with high separation were formed by the YORP effect.

Polishook et al. 2010. DPS meeting #42, p. 1055 - Yarkovsky effect dependent on asteroid's spin.

Polishook & Brosch 2009. Icarus 199 - Spin distribution of small main-belt asteroids is controlled by the YORP effect.





Space Weathering

   Surfaces of atmosphere-less bodies are modified with time by the 'space weathering' effect. This mechanism, caused by solar wind, cosmic rays and micrometeorite bombardment alter the top layer on planetary surfaces, causing it to display a 'weathered', darker and redder reflectance spectrum.

Space weathering mechanism is not yet understood: different bodies and materials present different amount of weathering, and current estimations of its timescale differ dramatically from one another, and range between 104 to 109 years.

The effect of space weathering on asteroids obscures their true nature. Understanding it will help us determine their true composition, origins, and their role in planet formation.


Image: lunar sample that shows the formation of a 'weathered' coating on the surface. Clark et al. 2002 Asteroids III, 585-599.


Polishook et al. 2014. Icarus 243 - No spectral variation on asteroid pairs.

Polishook et al. 2014. Icarus 233 - Fresh Surfaces Observed in the Main Belt on asteroid pairs!

Polishook et al. 2009. M&PS 44 - Looking for fresh surfaces by rotational spectroscopy.



Mining Astronomy

   Asteroidal data were 'mined' from the Palomar Transients Factory (PTF), a survey with exceptionally wide field of view (7.2 square degrees) on a 48'' telescope, dedicated for transient search. Our pipeline detects asteroids in the PTF images, constructs their lightcurves, and calculates their rotation periods. The pipeline was tested on data from four nights covering an area of ~21 deg2, and was able to detect 624 asteroids, of which 145 were previously unknown. Rotation periods for 173 asteroids were derived. 3 of the asteroids are probably binary asteroids. We estimate that implementing our search for all existing high-cadence PTF data will provide rotation periods for thousands of asteroids.


Image: Asteroids tracks on the PTF's large field of view.


Waszczak et al. including Polishook 2013. MNRAS 433 - A search for extended main-belt comets in the PTF archive.

Polishook et al., 2012. MNRAS 421 - Asteroids' and their rotation periods from the PTF archive.