Asteroid (3200) Phaethon is an active near-Earth asteroid and the parent body of the Geminid Meteor Shower. Because of its small perihelion distance, Phaethon's surface reaches temperatures sufficient to destabilize hydrated materials. We conducted rotationally resolved spectroscopic observations of this asteroid, mostly covering the northern hemisphere and the equatorial region, beyond 2.5-µm to search for evidence of hydration on its surface. Here we show that the observed part of Phaethon does not exhibit the 3-µm hydrated mineral absorption (within 2σ). These observations suggest that Phaethon's modern activity is not due to volatile sublimation or devolatilization of phyllosilicates on its surface. It is possible that the observed part of Phaethon was originally hydrated and has since lost volatiles from its surface via dehydration, supporting its connection to the Pallas family, or it was formed from anhydrous material.

Department of Astronomy and Planetary Sciences Northern Arizona University Flagstaff AZ 86011 USA

Institute of Astronomy Charles University CZ 18000 Prague 8 Czech Republic

JETS ARES NASA Johnson Space Center Houston TX 77058 3696 USA

Johns Hopkins University Applied Physics Laboratory Laurel MD 20273 USA

Lunar and Planetary Laboratory University of Arizona Tucson AZ 85721 0092 USA

Planetary Exploration Research Center Chiba Institute of Technology Narashino Japan

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